V.N. Yartsev - Ph.D.(Biol.), Senior Research Scientist, Pavlov Institute of Physiology, Russian Acad. Sci., St. Petersburg, Russia. E-mail: yartsev@infran.ru

Organ blood supply is known to depend both on the local and the central mechanisms of control, and the outcome of interplay between them is not always predictable. In previous studies carried out by other researchers using pre-stretched or spontaneously active isolated blood vessels from different organs of different species, acidosis was shown to cause dilatation [1]. Works performed in our laboratory on the rat mesenteric and tail arteries have shown inhibition of neurogenic vasoconstriction by acidosis [2, 3]. It is worthy of note that all studies mentioned above were carried out at normal temperature of about 36°С. The purpose of this research is to examine effects of acidosis upon the blood vessel tone at low temperatures since joint action of these factors often takes place in vivo. The experiments were carried out on the isolated segments of the rat tail artery. Neurogenic contraction of the vessel segment was evoked by periodic electrical field stimulation (conducted at a frequency of 5 Hz in the form of square wave pulses of 3 ms duration, 30 V amplitude, delivered as 6 s trains repeatedly, with a 3 min interval) of perivascular nerves before and after addition of noradrenaline in cumulative concentration (from 0.03 μM to 10 μM). In four series of experiments, the combination of temperature and pH in the organ bath was as follows: 36°С and 7.4, 36°С and 6.6, 25°С and 7.4, 25°С and 6.6. pH value of the solution (7.4 or 6.6, as required) was established by varying the amount of CO2 bubbled through the solution and was monitored throughout the experiment. Inhibitory effect of acidosis on the noradrenaline-evoked contraction observed at normal temperature was found to disappear at low temperature. Decrease in pH value from 7.4 to 6.6 at normal temperature lead to two-fold inhibition of neurogenic vasoconstriction, while the same decrease in pH value at low temperature lead to two-fold potentiation of the vasoconstriction. In addition, neurogenic vasoconstriction observed in the presence of noradrenaline, was more pronounced in acidic solution than in solution having normal pH value. This was shown for all concentrations of noradrenaline used in our study. Finally, noradrenaline in concentration from 0.03 μM to 0.5 μM potentiated the neurogenic contractile response diminished by cooling, and the degree of the potentiation was not changed by acidosis. We believe that paradoxical contractile effect of acidosis found in our experiments and manifested as an increase in the neurogenic tone of the rat tail artery, may be of importance for the thermoregulation, since it can provide decrease in heat emission at low temperatures.