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Abstract

Using the standing droplet technique in the renal proximal convolution and simultaneous microperfusion of the peritubular capillaries, the zero net flux transtubular concentration difference of taurocholate (DeltaC_TC^-) at 45 s was determined as a measure of active bile acid reabsorption in vivo. Starting with 0.1 mmol/liter taurocholate in both perfusates the control DeltaC_TC^- of 0.042 mmol/liter fell to 0.006 mmol/liter (P < 0.001) when the Na⁺ concentration in the perfusates was reduced to zero. Removal of bicarbonate from the perfusates to alter pH had no influence on DeltaC_TC^-. When glycocholate was added to the perfusates DeltaC_TC^- was decreased, while probenecid increased DeltaC_TC^-. These observations were extended by studies performed with brush border membrane vesicles derived from renal cortex. The initial (20 s) uptake of 0.01 mmol/liter taurocholate in the presence of a Na_o⁺ > Na_i⁺ gradient was stimulated twofold compared with its uptake in the absence of a Na⁺ gradient. Uptake of taurocholate was osmotically and temperature sensitive. Membranes preloaded with unlabeled glycocholate showed accelerated entry of labeled taurocholate (trans-stimulation) only in the presence of Na⁺. Replacement of Na⁺ in the media with K⁺, Li⁺, and choline⁺ decreased initial taurocholate uptake by 49, 53, and 62%, respectively. Stimulation of taurocholate transport by cation gradient diffusion potentials was unlikely inasmuch as the addition of valinomycin under K⁺ gradient conditions had no effect. A transmembrane pH gradient (pH_o < pH_i) did not influence initial uptake of taurocholate. Finally, in the presence of Na⁺ taurocholate transport showed cis-inhibition with unlabeled bile acids and saturation kinetics with respect to increasing taurocholate concentrations. The micropuncture and vesicle data indicate that the net transport of taurocholate in the proximal tubule is the result of an electroneutral Na⁺-taurocholate cotransport across the brush border membrane.