Action potential discharge in response to low extracellular potassium (K+) and activity-induced conduction velocity slowing are both thought be due to Na+/ K+-ATPase activation. We have examined whether the two phenomena are related in nerve fibres of the rat meninges, in vitro. The dura was perfused with physiological solution regulated at 35.5±0.5 °C. Conduction velocity slowing was determined in single units by stimulation at 0.5, 1, 2 and 4 Hz. All units exposed to low (nominally 0) K+ exhibited an initial increase in conduction latency as would be expected for a hyperpolarising shift in EK. The time to onset of this latency increase varied from 120s to 600s. Nine of 10 units responded to low extracellular K+. All 10 units exhibited more than 10% activity-induced slowing at 4Hz, but no correlation between slowing and response time or magnitude to low K+ was apparent.

In a separate series of experiments immunoreactive CGRP content in the meningeal perfusate was measured by ELISA. Low K+ produced no increase in iCGRP content over and above control (1.40±0.52-fold n.s., n=8).

Capsaicin (1mM) however increased the basal iCGRP content 19.79±8.98-fold (p<0.01, n=8). We assume either that low K+ evoked action potential discharge is insufficient to evoke CGRP release or that nerve fibres activated by low K+ do not contain CGRP.