Dynamics of circulating progesterone concentrations before and during luteolysis: a comparison between cattle and horses.

The research observes that the progesterone hormone, key in reproduction, behaves differently in cattle versus horses with more dynamic fluctuations in cattle due to the interplay between two other hormonal pulses. It elucidates the changes in progesterone levels in both animals following an overdose of prostaglandin F2alpha, a hormone that induces regression in the corpus luteum. Moreover, it proposes a new interpretation of the start of luteolysis in cattle.

Comparison between Progesterone Concentrations in Cattle and Horses

• The study shows that the hormone progesterone, crucial for reproduction, demonstrates different dynamics in cattle and horses. Specifically, fluctuations in progesterone levels are more dominant in cattle. This trait of cattle is attributed to the continual interplay between bursts of luteotropin (luteinizing hormone; LH) and luteolysin (prostaglandin F2alpha; PGF2alpha), two hormones that affect reproduction.

Impact of Prostaglandin F2alpha Overdose

• According to the researchers, administering a dose of PGF2alpha that achieves complete regression of a mature corpus luteum (a mass of cells that plays a vital role in reproduction) via a single treatment in either species is considered an overdose.
• In cattle, an overdose of PGF2alpha results in an immediate but non-natural increase in progesterone levels (within approx 30 minutes), after which there is a drop to a level below the original concentration. After two hours, there’s a dose-dependent rebound, followed by a steady decline until luteolysis (the process whereby the corpus luteum deteriorates) completes.
• A similar overdose in horses gets the same initial non-natural progesterone increase and subsequent complete luteolysis. However, unlike cattle, horses do not experience a rebound in progesterone levels.
• These observations indicate that previous misunderstandings of luteolysis have been based on overlooking the rebound phenomenon after an overdose of PGF2alpha.

Proposed Beginning of Luteolysis in Cattle

• The research suggests that natural pulses of PGF2alpha in cattle, but not in horses, result in a temporary decrease and resurgence in progesterone levels within a few hours. These rebounds are attributed to a combination of an LH pulse and the descending phase of a PGF2alpha pulse.
• There is a complete transitional progesterone rebound at the end of preluteolysis (before the corpus luteum starts to decline) and the start of luteolysis, which restores progesterone to its initial concentration. The study proposes that in cattle, luteolysis does not actually start until after this transitional rebound.
• During luteolysis, rebounds are incomplete and gradually diminish. An incomplete rebound during luteolysis in cattle is associated with a simultaneous rise in blood flow to the corpus luteum. Such an increase in blood flow does not occur in horses.