The effects of metergoline combined with PGF2alpha treatment on luteal function and gestation in pregnant bitches

This study was designed to determine the nature of the antiluteotrophic effect of metergoline on pregnant bitches, the occurrence of clinically evident side-effects, and the efficacy of PGF(2alpha) in initiating abortion after a course of metergoline therapy. Starting on Days 18 to 20 after the onset of diestrus, 8 adult pregnant beagle bitches were treated twice daily with 0.4 to 0.5 mg/kg po metergoline for 5 d. After receiving no treatment for the next 5 d, metergoline administration was repeated for a further 3 d, followed by twice-daily intramuscular injections of dinoprost tromethamine at 250 mug/kg. There was an overall trend for the plasma progesterone concentration to decrease during the first and second course of metergoline therapy and to rise during the intervening period of no treatment. None of the bitches aborted during or after the first 5 d of metergoline administration. No side-effects were evident during the metergoline therapy. After pretreatment with metergoline a mean of 4.8 injections of PGF(2alpha) was necessary to ensure complete abortion. All abortions were rapid and uneventful, except for the usual side-effects associated with PGF(2alpha). The plasma progesterone concentration at the onset of PGF(2alpha) treatment was positively correlated to the number of PGF(2alpha) injections needed to complete the abortion process. The plasma progesterone concentration was < 8 nmol/l for several days as a result of metergoline therapy in 6 of the 8 bitches. Only 1 bitch, however, aborted before PGF(2alpha) therapy was initiated. In the other 7 bitches PGF(2alpha) appeared to be necessary for abortion. The results suggest that the effect of metergoline has to be considered incompletely luteolytic at the doses used in this study. Even prolonged suppression of luteal function in early to mid-gestation, however, did not cause abortion without the previously documented luteolytic and/or ecbolic effects of PGF(2alpha). The average interestrus interval of cycles treated with metergoline and PGF(2alpha) was shorter (mean 182.2 d, SD 7.9 d, n = 6) than expected for this group of bitches (mean 211.4 d, SD 31.5 d, n = 7).