COMPARATIVE ASPECTS OF CORTISOL TRANSPORT: LACK OF FIRM BINDING TO PLASMA PROTEINS IN DOMESTIC RUMINANTS

Abstract

SUMMARY: The concentration of cortisol in peripheral plasma (μg./100 ml.) was 0·6 ± 0·1 (s.e.) in penned sheep accustomed to handling and 1·1 ± 0·2 in untrained grazing sheep, or about 1/10 of the basal level in man or the guinea-pig. Plasma cortisol in the goat (mean 1·2) and the ox (2·6 ± 0·3) was also low. Freely diffusible cortisol was distinguished from protein-bound hormone by gel filtration after equilibration of the plasma with ³H- or ¹⁴C-labelled steroid. The binding capacity (μg./100 ml.) of sheep plasma for cortisol at room temp. (1·7 ± 0·2) was only slightly above that of 5% ovine albumin solutions. It compared with 3·5 (2·2–4·6) in the goat, 6·1 ± 1·0 in the ox, 14–21 in the guinea-pig, 17–23 in normal human and 25–59 in human pregnancy plasma. These are minimal estimates, since partial dissociation of the transcortin-cortisol complex occurred under the test conditions. Sheep plasma showed no significant elevation in cortisol concentration or binding power during pregnancy, nor in response to oestrogen administration. At physiological cortisol levels and room temperature, 39 ± 5% of the cortisol in sheep plasma was protein bound, but values above 40% were associated with low plasma cortisol levels and hence confined to samples from trained or adrenalectomized sheep. ACTH administration or addition of very small amounts of cortisol to ovine plasma in vitro caused a sharp fall in this percentage. Moreover, at body temperature only 9 ± 2% of the plasma cortisol in the sheep was non-diffusible. This finding may account for the great sensitivity of target tissues to the changes of blood cortisol level in this species. In human pregnancy plasma 86–95% of plasma cortisol (> 14μg./100 ml.) was bound at room temperature and 60–74% at 37°. Published data and those now presented suggest that plasma level and biological half-life of cortisol in different animal species are directly related to binding capacity.