Muscarinic receptor subtypes in the heart--how many and why?

Muscarinic receptors are coupled to G proteins and to date, five subtypes have been characterized. These subtypes differ in their effects on second messenger production. Odd-numbered subtypes activate phospholipase C beta, and then they increase the production of inositoltrisphosphate and diacylglycerol. Even-numbered subtypes inhibit adenylylcyclase. The expression of individual subtypes is quite tissue specific. The heart tissue is realized as M2 predominant organ (in mammals; the muscarinic population in birds consist of M2 and M4 receptors). This article focuses on the possibility of existence of further muscarinic receptor subtypes in avian and mammalian heart in consequences with activation of phosphoinositid metabolism in the heart.