Complex interaction of ergovaline with 5-HT2A, 5-HT1B/1D, and alpha1 receptors in isolated arteries of rat and guinea pig.

Abstract

Vascular effects of ergovaline mediated by 5-hydroxytryptamine(HT)2A, 5-HT1B/1D, and alpha1 receptors were studied in isolated arterial preparations of rat and guinea pig. In rat tail artery ergovaline behaved as a potent contractile partial agonist showing an agonist potency (pEC50) of 8.86 +/- 0.03, a maximum response (Emax) of 59 +/- 2% with respect to 5-HT, and a partial agonist affinity (pK(P)) of 8.51 +/- 0.06. Ergovaline was equipotent with ergotamine (pEC50, 8.69 +/- 0.07; Emax, 52 +/- 4%; pK(P), 8.36 +/- 0.11). Contractile responses to ergovaline and ergotamine were surmountably antagonized by the 5-HT2A receptor antagonist ketanserin (3 nM). Antagonist affinity (apparent pA2) for ketanserin against ergovaline and ergotamine was 9.19 +/- 0.08 and 9.36 +/- 0.17, respectively. Ergovaline showed extremely slow on-set and off-set kinetics in rat tail artery. The construction of cumulative concentration-response curves required about 4 h, and the contractile response to ergovaline (30 nM), which completely abolished the subsequent contractile response to 5-HT (10 nM to 1 mM), could not be reversed by wash-out. In guinea pig iliac artery moderately precontracted with prostaglandin F2alpha (0.05 to 0.5 microM) ergovaline behaved as an agonist (pEC50, 7.71 +/- 0.10) with a potency similar to that of 5-HT (pEC50, 7.60 +/- 0.05). The contractile response to ergovaline was inhibited by the 5-HT1B/1D receptor antagonist GR127935 (10 nM). The apparent pA2 value for GR127935 against ergovaline was 8.90 +/- 0.12. Ergovaline (10 nM) produced no contractile response in guinea pig iliac artery when added before the PGF2alpha-induced precontraction but caused insurmountable blockade of the contractile response to the 5-HT1B/1D receptor agonist 5-carboxamidotryptamine (5-CT). The apparent pA2 value for ergovaline against 5-CT was 8.56 +/- 0.18. In rat thoracic aorta ergovaline (2 microM) activated alpha1 adrenoceptors only with low efficacy (Emax, 12 +/- 3%) but surmountably antagonized norepinephrine-induced contractions with a pK(P) of 7.07 +/- 0.12. It is concluded that the powerful constrictor effect of ergovaline mediated by activation of vascular 5-HT2A and 5-HT1B/1D receptors may explain the vascular symptoms of fescue toxicosis observed in livestock grazing tall fescue pastures infected with the endophytic fungus Neotyphodium coenophialum.