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1. The overall aim was to test whether clofilium has some potential as a positive inotrope for heart failure. We used Wistar Kyoto normotensive rats (WKY) and spontaneously hypertensive rats (SHRs) and studied the effects of clofilium on isolated blood vessels, left ventricular action potentials and left ventricular contractility. 2. Clofilium at < or = 10(-6) M had no effect on WKY portal vein contractions and at < or = 3 x 10(-4) M had no effect on WKY or SHR quiescent mesenteric and intralobar pulmonary arteries. 3. Clofilium at 10(7) - 10(-5) M prolonged the WKY left ventricular action potentials and with 10(-6) and 10(-5)M this included after-depolarizations. 4. Clofilium at < or = 3 x 10(-5) M augmented the peak force, prolonged the contractions and did not cause arrhythmias in the absence and presence of isoprenaline on left ventricle strips from 12-month-old WKY. 5. The 12-month-old SHR has hypertrophy of the left ventricle with reduced peak force and prolongation of relaxation. The effects of clofilium on 12-month-old SHR left ventricle contractility were similar to those in the age-matched WKY. 6 In summary, clofilium has positive inotropic effects on the rat left ventricle that are maintained in hypertrophy. Clofilium does not have effects on blood vessels that would be detrimental in heart failure. Clofilium prolongs the rat left ventricle action potential and causes after-depolarizations. The pro-arrhythmic potential of clofilium, however, makes it unlikely that it could be used as a positive inotrope in the treatment of heart failure.

1. The aim of this study was to determine whether the KA value and fractional occupancy-response relationship for 5-hydroxytryptamine (5-HT) at 5-HT2A-receptors were altered in a rat model of genetic hypertension. Thus, the effects of phenoxybenzamine, an irreversible blocker at 5-HT2A-receptors, on the responses of the aorta from spontaneously hypertensive rats (SHRs) and normotensive rats to 5-HT have been examined. The two strains of normotensive rats used were Wistar Kyoto (WKY) rats and Wistar rats bred in Auckland (WA rats). 2. The sensitivity to 5-HT was increased in aortae from hypertensive rats. The pD2 values for 5-HT during the first challenge were 5.54 +/- 0.08 (14), 5.43 +/- 0.05 (12) and 6.08 +/- 0.04 (12) on the aorta of WKY rats, WA rats, and SHRs, respectively. 3. The affinity for 5-HT was increased in hypertension. Phenoxybenzamine at 2 x 10(-8)M for 30 min caused nonparallel rightward shifts of 5-HT response curves and the KA values were 16.8 x 10(-6)M, 45.6 x 10(-6)M and 4.4 x 10(-6)M on the WKY rat, WA rat, and SHR aorta, respectively. 4. There was a loss of receptor reserve for 5-HT in aortae from hypertensive rats. On the WKY and WA rat aortae, 5-HT caused 50 and 95% maximal responses by occupying 10-20 and 45-60%, whereas on the SHR aorta 5-HT produced 50 and 95% maximal responses by occupying 20-30 and 75-85% of the available 5-HT2A receptors, respectively. 5. The sensitivity to phenylephrine was not altered in hypertension. The mean pD2 values for phenylephrine were 7.14 +/- 0.05 (22) and 7.11 +/- 0.06 (22) on the WKY rat and SHR aorta, respectively. 6. These results show that there is a selective increase in sensitivity to 5-HT on the aorta in a rat model of genetic hypertension. There is also an increase in affinity for 5-HT at the 5-HT2A-receptors and a loss of 5-HT2A-receptor reserve for 5-HT responses on the aorta of SHRs.

