Abstract

The present study explored the calcium source used in serotonin (5-HT)-induced contractions mediated by 5-HT2 receptor activation in the rat uterus and jugular vein in vitro. In the rat uterus, the calcium channel antagonists, nitrendipine and diltiazem, and the neuroleptic agents, trifluoperazine (TFP) and pimozide, antagonized potently and noncompetitively 5-HT-induced contractions. These data are compatible with the contention that 5-HT-induced contractions in uterine smooth muscle require extracellular sources of calcium. In contrast, neither diltiazem nor nitrendipine inhibited the contractile response to 5-HT in the rat jugular vein although 5-HT-induced contractions in the jugular vein required the presence of extracellular calcium. This difference in sensitivity to calcium channel antagonists between the uterus and jugular vein was not related to differences in the receptor occupancy-response curves for 5-HT as these were similar in the rat uterus and jugular vein. Furthermore, in the jugular vein, neither diltiazem nor nitrendipine were markedly effective in blocking contractions produced by potassium chloride, an agent that activates voltage-dependent calcium channels, suggesting that this tissue lacks calcium channels susceptible to blockade by conventional calcium channel antagonists. Although responses to 5-HT in the jugular vein were only affected marginally by calcium channel antagonists, TFP and pimozide produced concentration-dependent rightward parallel shifts of 5-HT-induced contractions in the jugular vein. Furthermore, both TFP and pimozide showed high affinity at 5-HT2 binding sites in rat brain cortical membranes and the apparent dissociation constant at 5-HT2 receptors in the jugular vein was 38 and 31 nM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)