Relationship between binding and biological effects of human growth hormone in rat adipocytes.

Insulin-like responses to human GH (hGH) were produced in adipocytes isolated from the epididymal fat of normal rats 3 h after excision of the tissues. Insulin-like responses consisted of increased oxidation of glucose and incorporation of its carbons into total lipid, increased oxidation of L-[1-14C]leucine, and antagonism of the lipolytic actions of epinephrine. Refractoriness to these effects of hGH in the fourth hour of incubation was produced by the addition of as little as 3 ng/ml hGH as soon as possible after excision of the tissues. These cells also responded to the delayed lipolytic effect of hGH in the presence of theophylline. The cells were found to have high affinity, low capacity, specific binding sites for 125I-labeled hGH. Monoiodination of hGH did not interfere with its capacity to produce biological responses. Specific binding equilibrated rapidly and appeared to saturate at about 100 ng/ml. In cells that were capable of exhibiting an insulin-like response to hGH, rat and ovine GH successfully competed with [125I]hGH for binding sites, but porcine insulin, at a concentration of 100 mU/ml, failed to reduce the binding of [125I]hGH, indicating that GH does not produce its insulin-like effects by interacting with the insulin receptor. Binding of [125I]hGH in cells that are refractory to the insulin-like effects of GH is indistinguishable from binding in responsive cells. Scatchard analysis of the data for both responsive and refractory cells gave linear plots consistent with a single class of about 20,000 receptors/cell, which become half-saturated at a concentration of approximately 20 ng/ml. This corresponds well with 30-50 ng/ml needed for half-maximal insulin-like responses and the 3-10 ng/ml ED50 for induction of refractoriness or lipolysis. It thus appears unlikely that there are appreciable spare receptors for insulin-like responses. These findings make it likely that refractoriness to the insulin-like effects of GH occurs at a postreceptor site.