Synchronization of Local Calcium Releases (LCRs) in Guinea Pig Single, Isolated SA Node Cells Contributes to Generation of Rhythmic Action Potential-Induced Ca2+ Transients

Abstract

Spontaneous, rhythmic LCRs are an important factor of pacemaker function of SA node cells (SANC). LCRs operate within the context of a coupled-clock system involving interactions of plasma membrane and sarcoplasmic reticulum (SR)-based calcium clocks. To date, the functional importance of this coupled-clock system has been demonstrated in rabbit SANC that beat spontaneously and rhythmically. The contribution of LCRs to action potential firing in guinea pig SANC, however, is controversial.We quantified the entire ensemble of LCRs in healthy appearing, single guinea pig SANC prior to and in the presence of beta-adrenergic stimulation (isoproterenol, 1µM). LCRs were measured by a high-speed 2D-camera and analyzed by a novel computer program capable of identifying LCRs in video-recordings.Although nine of the 22 SANC examined did not exhibit Ca2+ transients, LCRs were present in all cells. LCRs were smaller and more intense in cells without Ca2+ transients than in those with Ca2+ transients (rhythmic or dysrhythmic). Isoproterenol increased LCR size and induced rhythmic Ca2+ transients in majority of cells (6/9) without Ca2+ transients prior to isoproterenol. SANC that exhibited Ca2+ transients prior to isoproterenol generated approximately 80 LCRs/s. Cells in which Ca2+ transients were rhythmic had greater LCR size and amplitude than those with dysrhythmic Ca2+ transients. In cells with dysrhythmic Ca2+ transients, isoproterenol increased LCR size and amplitude and decreased LCR period, concurrently increasing the Ca2+ transient rhythmicity and frequency via presenting a larger and earlier occurring net Ca2+ signal to electrogenic Na+/Ca2+exchanger. Thus, all guinea pig SANC do indeed exhibit LCRs that contribute to generation of rhythmic action potential-induced Ca2+ transients.