Monolayer cultures of myocardial cells were prepared by trypsin dispersion of neonatal rat ventricles. The cells were cultured for 4-5 days by which time a synchronously contracting monolayer of some 1.0 × l06 cells per 6-cm diam petri dish had formed. The contraction frequency and Na influx of the cells were unaffected by tetrodotoxin (2 × 10-5 mg/ml) but both were markedly reduced by the addition of verapamil (10-9 M to 10-5 M). The effect of verapamil on both parameters occurred very rapidly. Although unresponsive to change in [Ca]0 between 0.3 mM and 3.0 mM, the contraction frequency of the cells declined rapidly as the [Ca]0 was reduced below 0.3 mM. On the other hand the beating rate of the cells was linearly related to [Na]0 between 40 mM and 170 mM. At values [Na]0 below 40 mM the cells ceased to contract. It is therefore apparent that both [Ca]0 and [Na]0 contribute to the maintenance of the contraction frequency of cultured myocardial cells, but the latter is by far the more important. There also appeared to be, under all conditions, a close relationship between verapamilsensitive Na influx and contraction frequency. For the greater part this relationship was linear although at higher Na influx values it appeared to show evidence of saturation.
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