Anita Vanderpooten

Growth hormone acutely decreases type III iodothyronine deiodinase in chicken liver

Growth hormone (GH) increases plasma T3 and decreases plasma T4 in 18‐day old chicken embryos, in newly hatched chicks and in adult chickens within 2 h after injection. The in vivo increase in T3 can be linked to an increased in vitro T3 recovery from liver homogenates incubated with T4. Specific type I and type III deiodinase tests (5′D‐I and 5D‐III), however, show that GH has no effect at all on the amount of hepatic type I enzyme (catalyzing T4 deiodination to T3) but acutely decreases the amount of type III enzyme (catalyzing T3 deiodination). This suggests that the GH‐induced increase in plasma T3 is not due to an increased T3 production, but is the result of a decreased T3 breakdown. The lack of a stimulatory effect of GH injection in 3‐day‐old fed chicks might be the combined result of a low hepatic type III enzyme level and a low GH receptor availability at that stage.

Pulsatility of plasma growth hormone and hepatic growth hormone receptor characteristics of broiler chickens divergently selected for abdominal fat content.

1. Plasma growth hormone (GH) pulsatility and hepatic GH receptor characteristics were compared in experimental lines of meat-type chickens selected for high (HF) or low (LF) abdominal fat content. 2. Mean GH concentration, baseline and amplitude of pulses were slightly, but not significantly, greater in LF chickens. Length and frequency of pulses were similar. 3. LF chickens exhibited higher plasma triiodothyronine (T3) concentrations. This difference between genotypes disappeared when the diet was supplemented with 1 mg/kg T3. 4. Specific binding of GH to liver membranes was higher for the fat line but was depressed by T3 supplementation to the same level in both lines. No difference was observed between lines for affinity constants. 5. It is concluded that direct selection for leanness has a less pronounced, if any, effect on GH pulsatility as compared with selection for food conversion efficiency; therefore, different physiological mechanisms are triggered to achieve leanness.