Glimepiride block of cloned beta-cell, cardiac and smooth muscle K(ATP) channels

Abstract

1. We examined the effect of the sulphonylurea glimepiride on three types of recombinant ATP-sensitive potassium (K(ATP)) channels. 2. K(ATP) channels share a common pore-forming subunit, Kir6.2, which associates with different sulphonylurea receptor isoforms (SUR1 in beta-cells, SUR2A in heart and SUR2B in smooth muscle). 3. Kir6.2 was coexpressed with SUR1, SUR2A or SUR2B in Xenopus oocytes and macroscopic K(ATP) currents were recorded from giant inside-out membrane patches. Glimepiride was added to the intracellular membrane surface. 4. Glimepiride inhibited Kir6.2/SUR currents by interaction with two sites: a low-affinity site on Kir6.2 (IC(50)= approximately 400 microM) and a high-affinity site on SUR (IC(50)=3.0 nM for SUR1, 5.4 nM for SUR2A and 7.3 nM for SUR2B). The potency of glimepiride at the high-affinity site is close to that observed for glibenclamide (4 nM for SUR1, 27 nM for SUR2A), which has a similar structure. 5. Glimepiride inhibition of Kir6.2/SUR2A and Kir6.2/SUR2B currents, but not Kir6.2/SUR1 currents, reversed rapidly. 6. Our results indicate that glimepiride is a high-affinity sulphonylurea that does not select between the beta-cell, cardiac and smooth muscle types of recombinant K(ATP) channel, when measured in inside-out patches. High-affinity inhibition is mediated by interaction of the drug with the sulphonylurea receptor subunit of the channel.

Figure 1

Molecular structures of tolbutamide, glibenclamide,glimepiride and meglitinide.

Figure 2

Inhibition of K_ATP currents by glimepiride. Macroscopic currents recorded from inside-out patches in response to a series of voltage ramps from −110 mV to +100 mV from oocytes coexpressing Kir6.2 and either SUR1, SUR2A or SUR2B. Glimepiride (1 μM) was added as indicated by the bars. The dashed line indicates the zero current level.

Figure 3

Dose-inhibition relationships for block of K_ATP currents by glimepiride. Glimepiride dose-response relationships for (a) Kir6.2/SUR1, (b) Kir6.2/SUR2A and (c) Kir6.2/SUR2B currents. The macroscopic conductance in the presence of glimepiride (G) is expressed as a fraction of its mean amplitude in the absence of the drug (G_c). The symbols represent the mean and the vertical bars indicate 1 s.e.mean. The lines are fit to equation 1 of the text using the following values. Kir6.2/SUR1 channels: IC₅₀₍₁₎=3.0 nM, h₁=0.64, IC₅₀₍₂₎=234 μM, h₂=1, L=0.20. Kir6.2/SUR2A channels: IC₅₀₍₁₎=5.4 nM, h₁=0.69, IC₅₀₍₂₎=104 μM, h₂=1, L=0.23. Kir6.2/SUR2B channels: IC₅₀₍₁₎=7.3 nM, h₁=0.6, IC₅₀₍₂₎=99 μM, h₂=1, L=0.23.

Figure 4

Block of Kir6.2ΔC36 currents by glimepiride. (a) Macroscopic currents recorded from inside-out patches in response to a series of voltage ramps from −110 mV to +100 mV from oocytes coexpressing Kir6.2ΔC36. Glimepiride (300 μM) was added as indicated by the bar. (b) Glimepiride dose-response relationships for Kir6.2ΔC36 currents. The macroscopic conductance in the presence of glimepiride (G) is expressed as a fraction of its mean amplitude in the absence of the drug (G_c). The symbols represent the mean, and the vertical bars indicate 1 s.e.mean. The line is fit to equation 4 of the text using IC₅₀=388 μM, h=0.72.