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

Objectives: 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 (I-to) in cardiomyocytes, the modulated expression of I-to was studied in cultured neonatal rat ventricular cells. These changes in vitro were compared with those in situ in acutely isolated ventricular myocytes. Methods: 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 I-to. Results: During development in situ from 5- to 15-day-old stages, I-to density was doubled at day 15 with a significant increase in membrane capacitance (C-m) of the myocytes. Some cells were incubated in serum-rich medium from day 6 to 15 during primary culture, revealing marked increases in both C-m and I-to density at day 15. However, no developmental changes in the C-m and I-to 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 C-m. aFGF (10-60 ng/ml) failed to stimulate I-to expression. 72-h treatment with bFGF significantly promoted the I-to density in a concentration-dependent manner; nevertheless, prolonged administration from day 6 to 15 did not induce a further increase, resulting in lower I-to density than in age-matched freshly isolated and serum-treated preparations. The increase in I-to 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 I-to activation and inactivation. Conclusions: bFGF and some other unknown serum factors may play important roles in the postnatal expression of I-to in neonatal cardiomyocytes. The developmental increase in I-to and postnatal cell hypertrophy of neonatal cardiomyocytes can be regulated independently.