L-type but not T-type calcium current changes during postnatal development in rabbit sinoatrial node

Although the neonatal sinus node beats at a faster rate than the adult. when a sodium current (IN.) present in the newborn is blocked, the spontaneous rate is slower in neonatal myocytes than in adult myocytes. This suggests a possible functional substitution Of IN by another current during development. We used ruptured [T-type calcium current (I-Ca.T)] and perforated [L-type calcium current (I-Ca,I-L)] patch clamps to study developmental changes in calcium currents in sinus node cells from adult and newborn rabbits. I-Ca,I-T density did not differ with age, and no significant differences were found in the voltage dependence of activation or inactivation. I-Ca,I-L density was lower in the adult than newborn (12.1 +/- 1.4 vs. 17.6 +/- 2.5 pA/pF, P = 0.049). However, activation and inactivation midpoints were shifted in opposite directions, reducing the potential contribution during late diastolic depolarization in the newborn (activation midpoints -17.3 +/- 0.8 and -22.3 +/- 1.4 mV in the newborn and adult, respectively, P = 0.001; inactivation midpoints -33.4 +/- 1.4 and -28.3 +/- 1.7 mV for the newborn and adult, respectively, P = 0.038). Recovery Of IC,.,L from inactivation was also slower in the newborn. The results suggest that a smaller but more negatively activating and rapidly recovering IC.,,L in the adult sinus node may contribute to the enhanced impulse initiation at this age in the absence of I-Na.