THE FUNCTIONAL SURFACE-CHARGE DENSITY OF A FAST K-CHANNEL IN THE MYELINATED AXON OF XENOPUS-LAEVIS

The action of Mg2+ on the putative xKv1.1 channel in the myelinated ax on of Xenopus laevis was analyzed in voltage clamp experiments. The ma in effect was a shift in positive direction of the open probability cu rve (16 mV at 20 mM Mg2+), calculated from measurements of the instant aneous current at Na reversal potential after 50-100 msec steps to dif ferent potentials. The shift was measured at an open probability level of 25% to separate it from shifts of other K channel populations in t he nodal region. The results could be explained in terms of screening effects on fixed charges located on the surface of the channel protein . Using the Grahame equation the functional charge density was estimat ed to -0.45 e nm(-2). Analyzing this value, together with previously e stimated values from other K channels, with reference to the charge of different extracellular loops of the channel protein, we conclude tha t the loop between the transmembrane S5 segment and the pore forming P segment determines the functional charge density of voltage-gated K c hannels.