Effects of purines on rabbit corpus cavernosum contractile activity

At least three well documented neuropharmacologic mediators participate in physiologic erection: inhibition or cessation of alpha-adrenergic transmission, increases in both cholinergic (acetylcholine) and non-adrenergic non-cholinergic (NANC) transmission. In-vitro studies of rabbit corporal smooth muscle reveal that adenosine tri-phosphate (ATP) has a variable effect on muscle tension depending on the level of basal tone. ATP has a pronounced relaxant effect on corporal smooth muscle at either high basal tension or pre-stimulated tension. ATP stimulates a contraction in corporal smooth muscle at low tension. The current studies compare the effects of a series of purines (adenine, adenosine, AMP, ADP, ATP and beta-gamma methylene ATP) on both basal tension and on field-stimulated relaxation. The results demonstrate that all purines relax both baseline tension (2g) and phenylephrine-stimulated contraction. Following phenylephrine pre-stimulation: beta-gamma methylene ATP was significantly more potent than ATP at inhibiting tension. ADP, AMP, adenosine, and adenine produced intermediate dose-response relaxation curves. At 2 grams baseline tension, adenosine, ADP, and AMP were equally potent relaxing agents, ATP was slightly less potent. Adenine and beta-gamma-methylene ATP induced similar dose-response curves which were significantly less potent and efficacious than adenosine, AMP, ADP, and ATP. Field stimulation of phenylephrine pre-contracted tissue strips produce relaxation at both low and high frequencies. None of the purines either facilitated or inhibited the corporal response to field stimulation. We conclude that field stimulated relaxation of rabbit corporal smooth muscle is independent of purinergic relaxation.