1 The effects of racemic cromakalim and the (+)-3R,4S and (-)-3S,4R isomers, BRL 38226 and levcromakalim respectively, on the membrane potential of the rat aorta have been determined. In addition, preliminary studies of the effects of BRL 38226 on the contractility of the rat aorta and portal vein have been made. 2 The rat aorta is hyperpolarized by racemic cromakalim at 10 μm and by levcromakalim at 1 μm, but is depolarized by BRL 38226 at 1 μm. 3 In the presence of depolarization with 20 μm KCl, racemic cromakalim and levcromakalim, but not BRL 38226, reversed the depolarization of the rat aorta. 4 In the presence of hyperpolarization with isoprenaline at 1 μm or in a CaCl2-free Krebs solution, racemic cromakalim and BRL 38226, but not levcromakalim reversed the hyperpolarization. 5 Glibenclarnide at 0.3 and 1 μm depolarized the rat aorta. Pretreatment with glibenclamide at 1 μm prevented the hyperpolarizing action of racemic cromakalim but did not alter the depolarizing action of BRL 38226. Pretreatment with BRL 38226 at 10 μm also prevented the hyperpolarizing action of racemic cromakalim. 6 Contractility studies demonstrated that BRL 38226 and glibenclamide have no effect on the resting tone of the rat aorta in normal Krebs solution. However in a CaCl2-free Krebs solution, BRL 38226 and glibenclamide, 0.1–100 μm, caused small contractions. BRL 38226 and glibenclamide increased the spontaneous contractile activity of the rat portal vein in normal Krebs solution. 7 The present study illustrates that levcromakalim opens the ATP-dependent potassium channels of the rat aorta to cause hyperpolarization. BRL 38226 and glibenclamide produce depolarization and contraction of the rat aorta and increase the contractile activity of the rat portal vein, possibly by blocking the ATP-dependent potassium channels.

1. The aim of the present study was to analyse the inhibitory effects of dihydroergotamine, cyproheptadine and ketanserin on the rat aorta contractile responses to 5-HT. Initially phenoxybenzamine treatment was used to determine whether spare receptors exist at the maximum responses to 5-HT. Then the reversibility of the inhibitory effects of phentolamine, dihydroergotamine, cyproheptadine and ketanserin against the fast and slow response of the rat aorta to 5-HT were determined. 2. Phenoxybenzamine caused non-parallel rightward shifts of the 5-HT concentration-response curves with reduced maximal responses. The KA values for 5-HT to produce fast and slow responses were 2-5 x 10(-5) M. These KA values are much higher than those previously reported for 5-HT at 5-HT2-receptors and suggest that the 5-HT2-receptor of the rat aorta is 'atypical'. The rat aorta has spare 5-HT-receptors for the maximal fast and slow responses to 5-HT. 3. Phentolamine, dihydroergotamine, cyproheptadine and ketanserin inhibited the fast and slow responses to 5-HT. Phentolamine and ketanserin and the higher concentrations of dihydroergotamine and cyproheptadine tested had greater inhibitory effects on the fast than slow 5-HT responses possibly because the fast 5-HT response did not always reach equilibrium in the presence of antagonists. 4. The inhibitory effects on 5-HT responses of phentolamine at 10(-6)-10(-5) M were readily reversible, those of cyproheptadine at 10(-9)-10(-8) M and ketanserin at 10(-8) M were slowly reversible and those of dihydroergotamine at 10(-9)-10(-8) M were irreversible by washing in drug-free Krebs. 5. The inhibitory effects of phentolamine and ketanserin were not altered by increasing the treatment time whereas some of the effects of cyproheptadine and the effects of dihydroergotamine on responses to 5-HT were increased by prolonging the contact time with the antagonist from 75 to 150 min. 6. The present study shows that at the rat aorta 5-HT2-receptor, phentolamine is a competitive readily reversible, cyproheptadine and ketanserin are competitive slowly reversible and dihydroergotamine is a competitive irreversible antagonist.

1. The aim of the study was to test the hypothesis that the offset of action of beta-adrenoceptor antagonists on the heart is related to their lipophilicity, with low and highly lipophilic drugs having a rapid and slow offset, respectively. The effects of beta-blockers with low (atenolol), moderate (celiprolol), high (propranolol) and very high (bopindolol) lipophilicity on the contractile responses of the rat right ventricle to isoprenaline were determined. 2. Atenolol at 10(-6) and 10(-5) M, celiprolol at 10(-7), 10(-6) and 10(-5) M, propranolol at 10(-8), 10(-7) and 10(-6) M and bopindolol at 2 x 10(-9) and 10(-8) M caused parallel rightward shifts of the isoprenaline response curves with no effect on maximum responses. The Schild plots for atenolol, celiprolol and propranolol had slopes that were not significantly different from 1, which is indicative of competitive reversible antagonism. The pKB values were 7.33, 7.78, and 8.79 for atenolol, celiprolol, and propranolol, respectively. The Schild plot for bopindolol had a slope that was significantly greater than 1. 3. Our hypothesis is supported as the effects of propranolol and bopindolol were more slowly offset than those of atenolol and celiprolol. Thus, the concentration-ratio of 141 in the presence of atenolol at 10(-5) M was reduced to 4 after the first wash, whereas the ratio of 100 in the presence of propranolol at 10(-7) M was only reduced to 45 after a similar wash. The ratio of 54 with celiprolol at 10(-6) M was reduced to 5, whereas the ratio of 70 with bopindolol at 10(-8) M was only reduced to 28 by the first wash. 4. The effects of bopindolol were very slowly or not reversible over two washes in the absence or presence of atenolol at 10(-6) M. It is suggested that bopindolol is a very slowly reversible beta-blocker, and that this contributes to its slow offset of action.

