Effects of aglepristone administration during the luteal phase of progesterone and LH concentrations of non-pregnant bitches

In the bitch, the mechanisms that regulate luteal function are stillunclear. To investigate the role of progesterone on the gonadalaxis the antiprogestinic aglepristone (Alizine Virbac, France)was administered to non-pregnant bitches during their lutealphase. Bitches were assigned to either control or treated groups.The first day of cytological diestrus was designed as day 0. Onboth days 21 and 22 of diestrus, control bitches (n = 4) receiveds.c. saline solution (0.3 ml/kg body weight) and treated ones(n = 4) aglepristone (10 mg/kg body weight). From day 0 today 70 of diestrus, blood samples were collected from each bitchtwice a week to evaluate luteal function through plasmaprogesterone (P4) concentrations. On day 23, blood sampleswere obtained every 20 min for 4 h to evaluate the hypotha-lamic-pituitary function through plasma LH concentrations.The administration of aglepristone affected progesterone secre-tion and shortened luteal phase of non-pregnant bitches. In fact,treated bitches had a shorter luteal phase than controls andcomplete luteolysis, arbitrarily defined as the failure of CL tosecrete progesterone so that blood levels fell below the 2.0 ng/ml, was observed on days 46.6 ± 11.4 and 66.3 ± 2.9(p < 0.05), respectively. In treated bitches, the mean plasmaLH concentrations under the curve were lower (p < 0.01) thanin controls (0.29 and 0.42 ng/min, respectively). The anti-luteotrophic action of aglepristone is likely because of the blockof P4 receptors at hypothalmo–pituitary and/or ovarian levels.