Is the neuronal ATP release from guinea-pig vas deferens subject to α2-adrenoceptor-mediated modulation?

Abstract

The effects of a variety of α 2-adrenoceptor agonists and antagonists were studied on stimulation-evoked release of endogenous ATP, measured by the luciferin-luciferase assay and on the release of [ 3H]noradrenaline from the guinea-pig vas deferens. The biphasic mechanical contraction of the guinea-pig smooth muscle was recorded concomitantly. The α 2-adrenoceptor agonist, xylazine (1 μM) inhibited the field stimulation-evoked (8 Hz, 0.1 ms, 480 shocks) release of ATP and [ 3H]noradrenaline and both phases of the contraction. The inhibitory effect of xylazine on the release of ATP, noradrenaline and muscle contraction was prevented by the selective α 2-adrenoceptor antagonist, CH 38083 [7,8-(methylenedioxi)-14α-alloberbanol, 1 μM]. In the presence of prazosin (0.1–1 μM) or WB 4101 [2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride, 0.1–1 μM], i.e. under the condition when the effect of noradrenaline on postjunctional α 1-adrenoceptors was excluded, the stimulation-evoked release of [ 3H]noradrenaline was significantly enhanced, however, the release of endogenous ATP and also both phases of contraction were reduced. In the presence of prazosin, xylazine was able to inhibit the stimulation-evoked release of ATP. In vas deferens dissected from reserpine pretreated (2 × 5 mg/kg, i.p.) guinea-pigs, the content of noradrenaline was 0.5% of control and there was no detectable evoked release of noradrenaline. Under this condition, the release of ATP evoked by electrical stimulation was still detectable, but the amount of ATP was much smaller than that measured from control animals. Xylazine did not reduce the release of ATP. Oxymetazoline, a relatively selective α 2-adrenoceptor agonist failed to inhibit the release of [ 3H]noradrenaline. On the other hand, it enhanced the evoked release of ATP and potentiated both phases of contraction at concentrations of 0.1 and 1 μM. In addition, it enhanced the resting release of ATP and contracted the smooth muscle in a concentration-dependent manner. Prazosin (1 μM) fully antagonized the effect of oxymetazoline, indicating the involvement of postjunctional α 1-adrenoceptors in the release of ATP. The effect of clonidine (1 μM) and dexmedetomidine (1 μM) was similar to that of oxymetazoline. The α 2-adrenoceptor-antagonists yohimbine (1 μM) and CH 38083 (0.1 μM) enhanced the release of both ATP and [ 3H]noradrenaline and also both phases of contraction, although their effect on ATP release was not significant. When prazosin and CH 38083 were administered in combination, i.e. when both α 1- and α 2-adrenoceptors were blocked, the release of noradrenaline was even higher (390% of control) and the release of ATP was reduced to 60% of control. Our findings provide direct evidence that the neuronal release of ATP is subject to modulation by prejunctional α 2-adrenoceptors in the guinea-pig vas deferens. In addition, the lack of inhibition of endogenous ATP release by xylazine in noradrenaline depleted animals suggests the existence of a distinct pool of the neuronal ATP, which is not depleted by reserpine and is modulated differently from noradrenaline.