The structure and function of the myocardial cell surface.

External to the lipid bilayer or unit membrane at the surface of the myocardial cell is the glycocalyx. This coat is approximately 50 nm thick and is composed for two layers, the surface coat (SC) and the external lamina (EL). The SC is an integral part of the sarcolemma and many of its glycoproteins penetrate into or through the lipid bilayer. The glycocalyx invaginates with the unit membrane to fill the transverse tubules of the cell. Both layers contain abundant fixed negatively charged sites and a prominent constitutent of both is sialic acid. Removal of sialic acid produces a large specific increase in sarcolemmal calcium permeability without perturbation of potassium permeability. Sialic acid also accounts for a component of negatively charged sites which, with other acidic mucopolysaccharides, contributes to cationic binding at the surface of the cell. Calcium bound at the surface seems to be of importance in the excitation-contraction (EC) coupling sequence whether as a source of "trigger" calcium for the sarcotubular system or as a direct activator of the myofilaments. The bound Ca appears to be in rapid equilibrium with Ca in the vascular and interstitial spaces and is the probable immediate source of the Ca that crosses the sarcolemma. The integrity of the glycocalyx appears to be necessary in the prevention of uncontrolled entry of Ca into the cell.