Distribution, surface density, and membrane area of diadic junctional contacts between plasma membrane and terminal cisterns in mammalian ventricle.

The surface density of diadic junctional complexes (DJC) between plasmalemma and terminal cisternal membrane, as well as the areas of plasmalemmal and cisternal membrane involved in DJC, have been determined morphometrically in external plasmalemmal envelope and T-system of rabbit, rat, and mouse ventricular heart muscle. In all three species, both the surface density and the plasmalemmal area of DJC are 4- to 6-fold greater in the T-system than in the external plasmalemmal envelope. The surface density and DJC membrane area per unit cell volume and per unit myofibrillar volume increase in the order rabbit less than rat congruent to mouse and are not related simply to basal heart rate or intraventricular pressure. The results show that if Ca2+ release is a function of terminal cisterns, then, in ventricular heart muscle, most of the Ca2+ thus released for myofibrillar activation must originate from cisterns associated with the T-system. They make it necessary to consider the possibility that the 20--50% of plasmalemma in the T-system that is involved in excitation-contraction coupling may be unavailable for other processes; and they show that interspecies differences in surface density and membrane area of DJC/unit cellular and myofibrillar volume correlate with differences in Ca2+- activated Ca2+ release in skinned fibers and with differences in rate of tension development described for intact rat and rabbit ventricular muscle.