Species-specific difference in distribution of voltage-gated L-type Ca2+ channels of cardiac myocytes

Ca2+ influx via sarcolemmal voltage-dependent Ca2+ channels (L-type Ca2+ ch annels) is the fundamental step in excitation-contraction (E-C) coupling in cardiac myocytes. Physiological and pharmacological studies reveal species -specific differences in E-C coupling resulting from a difference in the co ntribution of Ca2+ influx and intracellular Ca2+ release to activation of c ontraction. We investigated the distribution of L-type Ca2+ channels in iso lated cardiac myocytes from rabbit and rat ventricle by correlative immunoc onfocal and immunogold electron microscopy. Immunofluorescence labeling rev ealed discrete spots in the surface plasma membrane and transverse (T) tubu les in rabbit myocytes. In rat myocytes, labeling appeared more intense in T tubules than in the surface sarcolemma. Immunogold electron microscopy ex tended these findings, showing that the number of gold particles in the sur face plasma membrane was significantly higher in rabbit than rat myocytes. In rabbit myocyte plasma membrane, the gold particles were distributed as c lusters in both regions that were associated with junctional sarcoplasmic r eticulum and those that were not. The findings are consistent with the idea that influx of Ca2+ via surface sarcolemmal Ca2+ channels contributes to i ntracellular Ca2+ to a greater degree in rabbit than in rat myocytes.