Plasma testosterone levels in rats under various conditions.

A radioimmunoassay (RIA) for testosterone (T) was found to be suitable for determination in plasma of rats. Since the cross-reactive dihydrotestosterone levels were extremely low both in males (130.8 +/- 27.6 pmol/l) and females (189.4 +/- 41.3 pmol/l), the procedure measured T equally well, whether or not chromatography preceded the RIA. The method met all requirements of precision, sensitivity, accuracy and specificity. Basal levels of 5 male control groups (n approximately equal to 10) showed considerable variations from a low of 6.86 +/- 4.68 nmol/l to a high of 16.26 +/- 5.06 nmol/l. This high variability may be the reason, at least in part, for the fact that moderate stress did not produce always a significant decrease of T levels in males. However, after forced stress or administration of adrenaline plasma T was significantly (p less than 0.01) lower than in controls. Gonadectomy lowered extremely plasma T levels both in males and females, whereas adrenalectomy had no significant effect. These data demonstrate that the gonads are the main sources of T in both sexes. The significant (p less than 0.01) decrease of T levels after administration of adrenaline, dexamethasone and ACTH in adrenalectomized males indicated that these substances affect mainly the testicular synthesis of T. In cyclic females, administration of ACTH produced no significant change in plasma T levels. During pregnancy, there was a significant (p less than 0.01) increase of plasma T values from day 10 to day 18-19, followed by a decrease to day 22.