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

Abstract

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.

Phylogeny and effects of anoxia on hyperpolarization-activated, cyclic nucleotide-gated channel gene expression in the heart of a primitive chordate, the Pacific Hagfish (eptatretus stoutii)

“…Therefore, it is possible that regulation of HCN3a may play a role, amongst others, in controlling heart rate during prolonged anoxia and subsequent recovery; however, these hypotheses need to be tested using protein expression studies. The above discussion considered the HCN channel as the primary vertebrate cardiac pacemaker, yet a recent alternative hypothesis suggests that pacemaker activity involves a calcium clock (Rubenstein and Lipsius, 1989;Li et al, 1997;Hüser et al, 2000;Ju and Allen, 2000;Dobrzynski et al, 2007;Maltsev and Lakatta, 2008;Monfredi et al, 2013). In brief, this hypothesis proposes that spontaneous Ca 2+ sparks released from the sarcoplasmic reticulum via ryanodine receptors interact with local sodium-calcium exchanger proteins to cause an overall inward depolarizing current.…”

New insights into the molecular and cellular workings of the cardiac Na+/Ca2+ exchanger

“…The rise of cytoplasmic Ca 2ϩ causes an increased Ca 2ϩ extrusion by Na ϩ /Ca 2ϩ exchange in the diastole period and with that a "delayed" Na ϩ /Ca 2ϩ exchange current that promotes generation of the next action potential. Quantitatively, the possible contribution of this mechanism, if present, cannot be of large magnitude, because inhibition of sarcoplasmic reticulum Ca 2ϩ recycling by ryanodine has only modest negative chronotropic effects, ϳ19%, in true pacemaker cells (16). The second possibility, then, concerns the exchanger's Na ϩ leak function, which is predicted to continue unabated through the diastolic period (15).…”

Roles of hyperpolarization-activated current If in sinoatrial node pacemaking: insights from bifurcation analysis of mathematical models

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(27,35,41,55), yielding four principal hypotheses that attribute pacemaker depolarization primarily to 1) activation of the L-type Ca 2ϩ channel current (I Ca,L ) (58); 2) deactivation of the delayedrectifier K ϩ current (I K ) (39); 3) development of the hyperpolarization-activated cation current (I f ) (9); and 4) spontaneous Ca 2ϩ release from sarcoplasmic reticulum (SR) and subsequent activation of the Na ϩ /Ca 2ϩ exchanger current (I NaCa ) (28,30,33,34,45). Nevertheless, there is no dominant pacemaker mechanism, and the roles of individual time-dependent, voltage-gated channel currents in pacemaker generation remained to be clarified.

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