Luteolysis Following Carotid or Ovarian Arterial Injection of Prostaglandin F2α in Mares

Abstract

On the day of ovulation (Day 0), 20 pony mares were randomly assigned to one of five groups (4 mares/group) which were given intracarotid arterial (ICA) and intraovarian arterial (IOA) injections on Day 7 as follows: 1) vehicle ICA, vehicle IOA (control); 2) .125 mg PGF2α ICA, vehicle IOA; 3) vehicle ICA, .125 mg PGF2α IOA; 4) .250 mg PGF2α ICA, vehicle IOA; or 5) vehicle ICA, .250 mg PGF2α IOA. Peripheral plasma progesterone and LH concentrations were determined for 0 (pretreatment) and .1, .25, .50, .75, 1, 6, 12, 24, 48, 72 and 96 h posttreatment. The intervals (days) from treatment to estrus and ovulation, respectively, were shorter (P<0.05) for mares given IOA injection of .125 mg PGF2α (4.7, 12.8), ICA injection of .250 mg PGF2α (6.5, 14.8), or IOA injection of .250 mg PGF2α (2.8, 11.2), than for controls (11.8, 19.5). The intervals for mares given ICA injection of .125 mg PGF2α (9.2, 18.5) did not differ from controls. Compared to pretreatment samples (0 h), progesterone concentration did not change significantly throughout the blood sampling period in mares given ICA injection of .125 mg PGF2α; however, a significant reduction in progesterone concentration occurred by 24 h after treatment in mares given IOA injection of .125 mg PGF2α. Plasma LH concentration (ng/ml) was higher (P<0.05) at 12 h (3.4) than at 0 h for the .250 mg IOA group (0.8), but did not change significantly in the other groups. The correlation between progesterone and LH was significant for mares which were given .125 mg IOA (r=−.65) and .250 mg IOA (r=−.82). The IOA injection of PGF2α was more effective in causing luteolysis than ICA injection of PGF2α. These data indicate that the principal site of luteolytic action of exogenous PGF2α in the mare is at the ovarian rather than at the hypothalamic-pituitary level.