Changes of noradrenaline-induced contractility and gene expression in aorta of rats acclimated to heat in two different modes.

We investigated changes in responsiveness to noradrenaline (NA) and gene expression in aorta, and plasma nitrite/nitrate (NOx) levels in heat-acclimated rats. Male rats were exposed for 10 consecutive days to either 32 degrees C in the second half of the dark phase (HI) or to 32 degrees C constantly (HC). Controls were kept at 24 degrees C. After heat acclimation, thoracic aorta was excised and blood was taken in the first (Dark1) and second (Dark2) halves of the dark phase, and in the light phase. Contractile response to NA in HI was significantly depressed in Dark1 and Dark2, whereas in HC the contraction response to NA was significantly attenuated in all phases. Endothelial NO synthase (eNOS) mRNA expression was significantly enhanced in HI in Dark2. Inducible NOS mRNA was undetectable. mRNA expression of period2 was not modified by heat acclimation. Plasma NOx concentrations in HI were significantly elevated in Dark1 and Dark2, while in HC, the level was significantly raised in all phases. Our results suggest that in rats, heat acclimation improves distensibility of thoracic aorta due to attenuated responsiveness to NA, which might be attributable in part to an upregulated eNOS expression and an increase in circulating NO. The phases of the day when such changes occurred appear to be closely related to the periods when the animals were previously subjected to heat.