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BRL 34915 (cromakalim) activates ATP-sensitive K+ current in cardiac muscle
M C Sanguinetti1, A L Scott, G J Zingaro
1Department of Pharmacology, Merck Sharp & Dohme Research Laboratories, West Point, PA 19486.
Insights
The antihypertensive agent BRL 34915 (cromakalim) shortens cardiac action potential duration and effective refractory period by opening ATP-sensitive potassium channels. This effect, similar to hypoxia, is temperature-dependent and blocked by glyburide.
Area of Science:
- Cardiovascular Physiology
- Molecular Cardiology
- Pharmacology
Background:
- The antihypertensive agent BRL 34915 (cromakalim) is known to affect cardiac electrophysiology.
- ATP-sensitive potassium channels (KATP) play a crucial role in cardiac function and are implicated in responses to hypoxia.
- Understanding the precise mechanism of BRL 34915 is essential for its therapeutic application and for elucidating KATP channel function.
Purpose of the Study:
- To investigate the mechanism by which BRL 34915 influences action potential duration (APD) and effective refractory period (ERP) in isolated cardiac muscle.
- To determine if BRL 34915 activates ATP-sensitive potassium channels in the heart.
- To explore the temperature dependence and direct effects of BRL 34915 on cardiac ion currents.
Main Methods:
- Experiments were conducted on isolated papillary muscles from ferrets and guinea pigs to measure APD and ERP.
- Hypoxia was induced by perfusing muscles with N2-bubbled solution.
- Voltage-clamped guinea pig ventricular myocytes were used to directly assess BRL 34915's effects on K+ currents.
- Glyburide, a KATP channel inhibitor, was used to block or reverse the effects of BRL 34915 and hypoxia.
Main Results:
- BRL 34915 concentration-dependently shortened ERP and APD in cardiac muscles, an effect reversed by glyburide.
- Hypoxia also reduced APD and ERP, and these effects were prevented or reversed by glyburide.
- BRL 34915 significantly increased outward K+ currents in ventricular myocytes at 36°C but had no effect at 22°C.
- The action of BRL 34915 on K+ currents was temperature-sensitive and blocked by glyburide.
Conclusions:
- BRL 34915 activates ATP-sensitive potassium channels in the heart, similar to the effects of hypoxia.
- The profound temperature sensitivity suggests an indirect mechanism, possibly involving enzymatic modulation of channel gating.
- BRL 34915 and glyburide are valuable pharmacological tools for studying the role of ATP-sensitive potassium channels in cardiac function and dysfunction.
Abstract:
The mechanism by which the antihypertensive agent BRL 34915 (cromakalim) affects action potential duration (APD) and effective refractory period (ERP) in isolated cardiac muscle was investigated. BRL 34915 (greater than or equal to 3 microM) shortened ERP of ferret (Mustela putorius furo) and guinea pig (Cavia porcellus) papillary muscles in a concentration-dependent fashion. The reduction in ERP resulted from a decrease in APD. ERP and APD of papillary muscles were also reduced during hypoxia produced by bubbling the physiological bathing solution with N2 instead of O2. Reduction of APD during hypoxia has previously been attributed to activation of ATP-sensitive K+ channels in heart. Glyburide, an inhibitor of ATP-sensitive K+ channels, prevented or reversed the shortening of ERP and APD produced by hypoxia and BRL 34915, respectively. These results suggest that BRL 34915 acts by opening ATP-sensitive K+ channels in heart. The actions of BRL 34915 were temperature-dependent, decreasing ERP 64% at 37 degrees C, but having no effect at 22 degrees C. The effect of BRL 34915 on K+ currents was tested directly in voltage-clamped guinea pig ventricular myocytes. As observed with the papillary muscles, BRL 34915 was without effect at 22 degrees C. At 36 degrees C, BRL 34915 (after a delay) increased outward currents positive to, and less so at potentials negative to, the K+ current reversal potential. The normal inwardly rectifying current-voltage relationship for peak K+ currents during 200-msec pulses was changed to one that was nearly ohmic. The current activated by BRL 34915 was blocked by glyburide. The data support the hypothesis that BRL 34915, like hypoxia, activates ATP-sensitive K+ channels in the heart. Based upon the profound temperature sensitivity of BRL 34915 action, this activation may be indirect, perhaps by means of modulation of an enzymatic activity that regulates gating of these channels. BRL 34915 and glyburide will be valuable tools for studying the role of ATP-sensitive K+ channels in normal and abnormal cardiac function.