Na ⁺ -H ⁺ exchange in salivary secretory cells is controlled by an intracellular Na ⁺ receptor

Abstract

It recently has been shown that epithelial Na ؉ channels are controlled by a receptor for intracellular Na ؉ , a G protein (G o ), and a ubiquitin-protein ligase (Nedd4). Furthermore, mutations in the epithelial Na ؉ channel that underlie the autosomal dominant form of hypertension known as Liddle's syndrome inhibit feedback control of Na ؉ channels by intracellular Na ؉ . Because all epithelia, including those such as secretory epithelia, which do not express Na ؉ channels, need to maintain a stable cytosolic Na ؉ concentration ([Na ؉ ] i ) despite f luctuating rates of transepithelial Na ؉ transport, these discoveries raise the question of whether other Na ؉ transporting systems in epithelia also may be regulated by this feedback pathway. Here we show in mouse mandibular secretory (endpiece) cells that the Na ؉ -H ؉ exchanger, NHE1, which provides a major pathway for Na ؉ transport in salivary secretory cells, is inhibited by raised [Na ؉ ] i acting via a Na ؉ receptor and G o . This inhibition involves ubiquitination, but does not involve the ubiquitin protein ligase, Nedd4. We conclude that control of membrane transport systems by intracellular Na ؉ receptors may provide a general mechanism for regulating intracellular Na ؉ concentration.

Cevimeline-Induced Monophasic Salivation from the Mouse Submandibular Gland: Decreased Na+Content in Saliva Results from Specific and Early Activation of Na+/H+Exchange

“…Later the Ca 2ϩ -calmodulin complex was proposed to activate NHE, because no additional phosphorylation occurred in response to muscarinic stimulation in salivary gland (Robertson et al, 1997). In addition, intracellular Na ϩ concentration sensing mechanisms have been discussed (Ishibashi et al, 1999). In this study, it is likely that [Ca 2ϩ ] i alone was not involved in NHE up-regulation because we observed almost identical increases at both 30 M CVL and 3 M PLC (Figs.…”

A Slow pH-dependent Conformational Transition Underlies a Novel Mode of Activation of the Epithelial Na+/H+ Exchanger-3 Isoform

“…This may have resulted in variable intracellular Na ϩ contents at the time of assaying NHE. The observed time-dependent increase in activity may therefore have resulted from an increased driving force for forward Na ϩ /H ϩ exchange or from stimulation of a Na ϩ -sensitive G protein, as has been invoked for the regulation of NHE1 (21). To analyze this possibility, cells were initially preincubated in Na ϩ -free medium for 10 min and then subjected to acid loading and challenged with extracellular Na ϩ .…”

Stimulation of the Plasma Membrane Na+/H+ Exchanger NHE1 by Sustained Intracellular Acidosis

“…In sheep Purkinje fibers, Wu and Vaughan-Jones (23) show that pretreatment with Na ϩ -free solution reduces [Na ϩ ] i and leads to increased sarcolemmal NHE activity after the induction of intracellular acidosis and the reintroduction of extracellular Na ϩ . Such a reduction in [Na ϩ ] i may stimulate NHE1 activity through an increased thermodynamic driving force (via an increased transmembrane Na ϩ gradient) (23) or potentially through a Na ϩ -sensitive G protein (24). To explore the relative importance of extended exposure to intracellular acidosis versus that to Na ϩ -free solution (which likely produces a reduction in [Na ϩ ] i ), we performed additional experiments in which the duration of intracellular acidosis was extended by washout of NH 4 Cl with solution containing physiological [Na ϩ ] (ϳ140 mM) and the reversible NHE1 inhibitor cariporide.…”