Purinoceptor subtypes mediating contraction and relaxation of marmoset urinary bladder smooth muscle

The effects of adenosine triphosphate (ATP), adenosine diphosphate (ADP), α,β‐methylene‐ATP (α,β‐MeATP) and 2‐methylthio‐ATP (2‐MeSATP) on longitudinally orientated smooth muscle strips from marmoset urinary bladder were investigated by use of standard organ bath techniques. 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 greater than 0.1 g mg−1 of tissue and were subsequently utilized in the present investigation. On exposure to ATP, muscle strips exhibited a biphasic response, a rapid and transient contraction followed by a more prolonged relaxation. Both responses were found to be concentration‐dependent. ADP and 2‐MeSATP elicited a similar response (contraction followed by relaxation), whereas application of α,β‐MeATP only produced a contraction. The potency order for each effect was α,β‐MeATP>>2‐MeSATPATP>ADP (contractile response) and ATP=2‐MeSATPADP>>α,β‐MeATP (relaxational response). Desensitization with α,β‐MeATP (10 μM) abolished the contractile phase of the response to ATP, but had no effect on the level of relaxation evoked by this agonist. On the other hand, the G‐protein inactivator, GDPβS (100 μM) abolished only the relaxation response to ATP. Suramin (general P2 antagonist, 100 μM) shifted both the contractile and relaxation ATP concentration‐response curves to the right, whereas cibacron blue (P2Y antagonist, 10 μM) only antagonized the relaxation response to ATP. In contrast, the adenosine receptor antagonist, 8‐phenyltheophylline (10 μM), had no effect on the relaxation response curve to ATP. Incubation with tetrodotoxin (TTX, 3 μM) or depolarization of the muscle strip with 40 mM K+ Krebs failed to abolish the relaxation to ATP. In addition, neither Nω‐nitro‐L‐arginine (L‐NOARG, 10 μM) nor methylene blue (10 μM) had any effect on the relaxation response curve. However, tos‐phe‐chloromethylketone (TPCK, 3 μM), an inhibitor of cyclicAMP‐dependent protein kinase A (PKA), significantly (P