Characterization of growth hormone release in response to external heating. Comparison to exercise induced release.

The effects of increases in body temperature on growth hormone (GH)-release were studied in 10 young normal males in the fasting state as well as postprandially. The temperature increase of one degree centigrade was attained by external heating using thermostatically controlled water blankets covered by heat-reflecting aluminium foil. The increase in plasma GH after heating was partially suppressed in the non-fasting state reaching a mean of 7.9 +/- 3.5 (SEM), ng/ml, range 1.0-36 ng/ml. In contrast all subjects exhibited higher increases, mean 18.3 +/- 4.0 ng/ml, range 7-44 ng/ml, in response to heating when fasting. The results were compared in the same subjects to the plasma GH-responses obtained during exercise (450 kpm/min for 40 min) inducing a similar increase in body temperature of about one degree centrigrade. Nevertheless the response in plasma GH (8.4 +/- 3.3 ng/ml, range 0.4-34 ng/ml) was smaller than obtained by the heat test despite a rate of temperature increase on exercise which was about twice as high. Furthermore, the same exercise performed in a cold room under circumstances which precluded any major rises in core temperature resulted in complete inhibition of GH-release. The results indicate that exercise per se does not stimulate GH-secretion, indeed it may inhibit the response expected to be evoked by the exercise-induced rise in temperature. Evidence is also presented that it is core and not cutaneous temperature which modulated GH release. The procedure used for inducing the rise in temperature and plasma GH may be used as a simple, acceptable and safe clinical test for GH-insufficiency.