Effect of verapamil and of extracellular Ca and Na on contraction frequency of cultured heart cells.

Monolayer cultures of myocardial cells prepared by trypsin dispersion of neonatal rat ventricles (cultured for 4-5 days, ~1.0 x 10^6 cells per 6-cm diam petri dish)

Verapamil (10^-9 M to 10^-5 M), tetrodotoxin (2 x 10^-5 mg/ml), and varying concentrations of extracellular calcium ([Ca]0) and sodium ([Na]0)

Contraction frequency and Na influxsurrogate