We have investigated the mechanisms by which stimulation of cardiac muscarinic receptors result in paradoxical stimulatory effects on cardiac function, using cultured neonatal rat ventricular myocytes as a model system. Application of low concentrations of carbachol (CCh) (EC 50 Stimulation of postsynaptic muscarinic acetylcholine receptors (mAChRs)1 on cardiac cells is able to produce dual effects on the heartbeat. Classically, stimulation of the vagus nerve or the application of low concentrations of muscarinic cholinergic agonists produces characteristic negative chronotropic and inotropic effects on cardiac tissue and cells (1). Paradoxically, under appropriate conditions, activation of cardiac mAChRs elicits stimulatory effects on the rate of beating and contractile force of the heart. These latter effects often require higher concentrations of agonist and in some cardiac cell types are only seen after pretreatment with pertussis toxin (PTx) (2-4).It has been widely believed, based on both radioligand binding techniques as well as Northern blot analyses, that cardiac cells express only the m 2 mAChR subtype on their surface membranes (5-8). However, more recent findings suggest limits in the sensitivity of these techniques in detecting low levels of protein and mRNA (6). For example, it has been demonstrated that mRNA for mAChR subtypes that were missed by Northern blot studies were detected by use of reverse transcription followed by polymerase chain reaction (RT-PCR) in neuroblastoma cells as well as in guinea pig ventricular myocytes (7, 9). Previous work in this laboratory, employing the techniques of single-cell RT-PCR on cardiac myocytes isolated from adult rat ventricle has demonstrated the existence of m 1 mAChR mRNA in addition to m 2 mRNA (10, 11). Furthermore, we have demonstrated that the activation of m 1 mAChRs leads to an increase in muscle shortening and Ca 2ϩ transients. However, the signal transduction pathway mediating this stimulatory response is still unknown. Moreover, there is no report on the role of m 1 mAChRs in the regulation of heart rate.In the present studies, we have taken advantage of the ability of cultured neonatal rat ventricular myocytes to contract spontaneously and have utilized them as a cardiac cell model system to investigate the role of m 1 mAChRs in the regulation of rate of contraction. By applying a number of complementary approaches, including single-cell intracellular Ca 2ϩ concentration ([Ca 2ϩ ] i ) measurements, single-cell RT-PCR, immunocytochemistry and antisense technology, four specific questions were addressed. 1) Do cultured neonatal rat ventricular myocytes exhibit dual responses to mAChR stimulation in a manner similar to that observed in other cardiac cell preparations? 2) How many subtypes of mAChRs are expressed in cultured neonatal rat ventricular myocytes? 3) Which mAChR subtype mediates the stimulatory effects of muscarinic agonists on the rate of contraction? 4) What second messenger system is involved in generating this response?A prelim...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.