Effects of acetylcholine and cyclic nucleotides on the bullfrog atrial muscle

Effects of ACh on the slow inward current and tension components of bullfrog atrial muscle were examined with voltage-clamp technique. ACh decreased the phasic component of contractile tension and the slow inward current with a reduction of gCa. Contracture tensions were augmented by ACh and decreased by adrenaline. Intracellularly injected cGMP through microelectrode decreased the overshoot and the plateau level of action potential without affecting the resting membrane potential, indicating a decrease in the slow inward current. cAMP enhanced the overshoot and the plateau. ACh seems to reduce the slow inward current through an elevation of intracellular cGMP concentrations.