The Journal of General Physiology, Vol 68, 537-549, Copyright © 1976 by The Rockefeller University Press
Effect of verapamil and of extracellular Ca and Na on contraction frequency of cultured heart cells
D McCall
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 x 10(6) cells per
6-cm diam petri dish had formed. The contraction frequency and Na influx of
the cells were unaffected by tetrodotoxin (2 x 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 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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