Effects of Na⁺ on the muscarinic cholinergic receptor of rat ventricular myocytes

The effects of Na⁺ on α₂-adrenergic receptors coupled to inhibition of adenylate cyclase have been well studied. Effects of Na⁺ on other receptor types also linked to adenylate cyclase inhibition are not clear. We therefore studied the effects of Na⁺ on the muscarinic cholinergic receptor of rat ventricular myocardium. Intact ventricular myocytes bound the muscarinic receptor antagonist radioligand [³H]quinuclidinyl benzylate with high affinity (K_d 72 ± 18 pM) to a large number of sites (138,000 ± 25,000/cell). In ventricular membranes, Na⁺ (100 mM) increased receptor affinity for radioligand twofold and decreased receptor affinity for agonist (carbachol) twofold. Sodium was not required for and did not alter the pattern of cholinergic inhibition of adenylate cyclase in ventricular membranes. We conclude that Na⁺ has minimal effects on both binding and function of cardiac cholinergic receptors in rat. Our results suggest that monovalent cations are not important regulators of agonist binding and function of cardiac muscarinic cholinergic receptors.