V.N. Yartsev - Ph.D., Senior Research Scientist, Pavlov Institute of Physiology, Russian Acad. Sci., (St. Petersburg)
Hypothermic preconditioning nowadays is widely used to prevent brain injury due to ischemia-reperfusion . However the effect of cooling on the condition and properties of the blood vessels feeding the brain is not clearly understood. Internal carotid artery is known to play a key role in supplying blood to the brain , but we failed to find works dealing with the effect of cooling on the artery contractility. Previously, we have shown the profound potentiative effect of cooling on the rat tail artery constriction evoked by noradrenaline and the inhibitory effect on the artery constriction evoked by the neurogenic stimulation of the perivascular nerves . The primary concern of this research is to examine the effects of cooling on the noradrenaline-induced and the neurogenic tone of the rat internal carotid artery. The experiments were carried out on the isolated segments of the artery. Neurogenic contraction of the vessel segment was evoked by periodic electrical field stimulation (conducted at a frequency of 3, 10, and 40 Hz every 3 min) of perivascular nerves of before and after addition of noradrenaline in cumulative concentration (from 0.01 μM to 10 μM). In the first series of experiments, the temperature in the tissue bath was 36 °С while in the second series the temperature was reduced to 25 °С at 30 min from the beginning of the experiment and the temperature was kept at this level till the end of the experiment. pH value of the solution (7.4) was monitored throughout all experiments. In our experiments, noradrenaline-induced constriction of the internal carotid artery was considerably decreased by cooling (Fig. 1), as opposed to the tail artery constriction which was significantly decreased at low temperature. Neurogenic constriction of the internal carotid artery both in the absence and in the presence of noradrenaline in concentration from 0.01 μM to 10 μM was considerably decreased by cooling at all frequencies used in our experiments (Fig. 2). Noradrenaline-induced potentiation of the neurogenic constriction for the first time shown in our experiments on this artery was most prominent in the presence of high concentrations of noradrenaline (from 0.5 μM to 10 μM) and was inhibited at low temperature, the inhibition being maximal at the concentrations indicated. The rat internal carotid artery was shown to dilate at low temperature via decrease in the neurogenic and the noradrenaline-evoked tone of the artery. This might be of importance for the increase in the cerebral blood flow ensuring normal temperature of the brain despite low environmental temperatures.
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