Heat exposure elevates plasma immunoreactive growth hormone-releasing hormone levels in man

The effects of heat exposure on plasma levels of GH and GHRH were studied in six younger (31-46 yr) and six older (49-66 yr) normal men. For the GHRH RIA, 2-mL plasma samples were purified on Sep-Pak C18 cartridges, from which the mean recovery of synthetic GHRH-(1-44) was 62 +/- 10% (+/- SD; n = 8). Heat exposure (15 min) in a Finnish sauna bath at an ambient temperature of 72 C, led to an increase in plasma GH levels from 2 to 5 micrograms/L (P less than 0.01) at 30 min in the younger men. Their rectal temperature had risen by 0.2 C at 15 min. Plasma immunoreactive GHRH increased from 9 to 36 ng/L (P less than 0.05) 5 min after heat exposure, and it gradually fell to the initial levels by 45 min. The older men did not have a significant increase in plasma GH or GHRH levels in response to the heat exposure. Reverse phase high pressure liquid chromatography studies of plasma immunoreactive GHRH suggested that the major circulating GHRH immunoreactivity was GHRH-(1-40). We conclude that heat exposure-stimulated GH release in young adult men is mediated by GHRH, but in older men, GHRH and GH responses do not occur.