Properties of an endogenous steady current in rat muscle.

A vibrating probe was used to study a steady electric current generated by isolated, whole lumbrical muscles of the rat. Spatial mapping showed that current leaves the muscle in the synaptic region and re-enters in the flanking extrajunctional regions. The point of maximum outward current coincided precisely with the endplate region. As the probe was moved radially away from the endplate region, the current declined monotonically, and the results could be fit with a simple model. As the probe was moved axially away from the endplate region, the current declined and became inward over a distance of approximately 0.5 mm. The physiological mechanism by which the current is generated was also studied. alpha-Bungarotoxin and tetrodotoxin had no significant effect on the current, which suggests that acetylcholine channels and gated sodium channels are not involved in the generation of the current. Ouabain produced a slowly developing, partial inhibition of the current, reducing it by approximately 40% over a period of 30-40 min. Carbachol produced a large inward current at the endplate region. After the carbachol action was terminated with alpha-bungarotoxin, an outward current reappeared, and a transient "overshoot" developed. During the overshoot, which lasted approximately 30-40 min, the outward current was approximately doubled. This overshoot was completely abolished by ouabain. The overshoot is interpreted as reflecting the increased activity of electrogenic sodium pumping in the endplate region, caused by the influx of Na ions during carbachol application. Because of the very different actions of ouabain on the normal current and on the overshoot after carbachol application, we concluded that the normal outward current is not produced by electrogenic sodium pumping in the endplate region.