Secretion of synthesized steroid hormones

Once the various steroids have been formed in paticular subcellular compartments, they must be released into the peripheral blood circulation. There is evidence that some steroids are released by passive diffusion, as in the case of corticosterone, but for 18-hydroxylated corticosteroids, Na+/K+-ATPase activity is necessary [6,109]. The situation is more complicated, however, because the presence of proteins in the adrenal cortex, which act as non-classical receptors, may bind C2i steroids to different extents, thus reducing rates of steroid release (see Ref. 6). So far as pregnenolone is concerned, there is no barrier to its efflux from the mitochondria where it is formed from cholesterol [50], During incubation of rat testis [110], pregnenolone was found to travel from the mitochondria, through the ER and cytosol and then out into the medium. The release with time could be resolved into two components, one rapid and the second, much slower. More than 25% of the pregnenolone remained in the tissue after 150 min. incubation. This two-phase release may reflect the presence of two pools of steroid, the initial loss representing passive dif- 

Numerous other mechanisms, based on ultrastructural evidence, have been proposed [6] by which steroids may be secreted from their site(s) of synthesis. The steroids may be contained in secretory organelles or in lysosomes, these acting as vehicles of transport to the plasma cell membrane, where secretion occurs by 

The foregoing discussion has attempted to trace the ways in which cholesterol, derived from plasma lipoproteins, is converted into the various steroid hormones and how these are secreted back into the blood. Of necessity, many details have had to be omitted but it is hoped that this up-date has shown the complexities of steroid biosynthetic pathways and that earlier classical ideas have had to be modified as greater knowledge of intermediates, isoenzymes and multiple forms of cyt P-450s has become available. Perspectives for future studies are indeed exciting. 

Work performed in the author s laboratory was supported by AFRC (grant nos. AG 35/35 and 35/44) to whom grateful thanks are expressed. Mrs. D.M. Gower kindly prepared the manuscript for publication. 

and Gorban, A.M.S. (1980) In Recently Discovered Systems of Enzyme Regulation by Reversible Phosphorylation (Cohen. P.. ed.) p. 95. Elsevier/North Holland Biomedical Press, Amsterdam. 

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