Influence of heat stress on the reactivity of isolated chicken carotid artery to vasoactive agents

Cerebral ischaemia is considered to be an important cause of central nervous system dysfunction in heat stress. We hypothesized that heat stress would alter the reactivity of isolated carotid artery to vasoactive agents. Carotid arteries were isolated from broiler chickens maintained either at 23-24 degrees C with 55-65% humidity (control conditions) or exposed to 40 +/- 1 degrees C with 35% humidity for 4 h (heat stress). Contractions were elicited with vasoconstrictors such as 5-HT, phenylephrine, guanfacine and CaCl(2) (K(+)-depolarized) in endothelium-denuded arterial rings. Heat stress significantly increased the potency of 5-HT, but had no effect on the sensitivity of the vessel to phenylephrine or guanfacine. In contrast, it markedly decreased the potency and efficacy of CaCl(2). Vasodilator responses to ACh (endothelium-intact) and sodium nitroprusside (endothelium-denuded), however, were unaffected. Although cyclopiazonic acid (10 microm) significantly decreased 5-HT responses in both the conditions, the agonist was still more potent in heat stress. Extracellular Ca(2)(+) removal had no effect on contractions caused by 5-HT in control conditions, but it significantly decreased the agonist potency in heat stress. Interestingly, nifedipine (1 microm) markedly inhibited 5-HT-induced contractions both in control conditions and in heat stress, implying an inhibitory effect on both Ca(2)(+) influx and release. Thus, nifedipine had a markedly greater inhibitory effect on 5-HT-induced contractions in heat stress compared with control conditions. The results suggest that heat stress increased the vasoconstrictor responses to 5-HT by a mechanism that involved extracellular Ca(2)(+) influx through nifedipine-sensitive L-type calcium channels.