Developmental changes in Ca²⁺ currents from newborn rat cardiomyocytes in primary culture

Abstract

Electrophysiological characteristics of neonatal rat ventricular cardiomyocytes in primary culture were studied using the whole-cell patch-clamp recording technique. Cell size, estimated by measurement of membrane capacitance, was significantly increased throughout the culture from 22.4±5.4 pF at day 2 to 55.0±16.1 pF at day 7, reflecting the hypertrophic process which characterises postnatal cell development. The Ca²⁺ current was investigated at day 2 and 7 of the culture which constituted the early postnatal and maximally developed stages, respectively, of isolated cells in our experimental conditions. At 2 days of culture, two types of Ca²⁺ current could be distinguished, as also observed in freshly dissociated newborn ventricular cells. From their potential dependence and pharmacological characteristics, they could be attributed to the T- (I _Ca-T) and L-type (I _Ca-L) Ca²⁺ current components. After 7 days of culture, only the latterI _Ca-L was present and its density was significantly increased when compared to the density in 2-day-old cells, but lower than that obtained in freshly dissociated adult cells. As the age of the culture progressed, the steady-state inactivation curve was shifted toward negative potentials, in the direction of the inactivation curve obtained for adult cells. Compared to the serum-free control conditions, the density ofI _Ca-L was significantly increased in the presence of fetal calf serum throughout the culture. Consequently, the density ofI _Ca-L obtained in 7-day-old cells was similar to the density ofI _Ca-L obtained in freshly dissociated adult cardiac cells. These results show that in rat neonatal ventricular cardiomyocytes, the changes in Ca²⁺ current during development in primary culture can be compared to that observed in vivo during the first weeks of the postnatal period. The data suggest that the composition of the culture medium is a conditioning factor in the development of cardiac cells in culture. However, the determination and the role of specific factors contained in the serum need to be investigated. The data are also discussed in terms of a possible correlation between the expression and maturation of the Ca²⁺ current components and the capabilities of the neonatal cardiac cells to proliferate and/or to hypertrophy. For these reasons primary cultures of neonatal rat cardiac cells could constitute a valuable in vitro model for studies of postnatal development.