Intrarenal distribution and ATPase inhibiting activity of ouabain in dogs

Experiments were undertaken to substantiate the hypothesis that the mechanism of the direct effect of ouabain on the renal excretion of electrolytes is the result of inhibition of the transport enzyme, (Na, K)-ATPase. In dogs hydrated with saline, an injection of 3H-ouabain into the unilateral renal artery produced a continuing marked increase in excretion of water and sodium from the kidney, but not from the counter kidney. At maximal diuresis--90 min after ouabain injection, both kidneys were removed to assay microsomal ATPase activity and determine radioactivity distributed in subcellular structures. It was demonstrated that 3H-ouabain was deposited in the microsome fraction obtained from the injected kidney in concentrations ranging from 10(-7) to 10(-6) M/kg wet weight, and (Na, K)-ATPase activity of this fraction was inhibited as compared with that of the microsomal fraction obtained from control kidneys. Since (Na, K)-ATPase activity of renal microsomes was significantly inhibited in vitro by more than 10(-7) M of ouabain, ouabain concentration in microsomes obtained from the injected kidney was considered to be sufficient to inhibit ATPase activity. These findings indicate that ouabain diuresis under the present condition is closely related to direct inhibitory effect of ouabain on (Na, K)-ATPase activity of microsomes in tubular cells.