Muscarinic Agonists Induce Phosphorylation-independent Activation of the NHE-1 Isoform of the Na+/H+ Antiporter in Salivary Acinar Cells

Marli A Robertson,Michael Woodside,J Kevin Foskett,John Orlowski,Sergio Grinstein

doi:10.1074/jbc.272.1.287

Marli A Robertson, Michael Woodside + Show 3 more

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https://doi.org/10.1074/jbc.272.1.287

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Abstract

Cholinergic agonists stimulate isotonic fluid secretion in the parotid gland. This process is driven by the apical exit of Cl-, which enters the cells partly via Cl-/HCO-3 exchange across the basolateral membrane. Acidification of the cytosol by the extrusion of HCO-3 is prevented by the concomitant activation of the Na+/H+ exchanger (NHE), which is directly activated by cholinergic stimulation. Multiple isoforms of the NHE have been described in mammalian cells, but the particular isoform(s) present in salivary glands and their mechanism of activation have not been defined. Reverse transcriptase-polymerase chain reaction with isoform-specific primers was used to establish that NHE-1 and NHE-2, but not NHE-3 or NHE-4, are expressed in parotid glands. The presence of NHE-1 was confirmed by immunoblotting and immunofluorescence, which additionally demonstrated that this isoform is abundant in the basolateral membrane of acinar cells. The predominant role of NHE-1 in carbachol-induced Na+/H+ exchange was established pharmacologically using HOE694, an inhibitor with differential potency toward the individual isoforms. Because muscarinic agonists induce stimulation of protein kinases in acinar cells, we assessed the role of phosphorylation in the activation of the antiport. Immunoprecipitation experiments revealed that, although NHE-1 was phosphorylated in the resting state, no further phosphorylation occurred upon treatment with carbachol. Similar phosphopeptide patterns were observed in control and carbachol-treated samples. Together, these findings indicate that NHE-1, the predominant isoform of the antiporter in the basolateral membrane of acinar cells, is activated during muscarinic stimulation by a phosphorylation-independent event. Other processes, such as association of Ca2+-calmodulin complexes to the cytosolic domain of the antiporter, may be responsible for the activation of Na+/H+ exchange.

Highlights

Dant secretion of isotonic fluid, a process that is driven primarily by efflux of ClϪ and HCO3Ϫ across the apical membrane [1]
The purpose of the experiments described in this article was to identify the isoform(s) of the Naϩ/Hϩ exchanger present in acinar cells of the rat parotid gland, to explore their individual contribution to the uptake of Naϩ, and the regulation of pHi, and to define their mechanisms of activation during cholinergic stimulation
In agreement with earlier observations [4, 5], we found that muscarinic stimulation of parotid acinar cells in the presence of HCO3Ϫ induces a rapid and transient cytosolic acidification (Fig. 1A)

Summary

Introduction

Dant secretion of isotonic fluid, a process that is driven primarily by efflux of ClϪ and HCO3Ϫ across the apical membrane [1]. In isolated rat parotid acinar cells carbachol markedly activated Naϩ influx, and the initial rate of influx was inhibited by 75% in the presence of dimethylamiloride (DMA),1 a relatively specific inhibitor of Naϩ/Hϩ exchange [4, 5]. The purpose of the experiments described in this article was to identify the isoform(s) of the Naϩ/Hϩ exchanger present in acinar cells of the rat parotid gland, to explore their individual contribution to the uptake of Naϩ, and the regulation of pHi, and to define their mechanisms of activation during cholinergic stimulation.

Results

Conclusion