Abstract 494: Differences in Sarcoplasmic Reticulum Ca2+ Loading and Spontaneous Ca2+ Release in Permeabilized Sinoatrial Nodal and Ventricular Myocytes: Effect of cAMP.
To maintain their normal automaticity, sinoatrial nodal cells (SANC) have a unique property to generate spontaneous, local Ca2+ release (LCR) from sarcoplasmic reticulum (SR) under physiological conditions. To further characterize the mechanisms that underlie LCR’s, we compared SR Ca2+ loading and release in saponin permeabilized, rabbit SANC and ventricular myocytes (VM) over a wide range of free [Ca2+] in a physiological buffer containing 0.5 mM EGTA. The SR Ca2+ load, indexed as Ca2+ released by a rapid application of caffeine, was assessed by a confocal linescan image as the amplitude of Fluo-4 of Ca2+ fluorescent transient (F/F0); total spontaneous LCR signal mass during a 400 ms period over 100 μm was assessed by integrating the fluorescence of each LCR and summating the integral of all LCR’s. Over a range of free [Ca2+] from 50 to 200 nM SR Ca2+ load was the same in two cell types (Fig. A⇓), but at 250 nM SR load it doubled in VM, but not in SANC. Failure of SANC SR load to further increase may be attributed to the exaggerated spontaneous LCR: at 200 nM free Ca2+, spontaneous LCR signal mass was 4-times greater in SANC than in VM. A greater LCR signal mass in SANC than in VM at an equal or lesser SR Ca2+ load indicates that spontaneous Ca2+ release mechanisms differ in 2 cell types, possibly, on the basis of a recent report of high basal levels of cAMP/PKA phosphorylation in SANC. When VM were exposed to cAMP (10 μM) spontaneous LCR signal mass markedly increased (Fig. B⇓) at a constant Ca2+ load (150 nM [Ca2+]i Fig. A⇓). Thus, in physiological [Ca2+], spontaneous LCR’s differ in SANC vs VM, and this may be explained, in part at least, by high basal cAMP/PKA phosphorylation of Ca2+ cycling proteins in SANC.