Developmental differences in cardiac myocyte calcium homeostasis after steady-state potassium depolarization: Mechanisms and implications for cardioplegia

We previously reported developmental differences in the response of cytosolic free calcium ((Ca_i)) to extracellular potassium ((K_ex)) depolarization (cardioplegia) in juvenile (4 weeks post partum) and mature (12 to 18 months post partum) cardiac myocytes (rabbit). Our present study explored the physiologic basis for these observations. Single calcium-tolerant cardiac myocytes were isolated by sequential exposure to proteolytic agents, loaded with a fluorescent probe for calcium (fura-2) and (Ca_i) measured by standard fluorescence techniques. The response of (Ca_i) to (K_ex) depolarization (30 mmol/L) was determined in the presence of varying levels of extracellular calcium (Ca_ex), verapamil, ouabain, and amiloride. At nominal levels of (Ca_ex) (0 mmol/L), no significant increases in (Ca_i) from the juvenile (2.1% ± 3.3%) or mature (8.5% ± 3.1%) myocytes were seen. At increasing (Ca_ex), there was a progressive increase in the response of (Ca_i) to (K_ex) depolarization in mature animals' cells (191.8% ± 40.7%) but not in the juveniles' cells (28.0% ± 11.5%). Exposure to verapamil resulted in an approximate 80% reduction in relative increase in (Ca_i) in the mature compared With 60% in the juvenile myocytes. Finally, ouabain exposure resulted in a significant increase in the relative change of (Ca_i) in juvenile cells (30% to 126.4% ± 39.4%) but not in the adults' cells. This increase in the juvenile myocytes was blocked by amiloride. We conclude that increases in cardiac myocyte (Ca_i) after (K_ex) depolarization occur predominantly through the calcium channel in the mature animal and through sodium-calcium exchange in the juvenile.