SR Ca2+ store refill--a key factor in cardiac pacemaking.

This study presents a theoretical analysis of the role of store Ca(2+) uptake on sinoatrial node (SAN) cell pacemaking. Two mechanisms have been shown to be involved in SAN pacemaking, these being: 1) the membrane oscillator model where rhythm generation is based on the interaction of voltage-dependent membrane ion channels and, 2) the store oscillator model where cyclical release of Ca(2+) from intracellular Ca(2+) stores depolarizes the membrane through activation of the sodium-calcium exchanger (NCX). The relative roles of these oscillators in generation and modulation of pacemaker rate have been vigorously debated and have many consequences. The main new outcomes of our study are: 1) uptake of Ca(2+) by intracellular Ca(2+) stores increases the maximum diastolic potential (MDP) by reducing the cytosolic Ca(2+) concentration Ca(2+) and hence decreasing the NCX current; 2) this hyperpolarization enhances recruitment of key pacemaker currents (e.g. the hyperpolarization-activated HCN current (I(f)) and T-type Ca(2+) current (I(T-Ca))); 3) the resultant enhanced Ca(2+) entry during the pacemaker depolarization increases Ca(2+) causing advancement of the store Ca(2+) release cycle and increased NCX current. In overview, the novel feature of our study is an investigation of the role of store Ca(2+) uptake on SAN pacemaking. This occurs during the early diastolic period and causes enhanced I(f), I(T-Ca) and store release (and hence I(NCX)) during the later diastolic period. There is thus a symbiotic interaction between the two pacemaker "clocks" over the entire diastolic period, this providing robust and highly malleable SAN pacemaking. Accounting for store Ca(2+) uptake also provides insight into hitherto unexplained SAN behaviour, as we exemplify for the sinus bradycardia exhibited in catecholaminergic polymorphic ventricular tachycardia (CPVT).