Relationship between the biological and immunological activities of growth hormone circulating in normal rats

The ability of GH to induce refractoriness to its own insulin-like effect in adipose tissue is highly specific for GH. Moreover, refractoriness to the insulin-like action of GH can be induced with low concentrations of GH in the range of 10 ng/ml. In the present study, the ability of plasma from normal rats to induce refractoriness to the effect of GH on the production of 14CO2 from [14C]glucose in epididymal fat pads from hypophysectomized rats was examined as a measure of a GH-specific biological effect of the plasma. Plasma levels of GH were measured by RIA. Without exception, preincubation of fat pads with plasma having immunoreactive rat GH (rGH) levels over 50 ng/ml induced refractoriness to GH, whereas plasma having immunoreactive rGH levels below 15 ng/ml did not exert this effect. In fat pads preincubated with plasma having immunoreactive rGH levels between 10-50 ng/ml, the effect of a second exposure to GH varied in a dose-dependent manner. Also, plasma pools that were diluted with buffer or plasma from hypophysectomized rats were unable to induce refractoriness when the calculated or measured levels of immunoreactive rGH were below 15 ng/ml. The results suggest that in terms of the ability of GH to induce refractoriness to the insulin-like effect of the hormone, a very good correlation exists between the biological and immunological activities of GH in plasma from normal undisturbed rats.