The intracellular sodium activity of cardiac Purkinje fibres during inhibition and re‐activation of the Na‐K pump.

Abstract

The intracellular Na activity, .**GRAPHIC**. of sheep heart Purkinje fibers was continuously monitored using Na+-sensitive glass micro-electrodes. The effects of removal and restoration of external K and of application and removal of various cardioactive steroids, were investigated. The .**GRAPHIC**. increased in K-free solutions and rapidly recovered on addition of external K. The rate of this recovery depended on both the external K concentration, [K]o, and the .**GRAPHIC**. The rate of .**GRAPHIC**. recovery was half maximally activated at a [K]o of about 10 mM. If corrections are applied to allow for changes in the net passive Na influx at various [K]o, then this value is increased to approximately 12.5 mM. At a given [K]o, there appeared to be a linear relationship between the rate of .**GRAPHIC**. recovery and the level to which .**GRAPHIC**. had increased in K-free solution (over the range of .**GRAPHIC**. from 7.5 to 31 mM). Addition of the cardioactive steroids strophanthidin, acetylstrophanthidin, actodigin (AY 22,241) or dihydro-ouabain produced rapid changes of .**GRAPHIC**. At low concentrations, these compounds sometimes produced a small decrease in .**GRAPHIC**. while at concentrations above 10-7 M they produced a dose-dependent increase. The effects on .**GRAPHIC**. of both low and high concentrations of all these cardioactive steroids were readily reversible in 120 min. The time course of the .**GRAPHIC**. recovery mainly depended on the concentration of the cardioactive steroid applied, and on the level to which .**GRAPHIC**. had increased. Upon addition of a cardioactive steroid (above 10-7 M), .**GRAPHIC**. at first increased almost linearly with time. The rates of such an increase were measured during this period at various cardioactive steroid concentrations and used to produce dose-response curves. The concentrations that produced a half-maximum rate of .**GRAPHIC**. increase were near to 10-6 M for strophanthidin and acetylstrophanthidin, but near to 10-5 M for actodigin and dihydro-ouabain. The mean maximum rate of .**GRAPHIC**. increase produced by the addition of a high cardioactive steroid concentration was 0.49 .+-. 0.17 mM/min (.+-. SD, n = 21). This would indicate a net passive Na influx into the cells of approximately 2.8 pmol/cm2 s. This maximum rate of .**GRAPHIC**. increase could be achieved by the addition of 10-5 M strophanthidin or acetylstrophanthidin, but 10-4-10-3 M-actodigin or dihydro-ouabain was required to produce a similar rate of increase. The addition of these high cardioactive steroid concentrations produced an initially rapid increase of .**GRAPHIC**. After 15-30 min, this .**GRAPHIC**. increase slowed considerably. The .**GRAPHIC**. apparently reached a plateau in 2-4 h at levels much below those predicted for a Na electrochemical equilibrium across the cell membrane.