Cromakalim inhibits contractions of the rat isolated mesenteric bed induced by noradrenaline but not caffeine in Ca-free medium: Evidence for interference with receptor-mediated Ca mobilization

The effects of the K⁺ channel opener cromakalim on phasic contractions induced by noradrenaline and caffeine were studied in the rat isolated mesenteric bed. In the presence of 1.4 mM Ca²⁺, 1-s pulses of noradrenaline increased the perfusion pressure of the preparation concentration dependently (midpoint at 92 ± 10 μM noradrenaline). Cromakalim (0.3 and 1 μM) inhibited these contractions in a non-competitive manner. Contractions elicited by 1-s pulses of noradrenaline (100 μM) were inhibited by the dihydropyridine Ca²⁺ antagonist isradipine by maximally 24 ± 1%, indicating that only a minor component of this contraction depended on Ca²⁺ entry via dihydropyridine-sensitive Ca²⁺ channels. Cromakalim was a much more effective inhibitor of these contractions (maximum inhibition by 80%, midpoint of the inhibition curve at 171 ± 15 nM). The effect of cromakalim was stereoselective, inhibited by the sulphonylurea glibenclamide, and abolished in partially depolarizing media (KC1 = 35 and 50 mM). In Ca²⁺-free medium, cromakalim inhibited the contraction induced by noradrenaline (100 μM) by maximally 69 ± 4%, with a midpoint at 58 ± 14 nM. The effect of cromakalim was again stereoselective, inhibited by glibenclamide, and abolished in the presence of 50 mM KC1. Contractions induced by caffeine (10 and 100 μM) were not affected by cromakalim (1 μM). The results indicate that, in rat mesenteric resistance vessels, cromakalim interferes with the ability of noradrenaline, but not caffeine, to mobilize Ca²⁺ from intracellular stores. The antivasoconstrictor effect of cromakalim against noradrenaline is inhibited by glibenclamide and appears to be linked to the ability of cromakalim to hyperpolarize the cell membrane.