Dynamics of Circulating Progesterone Concentrations Before and During Luteolysis: A Comparison Between Cattle and Horses1

Abstract

The profile of circulating progesterone concentration is more dynamic in cattle than in horses. Greater prominence of progesterone fluctuations in cattle than in horses reflect periodic interplay in cattle between pulses of a luteotropin (luteinizing hormone; LH) and pulses of a luteolysin (prostaglandin F2alpha; PGF2alpha). A dose of PGF2alpha that induces complete regression of a mature corpus luteum with a single treatment in cattle or horses is an overdose. The overdose effects on the progesterone profile in cattle are an immediate nonphysiological increase taking place over about 30 min, a decrease to below the original concentration, a dose-dependent rebound 2 h after treatment, and a progressive decrease until the end of luteolysis. An overdose of PGF2alpha in horses results in a similar nonphysiological increase in progesterone followed by complete luteolysis; a rebound does not occur. An overdose of PGF2alpha and apparent lack of awareness of the rebound phenomenon has led to faulty interpretations on the nature of spontaneous luteolysis. A transient progesterone suppression and a transient rebound occur within the hours of a natural PGF2alpha pulse in cattle but not in horses. Progesterone rebounds are from the combined effects of an LH pulse and the descending portion of a PGF2alpha pulse. A complete transitional progesterone rebound occurs at the end of preluteolysis and the beginning of luteolysis and returns progesterone to its original concentration. It is proposed that luteolysis does not begin in cattle until after the transitional rebound. During luteolysis, rebounds are incomplete and gradually wane. A partial rebound during luteolysis in cattle is associated with a concomitant increase in luteal blood flow. A similar increase in luteal blood flow does not occur in mares.

Hormone concentrations temporally associated with contralateral and ipsilateral relationships between the CL and preovulatory follicle during the third follicular wave in heifers

“…During the descending portion, progesterone rebounds to the initial level from the positive effect of a pulse of LH [10,29]. These time-sensitive hormonal relationships are likely involved in the delay in the beginning of luteolysis in the three-wave IOIs.…”

“…Before the beginning of luteolysis, progesterone concentration decreases during the ascending portion of a pulse of a metabolite of PGF2a [29]. During the descending portion, progesterone rebounds to the initial level from the positive effect of a pulse of LH [10,29].…”

Natural Rescue and Resurgence of the Equine Corpus Luteum

“…Structure, echotexture [11], and function [9,10] of the CL in mares have been reviewed. Equine CL development occurs during about 5 days for the CL of an IOI, pregnancy, and PCL ( Fig.…”

“…In contrast, P4 continues to increase until luteolysis begins in heifers and/or cows, providing a species comparison that should be of interest to biologists [9]. After the gradual P4 decrease in mares during the IOI, the P4 decrease becomes more abrupt (luteolysis).…”

“…Reports for only three individual mares may indicate that CL rescue is rare, but like the discovery of many reproductive phenomena by transrectal ultrasonic imaging [8], awareness may lead to more frequent reports. The P4 rebound after the peak of a PGFM pulse in cattle is attributable in part to a pulse of luteinizing hormone (LH) (review [9]), and CL resurgence in pregnant mares is associated with the luteotropic properties of equine chorionic gonadotropin (eCG) from the endometrial cups (Section 4). Concentrations of LH were available for two of the three reports of CL rescue in mares, and each was temporally associated with a surge of LH.…”