Heterogeneous expression of Ca2+ handling proteins in rabbit sinoatrial node

We investigated the densities of the L-type Ca2+ current, i(Ca,L), and various Ca2+ handling proteins in rabbit sinoatrial (SA) node. The density Of i(Ca,L), recorded with the whole-cell patch-clamp technique, varied widely in sinoatrial node cells. The density Of i(Ca,L) was significantly (p<0.001) correlated with cell capacitance (measure of cell size) and the density was greater in larger cells (likely to be from the periphery of the SA node) than in smaller cells (likely to be from the center of the SA node). Immunocytochemical labeling of the L-type Ca2+ channel, Na+-Ca2+ exchanger, sarcoplasmic reticulum Ca2+ release channel (RYR2), and sarcoplasmic reticulum Ca2+ pump (SERCA2) also varied widely in SA node cells. in all cases there was significantly (p<0.05) denser labeling of cells from the periphery of the SA node than of cells from the center. In contrast, immunocytochemical labeling of the Na+-K+ pump was similar in peripheral and central cells. We conclude that Ca2+ handling proteins are sparse and poorly organized in the center of the SA node (normally the leading pacemaker site), whereas they are more abundant in the periphery (at the border of the SA node with the surrounding atrial muscle).