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Contractions in neonatal rabbit ventricular myocytes seem to depend predominantly on Ca2+ influx through the Na+-Ca2+ exchanger (NCX). Unexpectedly, neonates are sensitive to the negative inotropic effect of L-type Ca2+ channel blockers. L-type Ca2+ channel blockers depress contractile function indirectly in neonatal myocytes by shortening the action potential duration (APD), thereby decreasing the influx of activator Ca2+ through the NCX. Freshly isolated ventricular myocytes from adult and neonatal (1-5 d) rabbits were electrically stimulated (0.5 Hz; 35 degrees C) while action potential (AP) and Ca2+ transients (Indo-1) were recorded in the absence and presence of nifedipine (10 microM). In separate experiments, cells were voltage-clamped with a constant AP waveform (APD90=170 ms) to determine the effect of nifedipine on Ca2+ transients independent from effects on the AP. Voltage-clamp experiments confirmed that nifedipine blocks L-type Ca2+ current in neonatal myocytes. Nifedipine markedly reduced Ca2+ transient amplitude and APD in both adults (transient=20 +/- 7%; APD90=31 +/- 4% of control) and neonates (transient=38 +/- 10%; APD90=57 +/- 6% of control). When the AP was held constant by voltage clamping, nifedipine significantly reduced the amplitude of Ca2+ transients in adults (27 +/- 9% of control) but had no effect on Ca2+ transient amplitude in neonatal myocytes. These results are consistent with the concept that immature ventricular myocytes are less reliant on L-type calcium channels and are more dependent on NCX for contraction. The negative inotropic effect of L-type Ca2+ channel blockers in neonates is attributable to shortening of the AP.

Negative Inotropic Effect of Nifedipine in the Immature Rabbit Heart Is Due to Shortening of the Action Potential