1 We have studied the effects of bromoacetylalprenololmenthane (BAAM), a very slowly reversible β-adrenoceptor antagonist, on the responses of the left ventricle of 5 and 22 week old Wistar Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs) to isoprenaline. At 5 weeks the SHRs were prehypertensive and at 22 weeks they had hypertension-induced hypertrophy of the left ventricle. 2 The mean potency of isoprenaline on the left ventricle from 5 week old WKY was similar to that obtained on left ventricles from 22 week old WKY, and 5 and 22 week old SHRs. 3 Following treatment of the left ventricle with BAAM at 10−6m for 30 min, there was a parallel rightward shift of the isoprenaline concentration–response curves. Treatment with a higher concentration of BAAM (10−5m) caused non-parallel rightward shifts of the concentration–response curves, with a depression of the isoprenaline maximum responses. These data were used to derive affinity (KA) values for isoprenaline. The mean isoprenaline KA value on the left ventricle from 5 week old WKY was 2.44 × 10−6m, and similar KA values were obtained on the left ventricles from 22 week old WKY and 5 and 22 week old SHRs. On all the left ventricles tested, isoprenaline produced a half maximal response by occupying less than 1%, and a near maximal response by occupying about 5% of the available β1-adrenoceptors. 4 This study has shown that there are no differences in the cardiac responses to isoprenaline at β1-adrenoceptors, isoprenaline KA values or the β1-adrenoceptor reserve for isoprenaline on the SHR left ventricle in prehypertension or in the early stages of hypertension-induced hypertrophy.

1. The effects of potassium channel openers (cromakalim and pinacidil) and of calcium channel blockers (verapamil, diltiazem and flunarizine) on the contractile responses of the rat right ventricle have been determined. 2. On the electrically driven rat right ventricle cromakalim at 3 x 10(-7), 10(-6), 3 x 10(-6) M and 10(-5) M, and pinacidil at 10(-6), 10(-5) and 3 x 10(-5) M had no effect on the force responses to cardiac stimulation or to isoprenaline. 3. Pinacidil at 10(-4) M in a vehicle of 0.7% ethanol reduced the force responses to cardiac stimulation. This inhibitory effect was solely due to the ethanol. Pinacidil in 5 x 10(-4) M HCl (which had no effect alone) potentiated some of the responses to isoprenaline. 4. Verapamil, greater than or equal to 10(-6) M, diltiazem, greater than or equal to 10(-5) M, and flunarizine, greater than or equal to 10(-6) M, reduced the force responses to cardiac stimulation and to isoprenaline. 5. The present study has shown that calcium channel blockers, but not potassium channel openers, have inhibitory effects on the rat right ventricle.

