Comparative study of the actions of AP5 A and α,β‐methylene ATP on nonadrenergic, noncholinergic neurogenic excitation in the guinea‐pig vas deferens

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Abstract
1 The agonistic and antagonistic effects of two nucleotide analogues, P1,P5-di-(adenosine-5′) pentaphosphate (AP5A) and α,β-methylene ATP (α,β-Me ATP), have been compared in the guinea-pig isolated vas deferens. 2 In organ bath studies, both AP5A and α,β-Me ATP were approximately 100 times more potent than ATP in producing phasic contractions of the vas deferens smooth muscle. Repeated additions of either agonist (1–10 μm) produced desensitization to a subsequent addition of the test substance. AP5A and α,β-Me ATP were approximately equipotent in the production of desensitization. 3 After desensitization had been produced in the vas deferens by AP5A or α,β-Me ATP, excitatory responses elicited by ATP (100–150 μm) and nonadrenergic field stimulation (2–20 Hz) were blocked, whereas those elicited by carbachol (1–10μm) were augmented. 4 Intracellular electrical recordings demonstrated that AP5A and α,β-Me ATP produced similar effects on membrane activity of the vas deferens. Concentration-dependent depolarizations alone were produced by both substances until the voltage threshold for action potential discharge was attained; thereafter, action potential discharges were superimposed on the depolarization and an accompanying phasic contraction was recorded. Upon restoration of the membrane potential to its control value (5–10 min after addition of either AP5A or α,β-Me ATP), excitatory junction potentials (e.j.ps) elicited by field stimulation (up to 3 Hz) and spontaneous e.j.ps were reduced by AP5A (> 0.1 μm) in a concentration-dependent manner (as previously described for α,β-Me ATP). 5 The antagonistic effects of AP5A on mechanical responses elicited by field stimulation were more quickly reversed on washout of AP5A than were the effects of α,β-Me ATP, suggesting some dissimilarity in their mechanism of action at the receptor level. 6 The antagonistic effects of AP5A on the nonadrenergic contractile responses of the vas deferens were not produced by the structurally related P1,P4-di-(adenosine-5′) tetraphosphate (AP4A) even with cumulative concentrations up to 200 μm 7 Desensitization of P2-purinoceptors can be produced by some nucleotide analogues such as AP5A and α,β-Me ATP, whose activity may arise partly because of their structural conformation and stability.

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