Intracellular Free Potassium, Sodium and Chloride Measured with Ion-Selective Microelectrodes from Salivary Gland Cells of the Snail Planorbis Corneus

Double-barrelled, ion-selective microelectrodes (ISMEs) have been used to measure the intracellular free concentrations of K+ ([K+]1), Na+ ([Na+]1) and Cl- ([Cl-]1), together with membrane potentials (EM), from single salivary gland acinar cells of the pond snail Planorbis corneus. After adjustments had been made for the cross-sensitivities of the ion-exchangers to other intracellular ions, the mean concentrations were estimated to be: [K+]1, 42·9mmol1-1; [Na+]1, 2·4mmol1-1; and [Cl-]1, 10·3 mmol 1-1. The mean Nernstian equilibrium potentials for K+, Na+ and Cl- were calculated to be —88 mV, +74·4mV and —41 mV, respectively. The basolateral membrane of Planorbis salivary cells appears to be permeable to K+ and Na+ under resting conditions, because blocking the electrogenic Na+/K+ pump with K+-free saline or ouabain revealed the presence of a large passive efflux of K+ and an influx of Na+. Salivary gland cells also lose intracellular Cl- rapidly in CD-free saline (extracellular Cl- replaced by sulphate) which, along with other evidence, indicates a substantial resting permeability of the salivary cell membrane to Cl-. Stimulating gland cells with 10−4 mol 1-1 acetylcholine (ACh) led to a depolarization of EM1 a rise in [Na+]1 and a fall in [K+]1. This was followed by a transient hyperpolarization of EM and a recovery of [Na+]1 and [K+]1 to their original levels. There was no evidence that [Cl-]1 changes after stimulation with ACh. The mechanism of action of ACh on Planorbis salivary gland cells and its relevance for secretion are discussed.