GROWTH-HORMONE BUT NOT GONADAL-STEROIDS INFLUENCE LIPOPROTEIN-LIPASE AND HEPATIC LIPASE ACTIVITY IN HYPOPHYSECTOMIZED RATS

Lipoprotein lipase and hepatic lipase are involved in the degradation and cellular uptake of lipids in peripheral tissues and the liver. The se enzymes seem to be influenced by gonadal steroids in the rat as wel l as in man. Since gonadal steroids have been shown to influence the s ecretory pattern of GH and since the effect of gonadal steroids on sev eral metabolic functions may be dependent upon their effects on GH sec retion, the present study was undertaken to investigate the developmen tal regulation of heparin-releasable lipoprotein lipase and hepatic li pase activities in female and male rats, and to study the effects of g onadal steroids and different modes of GH administration to hypophysec tomized rats on these enzyme activities. Female and male Sprague-Dawle y rats from 20 to 65 days of age were studied. Hypophysectomy was perf ormed at 50 days of age and these rats were given replacement therapy with thyroxine and cortisone. Groups of hypophysectomized rats were tr eated with either oestradiol valerate (0.1 mg/kg per day) or testoster one enanthate (1 mg/kg per day). Bovine GH (1 mg/kg per day) was given to groups of hypophysectomized rats either by two daily subcutaneous injections or by continuous infusion using osmotic minipumps. Hormone treatment was given for 1 week. Lipoprotein lipase and hepatic lipase activities were measured in heparinized plasma. There was no differenc e in lipoprotein lipase activity between male and female rats at 20 to 45 days of age. Lipoprotein lipase activity decreased between 45 and 65 days of age in male rats but not in females and, at 65 days of age, lipoprotein lipase activity was higher in females compared with males . Hepatic lipase activity increased from 20 to 45 days of age in both female and male rats but at 45 and 65 days of age it was higher in fem ale than in male rats. Hypophysectomy decreased lipoprotein lipase and hepatic lipase activity in female rats. Neither oestradiol nor testos terone treatment had any effects in hypophysectomized rats. Treatment with bovine GH increased both lipoprotein lipase and hepatic lipase ac tivities irrespective of its mode of administration and these effects were not influenced by additional treatment with oestradiol or testost erone. After the last injection of GH, lipoprotein lipase activity was increased for 12 h. Hepatic lipase activity was increased at 2 and 4 h after the last GH injection but after 12 h the activity had decrease d, indicating that the time-course for the effects of GH on lipoprotei n lipase and hepatic lipase may be different. It is concluded that GH markedly influences post-heparin lipoprotein lipase and hepatic lipase activities. The lack of effects of gonadal steroids on these activiti es in hypophysectomized rats suggests that the gonadal steroids influe nce these lipases via their influence on GH release.