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 1993 and March 1994. The stallions were divided into 5 groups of 4 each, and, within groups, were randomly assigned to 1 of 4 treatments: 1) untreated controls; 2) once-a-day oral altrenogest (0.088 mg/kg BW) treatment for 150 d; 3) daily altrenogest treatment at the same dose for 240 d; and 4) daily oral altrenogest treatment for 240 d plus subcutaneous GnRH (80 microg) 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 behavior (libido) was measured as times to first erection and ejaculation. Serum LH and testosterone (T) were measured at various periods throughout the study. Altrenogest decreased serum concentrations of LH and T, TSW, daily spermatozoa output (DSO), the percentage of normal spermatozoa 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. The suppression appeared to be partially reversible because DSO, TSW and serum concentrations of LH increased after cessation of progestin treatment. Administration of GnRH during altrenogest treatment resulted in increased (P < 0.05) TSW, DSO and serum concentrations of LH but did not alter sperm morphology or behavior. In summary, the suppressive effects of altrenogest were apparently mediated primarily through a negative feedback inhibition of LH secretion.