Membrane Potential and Chemical Transmitter in Active Transport of Ions by Rat Skeletal Muscle

where [Na]o and [Na]i are the sodium concentrations in bathing fluid and musclefiber water respectively, E,~ the mean membrane potential of the muscle fibers (outside minus inside) measured in recovery fluid, and T, R, and F the absolute temperature and gas and Faraday constants, respectively. I t was found (Conway, Kernan, and Zadunaisky, 1961) that a critical energy barrier to the active transport of Na + by Narich muscles existed under stated metabolic conditions and that this determined the amount of this ion which could be excreted on reimmersion of Na-rich muscles in recovery fluid containing K +. If the calculated energy requirement exceeded a value of 2 ca l /meq Na +, no active excretion of this ion took place. When the ionic gradient was reduced by decrease of the NaC1 concentration of the recovery fluid, more Na + was excreted by the muscles over the 2 hr recovery period and a lower intracellular Na + concentration was achieved. Similarly, when the electrical energy gradient or membrane potential was reduced by increasing the K + concentration of the recovery fluid, more Na + was excreted by the muscles during the 2 hr recovery period. Reexamination of the data in this paper has shown reduction of the chloride concentration of the bathing fluid, [C1]o, to be a more effective way of reducing membrane potential and of increasing Na + excretion. Measured membrane potential, E,~, decreased by only 25 mv for a 10-fold increase in [K]o as KC1, but by 48 mv for a similar reduction of [C1]o as NaC1. The first part of the present communication deals with the influence of chloride concentration gradient across muscle membrane on E,~ and net transport of Na + by Na-rich rat muscles, and the second part describes how information obtained here was used in an at tempt to decide whether cholinergic agents increased K + permeability or stimulated a K pump in increasing Na + transport in muscle.