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 (a_K) in the neuraxis of cats under Dial. In resting conditions, a_K is stable (in range 2.0–3.0 mM) but may vary systematically with electrode position. Anoxia causes a progressive rise in a_K, 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 a_K, presumably caused by tissue trauma. In areas rich in afferent terminals, direct stimulation through a microelectrode causes a sharp increase in a_K, whose time course is consistent with diffusion from an instantaneous spherical source. Smaller increases in a_K 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 a_K during repetitive stimulation at rates > 0.5 s⁻¹. Although the observed changes in a_K fit reasonably a simple diffusion model at low frequencies (< 5 s⁻¹), a reuptake term (proportional to the change in a_K) is necessary to obtain a satisfactory fit over a wide range of frequencies (1–200 s⁻¹). The maximum a_K reached during repetitive stimulation does not increase linearly with frequency, owing to a corresponding falling-off in K⁺ release per impulse.