Developmental changes in calcium currents of rabbit ventricular cells

Abstract

We investigated the postnatal development of L-type Ca2+ current (I(Ca)) in enzymatically isolated adult (AD) and newborn (NB) (1-3-day-old) rabbit ventricular cells using the whole-cell, patch-clamp method. I(Ca) was recorded with C(s)+-rich pipettes and a Na+- and K+ free bath solution at 36°C to eliminate other currents. I(Ca) density (obtained by normalizing I(Ca) to the cell capacitance) was significantly higher in AD cells than in NB cells at potential levels between 0 and +50 mV with 1.8 mM Ca2+ as the charge carrier. There was no shift in the current-voltage relation between AD and NB cells. The maximum I(C(a) density was 9.9 ± 2.0 pA/pF at 14 ± 5 mV in AD cells (n = 11) compared with 5.6 ± 2.0 pA/pF at 13 ± 5 mV in NB cells (n = 7) (mean ± SD). Time to half inactivation (T( 1/2 ) showed a nearly U-shaped relation to membrane potentials from -10 to +30 mV with the shortest T( 1/2 ) at the potential giving the maximum I(Ca) density in both groups. T( 1/2 ) at 0 and +10 mV was slightly but significantly longer in NB cells (16.8 ± 4.6 and 13.5 ± 2.4 msec, respectively) than in AD cells (12.6 ± 3.0 and 10.6 ± 1.5 msec). Replacement of (Ca)2+ with equimolar Ba2+ caused a shift in the current-voltage relation of about 10 mV in the negative direction in both groups and an increase in current density with the maximum Ba2+ current density of 18.1 ± 8.1 pA/pF at 1 ± 6 mV for AD (n = 8) and 8.6 ± 1.5 pA/pF at -2 ± 4 mV for NB cells (n = 5). Ba2+ also caused a prominent prolongation of T( 1/2 ) in both groups, with a shortening of T( 1/2 ) as test potential increased from -10 to + 30 mV, suggesting that the inactivation for the Ba2+ current is mainly dependent on voltage. The voltage dependency of T( 1/2 ) was not different between the two groups. Along with the result of T( 1/2 ) for the Ba2+ current, there was no difference in the steady-state inactivation of I(Ca) for AD versus NB cells, suggesting that the shorter T( 1/2 ) for AD cells is mainly due to the increase in a Ca2+ dependent component of I(Ca) inactivation because of its higher I(Ca) density. The steady-state voltage dependence of activation of I(Ca) was also the same for AD versus NB cells. Lowering temperature from 36°C to room temperature caused a decrease in I(Ca) both in AD and NB cells by about 40%, but the AD/NB I(Ca) ratios were very similar at the two temperatures: the ratio was 1.77 at 36°C and 1.94 at room temperature. A similar AD/NB I(Ca) ratio was obtained at room temperature with 130 mM NaCl in the external solution. We conclude that L-type I(Ca) density (pA/pF) in rabbit ventricular cells increases after the postnatal period without changing the voltage-dependent activation and inactivation properties.