Development of dyadic junctional complexes between sarcoplasmic reticulum and plasmalemma in rabbit left ventricular myocardial cells. Morphometric analysis.

Dyadic junctional couplings between cardiac sarcoplasmic reticulum (SR) and plasma membrane presumably are implicated in release of Ca2+ from terminal cisterns of SR during excitation-contraction coupling. We measured the areas of SR and plasma membrane involved in such couplings in late embryonic and neonatal rabbit left ventricles during a developmental period characterized by rapid cell growth and rapid accumulation of myofibrils. By morphometric methods previously applied to adult hearts, it could be shown that from the inception of the nascent T-system the surface density of dyadic couplings at T-tubular plasmalemma exceeds by about four-fold that at the external plasmalemmal envelope. Before the development of a T-system (similar to or approximately 10 days after birth) surface density of dyads, as well as total dyad areas per unit cell volume and per unit myofibrillar volume, increase progressively during embryonic life until they approach constancy at near adult values one day after birth. Constancy of total dyadic membrane area per unit myofibrillar volume during neonatal cell growth confirms that the membrane area of the activating system and the volume of myofibrils to be activated accumulate in a constant proportion.