Effects of glibenclamide on cytosolic calcium concentrations and on contraction of the rabbit aorta

Abstract

1 Using fluorometry of fura-2 and rabbit aortic strips, we studied the effects of glibenclamide (GLB), a sulphonylurea anti-diabetic drug and an inhibitor of opening of K+ channels, on cystolic calcium concentrations ([Ca2+](i)) and on force development. 2 Both high K+-depolarization and noradrenaline (NA) increased [CA2+](i) and force, in a concentration-dependent manner, in the presence of extracellular Ca2+ (1.25 mM). However, force development in relation to [Ca2+](i) ([Ca2+](i)-force relationship) observed with NA was much greater than that observed with K+-depolarization. 3 Pretreatment with GLB (10-6-10-4 M) for 10 min partially inhibited, in a concentration-dependent manner, both [Ca2+](i) elevation and the force development induced by 118 mM K+-depolarization or NA 10-5 M in the presence of extracellular Ca2+. The [Ca2+](i)-force relationship induced by both 118 mM K+ physiological salt solutions and by NA 10-5 M in the GLB-treated strips overlapped that obtained in the non-treated strips, thereby suggesting that GLB has no effect on the Ca2+-sensitivity of the intracellular contractile apparatus. Only high concentrations (10-4 M) of GLB decreased [Ca2+](i) and the force, when applied after the force induced by 118 mM K+ PSS or NA 10-5 M reached the maximum level. 4 In the absence of extracellular Ca2+, NA induced a transient increase in [Ca2+](i) and in the force and these increases were inhibited when the vascular strips were pretreated with GLB for 10 min. The [Ca2+](i)-force relationship obtained in the GLB-treated strips overlapped that in the non-treated ones. 5 An ATP-sensitive K+ channel opener, cromakalim (10-5 M) reduced the increased [Ca2+](i) and force induced by 25 mM K+-depolarization and NA 10-5 M. Subsequent application of GLB concentration-dependently reversed this relaxant effect of cromakalim on the NA-induced contraction (IC50 = 2 x 10-7 M.) Complete reversal of the effect was observed with 10-5 M GLB. 6 We suggest that GLB inhibits both high K+-depolarization- and NA-induced contraction of the rabbit aorta, by decreasing [Ca2+](i) and with no effect on the [Ca2+](i)-force relationship. However, when NA-induced contractions were inhibited by a K+-channel opener, GLB reversed this inhibitory effect by inhibiting K+-channel opening and increasing [Ca2+](i).