Vasoconstriction in the viscera is one of the primary cardiovascular adjustments to heat stress. Because local temperature has been shown to influence vascular responsiveness to catecholamines and sympathetic nerve activity, we hypothesized that heating would alter reactivity in rat superior mesenteric arteries. Vascular rings were mounted in oxygenated Krebs solution for measurement of isometric tension and gradually heated to 43°C to mimic vascular resistance changes observed in vivo. Heating elicited graded increases in tension (Δ from 37°C to 43°C = 93 ±12 mg) that were significantly augmented by treatment with phenylephrine or KCl prior to heating (Δ from 37°C = 803 ± 61 mg and 1063 ± 111 mg, respectively). The contractile response to heating in phenylephrine-treated rings was markedly attenuated by the calcium channel antagonist nifedipine (Δ from 37°C = 225 ± 63 mg). In a separate group of rings, heating to 41°C also augmented maximal contractions to increasing concentrations of norepinephrine compared to values at 37°C. Finally, to further examine the role of calcium in mediating these enhanced responses, concentration-response curves to calcium were compared at 37 and 41°C. Heating significantly enhanced the contractile responses to increasing concentrations of calcium. Collectively, these results demonstrate that vasoconstrictor responses in mesenteric arterial rings are enhanced with heating, an effect mediated, in part, by a heat-induced increase in calcium sensitivity. This change in vascular reactivity may contribute to the hemodynamic adjustments to heat stress by modifying local regulation of vascular tone in the mesenteric artery.
|State||Published - 1997|