Proceedings of The Physiological Society

University College Dublin (2009) Proc Physiol Soc 15, PC194

Poster Communications

Simulation of the effects of BAPTA and EGTA on inactivation of ICa in a model of the rat ventricular myocyte

M. Pásek1,3, M. R. Fowler2, J. Šimurda3, G. L. Smith2, C. H. Orchard4

1. Institute of Thermomechanics - branch Brno, Czech Academy of Science, Brno, Czech Republic. 2. Faculty of Biomedical and Life Sciences, Glasgow, United Kingdom. 3. Department of Physiology, Masaryk University Brno, Brno, Czech Republic. 4. Department of Physiology and Pharmacology, University of Bristol, Bristol, United Kingdom.

In cardiac ventricular myocytes, the fast Ca2+ buffer BAPTA has a greater effect than the slow Ca2+ buffer EGTA on inactivation of the L-type Ca2+ current, ICa (e.g. Brette at al. 2004, Sham, 1997). The aim of the present study was to refine an existing model of the rat ventricular myocyte (Pasek et al. 2006) to reproduce the differential effect of BAPTA and EGTA on inactivation of ICa in the rat myocyte and to investigate its possible cause. The principal modifications to the model were: (i) reformulation of the description of sarcoplasmic reticulum (SR) Ca2+-release according to Shannon at al. (2004); (ii) reformulation of the description of voltage- and Ca2+-dependent inactivation of ICa to simulate the experimental data of Brette at al. (2004); (iii) modification of the parameters of the SR Ca2+-pump to give the relation between free Ca2+ in the cytosol and SR described by Shannon and Bers (1997); (iv) incorporation of equations controlling exchange of free EGTA and Ca2+-EGTA and of free BAPTA and Ca2+-BAPTA between the pipette, cytosol and dyadic space. In the model, both buffers could inhibit the cytosolic Ca2+-transient, and thus contraction. However, only BAPTA inhibited efficiently the rise of Ca2+ in the dyadic space, thus causing significant inhibition of Ca2+-dependent inactivation of ICa. The principal reason for the different potencies of BAPTA and EGTA in reducing the rise of Ca2+ in the dyadic space was their different rates of Ca2+ binding (BAPTA: kon= 500000 mM-1 s-1; EGTA: kon= 5000 mM-1 s-1). As a consequence, the concentration gradients of free BAPTA and Ca2+-BAPTA, and of free EGTA and Ca2+-EGTA, between the dyadic space and bulk cytosol were substantially different after the onset of depolarisation. In the case of BAPTA, the large gradients provided a large driving force, causing rapid transport of Ca2+-BAPTA molecules out of the dyadic space, and movement of free BAPTA into the dyadic space, so that BAPTA appeared to act as a fast ‘shuttle’. The slower rate of Ca2+ binding to EGTA and development of substantially smaller free EGTA and Ca2+-EGTA gradients between the dyadic space and cytosol reduced the ability of EGTA to affect Ca2+-dependent inactivation of ICa.

Where applicable, experiments conform with Society ethical requirements