Evidence for the kidney as an important source of 5′-monodeiodination activity and stimulation by somatostatin in Oreochromis niloticus L

Abstract

Radioimmunoassay of 3,5,3′-triiodothyronine (T 3) and thyroxine (T 4) in the thyroidal region of mature male Oreochromis niloticus revealed stores of T4 but negligible levels of T 3, yielding a very low T 3/T 4 ratio (0.3%). 5′-Deiodination (5-D) of T 4 into T 3 was examined in liver and kidney homogenates in vitro by radioimmunoassay of T 3 with T 4 as substrate. In both organs, the 5′-D activity was temperature dependent: at 4°C, T 3 production was below the level of detection and maximal in both tissues at 37°C; and at 45°C, the enzymatic activity was reduced. T 3 production seemed to reach a plateau after 60 min of incubation. The reaction required exogenous thiol cofactor (dithiothreitol) and was inhibited partially or completely by propylthiouracil depending on the concentrations used. Hepatic and renal 5′-D activities were stimulated by somatostatin (SRIF) within 4 hr, but a subsequent increase in plasma T 3 was observed only when SRIF was injected together with T 4, while the magnitude of rT 3 production decreased. It is concluded that almost all the circulating T 3 is provided by peripheral T 4 to T 3 conversion since T 3 RIA in thyroidal follicles demonstrated insignificant T 3 production. The kidney may contain the large part of the functional deiodinase which converts T 4 into T 3. As in mammals and unlike in other fishes, there is not only 5′-D activity, but also 5-D activity, and both may be influenced by SRIF.