The normal temperature of the rat testis in a scrotal position is approximately 32°C. In vitro incubation of normal testicular tissue at various temperatures has demonstrated a 32°C optimum for the incorporation of lysine and glucose into protein. A thermal optimum of 32°C was also found for the conversion of cholesterol to components of both the Δ4- and Δ5- steroidogenic pathways except for pregnenolone synthesis which has a positive temperature co-efficient between 28°C and 36°C. The Δ5-pathway may be more sensitive than the Δ4-pathway. In vivo inhibition of protein synthesis with cycloheximide simulated a temperature effect. Compared to saline injected control, tissue from inhibitor treated animals converted less cholesterol to steroids when incubated at 32°C, except for pregnenolone. In vitro cycloheximide treatment was less selective than inhibition of protein synthesis in vivo in that the synthesis of all steroid intermediates was suppressed below control levels. In addition, the hexose monophosphate shunt is less active at 36°C than at 32°C based on the in vitro yields of 14CO2 from glucose-1-14C and glucose-6-14C.
On the basic of these experiments, a hypothesis was proposed which can serve as a foundation for further study concerning the mode of action of temperature in regulating testicular function. The hypothesis is that a thermal environment above or below 32°C in the rat interferes with some aspect of protein synthesis, thus reducing the level of one or more enzyme related to the steroidogenic pathways beyond pregnenolone.