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Department of Physiology, Texas Tech University Health Sciences Center, Lubbock, Texas 79430
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+ (Ca2+i) 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 Ca2+i 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 (ICa,L and ICa,T) in both the presence and absence of BAPTA-AM. In contrast, ICa,L was unchanged by ryanodine. Slow inward current tails, presumed to be Na/Ca exchange current (INa/Ca), were abolished by BAPTA or ryanodine. The results suggest that a decrement of ICa,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 INa/Ca, as a consequence of the smaller action potential-induced Ca2+i transients, contributes to the effect of ryanodine.
L-type calcium current; T-type calcium current; sodium/calcium exchange current; indo 1; 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid; perforated-patch voltage-clamp technique
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