Cooling is a potent vasodilator of deep vessels in the rat

The objectives of this study were to determine the effect of cooling on smo oth muscle tone of the pulmonary artery and aorta and to clarify the basic mechanism of these responses. We recorded isometric tension in smooth muscl e strips of rat pulmonary artery and aorta in organ baths during stepwise c ooling. Cooling responses were tested before and after the addition of vari ous standard agents that interfere with known neurogenic (autonomic blocker s, tetrodotoxin) and myogenic mechanisms (calcium channel blockers) of rela xation. We also examined the hypothesis of the presence of a cooling-releas ed substance. Stepwise cooling (37 degreesC to 4 degreesC) of aortic smooth muscle induced reproducible graded relaxations that were inversely proport ional to temperature. Cooling-induced relaxation was not dependent on a neu ral mechanism nor the release of neurotransmitters or a cooling-released su bstance such as NO or CO. Cooling of pulmonary arterial and aortic smooth m uscle preparations induced a graded myogenic relaxation inversely proportio nal to the cooling temperature. The mechanism is not dependent on local ner vous or known mediators but related to a direct physico-chemical effect of cooling.