Do D 600 and diltiazem interact with serotonin receptors in rabbit vascular tissues?

This study investigates the interactive effects of Ca antagonists and responses to serotonin (5-HT) in isolated rabbit aorta and basilar artery. Isolated strips of rabbit aorta and of basilar artery were studied in organ baths containing 20 ml of Krebs-buffer solution. D 600 or diltiazem caused parallel rightward shifts of concentration-response curves for 5-HT in isolated rabbit aorta, but each reduced the 5-HT-induced maximum response in basilar artery. The slope values from Schild plot analyses in aorta were 1.11 +/- 0.05 and 1.22 +/- 0.37 in D 600 and diltiazem, respectively. Dibenamine pretreatment caused reduction of maximum response to 5-HT in both tissues. Such reduction was diminished markedly by the presence of D 600, diltiazem or 5-HT in aorta but not in basilar artery. Also, the antagonistic effects of D 600 and diltiazem were diminished by elevation of bath Ca in basilar artery but not in aorta. These results strongly suggest that the two Ca entry blockers used in this study might compete with 5-HT at 5-HT receptors in rabbit aorta and that the pharmacomechanical property of 5-HT receptors in rabbit aorta might be distinct from that in rabbit basilar artery.