Diurnal testosterone and 17alpha-hydroxyprogesterone in peripheral plasma of young post-pubertal bulls. A study by frequent sampling.

Testerone and 17alpha-hydroxyprogesterone were measured in the peripheral plasma of 6 young post-pubertal bulls of 1 year of age, by separate radioimmunoassays. Samples were collected every hour for 25 h at the beginning of each of four seasons. On a separate occasion blood samples were collected from one bull every 10 min for 2 h. As a result of the study, testosterone and 17alpha-hydroxyprogesterone appeared to be secreted episodically. These two steroid peaks showed good correlation. Each 24 h period showed its own characteristic pattern of pulsatile changes. Episodic secretion seems to be progressive rather than rapid and short lived. No real circadian rhythm was observed but at about 10.00 a.m. a trough in these steroid secretions occurred. This was followed by an increase in peripheral plasma concentration. These troughs occurred at all seasons after the morning feed and while semen was being collected in the performance test station.