INVOLVEMENT OF K-NAME AND INDOMETHACIN RESISTANT PART OF ADENOSINE-5'-O-(2-THIODIPHOSPHATE)-INDUCED RELAXATION OF PANCREATIC VASCULAR BED( CHANNEL PERMEABILITY CHANGES IN THE L)

1 We have previously demonstrated that adenosine-5'-O-(2-thiodiphospha te) (ADP beta S), a potent P2Y-purinoceptor agonist, relaxed pancreati c vasculature not only through prostacyclin (PGI(2)) and nitric oxide (NO) release from the endothelium but also through other mechanism(s). In this study, we investigated the effects of an inhibitor of the Na/K+ pump, of ATP-sensitive K+ (K-ATP) channels and of small (SKCa) or large (BKCa) conductance Ca2+-activated K+ channels. Experiments were performed at basal tone and during the inhibition of NO synthase and c yclo-oxygenase. 2 In control conditions, ADP beta S (15 mu M) induced an initial transient vasoconstriction followed by a progressive and su stained vasodilatation. In the presence of N omega-nitro-L-arginine me thyl ester (L-NAME, 200 mu M) the transient vasoconstriction was rever sed into a one minute vasodilator effect, which was then followed by a progressive and sustained vasodilatation similar to that observed wit h ADP beta S alone. The addition of indomethacin (10 mu M) did not sig nificantly modify the profile of ADP beta S-induced vasodilatation. 3 Ouabain (100 mu M) decreased basal pancreatic flow rate and did not mo dify ADP beta S-induced relaxation. This inhibitor of the Na+/K+ pump increased the pancreatic vasoconstriction induced by L-NAME or by the cn-administration of L-NAME and indomethacin. Ouabain did not modify e ither the L-NAME or the L-NAME/indomethacin resistant part of the ADP beta S vasodilatation. 4 The K-ATP inhibitor tolbutamide (185 mu M) di d not significantly modify basal pancreatic flow rate and ADP beta S-i nduced relaxation. This inhibitor which did not change L-NAME-induced vasoconstriction, significantly diminished the L-NAME resistant part o f ADP beta S-induced vasodilatation. Tolbutamide intensified the vasoc onstriction induced by the co-administration of L-NAME and indomethaci n. In contrast, the L-NAME/indomethacin resistant part of ADP beta S v asodilatation was not changed by the closure of K-ATP. 5 The SKCa inhi bitor apamin (0.1 mu M) did not significantly change pancreatic vascul ar resistance whatever the experimental conditions (in the absence or in presence of L-NAME or L-NAME/indomethacin). In the presence of L-NA ME, the closure of SKCa channels changed the one minute vasodilator ef fect of ADP beta S into a potent vasoconstriction and thereafter modif ied only the beginning of the second part of the L-NAME-resistant part of the ADP beta S-induced vasodilatation. In contrast, the L-NAME/ind omethacin resistant part of ADP beta S-induced relaxation remained unc hanged in the presence of apamin. 6 Charybdotoxin (0.2 mu M), an inhib itor of BKCa, increased pancreatic vascular resistance in the presence of L-NAME/indomethacin. In the presence of L-NAME, the closure of BKC a channels reversed the one minute vasodilator effect of AD beta PS in to a potent vasoconstriction and drastically diminished the sustained vasodilatation. In contrast the L-NAME/indomethacin resistant part of ADP beta S-induced relaxation was not modified by the presence of char ybdotoxin. Under L-NAME/indomethacin/charybdotoxin/apamin infusions, A DP beta S evoked a drastic and transient vasoconstriction reaching a m aximum at the second minute, which was followed by a sustained increas e in the flow rate throughout the ADP beta S infusion. The maximal vas odilator effect of ADP beta S observed was not modified by the additio n of apamin. 7 The results suggest that the L-NAME-resistant relaxatio n induced by ADP beta S in the pancreatic vascular bed involves activa tion of BKCa, K-ATP and to a lesser extent of SKCa channels, but the L -NAME/indomethacin resistant part of ADP beta S-induced relaxation is insensitive to the closure of K-ATP, SKCa and BKCa channels.