title: Physiological basis of a steady endogenous current in rat lumbrical muscle.
abstract: In an attempt to determine the mechanism by which rat skeletal muscle endplates generate a steady outward current, we measured the effects of several drugs (furosemide, bumetanide, 9-anthracene carboxylic acid [9-AC]) and changes in external ion concentration (Na+, K+, Cl-, Ba++) on resting membrane potential (Vm) and on the steady outward current. Each of the following treatments caused a 10-15-mV hyperpolarization of the membrane: replacement of extracellular Cl- with isethionate, addition of furosemide or bumetanide, and addition of 9-AC. These results suggest that Cl- is actively accumulated by the muscle fibers and that the equilibrium potential of Cl- is more positive than the membrane potential. Removal of external Na+ also caused a large hyperpolarization and is consistent with evidence in other tissues that active Cl- accumulation requires external Na+. The same treatments greatly reduced or abolished the steady outward current, with a time course that paralleled the changes in Vm. These results cannot be explained by a model in which the steady outward current is assumed to arise as a result of a nonuniform distribution of Na+ conductance, but they are consistent with models in which the steady current is produced by a nonuniform distribution of GCl or GK. Other treatments (Na+-free and K+-free solutions, and 50 microM BaCl2) caused a temporary reversal of the steady current. Parallel measurements of Vm suggested that in none of these cases did the electrochemical driving force for K+ change sign, which makes it unlikely that the steady current arises as a result of a nonuniform distribution of GK. All of the results, however, are consistent with a model in which the steady outward current arises as a result of a nonuniform distribution of Cl- conductance, with GCl lower near the endplate than in extrajunctional regions.
mesh_terms: D000818:Animals; D000873:Anthracenes / Q000494:pharmacology; D001369:Axons / Q000378:metabolism; D001464:Barium / Q000494:pharmacology; D001693:Biological Transport, Active / Q000187:drug effects; D002034:Bumetanide / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism*; D049832:Decapodiformes; D005665:Furosemide / Q000494:pharmacology; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects; D008954:Models, Biological; D009045:Motor Endplate / Q000378:metabolism; D009132:Muscles / Q000378:metabolism*; D051381:Rats
chemical_list: D000873:Anthracenes; D002712:Chlorides; D007473:Ion Channels; D002034:Bumetanide; D001464:Barium; C010030:9-anthroic acid; D005665:Furosemide