Barbiturates inhibit ATP‐K⁺ channels and voltage‐activated currents in CRI‐G1 insulin‐secreting cells

Abstract

1 Patch-clamp recording techniques were used to examine the effects of barbiturates upon the ATP-K+ channel, and voltage-activated channels present in the plasma membrane of CRI-Gi insulin-secreting cells.2 Thiopentone inhibited ATP-K' channel activity when applied to cell-attached patches or the intracellular or extracellular surface of cell-free patches. Secobarbitone and pentobarbitone were also effective inhibitors of ATP-K+ channels in cell-free patches, whereas phenobarbitone was ineffective. 3 The diabetogenic agent, alloxan, which is structurally related to the barbiturates also produced an inhibition of ATP-K+ channel activity in outside-out patches. 4 Whole-cell ATP-K+ currents were used to quantify the effects of the barbiturates: concentrationinhibition curves for thiopentone, secobarbitone and pentobarbitone resulted in IC50 values of 62, 250 and 360pM respectively. Phenobarbitone at a concentration of 1 mm was virtually ineffective. 5 Calculation of the apparent membrane concentrations for these drugs indicate that for a given degree of ATP-K+ channel inhibition a similar concentration of each barbiturate is present in the membrane. This suggests that hydrophobicity plays a primary role in their mechanism of action. The pH-dependence and additive nature of barbiturate block also indicates a membrane site of action. 6 Thiopentone, (100pM) was also found to inhibit differentially voltage-activated whole-cell currents.The relative potency of thiopentone at this concentration was 0.64, 0.38 and 0.12 for inhibiting Ca2", K+ and Na+ currents respectively when compared with its ability to inhibit the ATP-K+ channel.

Mg2+‐dependent inhibition of KATP by sulphonylureas in CRI‐G1 insulin‐secreting cells

“…In the presence of Mg2+, the IC5o for thiopentone was 69.4 tiM whilst in the absence of this cation the IC50 was 69.2 JsM with Hill coefficients of 1.25 and 1.13, respectively. These values are very similar to the previously-reported IC50 of 62 tiM for the inhibition of IK(ATP) (in the absence of intracellular Mg2+) in CRI-G1 cells (Kozlowski & Ashford, 1991 (Khan et al, 1993) and other insulin-secreting cells (Ashcroft & Ashcroft, 1992 prevented by removal of intracellular Mg2" (Kozlowski & Ashford, 1990). However, the difference in the inhibition produced by the sulphonylureas in the presence and absence of intracellular Mg2" cannot be accounted for solely in terms of run-down since under whole-cell recording conditions (where run-down can be quantified), removal of intracellular Mg2" leads to an equally significant loss of sulphonylurea potency.…”

“…We now show that the inhibitory effects of the sulphonylureas on KATp in an insulin-secreting cell line are dependent upon the presence of intracellular Mg2". In contrast, the barbiturate thiopentone, which is also an inhibitor of KATP (Kozlowski & Ashford, 1991), does not exhibit Mg2"-dependent inhibitory properties. Some of these results have been communicated to the British Pharmacological Society (Lee & Ashford, 1993).…”

Effect of MgATP on pinacidil‐induced displacement of glibenclamide from the sulphonylurea receptor in a pancreatic β‐cell line and rat cerebral cortex

“…The tested concentrations of quinine (20-500 JM), chlorpromazine (1O-100 JAM) and thiopentone (100-200fJM) inhibit the K-ATP-channels in insulin-secreting cells almost completely (Bokvist et al, 1990;Kozlowski & Ashford, 1991;Muller et al, 1991 (Table 2). …”

Thiopentone does not block ischemic preconditioning in the isolated rat heart

“…13,14 Kozlowski and Ashford investigated the effects of thiopentone on K ATP channels in insulin secreting cells. 9 They found that the K ATP channel activity is inhibited by thiopentone with an IC 50 value of 62 µM. Based on this finding, we hypothesized that thiopentone might block ischemic preconditioning like ketamine 6,7 which also possesses a K ATP channel blocking activity.…”

“…8 Thiopentone was reported to block K ATP channels in isolated cells at concentrations that may be clinically relevant. 9,10 Therefore, the present study tested the hypothesis that thiopentone may block the cardioprotective effects of ischemic preconditioning. To address this question, thiopentone was administered to isolated rat hearts just before the preconditioning ischemias.…”