Participation of electrogenic Na+-K+-ATPase in the membrane potential of earthworm body wall muscles

The effect of Na+-K+-ATPase inhibitor ouabain on the resting membrane poten tial (V-m) was studied by glass microelectrodes in isolated somatic longitu dinal muscles of the earthworm Lumbricus terrestris and compared with frog sartorius muscle. In earthworm muscle, V-m was -49 mV (inside negative) in a reference external solution with 4 mmol/l K+. The electrogenic participat ion of Na+-K+-ATPase was absent in solutions with very low concentrations o f 0.01 mmol/l K+, higher in 4 and 8 mmol/l K+ (4-5 mV) and maximal (13 mV) in solutions containing 12 mmol/l K+ where V-m was -46 mV in the absence an d -33 mV in the presence of 1x10(-4) M ouabain. The electrogenic participat ion of Na+-K+-ATPase was much smaller in m. sartorius of the frog Rana temp oraria bathed in 8 and 12 mmol/l K+. The results indicate that the Na+-K+-A TPase is an important electrogenic factor in earthworm longitudinal muscle fibres and that its contribution to V-m depends directly on the concentrati on of K+ in the bathing solution.