1. The force of the electrically driven rat right ventricle strip is increased by isoprenaline but not by procaterol, a beta 2-adrenoceptor agonist. The effects of some beta-adrenoreceptor antagonists on the responses of the strip to isoprenaline have been determined. 2. Metoprolol (a beta 1-adrenoreceptor-selective antagonist) at 3 x 10(-8)-3 x 10(-7) moll-1 and ICI 118,551 (a potent beta 2-adrenoreceptor-selective antagonist) at 10(-7)-10(-6) moll-1 had no effect on maximum responses to isoprenaline and caused parallel rightward shifts of the isoprenaline response curves. The pA2 values for metoprolol and ICI 118,551 were 7.86 and 7.40, respectively. Thus the isoprenaline responses of the rat right ventricle strip are due to stimulation of beta 1-adrenoreceptors. 3. Metoprolol at 10(-6) moll-1 acted as a dual antagonist of the responses of the ventricle strip to isoprenaline causing parallel rightward shifts of the concentration-response curves which was due to beta 1-adrenoreceptor antagonism and a depression of the maximum response to isoprenaline which was probably due to membrane stabilizing activity. 4. Low concentrations of (+/-)-pindolol (10(-8) moll-1), (+)-pindolol (10(-7) moll-1), (-)-pindolol (10(-8) moll-1) and mepindolol (10(-10) moll-1) had no effect on isoprenaline maximum responses and caused parallel rightward shifts of isoprenaline response curves. Both (+/-)- and (-)-pindolol at 10(-7) moll-1, mepindolol at 10(-9)-10(-7) moll-1 and bopindolol at 10(-9)-10(-7) moll-1 reduced the isoprenaline maximum responses and caused non-parallel rightward shifts of the isoprenaline response curves. 5. BAAM (an irreversible beta-adrenoreceptor antagonist) treatment (3 x 10(-6) moll-1) for 30 min followed by 60 min washout reduced the isoprenaline maximum responses and caused a non-parallel rightward shift of the isoprenaline response curve. The kA (dissociation constant) for isoprenaline was 1.10 x 10(-6) moll-1. Calculation of receptor occupancy demonstrated that in order to produce a maximal response of the rat right ventricle strip, isoprenaline had to occupy 73% of the beta 1-adrenoreceptors. 6. The use of the rat right ventricle strip, a tissue with a small beta 1-adrenoreceptor reserve for isoprenaline, allows a definitive characterization of reversible (e.g. metoprolol) and irreversible (e.g. BAAM) beta 1-adrenoreceptor antagonists. It is suggested that (+/-)- and (-)-pindolol, mepindolol and bopindolol may be slowly dissociating beta 1-adrenoreceptor antagonists.

The effects of labetalol and its R1R isomer, SCH 19927, on the accumulation of radioactivity from [3H]-noradrenaline, and on the subsequent spontaneous and nerve-evoked outflow of radioactivity have been investigated in the rat isolated right ventricle. In addition, the effect of these agents on the contractions of the electrically-driven rat right ventricle to isoprenaline and of the rat isolated aorta to phenylephrine and 5-hydroxytryptamine are reported. Labetalol and SCH 19927 (both at 10(-6)M) inhibited the accumulation of radioactivity from [3H]-noradrenaline by 26 and 37%, respectively. The spontaneous outflow of radioactivity, following loading of the ventricle with [3H]-noradrenaline, was increased by labetalol and SCH 19927 (both at 10(-6)M) by a cocaine and idazoxan-insensitive mechanism. The nerve-evoked outflow of radioactivity was increased by labetalol and SCH 19927 (both at 10(-6)M). The ability of labetalol and SCH 19927, to increase nerve-evoked outflow was maintained in the presence of cocaine (10(-5)M) or idazoxan (10(-7)M) but reversed in the presence of cocaine and idazoxan. It is suggested that labetalol and its R1R isomer act both to inhibit neuronal uptake of noradrenaline and as antagonists at prejunctional alpha 2-adrenoreceptors. Labetalol and SCH 19927 reduced the contractile responses associated with the nerve-evoked outflow of radioactivity probably mainly by acting as antagonists at postjunctional beta 1-adrenoreceptors. The contractile responses of the electrically-driven rat right ventricle to isoprenaline were inhibited by labetalol and SCH 19927. SCH 12297 (pA2 = 8.9) was 4 times more potent than labetalol (pA2 = 8.3) as a beta 1-adrenoreceptor antagonist. The ability of labetalol and SCH 19927 at greater than or equal to 10(-7)M to depress maximal responses to isoprenaline may represent membrane stabilizing activity. The contractile responses of rat aorta to phenylephrine were inhibited by labetalol and SCH 19927. Labetalol (pA2 = 7.5) was 4X more potent than SCH 19927 (pA2 = 6.9) as an alpha 1-adrenoreceptor antagonist. SCH 19927 and labetalol had no effect on contractile responses to 5-hydroxytryptamine.

