Preliminary study on the effect of temperature on the transport of thyroxine (T4) into the body tissues and sub-cellular fractions of liver and muscle of carp, Cyprinus carpio.

In order to determine if the inability of thyroxine to induce cellular effects at low temperature is mediated through a temperature-sensitive system for the translocation of T4 into the nucleus, the effect of temperature on the uptake of T4 by body tissues and sub-cellular fractions of carp liver and muscle was studied in vivo. A single injection of 125I-T4 (1 micro C/10 g body weight) was given intraperitoneally to juvenile carp maintained at 15 and 25 C. Uptake from the peritoneal cavity was rapid. All the tissues exhibited maximum radioactivity at 2 hour after the injection. Fish kept at 25 C showed another peak at 8 hour and those at 15 C at 48 hour after the single injection. Transfer of T4 from the cytoplasm to nuclei was not blocked at lower temperatures. For example, in liver at 8 hour, nuclei from fish tissues kept at lower or higher temperatures had equal amounts of radioactivity. Muscle nuclei had 15% more radioactivity than liver nuclei when expressed as radioactivity/g tissue. Since there are comparable amounts of activity in the nuclei at both temperatures, some other mechanism/s than a simple block in transport from cytoplasm to nuclei is operating. There are some indications that nutritional status of fish may be playing some role in this respect.