β-adrenergic stimulation modulates ryanodine receptor Ca²⁺ release during diastolic depolarization to accelerate pacemaker activity in rabbit sinoatrial nodal cells

It has long been recognized that activation of sympathetic β-adrenoceptors (β-ARs) increases the spontaneous beating rate of sinoatrial nodal cells (SANCs); however, the specific links between stimulation of β-ARs and the resultant increase in firing rate remain an enigma. In the present study, we show that the positive chronotropic effect of β-AR stimulation is critically dependent on localized subsarcolemmal ryanodine receptor (RyR) Ca²⁺ releases during diastolic depolarization (CRDD). Specifically, isoproterenol (ISO; 0.1 μmol/L) induces a 3-fold increase in the number of CRDDs per cycle; a shift to higher CRDD amplitudes (from 2.00±0.04 to 2.17±0.03 F/F₀; P<0.05 [F and F₀ refer to peak and minimal fluorescence]); and an increase in spatial width (from 3.80±0.44 to 5.45±0.47 μm; P<0.05). The net effect results in an augmentation of the amplitude of the local preaction potential subsarcolemmal Ca²⁺ transient that, in turn, accelerates the diastolic depolarization rate, leading to an increase in SANC firing rate. When RyRs are disabled by ryanodine, β-AR stimulation fails to amplify subsarcolemmal Ca²⁺ releases, fails to augment the diastolic depolarization rate, and fails to increase the SANC firing rate, despite preserved β-AR stimulation-induced augmentation of L-type Ca²⁺ current amplitude. Thus, the RyR Ca²⁺ release acts as a switchboard to link β-AR stimulation to an increase in SANC firing rate: recruitment of additional localized CRDDs and partial synchronization of their occurrence by β-AR stimulation lead to an increase in the heart rate.