1. The possibility of using contractility studies with the rat right ventricle strip to assess the effects of drugs on all aspects of noradrenergic transmission has been examined. 2. The force of the contractile responses to field stimulation at 0.5 and 2, but not 5 Hz, markedly decreased with time. About 55% of the tissues responded directly to (-)-isoprenaline, 1 X 10(-6)M (123 tissues from 220 tested) in the absence of other stimuli. In these tissues the force of the contractile responses remained constant and the rate increased with time. Thus it is essential to provide time controls in studies with rat right ventricle. 3. Under conditions in which the responses to (-)-isoprenaline, 1 X 10(-6)M, alone were unaffected the responses to field stimulation at 5 Hz were inhibited by 66 and 86% by 9 X 10(-6)M tetrodotoxin and 1 X 10(-5)M guanethidine, respectively. (+/-)-Propranolol and timolol (both at 1 X 10(-6)M), but not phentolamine, 1 X 10(-6)M, inhibited responses to (-)-isoprenaline, 1 X 10(-6)M alone and responses to field stimulation at 5 Hz. This demonstrates that the responses to field stimulation are largely due to activation of noradrenergic nerves, the released noradrenaline acting at postsynaptic beta-adrenoreceptors. 4. Although nortriptyline is a potent inhibitor of the neuronal uptake of noradrenaline, at 1 X 10(-6)M it had no effect on the contractile responses to field stimulation at 5 Hz and inhibited responses to (-)-isoprenaline, 1 X 10(-6)M, alone and at a higher concentration (1 X 10(-5)M) nortriptyline abolished both responses. It is suggested that the rat ventricle preparation may be useful in examining the effects of drugs on noradrenergic transmission in the heart.

1 The effects of prazosin, phentolamine and yohimbine on the accumulation of radioactivity from [3H]-noradrenaline, and on the subsequent spontaneous and field stimulation-evoked outflow of radioactivity have been investigated in the rat right ventricle in vitro. In addition the effects of these agents on contractions produced by exogenous amines or by field stimulation are reported. 2 Prazosin, phentolamine and yohimbine had no effect on the accumulation of radioactivity from [3H]-noradrenaline. 3 The decline in the spontaneous outflow of radioactivity, following loading of the tissue with [3H]-noradrenaline, was reduced by prazosin (1 times 10-8 or 1 times 10-7M) and 1 times 10-6M yohimbine. Phentolamine and 1 times 10-7M yohimbine had no effect on the spontaneous outflow of radioactivity. 4 Prazosin, phentolamine and yohimbine (all at 1 times 10-7M) reduced the decline in outflow of radioactivity evoked by field stimulation at 5Hz. In the presence of yohimbine, prazosin or phentolamine had no effect on the evoked outflow. The effect of prazosin, phentolamine and yohimbine alone probably represents antagonism at prejunctional α-adrenoreceptors. 5 The rate of beating and force of contractions evoked by 1 times 10-6M isoprenaline were not altered by prazosin, phentolamine or 1 times 10-7M yohimbine. The rate of beating evoked by noradrenaline (1 times 10-6M) was reduced by prazosin and phentolamine. Yohimbine (1 times 10-6M) reduced the rate of beating to isoprenaline and to noradrenaline. These effects of prazosin and phentolamine probably represent antagonism at postjunctional α1-adrenoreceptors and that of yohimbine a decrease in the excitability of the postjunctional membrane. 6 In the control tissues the force of contractions evoked by field stimulation at 5Hz declined slowly with successive stimulations at 30 min intervals. In the presence of prazosin (1 times 10-8 and 1 times 10-7M), phentolamine (1 times 10-7 and 1 times 10-6M) and 1 times 10-7M yohimbine the responses did not decline with successive stimulations. Yohimbine (1 times 10-6M) had no effect on the stimulation-evoked responses. 7 Phentolamine or yohimbine (both at 1 times 10-7M) had no additional effect on the stimulation-evoked responses in the presence of prazosin (1 times 10-7M). Prazosin (1 times 10-8M) and yohimbine (1 times 10-7M) had no further effect in the presence of phentolamine (1 times 10-6M). Prazosin or phentolamine (both at 1 times 10-7M) had no effect on stimulation-evoked responses in the presence of yohimbine (1 times 10-7M). 8 In the rat right ventricle, the ability of phentolamine and yohimbine to prevent the decline in the stimulation-evoked responses with successive stimulations is probably due to antagonism at prejunctional α-adrenoreceptors. However it is suggested that the ability of prazosin to prevent the decline in response is due to a release of endogenous noradrenaline and to antagonism at prejunctional α-adrenoreceptors.

