endophyte-infected tall fesuce impacts broodmare bloodflow

Researchers know that some types of tall fescue—particularly, those playing host to toxic endophytes called ergot alkaloids—can be bad news for pregnant mares. They know that mares consuming endophyte-infected tall fescue can develop fescue toxicosis, characterized by a lack of milk production, prolonged gestation, placental thickening, and weak or nonviable foals. They even know that ergovaline—one type of ergot alkaloid—can cause vasoconstriction in horses.

What they don’t know is exactly how the ergot alkaloids cause the problems seen in pregnant mares. One theory is that decreased blood flow to the uterus plays a role. Researchers previously determined that feeding horses ground endophyte-infected tall fescue results in vasoconstriction in the distal palmar artery, and such decreased blood flow to the uterus could have an impact.

So, researchers from the USDA Agricultural Research Service’s Forage-Animal Production Research Unit (ARS-FAPRU) and University of Kentucky Gluck Equine Research Center, both in Lexington, set out to evaluate whether a lack of placental bloodflow could contribute to fescue toxicosis development. Specifically, they evaluated how ergopeptine alkaloids (ergovaline, ergocryptine, ergocristine, ergocornine, and ergotamine) and ergoline alkaloids (lysergic acid and ergonovine), two ergot alkaloids found in endophyte-infected tall fescue, impact vasoactivity in the equine palmar artery and vein and uterine artery.

James Klotz, PhD, a scientist at ARS-FAPRU, and Karen McDowell, PhD, EMB, an associate professor at the Gluck Center, harvested palmar artery and vein segments of 23 cadaver horses’ forelimbs, as well as uterine arteries from 12 cadaver mares. They sliced the arteries and veins into cross-sections and incubated them in a buffer for 90 minutes. Then, to assess how the veins and arteries respond to the alkaloids, they applied:

  • Increasing concentrations of ergovaline, ergocryptine, ergocristine, ergocornine, and lysergic acid to the palmar samples; and
  • Increasing concentrations of norepinephrine, serotonin, ergotamine, and ergonovine to the palmar and uterine samples.

Prior to the alkaloid exposure, the team confirmed the presence of adrenergic receptors (adrenaline) and serotonergic receptors (serotonin) in each palmar artery and vein sample by adding increasing amounts of norepinephrine and serotonin. Klotz said they included this step because ergot alkaloids can interact with these receptors, causing blood vessel contraction. Because norepinephrine caused the most significant contraction of the two, they used that as a standard to compare the alkaloids’ effects.

They found that

  • Ergonovine and ergotamine induced a moderate contractile response in both palmar samples, approximately 40% and 20% of the maximum contraction norepinephrine caused, respectively;
  • Ergovaline produced the earliest contractile response in palmar vessels compared to the other four ergopeptine alkaloids tested, followed by ergotamine, while ergocristine produced the smallest contractile responses; and
  • Lysergic acid did not produce any vasoactivity in palmar samples.

The team found less pronounced contractile responses in the uterine artery after alkaloid exposure. The artery showed a significant contractile response when exposed to high norepinephrine concentrations, but none with serotonin, ergonovine, or ergotamine exposure.

“We were surprised by the lack of ergot alkaloid vasoactivity in the uterine artery preparations based on the traditional effects seen in pregnant mares,” said Klotz.

However, these findings agree with others from McDowell’s research group, which suggest that pregnant mares’ uterine arteries are less responsive to the endophyte-infected fescue seed’s vasoconstrictive effects than the palmar arteries.

“In light of this, we believe determining sites of ergot alkaloid vascular activity to be a step towards understanding how alkaloids cause decreased performance in equine production systems,” Klotz said.

These findings suggest that other factors beyond maternal vasoconstriction are likely involved in tall fescue’s effects on the equine fetus.

The study, “Tall fescue ergot alkaloids are vasoactive in equine vasculature,” was published in the Journal of Animal Science.