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

The rate of spontaneous diastolic depolarization (DD) of sinoatrial nodal cells (SANCs) that triggers recurrent action potentials (APs) is a fundamental aspect of the heart's pacemaker. Here, in experiments on isolated SANCs, using confocal microscopy combined with a patch clamp technique, we show that ryanodine receptor Ca²⁺ release during the DD produces a localized subsarcolemmal 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 SANC beating rate with an IC₅₀ of 2.6 μmol/L and abolishes the local Ca²⁺ transients that precede the AP upstroke. In voltage-clamped cells in which the DD was simulated by voltage ramp, 3 μmol/L ryanodine decreased an inward current during the voltage ramp by 1.6±0.3 pA/pF (SEM, n=4) leaving the peak of L-type Ca²⁺ current unchanged. Likewise, acute blockade of the NCX (via rapid substitution of bath Na⁺ by Li⁺ abolished SANC beating and reduced the inward current to a similar extent (1.7±0.4 pA/pF, n=4), as did ryanodine. Thus, in addition to activation/inactivation of multiple ion channels, Ca²⁺ activation of the NCX, because of localized sarcoplasmic reticulum Ca²⁺ release, is a critical element in a chain of molecular interactions that permits the heartbeat to occur and determines its beating rate.