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

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 cytosolic 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 (10-5M) reduced the increased [Ca2 + ]i and force induced by 25mm K+-depolarization and NA 10-SM. Subsequent application of GLB concentrationdependently reversed this relaxant effect of cromakalim on the NA-induced contraction (IC50 = 2x 10 7 M). Complete reversal of the effect was observed with 10IsM 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 +].