EFFECTS OF CHRONIC HYPERFILTRATION ON PROXIMAL TUBULE BICARBONATE TRANSPORT AND CELL ELECTROLYTES

The compensatory response to unilateral nephrectomy (UNX) was investig ated by a combination of renal clearance, microperfusion, electron mic roprobe, and morphological techniques. Filtration rate was significant ly elevated 21 days following UNX and associated with a marked stimula tion of bicarbonate and fluid absorption in the proximal tubule. Analy sis of kinetic data of bicarbonate transport demonstrated strong flow- dependent activation of bicarbonate absorption in both control and exp erimental condition. The bicarbonate level at which half-saturation (a pparent K-d) of transport occurred decreased uniformly at higher flow rates, but maximal transport rates (apparent V-max) in the proximal tu bule doubled in the remnant kidney. The flow dependence of bicarbonate transport in control and experimental conditions can be explained by an apparent unstirred layer effect modifying radial bicarbonate gradie nts in the tubule. Both Na/H-exchange and electrogenic H secretion con tribute to bicarbonate absorption, but only Na/H-exchange increased si gnificantly in proximal tubules of UNX rats. Cell ion concentrations a fter UNX were unchanged in cortical tubules, consistent with proportio nately enhanced apical and basolateral ion transport. Proximal tubule cell rubidium concentration measured after a 30-second rubidium infusi on as an index of basolateral Na,K-ATPase activity was unchanged in UN X rats. Inasmuch as cell, volume increased significantly (25%), these data are consistent with a proportionate and similar stimulation of ru bidium uptake and Na,K-ATPase activity.