Adrenal cortex circulating forms

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-... 

Biosynthesis of testosterone. The primary precursor is acetyl-CoA, and biosynthesis proceeds via cholesterol (see Terpenes, Steroids). Testes and adrenal cortex also utilize cholesterol which they receive as cholesterol esters from plasma lipoproteins (see Lipoproteins). A second androgen, more potent than testosterone, is 5a-dihydrotestosterone this is formed from circulating testosterone at peripheral sites, and not by the Leydig cells of the testes. Ring A of 5a-testosterone cannot be aromatized, so this hormone cannot serve as an estrogen precursor. 

Metabolism of Hormones. Hormones are regulatory substances they act in trace amounts. Being endogenous substances, they are produced by the organism itself. This is in contrast to the vitamins, which also are effective in very small amoimts, but which must be supplied from the environment. A few glands (e.g. thyroid) store some of the hormones that they produce others, however, release their hormones into the circulation at the same rate at which they form them (e.g. the adrenal cortex). 

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