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.
ASJC Scopus subject areas
- Cell Biology