The uptake of cortisol by rat tissues

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Abstract
The distribution ratio of C14 between tissues and plasma was determined 30 minutes after the intravenous injection of [4-C14]-cortisol into male rats. Liver had the highest ratio (about 6) and the ratios for kidney, spleen and intestine were considerably greater than 1Brain was the only tissue in which the ratio was less than 1. Cortisol was slowly concentrated by liver slices incubated at 4[degree]. After a period of 6 hours the cortisol concentration was about 6 times as high as that in the incubation medium. A large fraction of the tissue cortisol was not in equilibrium with that in the external solution. The activation energy for uptake varied from 340 cal. between 4[degree] and 10[degree] to 8000 cal. between 20[degree] and 30[degree]. The process followed Michaelis-Menten kinetics and at 4[degree] Vmax. was 33.4 [mu]mg/min/g wet weight with Km 100 [mu]g/ml. A constant distribution ratio of [4-C14]-cortisol between the particulate and the soluble fractions of liver homogenates (about 6:1) was attained within 5 minutes and the results could be expressed as an adsorption isotherm. These experiments, taken together with the characteristics of uptake of cortisol by slices at 4[degree], suggest that cortisol enters the cell by diffusion. A favorable diffusion gradient at 4[degree] appears to be maintained by the intracellular adsorption of free cortisol.

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