Effects of inhibitors of small- and intermediate-conductance calcium-activated potassium channels, inwardly-rectifying potassium channels and Na+/K+ ATPase on EDHF relaxations in the rat hepatic artery

1 In the rat hepatic artery, the SKCa inhibitors UCL 1684 (300 nM) completely blocked, and scyllatoxin (1 mu M) and d-tubocurarine (100 mu M) partially inhibited EDHF relaxations when each of them was combined with charybdotoxin (300 nM). 2 The IKCa inhibitors clotrimazole (3 mu M) and 2-chlorophenyl-bisphenyl-methanol (3 mu M) strongly depressed EDHF relaxations when each of them was combined with apamin (300 nM). The cytochrome P450 mono-oxygenase inhibitor ketoconazole (10 mu M) had no effect in the presence of apamin. 3 Ciclazindol (10 mu M), which abolishes EDHF relaxations in the presence of apamin, almost completely prevented the calcium ionophore (A23187) stimulated Rb-86(+) influx tin the Gardos channel (IKCa) in human erythrocytes. 4 The Na+/K+ ATPase inhibitor ouabain (500 mu M) and the K-IR blocker Ba2+ (30 mu M) neither alone nor in combination inhibited EDHF relaxations. Ba2+ was also without effect in the presence of either apamin or charybdotoxin. 5 In contrast to EDHF, an increase in extracellular [K+] from 4.6 mM to 9.6, 14.6 and 19.6 mM inconsistently relaxed arteries. In K+-free physiological salt solution, re-admission of K+ always caused complete and sustained relaxations which were abolished by ouabain but unaffected by Ba2+ 6 The present study provides pharmacological evidence for the involvement of SKCa and IKCa in the action of EDHF in the rat hepatic artery. Our results are not consistent with the idea that EDHF is K+ activating Na+/K+ ATPase and K-IR in this blood vessel.