Hyperthermia-induced vasoconstriction of the carotid artery, a possible causative factor of heatstroke.

Clinical and experimental studies indicate that hyperthermia can cause heatstroke with cerebral ischemia and brain damage. However, no study has examined the direct effects of heating carotid artery smooth muscle and tested the hypothesis that hyperthermia induces arterial vasoconstriction and, thereby, decreases cerebral blood flow. We recorded isometric tension of rabbit carotid artery strips in organ baths during stepwise temperature elevation. The heating responses were tested at basal tone, in norepinephrine- and KCl-precontracted vessels, and after electrical field stimulation. Stepwise heating from 37 degrees C to 47 degrees C induced reproducible graded contraction proportional to temperature. The responses could be elicited at basal tone and in precontracted vessels. Heating decreased the contractile responses to norepinephrine and electrical field stimulation but increased contraction to KCl. These responses were not eliminated by pretreatment with the neuronal blocker tetrodotoxin. Our results demonstrate that heating carotid artery preparations above 37 degrees C (normothermia) induced a reversible graded vasoconstriction proportional to temperature. In vivo this reaction may lead to a decrease in cerebral blood flow and cerebral ischemia with brain damage as in heatstroke. The heating-induced contraction is not mediated by a neurogenic process but is due to altered transcellular Ca2+ transport. Cooling, in particular of the neck area, therefore, should be used in the treatment of heatstroke.