Modulation of temperature-induced tone by vasoconstrictor agents

One of the primary cardiovascular adjustments to hyperthermia is a sympathe tically mediated increase in vascular resistance in the viscera. Nonneural factors such as a change in vascular tone or reactivity may also contribute to this response. Therefore, the aim of this study was to determine whethe r vascular smooth muscle tone is altered during heating to physiologically relevant temperatures >37 degrees C. Gradually increasing bath temperature from 37 degrees C (normothermia) to 43 degrees C (severe hyperthermia) prod uced graded contractions in vascular ring segments from rat mesenteric arte ries and thoracic aortae. In untreated rings these contractions were relati vely small, whereas hyperthermia elicited near-maximal increases in tension when rings were constricted with phenylephrine or KCI before heating. In p henylephrine-treated mesenteric arterial rings, the contractile responses t o heating were markedly attenuated by the Ca2+ channel antagonists nifedipi ne and diltiazem. Diltiazem also blocked the contractile responses to heati ng in thoracic aortic rings. These results demonstrate that hyperthermia ha s a limited effect on tension generation in rat vascular smooth muscle in t he absence of vascular tone. However, in the presence of agonist-induced to ne, tension generation during heating is markedly enhanced and dependent on extracellular Ca2+. In conclusion, these data suggest that local regulatio n of vascular tone can contribute to the hemodynamic adjustments to hyperth ermia.