Immunogold-labeled L-type calcium channels are clustered in the surface plasma membrane overlying junctional sarcoplasmic reticulum in guinea-pig myocytes - Implications for excitation-contraction coupling in cardiac muscle

Ca2+ release through ryanodine receptors, located in the membrane of the ju nctional sarcoplasmic reticulum (SR), initiates contraction of cardiac musc le. Ca2+ influx through plasma membrane L-type Ca2+ channels is thought to be an important trigger for opening ryanodine receptors ("Ca2+ induced Ca2-release"). Optimal transmission of the transmembrane Ca2+ influx signal to SR release is predicted to involve spatial juxtaposition of L-type Ca2+ ch annels to the ryanodine receptors of the junctional SR, Although such spati al coupling has often been implicitly assumed, and data from immunofluoresc ence microscopy are consistent with its existence, the definitive demonstra tion of such a structural organization in mammalian tissue is lacking at th e electron-microscopic level. To determine the spatial distribution of plas ma membrane L-type Ca2+ channels and their location in relation to underlyi ng junctional SR, we applied two high-resolution immunogold-labeling techni ques, label-fracture and cryothin-sectioning, combined with quantitative an alysis, to guinea-pig ventricular myocytes. Label-fracture enabled visualiz ation of colloidal gold-labeled L-type Ca2+ channels in planar freeze-fract ure electron-microscopic views of the plasma membrane. Mathematical analysi s of the gold label distribution (by nearest-neighbor distance distribution and the radial distribution function) demonstrated genuine clustering of t he labeled channels. Gold-labeled cryosections showed that labeled L-type C a2+ channels quantitatively predominated in domains of the plasma membrane overlying junctional SR. These findings provide an ultrastructural basis fo r functional coupling between L-type Ca2+ channels and junctional SR and fo r excitation-contraction coupling in guinea-pig cardiac muscle. (C) 2000 Ac ademic Press.