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Inactivation of glibenclamide-sensitive K+ channels in Xenopus oocytes by various calmodulin antagonists
H Sakuta1, M Sekiguchi, K Okamoto
1Department of Pharmacology, National Defense Medical College, Saitama, Japan.
Abstract:
In follicle-enclosed Xenopus oocytes, extracellular application of cromakalim (a K+ channel opener) or intracellular injection of cAMP induces the smooth outward K+ current which is inactivated by glibenclamide. We found that cromakalim- or cAMP-induced K+ currents in the oocytes were rapidly, reversibly and dose-dependently blocked by various drugs having a calmodulin antagonizing activity in common, namely, by a selective calmodulin antagonist (W-7), antipsychotics (trifluoperazine, chlorpromazine, haloperidol), an antidepressant (amitriptyline), a beta-adrenoceptor blocker (propranolol), a local anesthetic (lidocaine) and a calcium antagonist (prenylamine). W-7, trifluoperazine, chlorpromazine and prenylamine were relatively potent blockers. For example, IC50 values to block cromakalim (100 microM)-induced K+ currents were 12 microM for trifluoperazine and 16 microM for W-7, which were close to their IC50 values to inhibit Ca2+/calmodulin-dependent phosphodiesterase (an index of the potency of calmodulin antagonists). IC50 values to inhibit cAMP (20 pmol/oocyte)-induced K+ currents were 126 microM for prenylamine and 129 microM for chlorpromazine. The IC50 values of all drugs tested to block cromakalim or cAMP responses were significantly correlated with their calmodulin-antagonizing potencies. Isoproterenol-induced K+ currents in the oocytes were also dose-dependently inhibited by glibenclamide, W-7 and trifluoperazine. These results suggest the possibility that the activity of glibenclamide-sensitive K+ channels in follicle-enclosed oocytes are regulated by calmodulin or a calmodulin-dependent process.
Insights
Glibenclamide-sensitive potassium channels in Xenopus oocytes are regulated by calmodulin. Various drugs targeting calmodulin, including W-7 and trifluoperazine, effectively block these potassium currents, suggesting a novel regulatory mechanism.
Area of Science:
- Pharmacology
- Molecular Biology
- Cell Physiology
Background:
- Potassium (K+) channels play crucial roles in cellular function.
- Glibenclamide is known to affect certain K+ channels.
- Calmodulin is a key calcium-binding protein involved in cellular signaling.
Purpose of the Study:
- To investigate the role of calmodulin in regulating glibenclamide-sensitive K+ channels in Xenopus oocytes.
- To determine if drugs with calmodulin-antagonizing activity can modulate K+ currents induced by cromakalim or cAMP.
Main Methods:
- Extracellular application of cromakalim or intracellular injection of cAMP to induce K+ currents in follicle-enclosed Xenopus oocytes.
- Assessment of the inhibitory effects of various calmodulin antagonists (e.g., W-7, trifluoperazine) on induced K+ currents.
- Correlation analysis between drug potency and calmodulin-antagonizing activity.
Main Results:
- Cromakalim- or cAMP-induced K+ currents were dose-dependently blocked by drugs with calmodulin-antagonizing properties.
- Potent blockers included W-7, trifluoperazine, chlorpromazine, and prenylamine, with IC50 values comparable to their effects on phosphodiesterase.
- Isoproterenol-induced K+ currents were also inhibited by glibenclamide, W-7, and trifluoperazine.
Conclusions:
- Calmodulin or a calmodulin-dependent process likely regulates the activity of glibenclamide-sensitive K+ channels in Xenopus oocytes.
- This finding suggests a novel mechanism for K+ channel modulation involving calmodulin.
- The results highlight the potential of calmodulin antagonists in studying K+ channel function.