1. The present study was to ascertain whether drugs cause differential antagonism of the initial fast and secondary slow contractile responses to phenylephrine, 5-HT and methacholine of the rat portal vein. 2. Cocaine (5 x 10(-6) M) had no effect on the responses to methacholine. Cocaine potentiated the initial fast but not the secondary slow responses to phenylephrine suggesting that neuronal uptake limits the initial fast but not the secondary slow response to phenylephrine. Cocaine reduced the maximal responses to 5-HT suggesting that the responses to high concentrations of 5-HT are due, in part, to the release of noradrenaline. 3. In the presence of cocaine, idazoxan had no effect on responses to phenylephrine or 5-HT and prazosin had no effect on responses to 5-HT. Prazosin (3 x 10(-9)-10(-8) M) inhibited the responses to phenylephrine causing similar parallel rightward shifts of the concentration-response curves to the initial fast and secondary slow responses. 4. In the presence of cocaine, mianserin (10(-9)-10(-8) M), cyproheptadine, (10(-10)-10(-9) M), methysergide (10(-8) M), ketanserin (10(-10)-10(-9) M) and Ly 53857 (at 10(-9)-10(-8) M) had similar inhibitory effects on the initial fast and secondary slow responses to 5-HT, namely a parallel rightward displacement of the concentration-response curves. 5. Atropine (less than or equal to 10(-7) M) and pirenzepine less than or equal to 10(-6) M) had no effect on the responses to phenylephrine and 5-HT, but caused similar parallel rightward shifts of the concentration-response curves to both responses to methacholine. 6. The results of this study provide no evidence for differential antagonism of the initial fast and secondary slow responses of the rat portal vein to phenylephrine, 5-HT or methacholine.

1 The effects of amitriptyline, desipramine and mianserin on the accumulation of radioactivity from [3H]-noradrenaline, and on the subsequent spontaneous and field stimulation-evoked outflow of radioactivity have been investigated in the rat isolated right ventricle. In addition the effect of these agents on contractions produced by isoprenaline or by field stimulation are reported. 2 Amitriptyline, desipramine (both at ≥ 10-8M) and mianserin at ≥ 10-7M inhibited the accumulation of radioactivity from [3H]-noradrenaline. 3 The decline in the spontaneous outflow of radioactivity, following loading of the tissue with [3H]-noradrenaline, was not altered by amitriptyline, desipramine or mianserin. 4 Rauwolscine, amitriptyline, desipramine and mianserin (all at 10-6M) reduced or reversed the decline in outflow of radioactivity evoked by field stimulation at 5 Hz. Desipramine had no additional effects in the presence of cocaine (10-5M) and amitriptyline and mianserin had no further effect in the presence of cocaine and rauwolscine. The effect of desipramine probably represents inhibition of neuronal uptake and that of amitriptiyline and mianserin blockade of neuronal uptake and prejunctional α2-adrenoreceptors. 5 The rate of beating in the presence of isoprenaline (10-6M) was reduced by amitriptyline (10-7M), desipramine (10-6M) and mianserin Higher concentrations of the antidepressants greatly reduced or abolished the rate and force of beating in the presence of isoprenaline. 6 The force of contractions evoked by field stimulation at 5Hz was increased to a small extent by amitriptyline, desipramine and mianserin at 10-7M and reduced by the antidepressants at 10-5M. It is suggested that in the rat isolated right ventricle, the inability of antidepressants to potentiate contractile responses to field stimulation to a great extent as a consequence of blocking neuronal uptake and prejunctional α-adrenoreceptors is due to a decreased excitability of the postjunctional β-adrenoreceptors.

