Effects of Cold on Adrenoreactivity of the Rat Superior Mesenteric Artery

The effect of cold on the reactivity of the rat superior mesentericartery (SMA), estimated by the magnitude of contractile responsesof its segments to noradrenaline (NA, 0.01–10.0 µM) and electrical fieldstimulation (EFS) of the perivascular nerves at a frequency of 3,10 and 40 Hz in both the absence and presence of NA in the mediumat a temperature of 37 or 25°C, was studied in isometric experiments. Coolingdid not alter SMA tone evoked by NA at all its concentrations used.In both the absence and presence of low NA concentrations (0.01–0.10µM), cooling led to a significant decrease in the neurogenic SMAresponse at all EFS frequencies, while in the presence of NA ata high concentration (1–10 µM), cooling had no significant effecton 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 NAon the neurogenic SMA constriction. At a high concentration undernormal temperature conditions (37°C), NA reduced the neurogenicSMA constriction at all EFS frequencies, while under cooling conditions(25°C), it had a potentiating effect at low and a depressing effectat high EFS frequencies. These results demonstrate that the depressingeffect of cold on the neurogenic constriction of the rat SMA, occurringat low and disappearing at high NA concentrations, as well as onthe NA-evoked potentiation of the neurogenic vasoconstriction, is irrelevantto a decrease in the contractile effect of NA, which is the mainneurotransmitter in this artery. These effects may contribute toblood rerouting from the surface to the deeper regions of the body,thus reducing heat loss and improving thermoregulation.