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The interactions between UK-14304 and other vasoconstrictor agents were investigated using isolated canine mesenteric vascular rings mounted in tissue baths for the measurement of isometric contraction. In the mesenteric artery, exposure to UK-14304 caused a small contraction, producing 8% of the KCl maximal response. In the presence of either 20 mM KCl or 10(-9) M endothelin-1, which had small contractile effects, UK-14304 produced a biphasic concentration-response curve; 10(-7) M rauwolscine inhibited the responses to low concentrations of UK-14304 and 10(-7) M prazosin blocked the responses to high concentrations of UK-14304. In the presence of 10(-8) M Bay K 8644, UK-14304 elicited a monophasic concentration-dependent contraction that was antagonized by 10(-7) M prazosin, not by 10(-7) M rauwolscine. In the mesenteric vein, UK-14304 elicited concentration-dependent contractions, producing 63% of the KCl maximal response. The lower part of the biphasic concentration-response curve was inhibited by 10(-7) M rauwolscine and the upper part of the curve was antagonized by 10(-7) M prazosin. The presence in the medium of 20 mM KCl, 10(-11) M endothelin-1 or 10(-9) M Bay K 8644, which increased resting tension less than 10% of the KCl maximal response, markedly enhanced the responses to UK-14304 primarily at concentrations lower than 10(-6) M. The enhancement of responses were prazosin (10(-7) M)-resistant and rauwolscine (10(-7) M)-sensitive.(ABSTRACT TRUNCATED AT 250 WORDS)

Thapsigargin (TSG), a putative selective Ca(++)-ATPase inhibitor, has been used to study Ca++ mobilization in many non-excitable cell types. This study aims to determine whether TSG is effective as a selective microsomal Ca++ uptake inhibitor by studying its ability to affect repletion of the phenylephrine (PE)-sensitive Ca++ pool in rat aorta and dog mesenteric artery evaluated by contractility studies. TSG caused a concentration-dependent contraction that was dependent on the concentration of extracellular Ca++. Ca++ influx promoted by TSG was found to occur mostly through L-type Ca++ channels in the dog mesenteric artery but not in the rat aorta. When arterial rings, depleted of their PE-sensitive internal store, were allowed to replete their stores in normal Krebs' solution or in the presence of elevated K+ levels, it was found that repletion was significantly enhanced in the presence of elevated K+. In TSG-treated rings, however, repletion was significantly inhibited under both conditions as indicated by the subsequent PE-induced contraction in Ca(++)-free medium. While the rate of contraction induced by elevated K+ levels was slow immediately after pool depletion in controls, it was rapid in TSG-treated arterial rings. The slow onset of K+ contraction may reflect Ca++ uptake into the pool which was absent in TSG-treated arteries. Differences in the behavior of the two arteries were noted and these may reflect differences in the size of their Ca++ store and their coupling to the extracellular space. Single cells isolated from the dog mesenteric artery were also found to shorten in response to TSG to an amount comparable with that obtained from whole tissue experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

The distribution of [3H]ryanodine binding in subcellular fractions isolated from rat vas deferens (RVD) paralleled that of NADPH cytochrome C reductase activity indicating an endoplasmic reticulum origin of the binding sites. Scatchard analysis of [3H] ryanodine binding indicated an homogenous site with a Kd value of 6.4 nM. The maximum number of ryanodine binding sites was 488 fmol of [3H]ryanodine per milligram of protein. Norepinephrine (NE) or ATP endogenously released after electrical field stimulation (tetrodoxin-sensitive responses), both produced a biphasic contraction of the RVD when the action of the other was blocked. When NE was the agonist (prazosin-sensitive response), the initial transient contraction was suppressed by 30 microM ryanodine whereas the secondary component was abolished by 2 microM nifedipine. When ATP was the agonist (P2 tachyphylaxis-sensitive response), both phases of the contraction were suppressed by 2 microM nifedipine. However, the initial phasic component of the contraction induced by endogenously released ATP was also inhibited by 30 microM ryanodine except at high stimulation frequency (10 Hz). Exogenously added NE and alpha, beta methylene ATP produced concentration-dependent contractions of the RVD. The effect of both agonists was inhibited by 2 microM nifedipine whereas 30 microM ryanodine had little effect except at high concentrations of NE. We conclude that ryanodine binding sites reside in RVD endoplasmic reticulum. There was a lack of uniformity in the effect of ryanodine and nifedipine against alpha adrenoceptor stimulation and purinoceptor stimulation indicating a difference in the stimulation-contraction coupling process between these two modes of stimulation.