1 The effects of cocaine and lignocaine on the contractile responses to field stimulation and to exogenously applied agents, in the absence of other stimuli, have been investigated in the rat right ventricle using methods we have recently described (Doggrell & Vincent, 1981a). In addition the effects of 3H accumulation from (-)-[3H]-noradrenaline and on the spontaneous and field stimulation-induced overflow of 3H, following preloading of the tissue with (-)-[3H]-noradrenaline, are reported. 2 Cocaine, but not lignocaine, inhibited the accumulation of 3H from (-)-[3H]-noradrenaline. The spontaneous overflow of 3H, following preloading of the tissue with (-)-[3H]-noradrenaline was not altered by cocaine, 10 microM, or lignocaine, 100 microM. 10 microM Lignocaine had no effect on the overflow of 3H evoked by field stimulation at 5Hz. Lignocaine, 100 microM, increased and cocaine, 1 and 10 microM, reduced the decline in evoked release of 3H. This effect of lignocaine probably represents a decrease in nerve excitability and that of cocaine inhibition of neuronal uptake of noradrenaline. 3 Cocaine, 1 microM, reduced the rate of beat response to tyramine, 1 microM, alone probably by inhibiting the neuronal uptake process. 4 Cocaine, 1 and 10 microM, had no effect on the contractile responses to field stimulation (at 2 and/or 5 Hz). The rate of beat (-)-noradrenaline or (-)-isoprenaline, 1 microM, alone was decreased by cocaine, 10 microM. Lignocaine, 10 microM, reduced the force of contractions to field stimulation at 5 Hz and the responses to (-)-noradrenaline or (-)-isoprenaline alone. It is suggested that the inhibitory effects of cocaine and lignocaine on responses to (-)-isoprenaline in the rat right ventricle are due to a decreased postjunctional membrane excitability. The inability of 10 microM cocaine to potentiate contractile responses to endogenous or exogenous (-)-noradrenaline as a consequence of the inhibition of neuronal uptake is also probably due in part to decreased post-junctional excitability of the right ventricle.

1 The effects of KF-4317 on the accumulation of radioactivity from [3H]-noradrenaline, and on the subsequent spontaneous and noradrenergic nerve-evoked outflow of radioactivity have been investigated in the rat isolated right ventricle. In addition the effects of KF-4317 on the contractions of the electrically-driven directly muscle stimulated rat right ventricle to isoprenaline and of the rat isolated aorta to phenylephrine and 5-hydroxytryptamine are reported. 2 KF-4317 at 1μM had no effect on the ability of the rat right ventricle to accumulate radioactivity from [3H]-noradrenaline. 3 The spontaneous outflow of radioactivity, following loading of the ventricle with [3H]-noradrenaline, was increased by KF-4317 at 1μM by a cocaine-insensitive mechanism. 4 KF-4317 at 1μM had no effect on the noradrenergic nerve-evoked outflow or radioactivity, and therefore is not an α2-adrenoreceptor antagonist, but reduced the associated contractile response probably mainly by acting as an antagonist at postjunctional β1-adrenoreceptors. 5 KF-4317 caused a parallel rightward shift of the concentration-response curve of the electrically-driven directly muscle stimulated rat right ventricle to isoprenaline. However the inhibitory effect, × 9.0 and × 237.2 in the presence of 0.1 and 1μM KF-4317, was not closely concentration-related. At 1μM, KF-4317 also depressed the maximum responses to isoprenaline. This suggests that in addition to β1-adrenoreceptor antagonism, KF-4317 probably exerts membrane stabilizing activity. 6 The responses of the rat isolated aorta to phenylephrine were inhibited in a non-concentration related manner by KF-4317. Thus the responses were inhibited × 1.8, × 2.7 and × 4.7 in the presence of KF-4317 at 0.1, 1 and 1μM, respectively. The responses of the aorta to 5-hydroxytryptamine were inhibited to a small extent by KF-4317 at 1μM but potentiated to a small degree by KF-4317 at 10μM. We suggest that KF-4317 is an antagonist at α1-adrenoreceptors and 5-hydroxytryptamine receptors but that it also has an additional postjunctional action, in the rat aorta, to increase the sensitivity to both phenylephrine and 5-hydroxytryptamine.

