Calcium Fluxes, Ion Currents and Dihydropyridine Receptors in a New Immortal Cell Line from Rat Heart Muscle

A cell line (RCVC) in permanent culture was developed from adult rat ventricular cells; transformation was attained by incubation with conditioned media from UCHTI, a rat thyroid cell line. Immortalized ventricular cells have a doubling time of 20 h, contact inhibition of growth, and display some muscle markers such as a high glycogen content and positive immunoreaction for myoglobin, α-sarcomeric actin, α-actin and desmin. A microsomal fraction from these cells was shown to bind 3H-nitrendipine with a maximal capacity of 295 fmol/mg protein and an equilibrium dissociation constant of 0.7 nM. Nifedipine-sensitive 45Ca2+ influx was evident in partially depolarized cells (40 mM K+ in the incubation medium). An equivalent influx, induced by the calcium channel agonist BAYK-8644 and CGP-28392, was obtained in normally polarized cells. Patch clamp studies show slow inward currents that can be completely blocked by 5 μM nifedipine; cells were induced to further differentiation by culturing in a hormone supplemented medium for 30 days. Under this condition, fast, inactivating inward currents and a large outward current became apparent. After 40-60 days, the cells exhibit La3+-sensitive fast and slow inactivating inward currents that resemble T and L-type Ca2+ currents. This cell line appears to be a good model system for the investigation of cardiomyocyte differentiation in situ.