THE TESTIS IS NOT THE MAJOR SOURCE OF CIRCULATING FOLLISTATIN IN THE RAM

The aims of this study were to determine the plasma concentrations of follistatin in rams and to assess if the testis contributes to circula ting follistatin and if there is uptake or production of follistatin b y the head in rams. Catheters were inserted in the carotid artery, jug ular vein and spermatic vein of intact rams during the non-breeding se ason (experiment 1; n=5) and breeding season (experiment 2; n=4). In e xperiment 1, blood samples were collected from 5 rams every 10 min for 4 h, commencing 20-60 min after surgery. After 2 h of sampling 1 mu g gonadotrophin-releasing hormone (GnRH) was injected intravenously. In experiment 2, blood samples were collected from 4 of the rams used in experiment 1 by venipuncture 30 and 15 min before surgery and every 1 5 min throughout surgery. Commencing 1 h after surgery, matched sample s were taken from each of the vessels every 10 min for 4 h (1-4 h afte r surgery), then every hour for 20 h (4-24 h after surgery) and then e very 10 min for 4 h (24-28 h after surgery). In both experiments, foll istatin secretion was non-pulsatile and there were no significant diff erences between the concentrations of follistatin in any of the vessel s. There was a significant (P<0.05) increase in the concentrations of follistatin in each of the vessels throughout the 4 h of 10-min sampli ng in both experiments. In experiment 2 plasma concentrations of folli statin in the jugular vein were significantly (P<0.05) lower before su rgery than at other stages of the experiment. During the non-breeding season (experiment 1) the concentrations of follistatin in all vessels were about 2-fold higher (P<0.001) than during the breeding season (e xperiment 2). Concentrations of follistatin were measured in the testi cular tissue of the ram, bull, monkey and rat and were found to be 13. 6, 2.1, 2.5, 0.8 ng/g testis respectively. In experiment 3, blood samp les were collected every 15 min for 4 h from castrated rams (n=6) in t he absence of treatment with testosterone propionate (TP) and after 7 days of treatment with a physiological dose of TP during the breeding and non-breeding seasons. There was no effect of stage of breeding sea son or TP on the plasma concentrations of follistatin and these concen trations in the castrated rams were similar to the concentrations in t he intact rams in experiment 2. In experiment 4, the function of Leydi g cells was stimulated by administration of human chorionic gonadotrop hin but this had no effect on plasma concentrations of follistatin. Th ese experiments show that the concentrations of follistatin in the pla sma of rams are measurable, that the testis is not the major contribut or to circulating follistatin and that there is no significant uptake or production of follistatin by the head in rams. It appears that the contribution of the testis to circulating follistatin may vary with th e stage of the breeding season, being greater during the non-breeding season than the breeding season. The gonadotrophins and testosterone d o not appear to have a direct effect on the secretion of follistatin i n rams. The increase in concentrations of circulating follistatin duri ng surgery and more frequent blood sampling suggest a stress-related e ffect on the production of follistatin.