Overexpression of the Na/Ca exchanger and reduced SERCa function.

sing Na/Ca exchanger function during the cardiac cycle Dear Sir, are intriguing but depend greatly on the assumption that We have read with great interest Dr Isenberg’s editorial cytoplasmic [Na] is 9.6 mM. We will comment on this [1] published with our article [2] in Cardiovascular later. There are two further points to make regarding the Research. Dr Isenberg’s analysis highlights the numerous calculations that may not be immediately apparent to the problems in studying Na/Ca exchanger function and Ca Reader. Firstly, it is important to note that the model cell is regulation and stresses once again that these areas of not in a steady-state. The size and time-course of the Ca physiology are far from being completely understood. We transient are fixed and, as Dr Isenberg mentions in the feel, however, that some clarification is required on the legend, the model does not incorporate effects of I on following points that arise from his assessment of our x other Ca fluxes including the SR. Thus, there is no article: physiological relationship between one beat to the next. The Na/Ca exchanger normally removes the Ca which Use of AM-forms of fluorescent indicators vs. patchenters the cell to trigger Ca release (i.e. L-type Ca current). pipette loading Assuming that other sarcolemmal Ca extrusion mechaDr Isenberg’s criticism that the possible compartmennisms have a minimal importance, the net amount of Ca talisation of the AM form of Indo-1 may affect our handled by the Na/Ca exchanger must be in the direction quantification of Ca fluxes is justified. The issue is not new of Ca extrusion at the steady-state. This is not considered and there are many unresolved problems with the use of in Dr Isenberg’s analysis. Therefore the second point that this form of the indicator. Undoubtedly loading of the needs to be made is that the calculations are only a AM-forms is difficult to control and a possible overloading representation of net Ca fluxes mediated by the Na /Ca or loading into sub-cellular compartments cannot be exchanger. They do not provide a clue as to the net Ca excluded. In this respect, patch-pipette loading should be fluxes across the surface membrane during the cardiac preferred. However, the reader could be misled to believe cycle. that the latter technique should have been used in our In addition to the amount of Ca removed via the Na/Ca experiments. Patch-pipette loading is not a valid alternative exchanger to balance Ca entry via L-type Ca channels, to the AM-form of loading when the aim of the experisome more Ca is removed by the Na/Ca exchanger to ments is to investigate the physiological role of the Na/Ca balance Ca entry via the exchanger itself. It is the amount exchanger in the intact cell. Na/Ca exchanger function is of this Ca that is influenced by the overexpression of the dependent on extracellular and intracellular [Ca] and [Na]. Na/Ca exchanger and can affect the filling state of the SR. It is likely that [Ca] and [Na] modify the normal pip pip If there is more Ca entry, there is more SR Ca uptake to intracellular concentrations of these two ions. Any Ca increase SR Ca content. So if 2.5 times more exchanger is buffer (e.g. BAPTA) used in the pipette will also influence present, a significantly larger (2.5 times more?) Ca entry the cytoplasmic Ca buffering and so the cellular [Na] and follows. More SR Ca content results in a faster and larger [Ca] will not be physiologically relevant. SR Ca release at the next beat to change the Ca transient characteristics in favour of more Ca extrusion. Over Mechanisms by which overexpressed Na / Ca exchanger several beats a new steady-state SR Ca content is estabcould augment SR Ca filling lished in myocytes from the heart of transgenic mice (TG) The calculations presented in the editorial [1] on asseswhich is greater than in control myocytes. However, even in this condition, a balance between fluxes is required and *Corresponding author. Tel.: 144-171-352-8121 ext 3322/2097; fax: 144-207-351-8145. the integral of I at the end of the cardiac cycle should be x