Changes of the conduction velocity of isolated muscles induced by altered external potassium concentration

The propagation of action potentials along fibre bundles of fast and slow twitch rat muscles was tested by means of two separated different electrodes. In spite of marked differences in contractile properties between fast and slow twitch bundles, the conduction velocity (c.v.) was found to be similar in all preparations at room temperature. At 35 degrees C the c.v. was 2.76 +/- 0.36 m.s-1 in SOL and 3.0 +/- 0.77 m.s-1 in EDL muscles, respectively. An elevation of the extracellular K+ from 5 to 10 mM caused a reduction of the c.v. by 20 - 40% in limb and sternomastoid muscles. The effect increased with higher K+ concentrations. Repeated tetanic stimulation also induced a decrease of the c.v. The opposite effect occurred in solution with reduced external K+. In O K+ solution the c.v. of fast twitch fibres increased by almost 50%. The results show that extracellular K+ interacts with processes of the regeneration of the action potential along the fibres.