Effects of Cold on Adrenoreactivity of the Rat Superior Mesenteric Artery.

The effect of cold on the reactivity of the rat superior mesenteric artery (SMA), estimated by the magnitude of contractile responses of its segments to noradrenaline (NA, 0.01–10.0 µM) and electrical field stimulation (EFS) of the perivascular nerves at a frequency of 3, 10 and 40 Hz in both the absence and presence of NA in the medium at a temperature of 37 or 25°C, was studied in isometric experiments. Cooling did not alter SMA tone evoked by NA at all its concentrations used. In both the absence and presence of low NA concentrations (0.01–0.10 µM), cooling led to a significant decrease in the neurogenic SMA response at all EFS frequencies, while in the presence of NA at a high concentration (1–10 µM), cooling had no significant effect on this response. In the presence of NA at a low concentration, cooling insignificantly (at low EFS frequencies) and significantly (at high EFS frequencies) reduced the potentiating effect of NA on the neurogenic SMA constriction. At a high concentration under normal temperature conditions (37°C), NA reduced the neurogenic SMA constriction at all EFS frequencies, while under cooling conditions (25°C), it had a potentiating effect at low and a depressing effect at high EFS frequencies. These results demonstrate that the depressing effect of cold on the neurogenic constriction of the rat SMA, occurring at low and disappearing at high NA concentrations, as well as on the NA-evoked potentiation of the neurogenic vasoconstriction, is irrelevant to a decrease in the contractile effect of NA, which is the main neurotransmitter in this artery. These effects may contribute to blood rerouting from the surface to the deeper regions of the body, thus reducing heat loss and improving thermoregulation. [ABSTRACT FROM AUTHOR]