Glyburide Actions on the Dihydropyridine-Sensitive Ca 2+ Channel in Rat Vascular Muscle

The effects of glyburide, a purportedly selective ATP-sensitive K+ channel antagonist, were studied on dihydropyridine (DHP)-sensitive (L-type) Ca2+ channel currents in rat aortic muscle cells. Whole-cell voltage-clamp Ba2+ currents (IBa) were recorded at a series of test potentials (Vτ) from -30 to +60 mV during 300-ms voltage steps from a holding potential of –80 mV. Bay k8644 (1 µM) increased peak divalent cation currents from 47.2 ± 15.1 to 102.6 ± 13.4 pA, and the current-voltage relationship curve was shifted 10 mV to the left (n = 5). The combination of 10 µM glyburide with 1 µM Bay k8644 further increased Bay k8644-enhanced IBa in each cell (average of 223.7 ± 26.4 pA, n = 5), and caused a further 10 mV hyperpolarizing (leftward) shift of the activation curve. The kinetics of IBa were also changed (more rapid inactivation) by glyburide. These stimulatory actions of glyburide were reversed on washout. In contrast to this apparent synergism with Bay k8644, 10 µM glyburide alone inhibited (rather than potentiated) IBa by about 20% at Vτ of 0, + 10, and +30 mV. Increasing glyburide concentration to 30 µM further inhibited the IBa to about 40-50% of controls. With the pure agonist isomer, 0.5 µM Bay R5417, at theoretically the same concentration of the minus enantiomer as is present in Bay k8644, IBa increased from 137 ± 18.3 pA to 354.2 ± 12.4 pA (n = 4). However, the combination of 10 µM glyburide and 0.5 µM Bay R5417 failed to further enhance IBa (tendency to block) at any VT, implying that the glyburide-Bay k8644 interaction may depend on both the stronger agonist and weaker antagonist isomers. IBa enhanced by 10 µM glyburide seem to flow through L-type Ca2+ channels because they were significantly blocked by the DHP antagonists, nimodipine and nisoldipine. Nimodipine (3 µM)inhibited the peak IBa by 30%, and 3 µM nisoldipine blocked IBa by almost 70%. Because glyburide produced opposite effects (block) when combined with the pure agonist enantiomer of Bay k8644, similar to the glyburide alone, but opposite to enhancement of racemic Bay k8644 agonist action, it is likely that there are multiple sites for modulation of vascular muscle L-type Ca2+ channels. Possible explanations include the existence of subtypes of DHP receptors, and glyburide actions that depend on the balance of DHP agonist versus antagonist influence. Glyburide could either be weakening the Ca2+ antagonist actions or acting at a separate sulfonylurea site that interacts with DHP sites.