The role of bicarbonate and other buffers on isotonic fluid absorption in the proximal convolution of the rat kidney

Summary

The fluid reabsorption from the proximal convolution of the rat kidney was measured with the Gertz shrinking droplet technique. Simultaneously, the peritubular capillaries were perfused with artificial solutions. In some experimental series, fluid from the shrinking droplet was withdrawn and analysed for Cl⁻, Na⁺, and osmolality so that the transtubular transport of Na⁺, Cl⁻, and HCO ⁻₃ could be calculated. Capillary perfusate in some experiments was also withdrawn and its pH was measured. The following results were obtained: 1. With increasing concentration of HCO ⁻₃ in the capillary perfusate, the transtubular water, sodium, chloride, and bicarbonate reabsorption increased. 2. The sulfonamide buffers sulfamerazine and glycodiazine (Redul^®), which easily penetrate the tubular wall, could, in equimolar concentrations, substitute totally for the bicarbonate buffer in promoting isotonic fluid absorption. 3. Butyrate, propionate, and acetate were also effective; pyruvate, lactate, and paraaminohippurate, however, were not. 4. The effect of HCO ⁻₃ and glycodiazine on isotonic absorption was shown to depend exclusively on the concentration of the buffer anion and not on the concentration of undissociated acid or pH. From these data it is suggested that for proximal isotonic absorption of water, sodium, and chloride, the reabsorption of buffer anions via H⁺ secretion and nonionic diffusion may be essential. The H⁺ secretion or the buffer anion absorption across the luminal cell wall may secondarily influence the active Na⁺ transporting mechanism located at the basal cell site either by a luminal H⁺−Na⁺ exchange mechanism or by a lyotropic effect which would increase the Na⁺ permeability of the luminal cell site. Thereby more Na⁺ would be delivered to the Na⁺ pumping site and the rate of Na⁺ pumping would be augmented.