Shape, Size, and Distribution of Ca2+ Release Units and Couplons in Skeletal and Cardiac Muscles

Excitation contraction (e-c) coupling in skeletal and cardiac muscles involves an interaction between specialized junctional domains of the sarcoplasmic reticulum (SR) and of exterior membranes (either surface membrane or transverse (T) tubules). This interaction occurs at special structures named calcium release units (CRUs). CRUs contain two proteins essential to e-c coupling: dihydropyridine receptors (DHPRs), L-type Ca2+ channels of exterior membranes; and ryanodine receptors (RyRs), the Ca2+ release channels of the SR. Special CRUs in cardiac muscle are constituted by SR domains bearing RyRs that are not associated with exterior membranes (the corbular and extended junctional SR or EjSR). Functional groupings of RyRs and DHPRs within calcium release units have been named couplons, and the term is also loosely applied to the EjSR of cardiac muscle. Knowledge of the structure, geometry, and disposition of couplons is essential to understand the mechanism of Ca2+ release during muscle activation. This paper presents a compilation of quantitative data on couplons in a variety of skeletal and cardiac muscles, which is useful in modeling calcium release events, both macroscopic and microscopic ("sparks").