Changes of extracellular potassium activity in isolated spinal cord of frog under high Mg(2+) concentration.

Dorsal root potentials (DRPs) and changes of extracellular K(+) accompanying orthodromic stimulation were studied under high Mg(2+) concentration in isolated spinal cord of the frog. Under 20 mM MgSO(4) when synaptic transmission was blocked the amplitude of DRPs, evoked by a single volley in a dorsal root, decreased to 5-15% of the control and the transient increase of extracellular potassium concentration K evoked by 100 Hz orthodromic stimulation decreased to 10-15%. It is suggested that the DRPs, resistant to high Mg(2+) concentration, are produced by transient increase of K(+) that leaks from primary afferents.