Endophytic fungal toxin effect on adrenergic receptors in lateral saphenous veins (cranial branch) of cattle grazing tall fescue.

The objective of this research was to characterize the adrenergic receptor response in veins of cattle that grazed tall fescue (Festuca arundinacea Schreb.), with (E+) and without (E-) fungal endophyte (Neotyphodium coenophialum) infection. Measurement of contractile response of lateral saphenous veins to selective adrenergic receptor agonists (alphal, phenylephrine; alpha2, BHT-920) revealed enhanced reactivity (greater contractile response) only for the alpha2 adrenergic receptor in E+ pastured cattle. This response was consistent over a 3-yr grazing period. Significant contractility differences in response to BHT-920 occurred at concentrations of 3x10(-6) M and greater. Early research has reasoned that the enhanced reactivity is due to endophyte toxin effect on intracellular signaling systems. The enhanced contractile effect would alter blood flow dynamics and contribute to clinical signs and pathologic change in animals. The alpha2-adrenergic receptor is also involved in the control of many metabolic reactions. Thus, for drug therapy to be successful in reversing the adverse effects of E+ tall fescue exposure in cattle, effects on the alpha2-adrenergic tissue receptors must be neutralized.

Assessment of the mitogenic potential of the alkaloids produced by endophyte (Acremonium coenophialum)-infected tall fescue (Festuca arundinacea) on bovine vascular smooth muscle in vitro.

The objective of these experiments was to test the hypothesis that the major alkaloid classes found in endophyte-infected tall fescue could act as growth promoters for vascular smooth muscle. Bovine vascular smooth muscle cells (VSMC) from the dorsal metatarsal artery were grown in vitro and exposed to five concentrations (10(-6), 10(-8), 10(-9), 10(-11) and 0 M) of ergonovine, alpha-ergocryptine, ergovaline, and N-acetyl loline for 48 h. The mitogenic potential of the alkaloids was tested on both actively growing cells (serum fed, 10% fetal bovine serum) and quiescent cells (serum starved, 0.1% serum) to assess the need for serum constituents for alkaloid induced growth or toxicity. Ergonovine stimulated (P < .07) VSMC growth at 10(-6) M concentration in growing and quiescent culturs and at 10(-8) M concentration in quiescent cultures. alpha-Ergocryptine stimulated (P < .01) growth at 10(-6) M concentration in growing cultures and at 10(-8) and 10(-9) M concentrations in quiescent cultures. Ergovaline exhibited a dual activity on the growth of VSMC in culture, stimulating (P = .06) growth of quiescent cells at 10(-9) M concentration but inhibiting (P < .05) growth of growing cultures at concentrations of 10(-6) and 10(-9) M. This duality of activity was also noted for N-acetyl loline: N-acetyl loline stimulated (P < .05) growth of quiescent cultures at concentrations of 10(-8), 10(-9), and 10(-11) M but inhibited (P < .05) growth of growing cultures at concentrations of 10(-8) and 10(-9) M. The growth effects of the alkaloids in vitro on VSMC support in part the hypothesis that the alkaloids may contribute to the vascular complications noted in cattle grazing endophyte-infected tall fescue through hyperplasia of the intima. This would result in a decreased luminal diameter of the blood vessels and a resultant decrease in blood flow to the afflicted tissues. The diminished blood flow to tissues would result in tissue death and reduced ability to dissipate heat.

Evaluation of a dosing method for studying ergonovine effects in cattle.

We evaluated the feasibility of using miniosmotic pumps as a way to continuously treat cattle with a singular ergot alkaloid (ergonovine) of known content, thus mimicking the natural fescue toxicosis disease state, but allowing study of specific alkaloid effects. Dosing animals with increasing amounts of ergonovine via miniosomotic pumps, followed by daily acquisition of plasma samples for high-performance liquid chromatographic determination of the alkaloid, resulted in stepwise increases in plasma ergonovine concentration. However, despite the detectable blood concentration of ergonovine, calves did not have typical clinical signs of ergot alkaloid toxicosis. Similarly, serum prolactin concentration was unaffected by ergonovine in these cattle, implicating some other alkaloid of endophyte-infested fescue as causative of the usual prolactin-suppressive response. The results confirm use of this animal dosing method to study biological effects of singular purified alkaloids of known amount, without bioavailability concerns. Thus, this dosing method will facilitate studies to determine the harmful effects of individual alkaloids found in toxic tall fescue, and ultimately, to alleviate their costly effects in cattle, horses, and other species.

Vasoconstriction in bovine vasculature induced by the tall fescue alkaloid lysergamide.

Treatment of bovine lateral saphenous vein (cranial branch) and dorsal metatarsal artery with lysergamide (lysergic acid amide), an alkaloid in abundance in tall fescue, resulted in vasoconstriction similar to that previously shown for the ergot alkaloids ergonovine and ergotamine. Preincubation of tissues with lysergamide resulted in partial inhibition of the contractile response induced by the selective adrenergic agents phenylephrine and BHT-920 (P < .05), indicating partial agonist or antagonist activity of lysergamide at these receptors. Bovine vessels were strongly contracted by serotonin, and the response was markedly inhibited (P < .01) when tissues were preincubated with lysergamide. Studies with selective serotonergic agents indicated that lysergamide may have predilection for serotonin-2 receptors (5-HT2). Thus, lysergamide by itself has vasoconstrictor activity and acts as a partial agonist or antagonist at adrenergic and serotonergic receptors. Further studies are needed to establish the exact receptor effects of lysergamide. However, it is evident that this important alkaloid found in tall fescue infested with the endophytic fungus Acremonium coenophialum should be given consideration in studies designed to alleviate the fescue toxicosis syndrome in cattle.

Effects of phenothiazine and thiabendazole on bovine dorsal pedal vein contractility induced by ergonovine and serotonin; potential for alleviation of fescue toxicity.

Phenothiazine and thiabendazole were studied for their ability to antagonize venoconstriction induced by ergonovine, and the biogenic amine serotonin, in the isolated dorsal pedal vein of cattle. The two compounds are commercially available, approved for usage in cattle and have been reported to reverse some of the toxic effects associated with the intake of Acremonium coenophialum-infested fescue forage by cattle. Neither compound had any antagonistic activity against venoconstriction induced by ergonovine. However, thiabendazole did have some activity against venoconstriction induced by serotonin. Ergot alkaloids are known to cause venoconstriction through effects on biogenic amine receptors, including serotonergic receptors, and since thiabendazole has anti-serotonin activity, part of the reported beneficial effects of thiabendazole in alleviating fescue toxicity may be due to the anti-serotonin activity of the drug. Further work is needed to determine if phenothiazine and thiabendazole have any effect on other types of alkaloids that are present in A. coenophialum-infested fescue.