Purine Receptors in the Rat Anococcygeus Muscle

Adenosine triphosphate (ATP) caused contraction of the resting isolated rat anococcygeus muscle. Non-phosphorylated purines did not cause contraction of the resting muscle but did so in muscles in which the tone was raised by carbachol or guanethidine. Adenosine, (-)N6-phenylisopropyladenosine (PIA) and 5'-N-ethyl-carboxamide adenosine (NECA) were approximately equipotent, and these responses were not prevented by theophylline, quinidine, 2,2'-pyridylisatogen tosylate, phentolamine, methysergide, dipyridamole, hexobendine or indomethacin. The contractions became smaller as muscle tone progressively declined, and it is suggested that this effect may explain the apparent blockade of ATP responses by indomethacin reported previously. Adenosine, 2-chloroadenosine, ATP, PIA and NECA inhibited contractile responses of the anococcygeus to field stimulation of the excitatory adrenergic innervation. This inhibitory action was blocked by theophylline, and as PIA was easily the most potent purine tested, it may involve activation of an A1/Ri receptor. It is also argued, however, that the A/R scheme of classification may be inappropriate for the description of responses of intact tissues. As response to noradrenaline were not changed by the purines, the inhibitory effect on stimulation-evoked contractions is probably mediated at a presynaptic site. None of the purines tested had any effect on the neurally mediated inhibition of the anococcygeus which is seen when intrinsic tone is raised and the excitatory adrenergic nerves are blocked.