Vasoactive effects of ergot alkaloid exposure on ovine pedal artery and umbilical blood vessels
The incidence of ergot toxicity or ergotism has increased in Western Canada in recent years and resulted in significant economic losses to the livestock farming industry. Chronic ergot toxicity is known to cause severe vasoconstriction through the activation of alpha (α)-adrenergic and serotonergic receptors on vascular smooth muscle cells in end arterioles. Ergotism may also result in abortion. Despite extensive research in the mechanism of ergot toxicity, very little is known about the vascular effects of acute exposure to ergot alkaloids or the mechanism of these effects. Similarly, almost nothing is known about the effects of ergot alkaloids on the umbilical vasculature after 45-days of exposure to ergot alkaloids. Two studies were performed in sheep to address these two important questions in acute and chronic exposure. We hypothesized that acute single-dose exposure to ergot alkaloids increases the contractile response in the pedal artery through the activation of alpha1 (α1)- adrenergic and serotonergic receptors. We also hypothesized that terazosin (TE), a selective α1- adrenergic blocker, would abolish the effect of ergot exposure. Our last hypothesis was that a 45-day ergot exposure to ergot alkaloids increases the phenylephrine (PE) contractile response in the isolated umbilical artery and vein as well as the maternal pedal artery. In the first study, twelve adult sheep were randomly placed into control and exposure groups. The exposure group received a single oral dose of ergot alkaloids obtained from ground sclerotia (600 µg/kg BW) while the control group received only a water placebo. The PE and serotonin contractile responses were assessed in the pedal artery (dorsal metatarsal III artery) dissected six hours after exposure. The effect of an α1-adrenergic blocker (TE) was also evaluated. This study demonstrated that acute exposure to ergot alkaloids resulted in a significant increase in PE contractile response compared to the control group (Ctl EC50 = 1.74 x 10-6 M; Exp EC50 = 1.079 x 10-6 M, P = 0.046). However, there was no significant difference in serotonergic contractile response (P = 0.12) between the two groups. TE treatment resulted in a significant dose-dependent increase in EC50 in both the exposure and the control group (P < 0.05 for all treatments). Surprisingly, the effect of TE was significantly more pronounced in the ergot exposed group compared to the control group at two of the three concentrations of TE (TE 30 nM, P = 0.36; TE 100 nM, P < 0.001; TE 300 nM, P < 0.001). We concluded that the acute effects of ergot exposure are mainly mediated through the activation of α1-adrenergic receptors and not serotonergic receptors. TE appears to be more potent in blocking the PE contractile response in sheep exposed to ergot compared to the control group. This study may indicate that the dry gangrene seen in sheep, and likely other species, is related to the activation of α1-adrenergic receptor. This effect may be reversed by using TE especially during the early stages of the disease before cell death occurs. This study may also indicate that the acute single dose exposure scenario may be useful in the study of the vascular effects of ergot alkaloids. The objectives of the second study were to determine the effects of chronic ergot alkaloid exposure on PE contractile response in the umbilical vasculature, as well as the maternal pedal artery of pregnant sheep. Twelve adult pregnant sheep were utilized in this study, and were randomly placed into groups; six of which received a pelleted diet containing ergot (46 µg/kg BW) while the control group received the uncontaminated pelleted feed. This study showed that 45-days of oral exposure to ergot alkaloids resulted in a significant increase in PE contractile response in the umbilical artery (Ctl EC50 = 3.962 x 10-6 M; Exp EC50 = 1.161 x 10-6 M, P < 0.0001) and the umbilical vein (Ctl EC50 = 7.889 x 10-6 M; Exp EC50 = 6.801 x 10-7 M, P < 0.0001), but not in the maternal pedal artery (Ctl EC50 = 4.331 x 10-6 M; Exp EC50 = 4.856 x 10-6 M, P = 0.3927). A significantly lower fetal weight was also found in ergot exposed sheep compared to the control group (control, 3.3 ± 0.17 kg; exposure 2.07 ± 0.13 kg, P = 0.0002), (T-test, GraphPad Prism). This is the first study to report increased contractility in the umbilical vasculature after 45-days of oral ergot exposure. The mechanism of which is, at least in part, related to the activation of α1-adrenergic receptors. The concentration selected in this study is below the allowed limits set by the Canadian Food Inspection Agency in sheep which recommends that ergot levels in feed be less than 600 ppb. This study indicates that there is an urgent need to revisit these standards as negative effects can occur at lower concentrations.
ergot toxicity, sheep, vasoconstriction, pedal artery, umbilical blood vessels, adrenergic receptors
Master of Science (M.Sc.)