Involvement of K(+) channel permeability changes in the L-NAME and indomethacin resistant part of adenosine-5′-O-(2-thiodiphosphate)-induced relaxation of pancreatic vascular bed

1. We have previously demonstrated that adenosine-5′-O-(2-thiodiphosphate) (ADPβS), a potent P2Y-purinoceptor agonist, relaxed pancreatic 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 (SK(Ca)) or large (BK(Ca)) conductance Ca(2+)-activated K(+) channels. Experiments were performed at basal tone and during the inhibition of NO synthase and cyclo-oxygenase. 2. In control conditions, ADPβS (15 μM) induced an initial transient vasoconstriction followed by a progressive and sustained vasodilatation. In the presence of N(ω)-nitro-L-arginine methyl ester (L-NAME, 200 μM) the transient vasoconstriction was reversed into a one minute vasodilator effect, which was then followed by a progressive and sustained vasodilatation similar to that observed with ADPβS alone. The addition of indomethacin (10 μM) did not significantly modify the profile of ADPβS-induced vasodilatation. 3. Ouabain (100 μM) decreased basal pancreatic flow rate and did not modify ADPβS-induced relaxation. This inhibitor of the Na(+)/K(+) pump increased the pancreatic vasoconstriction induced by L-NAME or by the co-administration of L-NAME and indomethacin. Ouabain did not modify either the L-NAME or the L-NAME/indomethacin resistant part of the ADPβS vasodilatation. 4. The K(ATP) inhibitor tolbutamide (185 μM) did not significantly modify basal pancreatic flow rate and ADPβS-induced relaxation. This inhibitor which did not change L-NAME-induced vasoconstriction, significantly diminished the L-NAME resistant part of ADPβS-induced vasodilatation. Tolbutamide intensified the vasoconstriction induced by the co-administration of L-NAME and indomethacin. In contrast, the L-NAME/indomethacin resistant part of ADPβS vasodilatation was not changed by the closure of K(ATP). 5. The SK(Ca) inhibitor apamin (0.1 μM) did not significantly change pancreatic vascular resistance whatever the experimental conditions (in the absence or in presence of L-NAME or L-NAME/indomethacin). In the presence of L-NAME, the closure of SK(Ca) channels changed the one minute vasodilator effect of ADPβS into a potent vasoconstriction and thereafter modified only the beginning of the second part of the L-NAME-resistant part of the ADPβS-induced vasodilatation. In contrast, the L-NAME/indomethacin resistant part of ADPβS-induced relaxation remained unchanged in the presence of apamin. 6. Charybdotoxin (0.2 μM), an inhibitor of BK(Ca), increased pancreatic vascular resistance in the presence of L-NAME/indomethacin. In the presence of L-NAME, the closure of BK(Ca) channels reversed the one minute vasodilator effect of ADβPS into a potent vasoconstriction and drastically diminished the sustained vasodilatation. In contrast the L-NAME/indomethacin resistant part of ADPβS-induced relaxation was not modified by the presence of charybdotoxin. Under L-NAME/indomethacin/charybdotoxin/apamin infusions, ADPβS evoked a drastic and transient vasoconstriction reaching a maximum at the second minute, which was followed by a sustained increase in the flow rate throughout the ADPβS infusion. The maximal vasodilator effect of ADPβS observed was not modified by the addition of apamin. 7. The results suggest that the L-NAME-resistant relaxation induced by ADPβS in the pancreatic vascular bed involves activation of BK(Ca), K(ATP) and to a lesser extent of SK(Ca) channels, but the L-NAME/indomethacin resistant part of ADPβS-induced relaxation is insensitive to the closure of K(ATP), SK(Ca) and BK(Ca) channels