Effect of temperature and pH on the magnitude of the free fraction of cortisol in serum.

Concomitant effects of temperature and pH on changes in the magnitude of the free, unbound fraction of cortisol in human serum were studied at temperatures and pH ranging from 36 to 39 degrees C and from 6.9 to 7.4, respectively. Free fraction increased with increasing temperature and decreasing pH, the two factors acting synergically: a rise in temperature from 36 to 39 degrees C caused an about 30% increase in the free fraction at pH = 7.4, and at pH 6.9--over 2.2-fold. At highest temperature-acidity combination, i.e. at 39 degrees C, pH = 6.9, the free fraction was as much as 2.5-fold higher, relative to standard conditions (ph 7.4, 37 degrees C). This implies that in vivo the free, biologically active fraction of cortisol may increase due to e.g. hyperthermia and acidosis at a constant total concentration of this hormone.