The long and short of calcium-dependent automaticity in the sinoatrial node.

DEBATE ON THE IONIC BASIS of automaticity in the sinoatrial node (SAN) has persisted since the onset of the field of cardiac cellular electrophysiology, with the favored theory replaced or refined periodically (see Refs. 1, 16, 17, 21, 24–26). For many years, the accepted concept was that of a decaying repolarizing K current (IK2) against the background of a constant inward current (22). In the 1980s this was disproven [although the role of rapid delayed rectifier K current (IKr) deactivation has been explored (5, 14)] and replaced by a dominant role for a time-dependent, hyperpolarization-activated inward current (If) (6, 7, 10). In subsequent decades, various other inward membrane currents have been put forth as contributors or dominant players in cardiac automaticity (11, 23–25). Most recently, a strong argument has been made for a critical if not exclusive role of the Na /Ca exchanger (2, 17). The idea of Ca homeostasis and the Na /Ca exchanger being involved in SAN automaticity is not new (3, 13, 29). However, the most recent variation of this proposal, typically referred to as the “Ca clock” mechanism, greatly minimizes the role of membrane channels in initiating SAN automaticity or modulating rate in response to adrenergic stimulation. Instead, spontaneous and cyclical local Ca release from the sarcoplasmic reticulum (SR) is the primary mechanism driving both basal and adrenergically stimulated rate, with membrane channels having at best a minor modulatory role. In this model, the depletion of the Ca source driving the Na /Ca exchanger would be expected to result in the immediate cessation of automaticity. This has led to a spirited debate in the literature between the “membrane” (most often If) and “Ca clock” advocates (16), a debate that can create the impression that these are largely independent and mutually exclusive mechanisms. Proponents of a major role of If cite the fact that the hyperpolarizationactivated cyclic nucleotide-gated (HCN) gene family (the molecular correlate of If) is highly expressed in the SAN and other automatic tissues, that human mutations in HCN4 are associated with sinus rhythm abnormalities, and that the selective If blocker ivabradine slows sinus rate in humans (see Refs. 1, 8, 9). Proponents of the Ca clock mechanism cite the fact that If blockers only slow but do not stop automaticity, whereas ryanodine, which depletes SR Ca stores, can result in a complete cessation of automaticity. Also cited are reports that the rate response to adrenergic agonists is markedly attenuated following ryanodine. Confounding this interpretation is the observation that different laboratories find a wide range of results with ryanodine in isolated SAN cells, varying from no effect to full cessation of automaticity. When a more modest effect is observed, Ca clock proponents argue that the drug was not used at a sufficiently high concentration or for a sufficiently long time period (see Refs. 15, 17, 18). What these latter arguments do not always consider, beyond the potential nonspecific effects of high doses of ryanodine (19), is that 1) sustained Ca depletion may result in secondary effects on cellular homeostasis beyond simply eliminating an immediate Ca source to drive the Na /Ca exchanger and that 2) some of these secondary effects may in turn impact membrane channel function. In other words, the “membrane” and “Ca clock” mechanisms may be interdependent, and some of the apparent Ca dependence may be indirect. Recent observations (20, 28) that the SAN expresses a Ca -stimulated adenylyl cyclase isoform (AC1 or AC8), rather than the typical cardiac isoform (AC5 or AC6), provide support for such interdependence. In fact, when Ca is depleted with ryanodine, not only is the isoproterenol effect on rate greatly reduced (supporting the Ca clock mechanism), but the isoproterenol effect on If is also lost, but not that of membrane permeable cAMP (4), arguing for an effect of Ca homeostasis on adrenergic signaling and subsequent If responsiveness and thus supporting the interdependence of the two mechanisms. Also relevant are reports indicating that the inhibition of CaMKII reduces L-type Ca current and automaticity and that CaMKII activity is dependent on local Ca release (27). The article by Himeno et al. (12) in this issue of the American Journal of Physiology-Heart and Circulatory Physiology reexamines the question of Ca dependence of SAN automaticity using a fresh approach. Here the emphasis is on rapid (within a few seconds) depletion of intracellular Ca , thereby providing the possibility of separating direct and secondary effects of disrupted Ca homeostasis. In addition, they compare their experimental observations with the predictions of two computer models, one reliant on membrane channels (M model) to drive automaticity and the other dependent on local Ca release driving the Na /Ca exchanger (C model). As one might expect, the C model predicts that Ca chelation results in a rapid suppression of automaticity, whereas the M model predicts no effect of chelation on pacemaking. They then used 10 mM BAPTA in the pipet (rather than the more common introduction of membrane permeable BAPTA-AM) to rapidly reduce cytosolic Ca , resulting in the cessation of contraction, but not spontaneous activity, within a few seconds. High doses of SR blockers also failed to acutely alter pacemaking despite rapidly eliminating contractions. Taken together, these results argue against the Ca clock mechanism being the major driver of automaticity on a beat to beat basis. However, the authors also acknowledge that, when cytosolic Ca is depleted for an extended period, pacemaking is affected; automaticity ceased after 5 min of BAPTA dialysis through the pipet. While this is not the focus of the study by Himeno et al. (12) and the observation is not pursued, this result raises the intriguing possibility that sustained cytosolic Ca depletion leads to other effects that impact automaticity, possibly involving L-type Ca current rundown induced secondarily to the inhibition of CaMKII and/or the effects on the Ca -stimulated AC. Address for reprint requests and other correspondence: R. B. Robinson, Columbia Univ. Medical Ctr., Dept. of Pharmacology, 630 W. 168th St., Rm. PH7W-318, New York, NY 10032 (e-mail: [email protected]). Am J Physiol Heart Circ Physiol 300: H31–H32, 2011; doi:10.1152/ajpheart.01083.2010. Editorial Focus