Interaction between sulphate and molybdate in the sheep

An investigation was conducted to in vivo study the interaction between sulphate and molybdate during transport across the epithelium of the renal tubule and ileum in the sheep. In the kidney experiments, plasma concentration of sulphate and molybdate were progressively elevated by constant intravenous infusion. Kinetic analysis of the data indicated the presence of a common carrier system (C1) upon which a fully competitive inhibition between the two anions occurs for tubular reabsorption against the prevailing anion concentration in plasma. This has been characterized as active transport and the proximal tubule is postulated as a probable site for its occurrence. Further observation and analysis revealed the presence of a sulphate-activated system for molybdate tubular secretion. This has been characterized as facilitated diffusion and the distal tubule is postulated as a most probable site for its occurrence. There was no evidence to suggest a tubular secretion of sulphate. In the ileal experiments, a continuous perfusion technique was used to progressively increase the concentration of sulphate and molybdate in the lumen of the lower ileum. Kinetic study of the absorption data revealed the existence of two dissimilar machanisms for the transport of each anion across the ileal mucosa. One mechanism has shown the characteristics of a common carrier system (C1) where sulphate and molybdate exhibit a fully competitive inhibition for their transport across the Heal epithelium. This has been classified as active transport. The close similarity between kinetic parameters of C1 of the ileum and that of the renal tubule suggests that this is probably the same carrier system in both sites. The ileal absorption of sulphate by another mechanism has been observed to follow the anion concentration gradient; thus appeared to be diffusion. The sulphate absorption rates by this mechanism have been demonstrated to disobey Fick's law and to decrease with the progressive increase of molybdate concentration in the ileal lumen. Hence it has been declared facilitated diffusion mediated by a carrier system (Ds) It is also observed that Ds contributes to the majority of sulphate absorption from the ovine ileum up to 77%. The other route for molybdate ileal absorption (C2) has been found to be a sulphate-dependent system which is essentially activated by sulphate and proportionally accelerated with the progressive increase of sulphate concentration in the ileal lumen. It has shown the characteristics of active transport and contributed to about 78% in the total ileal absorption of molybdate. A mobile carrier model for the relationship between Ds and 02 is proposed. In this model, once Ds has attained a sulphate equilibrium sulphate is suggested to recycle to the mucosal membrane to be handled by C2. There a driving force or energy source may be found that enables the system to secrete sulphate back into the luminal pool in exchange for molybdate from that pool as countertransport. Unloaded carriers at the mucosal surface cannot bind molybdate but can bind sulphate only.