Role of New Subtypes of Adrenergic and Muscarinic Receptors in the Control ofCardiac Activity

Overview

Modulation of the basic characteristics of cardiac activity (frequency, power of contraction, rateof conductivity and threshold of stimulation) depend on the steady but ever changing influenceof two practically antagonistic nervous systems (sympathetic and parasympathetic). Themediators of the sympathetic nerve act via adrenoreceptors, the mediator of the parasympat-hetic nerve activates muscarine receptors. These type of receptors belong into the groupG-protein coupled receptors.At first it seemed that in the heart are only beta1-adrenergic receptors and M2-muscarinicreceptors. This idea had the advantage that both receptor systems have antagonistic effectsand this antagonism is manifested most markedly on the activity of adenylate cyclase. Whilebeta1-adrenoreceptors stimulate it, M2-muscarine receptors inhibit it.At present it is beyond doubt that adrenoreceptors are represented by sub-types alpha1, beta1,beta2. The expression of subtype beta3 known for its function in adipose tissue has not beenproved unequivocally but the existence of a further, fourth type of beta-adrenoreceptors,putative beta4 seems very probable. Similarly it is certain that M 2 muscarine receptors are notthe only subtype of muscarine acetylcholine receptors in cardiac tissue.The findings of new subtypes are not merely a morphological characteristics but differentsubtypes play a different role in the modulation of characteristics of cardiac activity, wherebya different activation, depending on the amount of mediator in their environment, is possible.Moreover receptor are not static structures but are able to change their number (and thus alsothe magnitude of response) and this applies to all receptor subtypes found in the heart.

Key words:
cardiac activity - receptors - sympathetic nerve - parasympathetic nerve - G-protein