Effects of Bay K 8644 on L-type calcium current from newborn rat cardiomyocytes in primary culture

We examined the effects of the dihydropyridine agonist Bay K 8644 on L-type calcium channels in newborn rat ventricular cardiomyocytes during their development in primary culture. Experiments were performed 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. In the presence of racemic Bay K 8644 (10(-6) M), L-type calcium current (ICa-L) density recorded by perforated patch-clamp technique was increased by 127 +/- 4% (n = 8) in 2-day-old cells. The increase was only 103 +/- 5% (n = 10) in 7-day-old cells, resulting in a current density similar to that observed in freshly-dissociated adult cells (90 +/- 7%; n = 10). At every stage of the culture Bay K 8644 increased ICa-L with a 10-mV shift of the peak current towards hyperpolarized levels but without change in activation threshold and reversal potential. This shift can be explained by the corresponding change in steady-state activation and inactivation relationships towards negative potentials. The potentiating effect of Bay K 8644 was further studied as a function of phosphorylation levels of calcium channels. When calcium channels were phosphorylated by dibutyryl cyclic AMP (2 x 10(-4) M), increase of ICa-L density by Bay K 8644 was comparable at every stage of Cell Culture. However, direct activation of beta-receptors by isoproterenol did not increase ICa-L density in 2-day-old cultured cells as it did in 7-day-old cells, while direct activation of Adenylate Cyclase by forskolin similarly increased ICa-L, at both stages of culture. From these results, it can be suggested that the higher increase of ICa-L density by Bay K 8644 in 2-day- than in 7-day-old cultured cells could be interpreted as the result of a difference in the phosphorylation level of calcium channels for each stage of development. The possible increase in the basal phosphorylation level of calcium channels during culture of neonatal cardiac cells is discussed with respect to changes in functional properties of calcium channels during postnatal maturation of these cardiac cells.