ANOXIC SUPPRESSION OF NA+-K+-ATPASE AND CONSTANT MEMBRANE-POTENTIAL IN HEPATOCYTES - SUPPORT FOR CHANNEL ARREST

The maintenance of ion gradients across the plasma membrane by the Na+-K+-ATPase has been shown to utilize a large fraction of the total cellular energy demand. In view of the importance of ion gradients to cellular function, and the remarkable anoxia tolerance of Chrysemys picta bellii (western painted turtle) and hepatocytes isolated from this species, it was of interest to determine if in response to anoxia 1) ion gradients were maintained and 2) if the activity of the plasma membrane Na+-K+-ATPase changed to aid in ion gradient maintenance. From normoxic hepatocyte suspensions the ouabain-inhibitable Rb-86+ uptake (a measure of Na+-K+-ATPase activity) was determined, and the rate of ATP utilization was 19.1 mumol ATP.g cells-1.h-1 or 28% of the total normoxic cellular ATP turnover. In response to anoxic incubation the activity of the pump decreased by 75% to 4.8 mumol ATP.g cells-1.h-1 and this comprised 74% of the total anoxic ATP turnover. Presently, it is not known whether the observed reduction in Na+-K+-ATPase activity is regulated by 1) allosteric modification, 2) endocytosis from the membrane, or 3) reduced Na+ influx. Plasma membrane potential was measured during anoxia, using the distribution of Cl-36-, and was not significantly different from the normoxic measurement, -30.6 +/- 3.9 and -31.3 +/- 5.8 mV, respectively. Therefore, the plasma membrane ion gradient is maintained during anoxia, and since the activity of the NA+-K+-ATPase decreases, the influx of ions must also decrease. The combination of a decrease in Na+-K+-ATPase activity and a constant membrane potential in response to anoxia supports the channel arrest concept.