EFFECTS OF ALTRENOGEST ON TOTAL SCROTAL WIDTH, SEMINAL CHARACTERISTICS, CONCENTRATIONS OF LH AND TESTOSTERONE AND SEXUAL-BEHAVIOR OF STALLIONS

Twenty stallions (3 to 18 yr old) were used in a study between June 19 93 and March 1994. The stallions were divided into 5 groups of 4 each, and, within groups, were randomly assigned to I of 4 treatments: 1) u ntreated controls; 2) once-a-day oral altrenogest (0.088 mg/kg BW) tre atment for 150 d; 3) daily altrenogest treatment at the same dose for 240 d and 4) daily oral altrenogest treatment for 240 d plus subcutane ous GnRH (80 mu g) every 4 h from Days 151 to 240. Total scrotal width (TSW) was recorded and semen was collected and evaluated for gel free volume, concentration, sperm motility and sperm morphology. Sexual be havior (libido) was measured as times to first erection and ejaculatio n. Serum LH and testosterone (T) were measured at various periods thro ughout the study. Altrenogest decreased serum concentrations of LH and T, TSW, daily spermatozoa output (DSO), the percentage of normal sper matozoa and libido. There was a significant decrease in sperm motility in the Alt-240 and Alt-240 + GnRH group, but not the ALT-150 group. T he suppression appeared to be partially reversible because DSO, TSW an d serum concentrations of LH increased after cessation of progestin tr eatment. Administration of GnRH during altrenogest treatment resulted in increased (P < 0.05) TSW, DSO and serum concentrations of LH but di d not alter sperm morphology or behavior. In summary, the suppressive effects of altrenogest were apparently mediated primarily through a ne gative feedback inhibition of LH secretion. (C) 1997 by Elsevier Scien ce Inc.