1 The effects of 6-hydroxydopamine treatment in vitro for varying times on the overflow of 3H (following preloading of the tissue with (–)-[3H]-noradrenaline) and the accumulation of 3H from (–)-[3H]-noradrenaline have been investigated in the rat anococcygeus muscle. In addition the effects on contractile responses to field stimulation and to other stimulants are reported. 2 Following preloading of the rat anococcygeus muscle with 5 times 10-7M (–)-[3H]-noradrenaline for 1 h, the overflow of 3H was increased by 6-hydroxydopamine (1 times 10–3M). At the end of the overflow period the tissue content of 3H was reduced by 95% from tissues treated with 6-hydroxydopamine for ≥ 30 min. The rat anococcygeus muscle accumulated 3H from a medium containing 5 times 10–8M (–)-[3H]-noradrenaline. The ability to accumulate 3H was reduced after incubation in 6-hydroxydopamine for 5 min and abolished by longer incubations. These effects of 6-hydroxydopamine probably represent depletion of endogenous (–)-noradrenaline stores and destruction of noradrenergic neurones. 3 6-Hydroxydopamine induced contractions of the rat anococcygeus that were abolished by phentolamine (1 times 10-6M). 4 The excitatory contractile responses to field stimulation were reduced after exposure to 6-hydroxy-dopamine for 5 min and were abolished after longer exposures. 6-Hydroxydopamine had no effect on the inhibitory response to field stimulation. The contractile responses to (–)-noradrenaline were potentiated after 6-hydroxydopamine. 5 Following 6-hydroxydopamine treatment for 5 min the responses to tyramine were potentiated. This potentiation was probably due to the release of endogenous (–)-noradrenaline. The responses to tyramine were reduced after longer treatments (30–120 min). 6 Short incubations of the anococcygeus muscle with 6-hydroxydopamine or (–)-noradrenaline had no effect on the subsequent responses to acetylcholine whereas after a 180 min treatment the responses were reduced. This effect was possibly due to fatigue of the muscle. Incubations with (–)-noradrenaline for 60 min had no effect on the responses to field stimulation or to (–)-noradrenaline. After longer pretreatments the responses were reduced. Thus after long exposures to (–)-noradrenaline, and probably to 6-hydroxy-dopamine, responses may be reduced by fatigue or by desensitization at α-adrenoreceptors. 7 These results suggest that, in the rat anococcygeus muscle, a selective abolition of noradrenergic transmission was obtained by treating the tissue with 6-hydroxydopamine for 30–60 min. The contractile responses to tyramine in this tissue may, in part, be due to a direct postjunctional action.

1 The rat isolated aorta is contracted by 5-hydroxytryptamine (5-HT) with an initial fast and secondary slow response. The secondary slow responses were greater than the initial fast. The effects of antagonists (prazosin, idazoxan, phentolamine, dihydroergotamine, methysergide, cyproheptadine, mianserin and ketanserin) on these initial fast and secondary slow contractile responses to 5-HT were studied. 2 Prazosin and idazoxan at 10(-7)M had no effect on responses to 5-HT. Thus all concentrations of 5-HT contract the aorta by stimulating receptors distinct from alpha-adrenoreceptors. 3 Phentolamine, dihydroergotamine, methysergide and cyproheptadine had similar inhibitory effects on the initial fast and secondary slow responses to 5-HT. Phentolamine at 10(-6)M caused parallel, whereas dihydroergotamine (10(-9)-10(-8)M), methysergide (10(-9)-10(-8)M) and cyproheptadine (3 X 10(-11)-10(-8)M) caused non-parallel rightward shifts of the concentration-response curves. 4 Mianserin and ketanserin had greater inhibitory effects on the initial fast than secondary slow responses. Mianserin (10(-9)-10(-7)M) caused a non-parallel and parallel rightward shift of the concentration-response curves to the initial fast and secondary slow responses to 5-HT, respectively. Ketanserin (10(-8)-10(-7)M) produced non-parallel rightward shifts of the concentration-response curves. 5 One possible explanation of these results is that the rat aorta has no spare 5-HT2 receptors and that in inhibiting responses to 5-HT: (a) phentolamine is a competitive antagonist, (b) mianserin and ketanserin are slowly dissociating antagonists, and (c) dihydroergotamine, methysergide and cyproheptadine are irreversible antagonists. Furthermore, the competitive and irreversible antagonists have similar effects on initial fast and secondary slow responses to 5-HT, whereas the slow dissociating antagonists (mianserin and ketanserin) have a greater inhibitory effect on the initial fast than secondary slow response.