Inhibitory responses to exogenous adenosine in murine proximal and distal colon

1 The aims of the present study were firstly, to characterize pharmacologically the subtypes of P-1 purinoreceptors involved in the inhibitory effects induced by exogenous adenosine in longitudinal smooth muscle of mouse colon, and secondly, to examine differences in the function and distribution of these receptors between proximal and distal colon. 2 Adenosine (100 mu M-3mM) caused a concentration-dependent reduction of the amplitude of spontaneous contractions in the proximal colon, and muscular relaxation in the distal colon. In the proximal colon, adenosine effects were antagonized by a selective A(1) receptor antagonist, 1,3-dipropyl- 8-cyclopentylxanthine (DPCPX, 10 nM), but were not modified by 3,7-dimethyl-1-propargylxanthine (DMPX, 10 mu M) or by 9-chloro-2-(2-furanyl)-5-((phenylacetyl)amino)-[1,2,4]triazolo[1,5-c]quinazoline (MRS 1220, 0.1 mu M), selective A(2) and A(3) receptor antagonists, respectively. In the distal colon, adenosine effects were antagonized by DPCPX, DMPX, and by a selective A(2B) receptor antagonist, 8-[4-[((4-cyanophenyl)carbamoylmethyl)oxy]phenyl]-1,3-di(n-propyl) xanthine (MRS 1754, 10 mu M), but not by 8-(3-chlorostyryl)-caffeine (CSC, 10 mM), a selective A(2A) receptor antagonist, or by MRS 1220. 3 Tetrodotoxin (TTX 1 mu M), the nitric oxide (NO) synthase inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME, 100 mu M), or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 mu M), an inhibitor of soluble guanylyl cyclase, reduced adenosine effects only in distal colon. In addition, L-NAME induced a further reduction of adenosine relaxation in the presence of DPCPX, but not in the presence of MRS 1754. 4 From these results we conclude that, in the murine proximal colon, adenosine induces inhibitory effects via TTX-insensitive activation of A(1) receptor. In the distal colon, adenosine activates both A(1) and A(2B) receptors, the latter located on enteric inhibitory neurons releasing NO.