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The partial beta adrenoceptor agonist properties of cicloprolol, xamoterol and pindolol have been compared in vivo (anesthetized catecholamine-depleted or pithed rats) and in vitro (guinea pig or rat right atria and guinea pig tracheal muscle preparations) conditions. All three compounds increased heart rate in the former preparations, and their intrinsic activities relative to isoproterenol were 0.7, 0.65 and 0.45, respectively. The positive chronotropic effects of cicloprolol or xamoterol were competitively antagonized by betaxolol or propranolol; however, part of those induced by pindolol were resistant to these beta adrenoceptor antagonists. None of these compounds increased the spontaneous beating rate of isolated guinea pig atria; however, xamoterol only increased heart rate in isolated rat atria, and its intrinsic activity with respect to isoproterenol was 0.4. Pindolol, xamoterol and cicloprolol behaved as competitive beta-1 adrenoceptor antagonists against isoproterenol-induced tachycardia in a pithed rat model. In order to mimic the intrinsic effects of the partial agonist drugs, control dose-response curves for isoproterenol were determined in pithed rats in which the base-line heart rate was elevated by thoracic spinal cord stimulation. In this in vivo preparation, xamoterol and pindolol were more potent beta-1 adrenoceptor antagonists than cicloprolol; however, cicloprolol and xamoterol, in contrast to pindolol, were selective for beta-1 adrenoceptors. In isolated spontaneously beating guinea pig right atria, cicloprolol and xamoterol were equipotent beta-1 adrenoceptor antagonists but were about 50 times less potent than pindolol. In isolated rat atria, the beta-1 adrenoceptor antagonist potency of xamoterol was greater (pA2 = 8.7) than in guinea pig atria (pA2 = 7.8). The potencies of cicloprolol and pindolol did not vary between these species. In catecholamine-depleted rats, high i.v. doses of cicloprolol had vasodilator activity that was partly mediated by beta-2 adrenoceptors. In carbachol-contracted guinea pig trachea, cicloprolol and xamoterol, in contrast to pindolol, were relatively inactive against isoproterenol-induced relaxation. In conclusion, cicloprolol and xamoterol, similarly to pindolol, behave as agonists and antagonists of beta-1 adrenoceptors. However, only cicloprolol and xamoterol show an elevated degree of selectivity toward the beta-1 adrenoceptor subtype.

In pithed rats, blood pressure dose-response curves to i.v. cirazoline, methoxamine and phenylephrine (full alpha-1 adrenoceptor agonists) exhibited higher maxima than those to B-HT 920, M-7, UK-14,304 (full alpha-2 adrenoceptor agonists) and indanidine (Sgd 101/75: partial alpha-1 adrenoceptor agonist). For an 80 mm Hg increase in blood pressure, full alpha-1 adrenoceptor agonists enhanced total peripheral, renal and mesenteric vascular resistances significantly more than alpha-2 adrenoceptor stimulants or indanidine. In contrast, all compounds produced a similar degree of hindquarter vasoconstriction, suggesting that both types of alpha adrenoceptors have the same functional importance in this skeletal muscle vascular bed. Application of a multivariate discriminant analysis to the drug-induced changes in the total peripheral and mesenteric vascular resistances associated with a pressor effect of 80 mm Hg allowed their assignment to two distinct groups corresponding to the full alpha-1 and the full alpha-2 adrenoceptor agonists plus indanidine. All investigated compounds in low doses increased cardiac output, which returned to base-line values after high doses of alpha-1 but plateaued after high doses of alpha-2 adrenoceptor agonists or indanidine. alpha-1 adrenoceptor agonists decreased whereas alpha-2 stimulants and indanidine successively increased and then decreased renal blood flow. Finally, all investigated compounds increased hindquarter blood flow at low doses but decreased it at high doses. The ratios of the doses of cirazoline required to produce a 100% rise in systemic and local vascular resistances in the presence or in the absence of prazosin were of similar magnitude. This was also true for M-7 when studied in the presence or in the absence of yohimbine. These findings suggest pharmacological identity within alpha-1 as well as within alpha-2 adrenoceptor populations in all investigated vascular beds. Finally, the calcium entry blocker diltiazem did not affect the increases in systemic and regional resistances evoked by cirazoline but depressed profoundly the effects of M-7 and indanidine. In conclusion, full alpha-1 and alpha-2 adrenoceptor agonists can be discriminated easily on the basis of their systemic and regional hemodynamics in the pithed rat. That the hemodynamic effects of the partial alpha-1 adrenoceptor agonist indanidine are similar to those of alpha-2 adrenoceptor agonists and susceptible to calcium channel blockade suggests that the alpha-1 adrenoceptors stimulated by this drug have the same coupling modality as alpha-2 adrenoceptors and share with the latter the same functional expression when stimulated.

In conscious spontaneously hypertensive rats, pergolide (50.0 micrograms/kg s.c.) produced a sustained decrease in tail artery pressure which was blocked by haloperidol (1.0 mg/kg s.c.) pretreatment. In anesthetized spontaneously hypertensive rats this effect was accompanied by a fall in total peripheral resistance inasmuch as pergolide did not significantly change cardiac output. In anesthetized normotensive rats, pergolide (30.0 micrograms/kg i.v.) also lowered blood pressure. This effect was not significantly modified by adrenalectomy, methysergide, idazoxan (alpha-2 adrenoceptor antagonist), vagotomy alone or plus ligation of carotid arteries or plus atenolol, but was entirely prevented by domperidone or sulpiride pretreatment and was reverted to a pressor response (due to stimulation of alpha adrenoceptors and 5-hydroxytryptamine receptors) by blockade of ganglionic transmission with chlorisondamine. Pergolide given either i.v. or into the cisterna magna or the lateral cerebral ventricle produced changes in blood pressure of the same magnitude. In intact or adrenalectomized rats, i.v. pergolide significantly lowered plasma norepinephrine concentration. Furthermore, in saline but not sulpiride-pretreated pithed rats, pergolide reduced the pressor responses and the accompanying increases in plasma norepinephrine evoked by electrical stimulation of the spinal cord. However, pergolide failed to modify the vascular reactivity to several pressor agents and lacked beta-2 and DA-1 dopamine receptor agonist properties. These results indicate that the decrease in blood pressure produced by pergolide can be accounted for by an inhibition of sympathetic tone resulting from stimulation of peripheral neuronal dopamine receptors. A possible central contribution remains to be substantiated. The pronounced bradycardia produced by pergolide (30.0 micrograms/kg i.v.) in anesthetized intact rats was partly reduced by vagotomy, methylatropine, domperidone, sulpiride, idazoxan, phentolamine or atenolol. The effects of pergolide in vagotomized rats were further diminished by domperidone but they were blocked by the combination of phentolamine or idazoxan plus domperidone. In rats pretreated with atenolol or in rats with the cervical section of spinal cord and the low level of heart rate increased with an isoprenaline infusion, the decrease in heart rate produced by pergolide was abolished by domperidone, methylatropine or idazoxan. In pithed rats, pergolide changed neither the base-line heart rate nor the tachycardia to exogenous norepinephrine nor the bradycardia evoked by carbachol or electrical stimulation of the peripheral cervical vagus.(ABSTRACT TRUNCATED AT 400 WORDS)