Ca depletion and repletion in rat heart: age-dependent changes in the sarcolemma

Abstract

Rat hearts perfused with Ca (Ca)-free medium exhibit an age-dependent response to the Ca paradox [chizzonite and Zak, 1981]. Below 6 days the hearts were insensitive to Ca depletion. By 15 days the hearts developed a sensitivity to Ca depletion monitored by the development of contracture and release of intracellular enzymes on return to Ca. The present study compared the structure of the cell surface in the neonatal rat and the adult rat after Ca depletion and repletion. The study was carried out to see if separation of the layers of the glycocalyx (surface coat-external lamina) correlated with the age dependence of the Ca paradox. Langendorff-perfused 3-day hearts exposed for 20 min to < 10-8 M Ca had an intact glycocalyx in 95% of their cells. By 20 days the hearts had separation between layers of glycocalyx over 38.5 .+-. 1.6% of their surface in all cells (40) examined. Adult hearts had separation of external lamina in 65.9 .+-. 4.6% of their surface. La used as a cytochemical probe for intact membrane permeability was excluded from the cytoplasm of 3-day hearts after both Ca depletion and repletion. Twenty- to 26-day-old hearts on the other hand had La deposits throughout the cytoplasm. In addition, intracellular enzyme release (creatine kinase) after Ca depletion and repletion was negligible at 3 days but rose to 1152 .+-. 163 IU/g dry wt for 22-day-old hearts. The parallelism between the development of ultrastructural changes in the cell surface and the pattern of increasing functional sensitivity to Ca loss in the neonate supports the idea of a role for the glycocalyx in the regulation of Ca permeability.