Intracellular calcium transients from newborn rat cardiomyocytes in primary culture

Resting and transient levels of intracellular free calcium concentrations were recorded in indo-1 loaded neonatal rat ventricular cardiomyocytes in primary culture by means of an interactive laser cytometer. The calcium transients were induced by high potassium and caffeine applications. The resting level of intracellular calcium remained constant (about 140 nM) throughout the culture (up to 7 days). The calcium transients induced by 100 mM K+ changed during culture from a low, cobalt sensitive response at 2 days, to a strong biphasic response at 7 days. At 2 days the response was fully blocked by cobalt. At 7 days the transient phase was abolished by cobalt and ryanodine, whereas the second sustained phase was only partially blocked. The calcium transient induced by caffeine was present as early as the first days, and increased with the age of the culture. This transient was blocked by ryanodine. The calcium influx through sarcolemmal calcium channels could be responsible for intracellular calcium transients in 2 day-old cells, whereas in 7 day-old cells, they seem to be only the trigger for sarcoplasmic reticulum calcium release via a mechanism such as 'calcium-induced calcium-release'. Other mechanisms, such as the sodium-calcium exchange mechanism activated by sarcolemmal depolarisation, seem to be implicated too and therefore could explain the sustained level of intracellular calcium during 100 mM K+ stimulation. The developmental changes through differentiation and maturation of myocytes in culture could account for the age dependent evolution of the responses obtained. From these results it is possible to conclude that calcium movements implicated in the excitation-contraction coupling mechanism in the development of rat neonatal cardiomyocytes are similar in primary culture and in the postnatal period in vivo.