Elicitation of retinal spreading depression by barium ions

The association of potassium ions with the occurrence and propagation of spreading depression is well known. The effects of barium ions, a potassium-channel blocker, on spreading depression in isolated retina are described. Pulses of 1-4 mM BaCl2, when applied to the retina, first induce the reaction, then hinder the propagation of the wave which finally stops. The threshold level which triggers the reaction is 0.4-0.6 mM BaCl2 but it varies with the composition of the superfusing Ringer solution. For example, in retinas superfused with low NaCl, or NaCl partially substituted by sodium isethionate, spreading depression may be evoked by barium ions at micromolar concentrations, without changing the velocity of spreading. The delayed blocking actions observed with higher doses may predominantly affect the recovery processes of the wave front, and are related to the refractory period of a preceding spreading depression.