THE DISTRIBUTION AND METABOLISM OF LABELLED CORTICOSTEROIDS IN THE DUCK (ANAS PLATYRHYNCHOS)

Abstract

SUMMARY After the injection of [1,2-3H]corticosterone into ducks, no unaltered free [1,2-3H]corticosterone was found in either bile, gut contents or cloacal fluid. All the [1,2-3H]corticosterone metabolites in cloacal fluid had similar chromatographic mobilities. A portion of these metabolites was steroid ester sulphate but enzymic hydrolysis suggested that only a small fraction of the sulphate was conjugated to [1,2-3H]corticosterone. The amount of ester sulphate in samples of cloacal fluid, however, was highly variable. After the injection of [1,2-3H]aldosterone one-fifth of the total radioactivity in urine was free [1,2-3H]aldosterone but none was found in the bile and gut contents. Chromatographic purification of the [1,2-3H]aldosterone metabolites in cloacal fluid showed two significant peaks and both peaks contained steroid ester sulphates. Enzymic hydrolysis of the tritiated material showed that each peak also contained a glucuronide of [1,2-3H]aldosterone. The effect of the entero-hepatic circulation on the excretion of tritiated metabolites by the kidney was also examined. Diversion of bile from the duodenum prevented the entry of radioactive material into the gut. The reduced reabsorption of tritiated corticosterone and tritiated aldosterone metabolites from the gut, however, did not affect the amount of tritium excreted by the kidneys. In the case of both hormones, some of the reabsorbed metabolite which had been extracted by the liver was released and re-entered the peripheral blood via the hepatic circulation. The stress of surgery in both the control birds and the birds with bile catheters caused the distribution of [1,2-3H]corticosterone and the tritiated corticosterone metabolites to change; a significantly smaller percentage of the injected radioactivity was recovered from the tissues and excretory fluids. This response to stress was not observed in the [1,2-3H]aldosterone injected birds. When partially purified metabolites of [1,2-2-3H]corticosterone and [1,2-3H]aldosterone were injected into the lumen of the gut, the rate of uptake of the tritiated corticosterone metabolites was higher than that of the tritiated aldosterone metabolites. But, when the metabolites of [1,2-3H]corticosterone and [1,2-3H]aldosterone were injected intravenously, and the rate of the intestinal reabsorption of the metabolites was not limiting, the rates of tritium excretion by the kidneys were the same. The volumes of distribution and metabolic clearance rates of the partially purified [1,2-3H]corticosterone and [1,2-3H]aldosterone metabolites were significantly different from one another and the metabolites could be readily distinguished physiologically as well as chromatographically. The pattern of distribution and the disappearance from plasma of i.v. administered synthetic [1,2-3H]corticosterone-21-SO4 was also distinguishable from the naturally occurring metabolites of [1,2-3H]corticosterone.