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Cardiovascular Division, Brigham and Women's Hospital and Harvard Medical School, Boston, MA 02115, U.S.A.

Departments of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA 02115, U.S.A.

1. Uptake of calcium was studied in spontaneously contracting monolayers of cultured chick embryo ventricular cells. Ca exchange could be separated into two components: a rapid phase with a rate constant of 3·91/min, accounting for 1·6 nmol/mg protein; and a slower phase with a rate constant of 0·069/min, accounting for 2·7 nmol/mg protein.

2. Negatively inotropic concentrations of the slow Ca channel blocker verapamil inhibited the rapid phase of Ca uptake partially, with a maximum inhibition of 30-40% observed at concentrations of verapamil which completely inhibited contraction.

3. The component of Ca uptake not inhibited by verapamil could be stimulated up to 25-fold by elevation of intracellular Na concentration and reduction of extracellular Na concentration, and thus appeared to represent at least in part Ca uptake via Na—Ca exchange.

4. Ca uptake by cultured cells could be almost completely inhibited by exposure to LaCl₃, 1 mmol/l, within 5 s. This same concentration of La completely inhibited contraction within 5 s.

5. During efflux of ⁴⁵Ca from cells, exposure to La (1 mmol/l) slightly inhibited efflux initially with more marked inhibition of Ca influx after 3 min of La exposure. There was no evidence for a component of superficial La-displaceable Ca, and thus the rapid phase of Ca uptake probably is due to an intracellular rapidly exchanging Ca pool. Efflux of Ca from this rapidly exchanging intracellular Ca pool was not significantly altered by exposure to Na-free choline chloride solutions.

6. We conclude that rapid Ca uptake in these cultured myocardial cells is due primarily to Ca influx via the slow Ca channel and via Na—Ca exchange. In the presence of physiological [Ca²⁺]_i, efflux of Ca from this intracellular Ca pool does not appear to be due to Na—Ca exchange.

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