Ionic basis of ryanodine's negative chronotropic effect on pacemaker cells isolated from the sinoatrial node.

Spontaneous electrical activity and indo 1 fluorescence ratios were recorded simultaneously in cultured pacemaker cells isolated from the rabbit sinoatrial node. Ryanodine (10 μM) reduced the amplitude of action potential-induced intracellular Ca2+([Formula: see text]) transients by 19 ± 3%, increased the time constant for their decay by 51 ± 5%, and slowed spontaneous firing by 32 ± 3%. 1,2-Bis(2-aminophenoxy)ethane- N, N, N', N'-tetraacetic acid (BAPTA)-acetoxymethyl ester (AM; 25 μM) inhibited the [Formula: see text] transients and slowed spontaneous firing by 28 ± 4%. Ryanodine did not alter hyperpolarization-activated or time-independent inward current, but it reduced the sum of L- and T-type Ca2+ currents ( I Ca,L and I Ca,T) in both the presence and absence of BAPTA-AM. In contrast, I Ca,L was unchanged by ryanodine. Slow inward current tails, presumed to be Na/Ca exchange current ( I Na/Ca), were abolished by BAPTA or ryanodine. The results suggest that a decrement of I Ca,T, due to reduction of the intracellular Ca2+ concentration or a direct effect of ryanodine on T-type Ca2+channels, contributes to the negative chronotropic effect. Another possibility, based primarily on theory and results in other preparations, is that a reduction of I Na/Ca, as a consequence of the smaller action potential-induced[Formula: see text] transients, contributes to the effect of ryanodine.