Sinoatrial nodal cell ryanodine receptor and Na+-Ca2+ exchanger - Molecular partners in pacemaker regulation

The rate of spontaneous diastolic depolarization (DD) of sinoatrial nodal c ells (SANCs) that triggers recurrent action-potentials (APs) is a fundament al aspect of the heart's pacemaker. Here, in experiments on isolated SANCs. using confocal microscopy combined with a patch clamp technique, we show t hat ryanodine receptor Ca2+ release during the DD produces a localized subs arcolemmal Ca2+ increase that spreads in a wavelike manner by Ca2+-induced Ca2+ release and produces an inward current via the Na+-Ca2+ exchanger (NCX ). Ryanodine, a blocker of the sarcoplasmic reticulum Ca2+ release channel, in a dose-dependent manner reduces the SANG beating rate with an IC50 of 2 .6 mu mol/L and abolishes the local Ca2+ transients that precede the AP ups troke. In voltage-clamped cells in which the DD was simulated by voltage ra mp, 3 mu mol/L ryanodine decreased an inward current during the voltage ram p by 1.6 +/-0.3 pA/pF (SEMI n = 4) leaving the peak of L-type Ca2+ current unchanged. Likewise, acute blockade of the NCX (via rapid substitution of b ath Na+ by Li+) abolished SANG beating and reduced the inward current to a similar extent (1.7 +/-0.4 pA/pF, n = 4), as did ryanodine. Thus, in additi on to activation/inactivation of multiple ion channels, Ca2+ activation of the NCX, because of localized sarcoplasmic reticulum Ca2+ release, is a cri tical element in a chain of molecular interactions that permits the heartbe at to occur and determines its beating rate.