Effectiveness of a two-dose regimen of prostaglandin administration in inducing luteolysis without adverse side effects in mares

Abstract

Our objectives were to determine whether repeated administration of prostaglandin F2α (PGF2α) to simulate the endogenous mode of secretion would be more effective than a single injection in inducing luteolysis and enable use of smaller doses less likely to cause adverse side effects. The main study comprised 43 dioestrous mares, who were given i.m. either a single 10 mg dose of natural PGF2α (n = 22) or 2 doses of 0.5 mg PGF2α 24 h apart (n = 21). The intensity of side effects was assessed in 8 dioestrous mares given 5, 1.5, 0.5 or 0 mg PGF2α in consecutive cycles. Two doses of 0.5 nig PGF 2α 24 h apart caused lysis of the corpus luteum in all mares, whether this was determined from a fall in plasma progesterone concentrations or reproductive tract/behavioural changes; and when 10 mg PGF2α was given, the corpus luteum was lysed in 17 of 22 mares i.e. a lower proportion (P = 0.0485). A single dose of 0.5 mg PGF2α was no more effective than saline in inducing luteolysis.The intensity of side effects of PGF2α increased with dose. Although the 0.5 mg dose was no more likely than saline to cause sweating or muscle spasms, it raised plasma cortisol concentrations and prevented the decline in heart rate seen after saline. We conclude that a 2 dose regimen of administration increases the luteolytic efficacy of PGF2α and thereby provides a way to minimise adverse side effects.