Peculiar Effects of Muscarinic M1, M2, and M3 Receptor Blockers on Cardiac Chronotropic Function in Neonatal Rats

Role of Muscarinic M1, M2, and M3 Receptors in the Regulation of Electrical Activity of Myocardial Tissue of Caval Veins during the Early Postnatal Ontogeny

We studied the influence of blockers of muscarinic M1, M2, and M3 receptors on the effect of acetylcholine in the myocardial tissue of caval veins in rats at the early stage of ontogeny. The experiments were performed on isolated preparations of the right superior vena cava working under their own rhythm. Action potentials were recorded using the standard microelectrode technique. Acetylcholine (1 μM) suppressed automatic activity in the superior vena cava myocardium. Preliminary perfusion of the preparation with non-selective blocker atropine (1 μM) completely abolished the effect of acetylcholine, treatment with M2 receptor blocker AQ-RA 741 (1 μM) led to partial suppression of the effect of acetylcholine. Blockers of M1 and M3 receptors pirenzepine (1 μM) and 4DAMP (0.1 μM) did not suppress the effect of acetylcholine. Thus, the effect of acetylcholine is predominantly realized via M2 receptors, but M3 receptors can also partially mediate its effect in the superior vena cava myocardium in rats at the early stages of ontogeny.

Age-related peculiarities of inotropic response of rat myocardium to selective block of M1-cholinoreceptors

In vitro effect of M1-cholinoreceptor blockade on the cardiac inotropic function was examined in rats aging 1, 3, 6, 8, and 20 weeks. In 1- and 3-week old rat pups, the sympathetic control of the heart has not developed, the age of 7-8 weeks being pubertal. Adult 20-week rats were used as the controls. In rats of all age groups, preliminary blockade of M1-cholinoreceptors did not prevent the inhibitory effect of carbacholine on contractility of the atrial and ventricular myocardium. The inhibitory effect of pirenzepine on the contractile force of ventricular myocardium was revealed in 6-week rats.