Ca depletion of the sarcolemma--ultrastructural changes

In the adult cardiac myocyte the functional and structural integrity of the sarcolemma depends on the presence of calcium ions in the extracellular fluid. Ultrastructural damage to the myocardial cell after Ca is repleted following brief periods of Ca depletion includes (1) sarcolemmal disruption, (2) mitochondrial damage and (3) severely contracted myofibrils. Ca depletion is the necessary first step in the generation of the paradox, presumably altering the sarcolemmal permeability to Ca. The structural basis for the altered membrane permeability is not known. In the adult rabbit heart two major sarcolemmal defects occur with Ca depletion: (1) 90% of Ca-depleted cells have separation of the layers of the glycocalyx over 30% of their surface. (2) Within the bilayer the intramembrane particles on the P face appear to be reoriented. Hypothermia and cadmium substitution protect against the functional manifestations of the paradox and prevent separation of the glycocalyx and the intramembrane particle changes as well. While the paradox is found in many species, the adult rat myocardium appears to be functionally more sensitive than the adult rabbit. With Ca depletion the adult rat has 65.9 +/- 4.6% of its myocardial cell surface separated while the adult rabbit has 33.4 +/- 2.1%. On the other hand, both the rat and rabbit are insensitive to the paradox before 6 days of age. Three-day old rat hearts exposed for 20 min to [Ca] less than 10(-8) mol l-1 show an intact glycocalyx in 95% of their cells. By 20 days all cells show separation of the glycocalyx over 38.5% +/- 1.6% of their surface.(ABSTRACT TRUNCATED AT 250 WORDS)