Coupling of aerobic metabolism to active ion transport in the kidney.

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Abstract
The redox state of mitochondrial NAD was monitored fluorometrically as a function of active ion transport work in the isolated doubly perfused bullfrog kidney. Initial experiments to measure the O2 consumption (QO2) of small pieces from the bullfrog kidney gave a basal QO2 = 3.0 (.+-. 0.43) nmol O2/mg dry wt per min. Addition of 50 .mu.M-ouabain inhibited QO2 by 72.7%. Subsequent addition of the mitochondrial uncoupler 1799 stimulated QO2 by 226%; cyanide totally inhibited respiration. Ion transport functional parameters and NADH fluorescence were simultaneously monitored during systematic reductions in perfusate PO2 to test the sufficiency of O2 delivery to the isolated perfused frog kidney. No significant changes in transport functions or fluorescence were observed until the PO2 dropped to 184 mm Hg or below. O2 tensions of 184 mm Hg or below caused decreases in G.F.R. (glomerular filtration rate) and transport functions accompanied by an increase in NADH fluorescence. The lack of changes in kidney function in the PO2 range 550-340 mmHg suggested that the tissue is adequately oxygenated at the normal perfusate PO2 of 550 mmHg. The relationship between active transport rate and NAD redox levels was studied by increasing transport work (via increased G.F.R. or ADH [antidiuretic hormone] or by decreasing transport work (via decreased G.F.R. or ouabain) while simultaneously monitoring the NAD redox state of the intact tissue fluorometrically. An increase in work caused a net oxidation of NAD; a decrease in work caused a reduction of NAD. The NADH fluorescence responses may be indicative of mitochondrial active to passive transitions in response to changes in active transport work. The aerobic production of ATP and the normally functioning Na-K-ATPase appear essential to maintain active transport and elicit the appropriate state transitions. ATP (and possibly ADP and Pi) may be part of the coupling mechanism linking active ion transport and elicit the appropriate state transitions. ATP (and possibly ADP and Pi) may be part of the coupling mechanism linking active ion transport and aerobic metabolic rate in the kidney.

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