Influx of testosterone-binding globulin (TeBG) and TeBG-bound sex steroid hormones into rat testis and prostate.

The availability of testosterone and estradiol to Sertoli and prostate cells is dependent upon 1) the permeability properties of the blood-tubular barrier (BTB) of the testis or prostate cell membrane, and 2) sex steroid binding to plasma proteins, such as albumin or testosterone-binding globulin (TeBG). Sex steroid influx into these tissues was studied after in vivo arterial bolus injections of [3H]testosterone or [3H]estradiol in anesthetized rats. Both testosterone and estradiol were readily cleared across the BTB or prostate cell membrane in the absence of plasma proteins and in the presence of human pregnancy serum, in which testosterone or estradiol are 80-95% distributed to TeBG. The extravascular extraction of [3H]TeBG across the BTB or prostate plasma membrane [73 +/- 2% (+/- SE) and 92 +/- 9%, respectively] was significantly greater than extraction of [3H]albumin or other plasma space markers and indicative of a rapid first pass clearance of TeBG by Sertoli or prostate cells. In summary, these studies indicate that 1) testosterone and estradiol are readily cleared by Sertoli and prostate cells; 2) albumin- and TeBG-bound sex steroids represent the major circulating pool of bioavailable hormone for testis or prostate; and 3) the TeBG-sex steroid complex may be nearly completely available for influx through the BTB or prostate plasma membrane.