ANOXIC SUPPRESSION OF NA-K+-ATPASE AND CONSTANT MEMBRANE-POTENTIAL INHEPATOCYTES - 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 cel lular energy demand. In view of the importance of ion gradients to cel lular function, and the remarkable anoxia tolerance of Chrysemys picta bellii (western painted turtle) and hepatocytes isolated from this sp ecies, it was of interest to determine if in response to anoxia 1) ion gradients were maintained and 2) if the activity of the plasma membra ne Na+-K+-ATPase changed to aid in ion gradient maintenance. From norm oxic hepatocyte suspensions the ouabain-inhibitable Rb-86+ uptake (a m easure 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 normo xic cellular ATP turnover. In response to anoxic incubation the activi ty of the pump decreased by 75% to 4.8 mumol ATP.g cells-1.h-1 and thi s comprised 74% of the total anoxic ATP turnover. Presently, it is not known whether the observed reduction in Na+-K+-ATPase activity is reg ulated by 1) allosteric modification, 2) endocytosis from the membrane , or 3) reduced Na+ influx. Plasma membrane potential was measured dur ing anoxia, using the distribution of Cl-36-, and was not significantl y 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.