Extracellular accumulation of K+ evoked by activity of primary afferent fibers in the cuneate nucleus and dorsal horn of cats.

Abstract

K+-selective microelectrodes were used to measure extracellular K+ activity (aK) in the neuraxis of cats under Dial. In resting conditions, aK is stable (in range 2.0–3.0 mM) but may vary systematically with electrode position. Anoxia causes a progressive rise in aK, but only after a delay of 2–3 min and a maximum is not reached even after 15 min. Movements of the electrode are associated with transient increases in aK, presumably caused by tissue trauma. In areas rich in afferent terminals, direct stimulation through a microelectrode causes a sharp increase in aK, whose time course is consistent with diffusion from an instantaneous spherical source. Smaller increases in aK are produced by stimulating afferent fibers more peripherally, but definite changes can be recorded even with single pulses, and there is a cumulative, maintained rise in aK during repetitive stimulation at rates > 0.5 s−1. Although the observed changes in aK fit reasonably a simple diffusion model at low frequencies (< 5 s−1), a reuptake term (proportional to the change in aK) is necessary to obtain a satisfactory fit over a wide range of frequencies (1–200 s−1). The maximum aK reached during repetitive stimulation does not increase linearly with frequency, owing to a corresponding falling-off in K+ release per impulse.