Influence of postnatal changes in action potential duration on Na-Ca exchange in rabbit ventricular myocytes.

Cardiac Na-Ca exchanger (NCX) expression and current density are significantly greater in newborn rabbit hearts compared with adults. However, the relatively short action potential (AP) at birth may limit the impact of increased NCX expression by diminishing Ca2+ entry via Na-Ca exchange current (INaCa). To address the interdependence of AP duration and NCX activity, we voltage-clamped newborn (NB, 1-5 day), juvenile (JV, 10-14 day) and adult (AD) rabbit myocytes with a series of APs of progressively increasing duration (APD90: 108-378 ms) under nominally chloride-free conditions. In each age group we quantified an increase in outward (QExout) and inward (QExin) Ni2+-sensitive charge movement in response to AP prolongation. QExout and QExin measured during age-appropriate APs declined postnatally [QEXout: NB (2 day) 0.19 +/- 0.02, JV (10 day) 0.10 +/- 0.01, AD 0.04 +/- 0.002; QEXin: NB -0. 2 +/- 0.01, JV -0.11 +/- 0.02; AD -0.04 +/- 0.003 pC/pF] despite the significantly shorter APD90 of newborn myocytes (NB 122 +/- 10; AD 268 +/- 22 ms). When Ca2+ fluxes by other transport pathways were blocked with nifedipine, ryanodine and thapsigargin, age-appropriate APs elicited contractions in NB and JV but not AD myocytes (NB 4.8 +/- 0.5, JV 1.2 +/- 0.3% resting length). These data demonstrate that a shorter AP does not negate the impact of increased NCX expression at birth.