The calculation of intracellular ion concentrations and membrane potential from cell-attached and excised patch measurements. Cytosolic K⁺ concentration and membrane potential in Vicia faba guard cells

Ion channel activity in cell-attached patch recordings shows channel behavior under more physiological conditions than whole-cell and excised patch measurements. Yet the analysis of cell-attached patch measurements is complicated by the fact that the system is ill defined with respect to the intracellular ion activities and the electrical potential actually experienced by the membrane patch. Therefore, of the several patch-clamp configurations, the information that is obtained from cell-attached patch measurements is the most ambiguous. The present study aims to achieve a better understanding of cell-attached patch measurements. Here we describe a method to calculate the intracellular ion concentration and membrane potential prevailing during cell-attached patch recording. The first step is an analysis of the importance of the input resistance of the intact cell on the cell-attached patch measurement. The second step, and actual calculation, is based on comparison of the single channel conductance and reversal potential in the cell-attached patch and excised patch configurations. The method is demonstrated with measurements of membrane potential and cytosolic K⁺ concentrations in Vicia faba guard cells. The approach described here provides an attractive alternative to the measurement of cytosolic ion concentrations with fluorescent probes or microelectrodes.