NA+ CA2+ EXCHANGE CURRENT AND CONTRACTIONS MEASURED UNDER CL--FREE CONDITIONS IN DEVELOPING RABBIT HEARTS/

Previous studies suggesting a greater functional role of cardiac Na+/C a2+ exchange at birth were performed using tightly buffered free cytos olic Ca2+ concentration ([Ca2+](i)). Because Na+/Ca2+ exchange current (I-NaCa) is influenced by physiological fluctuations in [Ca2+](i), we used conditions of minimally buffered [Ca2+](i) to simultaneously rec ord I-NaCa and cell contractions in single ventricular myocytes isolat ed from 1- to 27-day-old and adult rabbits. With conventional Cl--cont aining solutions, Ni2+-sensitive outward and inward charge movements w ere unbalanced, suggesting the presence of a contaminating current (pr esumably the Ca2+-activated Cl- current). Removing Cl- abolished this discrepancy in all age groups and allowed for the accurate quantitatio n of I-NaCa. Under Cl--free conditions, outward and inward charge move ments were high at birth (4 days: 0.42 +/- 0.03 and -0.38 +/- 0.04 pC/ pF, respectively) and decreased postnatally (adult: 0.08 +/- 0.01 and -0.07 +/- 0.01 pC/pF, respectively). Newborn but not adult myocytes co ntracted during depolarizations in the presence of nifedipine, ryanodi ne, and thapsigargin. The magnitudes of outward charge movement (Ca2influx) and cell shortening exhibited similar voltage dependence, cons istent with I-NaCa-mediated contractions. These results indicate that I-NaCa can directly support contraction in newborn rabbit ventricular myocytes.