Growth hormone and lipolysis: a reevaluation.

A recent report claimed that the lipolytic activity found in clinical grade human GH (hGH) was entirely attributable to an acidic peptide contaminant. Using the same preparations as studied in the earlier report, we found that delayed lipolytic activity, as demonstrated in the presence of dexamethasone, was fully evident in the highly purified hGH preparation which had been reported to be devoid of lipolytic activity. We also studied the acidic peptide contaminant in which all of the lipolytic activity of clinical grade hGH was alleged to reside and found its lipolytic activity was equivalent to that of the highly purified hGH. Both compounds increased lipolysis in the presence of 0.1 micrograms/ml dexamethasone after a lag period of 1 h. The minimum effective concentration of both fell within the range of 1 and 10 ng/ml, and both were capable of producing a 2- to 3-fold increase in glycerol production. When saturating amounts of each were added simultaneously they produced no greater effect than either alone, although the lipolytic rate was far from saturated as evidenced by the more than 2-fold further increase in glycerol production when 100 ng/ml epinephrine was added along with the combined peptides. The lipolytic response to highly purified hGH was comparable to that seen with bacterially synthesized hGH which should be free of pituitary peptide contaminants. The acidic peptide appeared to be a variant of hGH as judged by its ability to produce both an insulin-like response and refractoriness. Furthermore, the acidic peptide was indistinguishable from hGH in competing for specific binding sites on adipocytes. Although an ability to increase glycerol production in rat adipose tissue in vitro thus appears to be an intrinsic property of hGH, the physiological significance of this effect remains to be established.