Heterogeneous inhibitory effect of nitrendipine on 5-HT-responses in rabbit vascular tissues

1. Sensitivity of 5-HT2 receptor mediated vascular responses to calcium channel antagonism was investigated in the rabbit thoracic aorta, common carotid artery and main pulmonary artery considering tissue-state of receptor reserve and agonist efficacy. 2. Although 5-HT2 receptor reserve seems to be similar in these preparations, nitrendipine had a heterogeneous inhibitory effect on the contractile response to 5-HT. 3. Reducing 5-HT2 receptor reserve by phenoxybenzamine caused an increase in the inhibitory effect of nitrendipine in these vascular tissues. 4. The vasoconstrictor response induced by 5-carboxyamidotryptamine (5-CT), which is a less potent agonist, showed greater sensitivity to nitrendipine antagonism than the response to 5-HT. 5. The results suggest that efficacy of agonists and tissue-state of receptor reserve seem to be important factors which are partially responsible for the inhibitory effect of nitrendipine on the contractile responses mediated by 5-HT2 receptors.