Modulated expression of transient outward current in cultured neonatal rat ventricular myocytes: comparison with development in situ

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Developmental changes of cardiac ion channels have been characterized in freshly isolated mammalian heart cells. To investigate the regulatory factors of postnatal development in transient outward current (Ito) in cardiomyocytes, the modulated expression of Ito was studied in cultured neonatal rat ventricular cells. These changes in vitro were compared with those in situ in acutely isolated ventricular myocytes. Ventricular cells were enzymatically isolated from day-old Wistar rats and cultured under various growth conditions from day 6 to 15. Whole-cell patch-clamp recording was used to study the functional expression of Ito. During development in situ from 5- to 15-day-old stages, Ito density was doubled at day 15 with a significant increase in membrane capacitance (Cm) of the myocytes. Some cells were incubated in serum-rich medium from day 6 to 15 during primary culture, revealing marked increases in both Cm and Ito density at day 15. However, no developmental changes in the Cm and Ito density were observed in serum-free medium. Under the serum-free condition, neither the addition of acidic fibroblast growth factor (aFGF) nor basic FGF (bFGF) to culture medium influenced the Cm. aFGF (10-60 ng/ml) failed to stimulate Ito expression. 72-h treatment with bFGF significantly promoted the Ito density in a concentration-dependent manner; nevertheless, prolonged administration from day 6 to 15 did not induce a further increase, resulting in lower Ito density than in age-matched freshly isolated and serum-treated preparations. The increase in Ito in cultured cells induced by serum and bFGF may be attributable to paralleling changes in the ionic selectivity of the channel, but was not caused by changes in the voltage-dependence of steady-state Ito activation and inactivation. bFGF and some other unknown serum factors may play important roles in the postnatal expression of Ito in the neonatal cardiomyocytes. The developmental increase in Ito and postnatal cell hypertrophy of neonatal cardiomyocytes can be regulated independently.