Effects of thapsigargin and cyclopiazonic acid on intracellular calcium activity in newborn rat cardiomyocytes during their development in primary culture.

The effects of specific inhibitors of sarcoplasmic reticulum (SR) calcium ATPase, thapsigargin (TG), and cyclopiazonic acid (CPA) were investigated on the resting and transient levels of intracellular free calcium concentrations recorded in Indo-1-loaded ventricular myocytes of newborn rat heart in primary culture. The calcium transients were induced by caffeine (10 mM) or high potassium (100 mM) solutions. In 2 day- as in 7-day-old cultured cells, the calcium transients induced by 10 mM caffeine were blocked dose dependently by TG and CPA. The dose-response curves suggest that TG was more efficient than CPA and that both drugs were more efficient in 7-day- than in 2-day-old cells. The calcium transients induced by 100 mM K+ were also strongly inhibited by these agents. The lack of effect on sarcolemmal calcium currents, as shown by whole-cell patch-clamp experiments, suggests that these drugs affect only SR function. In cells exhibiting spontaneous activity, the associated calcium transients were not affected by TG or CPA at the beginning of the culture (2-day-old cells) but were fully blocked at the end (7-day-old cells). These results confirm that TG and CPA specifically inhibit the cardiac SR Ca2+ pump without affecting the sarcolemmal calcium current. Their blocking effect of the calcium transients as a function of the developmental stage of neonatal cardiac cells in culture suggests an increasing role of the SR in the regulation of intracellular calcium. This argues for developmental changes of the SR through the differentiation and maturation of newborn cardiomyocytes at the early stage of the postnatal life, leading to a predominant role of the SR in excitation-contraction coupling mechanisms in adult cells.