EVIDENCE FOR THE ROLE OF THE SECRETORY PATTERN OF GROWTH-HORMONE IN THE REGULATION OF SERUM CONCENTRATIONS OF CHOLESTEROL AND APOLIPOPROTEIN-E IN RATS

Adult male Sprague-Dawley rats were hypophysectomized and connected to an automatic i.v. infusion system. The same daily dose of human GH (hGH) was given either as eight daily pulses (3-h intervals) to mimic the male specific secretory pattern of GH or as a continuous infusion of GH, to mimic the female secretory pattern. Hypophysectomized rats received i.v. replacement therapy with L-thyroxine and cortisol. The rats were treated for 5 days. The serum cholesterol concentration was higher when hGH was given continuously than when hGH was given as eight daily pulses. The concentration of high-density lipoprotein (HDL)-cholesterol was not influenced by intermittent GH treatment, but increased when hGH was given as a continuous infusion. The serum concentration of apolipoprotein (Apo) E increased following treatment with a continuous infusion of hGH, whereas eight daily pulses of hGH had no effect. The serum concentration of ApoA-I was unaffected by hGH treatment. The serum concentration of ApoB decreased to the same degree whether hGH was given as a continuous infusion or as eight daily pulses. The serum concentration of triglycerides was not affected by hGH treatment. These results indicate that the higher serum HDL-cholesterol and serum ApoE concentrations of female rats may be due to their more continuous secretion of GH. In contrast, the effects of GH on the serum concentration of ApoB, which is not sexually differentiated, may be independent of the mode of GH secretion.