Degree Name

PhD (Doctor of Philosophy)

Program

Biomedical Sciences

Date of Award

August 1999

Abstract

The signaling mechanisms for muscarinic receptor-mediated vasoconstriction in coronary resistance arteries were studied in KCl-arrested isolated rat hearts perfused at a constant flow rate. The cholinergic agonists acetylcholine and bethanechol were given by bolus injection or constant infusion. The coronary vascular resistance was monitored by measuring the changes in perfusion pressure. The selective muscarinic agonist bethanechol caused a similar vasoconstrictor response as ACh, but with less potency and efficacy. Bolus injection of bethanechol evoked a phasic vasoconstriction in a dose-dependent manner, while infusion of bethanechol evoked a tonic vasoconstriction without producing tachyphylaxis. Coronary vascular responses to bethanechol were further examined in the absence and presence of a specific inhibitor or blocker for the potential signaling components. The bethanechol-induced phasic vasoconstriction was eliminated by perfusion with a Ca2+ -free medium. The maximal vasoconstriction to bethanechol was suppressed by 31% in the presence of the Ca2+ -dependent Cl- -channel blocker A-9-C. The L-type voltage-operated Ca2+ channel blocker nifedipine decreased the maximal constrictor response to bethanechol by 59%, while the putative receptor-operated Ca 2+ channel blocker SK&F 96365 converted this vasoconstriction into vasodilation which was not affected by the nitric oxide synthase inhibitor L-NAME. Coronary vascular responses to bethanechol were almost abolished by a combination of nifedipine and SK&F 96365. The protein kinase C inhibitor chelerythruine reduced bethanechol-evoked peak vasoconstriction by 79%. The bethanechol-induced tonic vasoconstriction was rapidly converted into vasodilation by the concomitant infusion of SK&F 96365 or nifedipine, but the simultaneous infusion of chelerythrine gradually attenuated this response. These data suggest that the novel receptor-operated Ca2+ channel, voltage-operated Ca2+ channel, and protein kinase C are the most crucial signaling components for muscarinic receptor-mediated coronary vasoconstriction in the isolated rat heart. Extracellular Ca 2+ influx via receptor-operated Ca2+ channels and voltage-operated Ca2+ channels is essential to both phasic and tonic vasoconstrictor responses to bethanechol. Protein kinase C plays a pivotal role in the regulation of bethanechol-evoked vasoconstriction by sensitizing the contractile apparatus and modulating the activity of Ca 2+ channels.

Document Type

Dissertation - Open Access

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