Key result
High outward charge movements at birth directly support newborn myocyte contraction before dropping ~81% postnatally.
Why the study?
Previous studies suggested a greater functional role of cardiac Na+/Ca2+ exchange at birth but used tightly buffered free cytosolic Ca2+ concentration, which may not reflect physiological fluctuations.
The study demonstrates that Na+/Ca2+ exchange current can directly support contraction in newborn rabbit ventricular myocytes under physiological conditions.
Answers are grounded in this paper and cite their sources.
Should not alter neonatal management; leaves open NCX role in human infant contraction.
Previous studies suggesting a greater functional role of cardiac Na+/Ca2+ exchange at birth were performed using tightly buffered free cytosolic Ca2+ concentration ([Ca2+]i). Because Na+/Ca2+ exchange current (INaCa) is influenced by physiological fluctuations in [Ca2+]i, we used conditions of minimally buffered [Ca2+]i to simultaneously record INaCa and cell contractions in single ventricular myocytes isolated from 1 to 27-day-old and adult rabbits. With conventional Cl(-)-containing solutions. Ni(2+)-sensitive outward and inward charge movements were unbalanced, suggesting the presence of a contaminating current (presumably the Ca(2+)-activated Cl- current). Removing Cl- abolished this discrepancy in all age groups and allowed for the accurate quantitation of INaCa. Under Cl(-)-free conditions, outward and inward charge movements 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 contracted during depolarizations in the presence of nifedipine, ryanodine, and thapsigargin. The magnitudes of outward charge movement (Ca2+ influx) and cell shortening exhibited similar voltage dependence, consistent with INaCa-mediated contractions. These results indicate that INaCa can directly support contraction in newborn rabbit ventricular myocytes.
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Haddock et al. (1997) studied this question. Cl(-)-free conditions vs. conventional Cl(-)-containing solutions was evaluated on Na+/Ca2+ exchange current (INaCa) and cell contractions. Cl(-)-free conditions revealed outward charge movements were high at birth and decreased postnatally, directly supporting contraction in newborn myocytes.
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