The tissue distribution of recombinant human inhibin A (rh-inhibin A) and rh-activin A was determined in immature female Sprague Dawley-derived rats after iv administration of radiolabeled proteins. [I25I]rh-Inhibin A and [125I]rh-activin A diverge in their distribution to tissues of the immature female rat as examined histologically (whole body autoradiography and thin section analysis) and by computing the percent dose and tissue to blood ratios for individual tissues. [I25I]rh-inhibin A accumulated in the spleen, adrenal, bone marrow, and ovary after iv injection. Iodinated rh-inhibin A was also found in the anterior and posterior pituitary. [I25I]rh-activin A was found in the ovary and pituitary after iv injection. Little specific binding was found in the spleen or adrenal. The bone marrow accumulated some [125I]rh activin A which was competed by rh-activin A. The primary route of excretion for radioactivity was the kidney, with the label appearing in the bladder by 10 min after iv injection. Not only do rh-inhibin A and rh-activin A have different pharmacokinetics, but fewer tissues accumulate radioactive rh-activin A than rh-inhibin A.
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