Impact Of Hypoxia In The Expression And Regulation Of The TASK‐1 Potassium Channel In Cardiac Myocytes

Abstract

TASK‐1 is a member of the two‐pore domain potassium channel family and strongly expressed in the heart. However, little is known about the roles and regulation of TASK‐1 channels in cardiac hypoxia. The present study was designed to investigate whether hypoxia modulated the expression and function of TASK‐1 and whether protein kinase C (PKC) was critical in hypoxia‐mediated processes in adult rat cardiac myocytes. The effects of acute hypoxia on the TASK‐1 channel function were chemically induced by sodium cyanide (NaCN). Activity of TASK‐1 in cardiomyocytes was investigated by the whole‐cell patch‐clamp technique. The current flowing through TASK‐1 channels in cardiac myocytes was quantified by analyzing the effects of extracellular pH or the selective TASK‐1 inhibitor methanandamide on the steady‐state current‐voltage relation. The voltage protocol, which included voltage ramps from +40 to −100 mV preceded by a test pulse from 0 to +40 mV, was designed to inactivate voltage‐dependent ion channels. In addition, a blocker cocktail, that had no effect on TASK‐1 channels expressed in HEK293T cells, was used to eliminate a number of ion channels in cardiac myocytes. Our data show that exposure of cardiac myocytes to NaCN for 5 minutes reduced the steady‐state outward current at +40 mV, an effect that was abolished by pretreatment with methanandamide. Either methanandamide or extracellular acidification (from pH 7.4 to 6.4) decreased the outward current, identifying TASK‐1 as the channel mediating this effect. Interestingly, the NaCN‐induced reduction in the outward current was largely prevented by pretreatment with the PKC selective inhibitor bisindolylmaleimide. To assess the expression and protective role of TASK‐1 in hypoxia, the prolonged hypoxia (4 hours) was employed by incubating cells in an airtight chamber in which O2 was replaced by N2 with glucose‐free Tyrode's solution. While 4 hours of hypoxia caused a significant reduction in the viability of cardiac myocytes, the inhibition of cardiac TASK‐1 channels by methanandamide further increased hypoxia‐induced cell death. Western blot analysis showed that 4 hours of hypoxia led to a decrease in the protein expression of TASK‐1 in cardiac myocytes. In summary, we provided the evidence that TASK‐1 channels in cardiomyocytes is regulated by hypoxia‐induced PKC signaling in cardiomyocytes. Our data are also supportive for a possible role of TASK‐1 channels in the cardioprotection against hypoxia.