Effect of the intracellular calcium concentration chelator BAPTA acetoxy-methylester on action potential duration in...

Intracellular calcium concentration ([Ca²⁺]_i) is often buffered by using the cell-permeant acetoxy-methylester form of the Ca²⁺ chelator BAPTA (BAPTA-AM) under experimental conditions. This study was designed to investigate the time-dependent actions of extracellularly applied BAPTA-AM on action potential duration (APD) in cardiac cells. Action potentials were recorded from enzymatically isolated canine ventricular myocytes with conventional sharp microelectrodes. The effect of BAPTA-AM on the rapid delayed rectifier K⁺ current (I_Kr) was studied using conventional voltage clamp and action potential voltage clamp techniques. APD was lengthened by 5 μM BAPTA-AM - but not by BAPTA - and shortened by the Ca²⁺ ionophore A23187 in a time-dependent manner. The APD-lengthening effect of BAPTA-AM was strongly suppressed in the presence of nisoldipine, and enhanced in the presence of BAY K8644, suggesting that a shift in the [Ca²⁺]_i-dependent inactivation of L-type Ca²⁺ current may be an important underlying mechanism. However, in the presence of the I_Kr-blocker dofetilide or E-4031 APD was shortened rather than lengthened by BAPTA-AM. Similarly, the APD-lengthening effect of 100 nM dofetilide was halved by the pretreatment with BAPTA-AM. In line with these results, I_Kr was significantly reduced by extracellularly applied BAPTA-AM under both conventional voltage clamp and action potential voltage clamp conditions. This inhibition of I_Kr was partially reversible and was not related to the Ca²⁺ chelator effect BAPTA-AM. The possible mechanisms involved in the APD-modifying effects of BAPTA-AM are discussed. It is concluded that BAPTA-AM has to be applied carefully to control [Ca²⁺]_i in whole cell systems because of its direct inhibitory action on I_Kr.