El. Squires et al., EFFECTS OF ALTRENOGEST ON TOTAL SCROTAL WIDTH, SEMINAL CHARACTERISTICS, CONCENTRATIONS OF LH AND TESTOSTERONE AND SEXUAL-BEHAVIOR OF STALLIONS, Theriogenology, 48(2), 1997, pp. 313-328
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.