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⁺]_i) and Cl⁻ ([Cl⁻]₁), together with membrane potentials (E_M), 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.