Evidence of Facilitory Effect of Growth Hormone on Tissue Vitamin A Uptake in Rats

Abstract

The proposed study was designed to investigate the effect of growth hormone (GH) on liver vitamin A release and uptake at the tissue level. Labeled vitamin A (retinol) was injected in hypophysectomized (hypox) rats, and plasma radioactivity was monitored at intervals for up to 6 hours and at 24 hours following injection. Controls (intact sham-operated) received similar treatment. The results showed that the radioactivity reached its maximum levels in both groups between the third and fourth hours following injection and declined thereafter at different rates in each group. At 24 hours following injection, plasma radioactivity in the intact rats was less than 10% of its maximum levels while in the hypox rats for the same time period it remained at 80--90% of its maximum levels. A single injection of GH at the third hour after injection in the hypox rats decreased significantly (P less than 0.01) plasma radioactivity while radioactivity in the testes and kidneys and in urine collected for the 24 hours increased significantly (P less than 0.01). Hypophysectomy had no effect of vitamin A absorption and liver vitamin A release to the blood. Also, hypophysectomy had no significant effect on plasma and tissue retinol-binding protein levels and its binding capacity to retinol. It is concluded that a facilitory effect of GH at the tissue level may be an important factor in vitamin A metabolism.