EFFECTS OF INTRACELLULAR CA2+ CHELATING COMPOUNDS ON INWARD CURRENTS CAUSED BY CA2+ RELEASE FROM SARCOPLASMIC-RETICULUM IN GUINEA-PIG ATRIAL MYOCYTES

Ca2+ release from the sarcoplasmic reticulum (SR) of mammalian cardiac myocytes occuring either due to activation by a depolarization or the resulting transmembrane Ca2+ current (I(Ca)), or spontaneously due to Ca2+ overload has been shown to cause inward current(s) at negative membrane potentials. In this study, the effects of different intracellular Ca2+ chelating compounds on I(Ca)-evoked or spontaneous Ca2+-release-dependent inward currents were examined in dialysed atrial myocytes from hearts of adult guinea-pigs by means of whole-cell voltage-clamp. As compared to dialysis with solutions containing only a low concentration of a high affinity ethylene glycol-bis(beta-aminoethylether) N,N,N',N'-tetraacetic acid (EGTA) like chelator (50-200-mu-M), inward membrane currents (at -50 mV) due to evoked Ca2+ release, spontaneous Ca2+ release or Ca2+ overload following long-lasting depolarizations to very positive membrane potentials are prolonged if the dialysing fluid contains a high concentration of a low affinity Ca2+ chelating compound such as citrate or free adenosine 5'-triphosphate (ATP). Without such a non-saturable Ca2+ chelator in the dialysing fluid, Ca2+-release-dependent inward currents are often oscillatory and show an irregular amplitude. With a low affinity chelator in a non-saturable concentration, discrete inward currents with constant properties can be recorded. We conclude that the variability in Ca2+-release-dependent inward current seen in single cells arises from spatial inhomogeneities of intracellular Ca2+ concentration ([Ca2+]i) due to localized saturation of endogenous and exogenous high affinity Ca2+ buffers (e.g. [2]). This can be avoided experimentally by addition of a non-saturable buffer to the intracellular solution. This condition might be useful, if properties of Ca2+ release from the SR and/or the resulting membrane current, like for example arrhythmogenic transient inward current, are to be investigated on the single cell level.