Molecular architecture of membranes involved in excitation-contraction coupling of cardiac muscle.

Peripheral couplings are junctions between the sarcoplasmic reticulum (SR) and the surface membrane (SM). Feet occupy the SR/SM junctional gap and are identified as the SR calcium release channels, or ryanodine receptors (RyRs). In cardiac muscle, the activation of RyRs during excitation-contraction (e-c) coupling is initiated by surface membrane depolarization, followed by the opening of surface membrane calcium channels, the dihydropyridine receptors (DHPRs). We have studied the disposition of DHPRs and RyRs, and the structure of peripheral couplings in chick myocardium, a muscle that has no transverse tubules. Immunolabeling shows colocalization of RyRs and DHPRs in clusters at the fiber's periphery. The positions of DHPR and RyR clusters change coincidentally during development. Freeze-fracture of the surface membrane reveals the presence of domains (junctional domains) occupied by clusters of large particles. Junctional domains in the surface membrane and arrays of feet in the junctional gap have similar sizes and corresponding positions during development, suggesting that both are components of peripheral couplings. As opposed to skeletal muscle, membrane particles in junctional domains of cardiac muscle do not form tetrads. Thus, despite their proximity to the feet, they do not appear to be specifically associated with them. Two observations establish the identify of the structurally identified feet arrays/junctional domain complexes with the immunocytochemically defined RyRs/DHPRs coclusters: the concomitant changes during development and the identification of feet as the cytoplasmic domains of RyRs. We suggest that the large particles in junctional domains of the surface membrane represent DHPRs. These observations have two important functional consequences. First, the apposition of DHPRs and RyRs indicates that most of the inward calcium current flows into the restricted space where feet are located. Secondly, contrary to skeletal muscle, presumptive DHPRs do not show a specific association with the feet, which is consistent with a less direct role of charge movement in cardiac than in skeletal e-c coupling.