Abstract
Cholinergic modulation of heart rate in isolated spontaneously beating single cells from the rabbit sino-atrial node was investigated by measuring transmembrane ionic currents using the nystatin-perforated patch whole-cell voltage-clamp technique. Carbamylcholine (CCh), a stable analogue of acetylcholine (ACh), significantly inhibited L-type calcium currents (Ica(L) which had been augmented by beta-adrenergic stimulation. In addition, CCh activated a potassium outward current (IK(ACh)). Both effects were blocked by atropine. The possible involvement of nitric oxide (NO) in these responses was evaluated by inhibiting NO synthesis. In the presence of NG-monomethyl-L-arginine (L-NMMA, 100 microM) or nitro-L-arginine methyl ester (L-NAME, 1 mM), two specific inhibitors of nitric oxide synthase (NOS), CCh no longer inhibited ICa(L). IK(ACh) could still be activated. Co-incubation of cells in L-NAME or in L-NMMA with arginine (the endogenous substrate of NOS) restored the CCh-induced attenuation of ICa(L), indicating that L-NAME or L-NMMA did not interfere directly with the muscarinic action of CCh on ICa(L). Effects of the NO-releasing agent molsidomine (SIN-1) on CCh-induced changes in ICa(L) were also investigated. After ICa(L) had been augmented by beta-adrenergic stimulation, SIN-1 (0.1 mM) inhibited ICa(L); however, SIN-1 had no further inhibitory effect after a maximal CCh concentration had been applied. These findings suggest that NO generation is an obligatory process in cholinergic inhibition of ICa(L) in mammalian cardiac pacemaker tissue.