EFFECT OF SYNTHETIC GONADOTROPHINRELEASING HORMONE (Gn-RH) ON CIRCULATING LUTEINIZING HORMONE (LH) AND TESTOSTERONE IN YOUNG POST-PUBERTAL BULLS

Abstract

The reproducibility of LH and testosterone responses after Gn-RH treatment in young post-pubertal bulls was tested. Ten bulls were injected im (250 \g=m\g) twice with an interval of 4 weeks. Blood samples were collected every 15 min for 2\ m=1/ 2\ h and then on the second occasion, every hour. LH increases very rapidly and reaches an initial peak by 30 min after Gn-RH injection. This increase over basal level was 25 to 30-fold. After a plateau of high levels lasting 40 to 50 min, a second peak of LH concentration occurs. LH then decreases and returns to basal levels by 6 to 7 h after treatment. The increase of plasma testosterone was more prolonged and of smaller magnitude than that of LH: 3 to 4-fold. There was a large and significant variation between bulls; but with LH, a given individual responded in a consistent manner on the two separate occasions. It is concluded that LH response to Gn-RH treatment might be useful and should be tested as of diagnostic value for clinical or breeding purposes. The testosterone response does not seem to be as valuable and reliable as that of LH.The availability of synthetic gonadotrophin-releasing hormone (Gn-RH) has provided the opportunity to evaluate the pituitary mobilizable reserve of LH gonadotrophin hormone and the indirect response of interstitial tissue to this

Plasma LH and testosterone responses to synthetic gonadotrophin-releasing hormone (GnRH) or dexamethasone-GnRH combined treatment and their relationship to semen output in bulls

“…Mean patterns of LH and testosterone after GnRH challenge in the present study are in good agreement with previous reports for dairy bulls of the same age (Thibier, 1976b(Thibier, , 1977aChantaraprateep & Thibier, 1979a). The mean total responses as estimated by the area under the curve do not vary significantly between the ages of 12 and 13 months in accordance with previous studies with fewer bulls (Thibier, 1976b;Chantaraprateep & Thibier, 1979a (Chantaraprateep & Thibier, 1979a (Thibier, 1976a(Thibier, , 1977aChantaraprateep & Thibier, 1979a (Thibier, 1977b;Chantaraprateep & Thibier, 1979b).…”

“…The mean total responses as estimated by the area under the curve do not vary significantly between the ages of 12 and 13 months in accordance with previous studies with fewer bulls (Thibier, 1976b;Chantaraprateep & Thibier, 1979a (Chantaraprateep & Thibier, 1979a (Thibier, 1976a(Thibier, , 1977aChantaraprateep & Thibier, 1979a (Thibier, 1977b;Chantaraprateep & Thibier, 1979b). It was suggested by in-vivo data (Thibier, 1977b) (Rawlings, Hafs & Swanson, 1972;Thibier, 1975b;Sitarz, Erb, Martin & Singleton, 1977) but the frequency of blood or semen sampling was too low to allow any definite conclusion.…”

“…This could give further insight on the hormonal control of spermatogenesis and on its practical use as an indicator of the sperm output ability, or even of fertility, as suggested by Bindon & Turner (1974), Bindon, Chang & Evans (1974), Thibier (1977a) and Land (1978). Because hormone concentrations fluctuate randomly during the day (Thibier, 1975a(Thibier, , 1976a, the magnitude of the LH and testosterone responses in each individual to a uniform challenge was used for assessment (Thibier, 1977a Individual bulls respond to a single GnRH challenge in a repeatable manner when compared to their contemporaries after puberty (Thibier, 1976b(Thibier, , 1977a or even before and after the onset of puberty (Chantaraprateep & Thibier, 1979a These data on semen characteristics were used to classify bulls into so-called good, medium and poor semen producers (see Thibier & Colchen-Bourlaud, 1972b;Colchen-Bourlaud & Thibier, 1973 The other 29 bulls were treated i.m. with 20 mg dexamethasone (Intervet) at 08:00 h and 5 h later, 0-25 mg GnRH was administered (i.m.).…”

“…Hormonal assays LH and testosterone concentrations were measured by radioimmunoassay as previously described (Thibier, 1975(Thibier, a, 1976b.…”

“…For an estimate of the amount of LH or testosterone released after GnRH or dexamethasone-GnRH treatment, a so-called total response has been used (Thibier, 1976b) Snedecor & Cochran (1956).…”