P1078Correlation between magnitude of cardiac ionic currents and cell membrane capacity: always positive?

Abstract

Introduction: The magnitude of cardiac ionic currents is generally considered as being directly proportional to the membrane area of investigated cell which may be determined by measurement of the cell membrane capacity. Therefore, the measured values of ionic currents are usually converted to the current density (by dividing measured current by estimated cell membrane capacity) to reduce differences among cells caused by their varying membrane area. This strategy is asserted in patch clamp studies on cardiomyocytes and, in most cases, also on cell lines heterogeneously expressing cardiac ionic channels. Our recent pilot data has shown that the correlation may be absent or even inverse in some cases which may lead to misinterpretation of the evaluated data. Purpose: We aimed to investigate the assumed correlation between the magnitude of cardiac ionic currents (absolute value) and membrane capacity of respective cells. Methods: Experiments were performed by the whole cell patch clamp technique on enzymatically isolated rat atrial myocytes (two inward rectifier potassium currents were investigated, namely IK1 and acetylcholine-sensitive current IK(Ach); at 23 ? 1?C) and on Chinese hamster ovary (CHO) cells transiently expressing human slowly activated delayed rectifier (IKs) channels (KCNQ1, KCNE1, Yotiao; at 37?C). Results: The significance test indicated that the Pearson’s coefficients of correlation (r) between the magnitude of inward component of IK1 or of acetylcholine-induced IK(Ach) and the capacity of respective rat atrial myocytes were significant (r = 0.79 and 0.64, n = 27 and 20, P ? 0.001 and 0.002, respectively). Surprisingly, analogical significant correlation was not apparent in the case of inward component of constitutively active IK(Ach) measured in rat atrial cells (r = 0.17, n = 56). In the case of CHO cells, the magnitude of expressed human IKs and the capacity of CHO cells did not significantly correlate as well. Furthermore, even negative correlation was indicated, i.e. the higher was the cell capacity, the smaller was this current (r = -0.22, n = 18). Conclusions: We proved the expected significant positive correlation between the magnitude of big ionic currents, namely IK1 and acetylcholine-induced IK(Ach), and the membrane area of rat atrial myocytes. On the contrary, no significant correlation was apparent in the case of small constitutively active IK(Ach). Importantly, even negative (but insignificant) correlation was observed in the case of human IKs transiently expressed in CHO cells. Hence, the usually used conversion of the current magnitude to the current density using the membrane capacity of respective cell should be used with caution, namely in the case of small ionic currents in cardiomyocytes and, especially, in the case of currents measured in cell lines. The cause of missing or even inverse proportionality between membrane current and capacity remains to be clarified.