Relationship between increased binding and insulin-like effects of human growth hormone in adipocytes from young fa/fa rats.

Binding and insulin-like effects of human GH (hGH) in inguinal adipocytes from young lean Fa/fa and obese fa/fa Zucker rats were studied. The binding of [125I]hGH per unit surface area is 2-fold higher in adipocytes prepared from 16- and 30-day-old fa/fa rats than in cells from lean littermates. A 3-h preincubation of the cells increases the hGH-binding capacity without changing the affinity of the binding, regardless of genotype. Freshly isolated adipocytes from 30-day-old lean rats fail to respond to hGH, whereas after preincubation of the cells, hGH produces a maximal 105% increase in glucose transport and a maximal 40% increase in glucose oxidation. In contrast, freshly isolated adipose cells from fa/fa rats are already responsive to hGH and the amplitude of the response is markedly elevated in preincubated cells, with a 430% stimulation of glucose transport. The concentration of hGH (1 nM) that inhibits 50% of [125I]hGH binding and that which produces half-maximal stimulation of glucose transport as well as glucose metabolism are not different, suggesting the absence of spare receptors for these insulin-like effects of hGH. Plots of GH effects on glucose transport as a function of receptor occupancy are linear, with a change in the slope after preincubation. Our results suggest a strong correlation between binding of hGH and actions on glucose transport and glucose metabolism in adipocytes of young Zucker rats.