Seasonal changes of the 5′-monodeiodination activity in kidney and skin homogenates of male Rana ridibunda: Relation to plasma thyroxine (T 4) and testosterone

Abstract

Plasma concentrations of thyroxine (T 4) and testosterone were studied together with the in vitro 5′-monodeiodination activity in liver, kidney, and skin homogenates in male Rana ridibunda during 12 consecutive months. Plasma T 4 levels increase in February, drop in April, and increase again in May. In July a sharp decrease is seen and the values remain low during the rest of the year. Testosterone plasma levels also exhibit month to month variations. A maximum is found in February and March, and minimal values are measured in August, September, and October. A 5′-monodeiodination activity with a resulting 3,3′,5-triiodothyrinine (T 3) production could be demonstrated in kidney and skin homogenates but not in liver homogenates. Addition of 1,4-dithio- dl-threitol (DTT) and EDTA to the homogenates significantly increases the T 3 production. In kidney homogenates incubated with DTT and EDTA a significant annual variation in the T 3 production is seen. This T 3 production is very high in April, May, and June and is followed by a minimum in July and August. It is concluded that the opposite changes in 5′-monodeiodination activity and plasma T 4 concentrations may indicate a peripheral monodeiodination of T 4 to T 3. During reproduction, which is preceded by a testosterone peak, the increase in the 5′-monodeionation activity may reflect a greater need for T 3.