The effect of changing extracellular potassium concentration on synaptic transmission in isolated spinal cords.

Hemisected spinal cords of infant mice were exposed in vitro to varying concentrations of K+ in the bathing fluid. Interstitial potassium concentration ([K+]o) as well as dorsal and ventral root (DR and VR) potentials evoked by DR stimulation were recorded. [K+]o in spinal tissue was made to change from its control level of 3.5 mM by superfusing the preparation with modified artificial cerebrospinal fluid (ACSF). [K+]o in tissue followed bath [K+] in the range from 2.0 to 13 mM, but diverged from bath concentration at levels at or below 1.0 mM. Relatively high tissue [K+]o during exposure to low bath [K+] was attributed to leakage of K+ ions from cells. Between 2.0 and 5.0 mM [K+]o a shallow but consistent positive correlation was found between [K+]o and dorsal root reflex (DRR) amplitude, while segmental reflex (VRR) amplitude changed little. Outside the range of 2.0-5.5 mM DR and VR responses were severely depressed in both elevated and lowered [K+]o. In spite of depressed reflex responses, transient stimulus-evoked elevations of [K+]o were strikingly increased when the resting [K+]o fell below 2.0 mM, suggesting enhancement of voltage-dependent K+-current.