Developmental changes in membrane Ca²⁺ and K⁺ currents in fetal, neonatal, and adult rabbit ventricular myocytes

Whole-cell calcium current (I(Ca)) and inwardly rectifying potassium current (I(K1)) were studied in 21-day fetal, 28-day fetal (total gestation, 31 days), 2-5-day neonatal, and adult rabbit ventricular myocytes isolated by enzymatic dissociation. Whole-cell peak I(Ca) and I(K1) at -100 mV increased significantly after birth. Cell size approximated from cell membrane capacitance also increased with age, with the most significant increase occurring after birth. When normalized to cell surface area, peak I(Ca) density increased from day 21 of gestation to the neonate and then increased again from neonate to adult. In all age groups, peak I(Ca) occurred at a test potential of +10 mV, and the shape of the Ca²⁺ current-voltage relation did not change with age. These findings suggest that there are no significant developmental changes in the voltage dependence of I(Ca). Therefore, the measured age-related increase in Ca²⁺ current density may result from increased channel expression. I(K1) also exhibited a pattern of increasing current density with age. For I(K1), the increase in current density was most rapid between day 21 and the perinatal period and much slower after birth. These results demonstrate that I(Ca) and I(K1) undergo significant changes during late fetal and postnatal development.