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1 Department of Pharmacology, Columbia University, New York, New York 10032; 2 Department of General Physiology and Biochemistry, Universita di Milano; and Istituto Nazional Milano University Unit, di Fisica della Materia, 20133 Milan, Italy
Although the neonatal
sinus node beats at a faster rate than the adult, when a sodium current
(INa) 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
INa by another current during development. We
used ruptured [T-type calcium current (ICa,T)]
and perforated [L-type calcium current
(ICa,L)] patch clamps to study developmental
changes in calcium currents in sinus node cells from adult and newborn
rabbits. ICa,T density did not differ with age,
and no significant differences were found in the voltage dependence of
activation or inactivation. ICa,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 ICa,L from inactivation was also
slower in the newborn. The results suggest that a smaller but more
negatively activating and rapidly recovering
ICa,L in the adult sinus node may contribute to
the enhanced impulse initiation at this age in the absence of
INa.
Ca current; automaticity; development; diastolic depolarization
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