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

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Abstract
Double-barrelled, ion-selective microelectrodes (ISMEs) have been used to measure the intracellular free concentrations of K+ ([K+]1,), Na+ ([Na+]i) 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, 42.9 mmoll−1; [Na+]1, 2.4 mmoll−; and [Cl−]i, 10.3 mmoll−. The mean Nernstian equilibrium potentials for K+, Na+ and Cl− were calculated to be −88 mV, +74.4 mV 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 Cl−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 moll−1 acetylcholine (ACh) led to a depolarization of EM, 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.