PURINOCEPTOR SUBTYPES MEDIATING CONTRACTION AND RELAXATION OF MARMOSET URINARY-BLADDER SMOOTH-MUSCLE

1 The effects of adenosine triphosphate (ATP), adenosine diphosphate ( ADP), alpha,beta-methylene-ATP (alpha,beta-MeATP) and 2-methylthio-ATP (2-MeSATP) on longitudinally orientated smooth muscle strips from mar moset urinary bladder were investigated by use of standard organ bath techniques. 2 After being mounted in superfusion organ baths, 66.7% (n =249) of marmoset detrusor smooth muscle strips developed spontaneous tone, 48.2% of all strips examined developed tone equivalent to greate r than 0.1 g mg(-1) of tissue and were subsequently utilized in the pr esent investigation. 3 On exposure to ATP, muscle strips exhibited a b iphasic response, a rapid and transient contraction followed by a more prolonged relaxation. Both responses were found to be concentration-d ependent. ADP and 2-MeSATP elicited a similar response (contraction fo llowed by relaxation), whereas application of alpha,beta-MeATP only pr oduced a contraction. The potency order for each effect was alpha,beta -MeATP>>2-MeSATP greater than or equal to ATP>ADP (contractile respons e) and ATP=2-MeSATP greater than or equal to ADP>>alpha,beta-MeATP (re laxational response). 4 Desensitization with alpha,beta-MeATP (10 mu M ) abolished the contractile phase of the response to ATP, but had no e ffect on the level of relaxation evoked by this agonist. On the other hand, the G-protein inactivator, GDP beta S (100 mu M) abolished only the relaxation response to ATP. Suramin (general P2 antagonist, 100 mu M) shifted both the contractile and relaxation ATP concentration-resp onse curves to the right, whereas cibacron blue (P2Y antagonist, 10 mu M) only antagonized the relaxation response to ATP. In contrast, the adenosine receptor antagonist, 8-phenyltheophylline (10 mu M), had no effect on the relaxation response curve to ATP. 5 Incubation with tetr odotoxin (TTX, 3 mu M) or depolarization of the muscle strip with 40 m M K+ Krebs failed to abolish the relaxation to ATP. In addition, neith er N-omega-nitro-L-arginine (L-NOARG, 10 mu M) nor methylene blue (10 mu M) had any effect on the relaxation response curve. However, tos-ph echloromethylketone (TPCK, 3 mu M), an inhibitor of cyclicAMP-dependen t protein kinase A (PKA), significantly (P<0.01) shifted the curve for the ATP-induced relaxation to the right. 6 It is proposed that marmos et detrusor smooth muscle contains two receptors for ATP, a classical P2X-type receptor mediating smooth muscle contraction, and a P2Y (G-pr otein linked) receptor mediating smooth muscle relaxation. The results also indicate that the ATP-evoked relaxation may occur through the ac tivation of cyclicAMP-dependent PKA.