Presynaptic inhibitory action of adenosine on neuromuscular transmission in the canine cavernous carotid artery.

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Abstract
We investigated the effect of adenosine on neurogenic contraction of the canine cavernous carotid artery, using an isometric tension recording device and transmural nerve stimulation. Adenosine, in concentrations under 10(-5)M, had no relaxing effect on the contractions produced by high [K]o solution or 10(-5)M norepinephrine. Transmural nerve stimulation (stimulus: 1 msec duration, 100V intensity) evoked a frequency-dependent contraction, which was abolished by 3 X 10(-7)M tetrodotoxin. Adenosine in concentrations of 10(-6)M and 10(-5)M, inhibited the neurogenic contraction at each frequency, more so in the low frequency range. This inhibitory effect of adenosine was significantly antagonized by 10(-5)M theophylline. Pretreatment with 2 X 10(-8)M dipyridamole had no effect on neurogenic contractions, but augmented the inhibitory effect of adenosine. 10(-5)M theophylline did not augment the neurogenic contractions. The findings that both dipyridamole and theophylline failed to affect the neurogenic contractions in the absence of adenosine suggests that the presynaptic autoinhibition mechanism of adenosine may not be involved in neuromuscular transmission in this tissue. These results suggest that there is a presynaptic adenosine receptor in the nerve terminal which inhibits the release of neurotransmitter in canine cavernous carotid artery. It is also probable that the vasodilating effect of adenosine in the cavernous carotid artery is mainly due to its inhibitory effect on neurotransmission rather than to a direct relaxing effect on smooth muscle.