Abstract
Summary
This study was designed to investigate temporal effects of environmental heat stress on the release and synthesis of hypothalamic GHRF in rats. The experiments consisted of five treatment groups; one group of normals at 25° ± 1° and four groups, each exposed to 35.5° ± 0.5° for 1, 4, 96, and 240 hr, respectively. GHRF activity was measured by in vivo depletion of pituitary GH content in immature rats. Vasopressor and antidiuretic activities of crude milk median eminence extracts were abolished by treating the extracts with thioglycollic acid. Exposures for 1, 4, and 96 hr significantly (p < 0.01) elevated GHRF activity as compared to the control which returned to a normal level by 240 hr. No significant difference existed in GHRF activity for two consecutive periods of exposures, indicating a continuous synthesis of GHRF. Enhanced GHRF activity on heat exposure has been discussed in light of the increased endogenous turnover of hypothalamic norepinephrine on heat exposure and the enhanced firing rate of the hypothalamic neurons. Synthesis of GHRF to near normal levels followed by an initial depletion after a prolonged heat stress has been visualized in terms of loss of peripheral stimulation through acclimatization and/or an increase in the somatotrophin release-inhibiting factor.
The senior author gratefully acknowledges the receipt of the fellowship from the National Academy of Sciences under the tenure of which this manuscript was written. Dr. George C. Bucher, Deputy Associate Director for Sciences at the George C. Marshall Space Flight Center, is also cited for his encouragement and assistance. The manuscript would not have been completed without Dr. Bucher's help.
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