Renal handling of secretin in dogs: free and stop flow analyses

Renal handling of secretin was studied in anesthetized dogs. Highly purified porcine secretin was infused into the renal artery and its concentration in plasma and urine was measured by a radioimmunoassay. Free flow experiments demonstrated that secretin appeared in urine above 10 ng/ml of plasma secretin and that its urinary excretion was increased in proportion to the plasma level of the hormone. Clearance of secretin was less than 0.05% of creatinine clearance and nearly constant over the plasma range of 10-170 ng/ml. In stop flow studies secretin appeared in urine in the same manner as inulin when both substances were injected simultaneously. This indicates that the hormone is filtered through glomerular capillary membranes. The maximal fall of secretin concentration during an infusion of the hormone appeared around the distal portion of the nephron where the dip of Na concentration was present. This suggest, although not entirely excludes the possibility of proximal reabsorption of secretin, that the hormone is reabsorbed around the distal portion of the nephron. It may be concluded that secretin, once filtered by the glomerulus, is reabsorbed by the tubular epithelium.