Renal handling of zinc: effect of cysteine infusion

Zinc clearance studies in anesthetized dogs were performed during hydropenia, mannitol infusion, and infusion of mannitol plus ZnSO4, ZnCL2, or cysteine. Mannitol expansion caused no significant change in Zn clearance. ZnSO4 infusion increased filtered Zn 13-fold without changing clearance. Zn excretion increased only sixfold, indicating increased net Zn reabsorption. Cysteine infusion increased urinary Zn excretion 86-fold, indicating net tubular Zn secretion, some of which derived from nonplasma sources. Stop-flow studies localized Zn reabsorption to the distal nephron during infusion of mannitol and mannitol plus ZnSO4 or ZnCl2. Net Zn secretion was shown to occur in the proximal tubule during cysteine infusion with reversal of the distal reabsorption pattern seen during ZnSO4 and ZnCl2 infusion. Despite increased urinary Zn excretion during ZnSO4 infusion, calcium excretion was unaltered. During cysteine infusion dissociation of tubular handling of CA2+ and Zn occurred in both the proximal and distal tubule. These experiments demonstrate that the nephron under these experimental conditions is capable of both proximal secretion and distal reabsorption of Zn.