Cation exchange and glycoside binding in cultured rat heart cells

The Na/K-exchange characteristics, ouabain-binding kinetics, and Na pump turnover rates of synchronously contracting monolayers of neonatal rat myocardial cells were studied. The cells exchange Na rapidly (T(1/2) = 35 sec) with a mean Na flux of approximately 25 (pmol/cm²)/sec. The half time (T(1/2)) of K exchange is much longer (12 min); the mean K flux is 13 (pmol/cm²)/sec. Active Na/K transport, as measured by K influx, is relatively ouabain sensitive, and 10^-6 M ouabain produces half-maximal inhibition. Ouabain (10^-2 M) inhibits 60% of the Na efflux and 75% of the K influx. The cells bind [³H]ouabain rapidly (T(1/2)) = 8 min), but release it very slowly (T(1/2)) = 11 hr), and both the amount bound and the rate of binding were inversely proportional to extracellular K. Specific [³H]ouabain binding demonstrates saturation reaching a maximum of 1.6 x 10⁶ molecules per cell at 2 x 10^-7 M [³H]ouabain. From cell surface area and ouabain-sensitive flux measurements, the Na pump density was calculated at 720/μm² with an individual pump turnover rate of 50/sec. Thus the studies indicate that despite their neonatal origin, the behavior of the Na pump in these cells is very similar to that in other mammalian tissues.