Absence of uterokinetic effects of prostaglandin F 2α on oxytocin-reactive uterus in the mare

Abstract

In most cyclic females, prostaglandin F 2α (PGF 2α) triggers a uterine motility response resembling that of oxytocin (OT). To determine if PGF 2α is a uterokinetic substance in the cycling mare, uterine motility was measured by intrauterine balloon technique in 12 conscious, normally cyclic mares. After 60 min of saline infusion, continuous intravenous (i.v.) infusion with OT (1 i.u./min) was followed by PGF 2α (200 μg/min) for 60 min each. The experiment was repeated 3 wk later except with PGF 2α preceeding OT. A second group of mares was administered OT (60 i.u.) either i.v., intramuscularly (i.m.), or intrauterinely (i.u.). Plasma samples were studied for progesterone concentration. Control uterine motility for the first group of mares was (mean ± SEM) 545.83 ± 45.10 mm 2. Significant (P<0.05) elevation in uterine motility was recorded for OT (1118.60 ± 70.56 mm 2) regardless if PGF 2α preceded OT infusion or vice-versa. No significant difference (P>0.05) was seen in motility after PGF 2α (423.33 ± 31.12 mm 2) infusion. The uterokinetic effect of OT was greatest when OT was administered i.v. (1696.50 ± 195.46 mm 2) followed by i.m. (819.82 ± 39.96 mm 2), and it was least effective when administered i.u. (607.83 ± 21.56 mm 2) as compared to control uterine motility (279.78 ± 22.33 mm 2). Skin electrical resistance values rose from 0 to 2000 ohms with PGF 2α infusion (but not with OT), indicating that PGF 2α was bioactive. It was concluded that PGF 2α was not a uterokinetic substance in the cyclic mare.