Cardiac Glycoside Chronotropic and Arrhythmogenic Effects in Sinoatrial Nodal Pacemaker Cells (SANC) Occur Along a Continuum of Electrochemical Gradients of Na+ (ENa) and Ca2+ (ECa)

Abstract

Cardiac glycosides reduce ENa and ECa (due to an increase of Na+i via Na-K pump inhibition, and of Ca2+i, due to a secondary reduction in Ca efflux for Na influx via NaCa exchange). Here we show that exposure of single rabbit SANC to the cardiac glycoside, digoxigenin (10-20μM) results in a continuum of time-dependent effects. Within 30s to 1 min, the rate of rhythmic spontaneous action potentials (AP) increases by 20% (n=3) and this is associated with an earlier occurrence (reduced period) of local sub-membrane Ca2+ releases (LCR's) during diastolic depolarization, detected by confocal Ca2+ imaging. Approximately 1-3 minutes following AP rate acceleration, LCR period lengthens by 40%, accompanied by a similar reduction in the rhythmic AP rate. The changes in LCR period during the biphsic changes in rhythmic AP firing rate increase are highly correlated with the changes in AP cycle length (R2=0.98). A progressive increase in the steady level of diastolic Ca2+ beneath the surface membrane then ensues usually within 4 to 6 additional minutes, LCR's became undetectable, and dysrhythmic and chaotic AP firing occurs. Numerical model simulations (Maltsev-Lakatta model, AJP 2009) in which Nai was increased progressively 5-15mM during glycoside exposure reproduced the experimental results. That rate and rhythm regulation of SANC AP firing during cardiac glycoside exposure occurs along a continuum of ENa/ECa is in agreement with repeated observations over the last decade, showing that the SANC spontaneous AP firing rate is critically dependent on the timing of acute changes in sub-membrane ECa during DD caused by LCR occurrence (LCR period) that accelerates DD by activation of an inward Na/Ca exchange current.