Muscarinic activation of Na+-dependent ion transporters and modulation by bicarbonate in rat submandibular gland acinus

Involvement of sodium-glucose cotransporter-1 activities in maintaining oscillatory Cl- currents from mouse submandibular acinar cells

In salivary acinar cells, cholinergic stimulation induces elevations of cytosolic [Ca2+]i to activate the apical exit of Cl- through TMEM16A Cl- channels, which acts as a driving force for fluid secretion. To sustain the Cl- secretion, [Cl-]i must be maintained to levels that are greater than the electrochemical equilibrium mainly by Na+-K+-2Cl- cotransporter-mediated Cl- entry in basolateral membrane. Glucose transporters carry glucose into the cytoplasm, enabling the cells to produce ATP to maintain Cl- and fluid secretion. Sodium-glucose cotransporter-1 is a glucose transporter highly expressed in acinar cells. The salivary flow is suppressed by the sodium-glucose cotransporter-1 inhibitor phlorizin. However, it remains elusive how sodium-glucose cotransporter-1 contributes to maintaining salivary fluid secretion. To examine if sodium-glucose cotransporter-1 activity is required for sustaining Cl- secretion to drive fluid secretion, we analyzed the Cl- currents activated by the cholinergic agonist, carbachol, in submandibular acinar cells while comparing the effect of phlorizin on the currents between the whole-cell patch and the gramicidin-perforated patch configurations. Phlorizin suppressed carbachol-induced oscillatory Cl- currents by reducing the Cl- efflux dependent on the Na+-K+-2Cl- cotransporter-mediated Cl- entry in addition to affecting TMEM16A activity. Our results suggest that the sodium-glucose cotransporter-1 activity is necessary for maintaining the oscillatory Cl- secretion supported by the Na+-K+-2Cl- cotransporter activity in real time to drive fluid secretion. The concerted effort of sodium-glucose cotransporter-1, Na+-K+-2Cl- cotransporter, and apically located Cl- channels might underlie the efficient driving of Cl- secretion in different secretory epithelia from a variety of animal species.

Two-photon microscopy and fluorescence lifetime imaging reveal stimulus-induced intracellular Na+ and Cl− changes in cockroach salivary acinar cells

The intracellular ion homeostasis in cockroach salivary acinar cells during salivation is not satisfactorily understood. This is mainly due to technical problems regarding strong tissue autofluorescence and ineffective ion concentration quantification. For minimizing these problems, we describe the successful application of two-photon (2P) microscopy partly in combination with fluorescence lifetime imaging microscopy (FLIM) to record intracellular Na+ and Cl− concentrations ([Na+]i, [Cl−]i) in cockroach salivary acinar cells. Quantitative 2P-FLIM Cl− measurements with the dye N-(ethoxycarbonylmethyl)-6-methoxy-quinolinium bromide indicate that the resting [Cl−]i is 1.6 times above the Cl− electrochemical equilibrium but is not influenced by pharmacological inhibition of the Na+-K+-2Cl− cotransporter (NKCC) and anion exchanger using bumetanide and 4,4′-diisothiocyanatodihydrostilbene-2,2′-disulfonic acid disodium salt. In contrast, rapid Cl− reuptake after extracellular Cl− removal is almost totally NKCC mediated both in the absence and presence of dopamine. However, in physiological saline [Cl−]i does not change during dopamine stimulation although dopamine stimulates fluid secretion in these glands. On the other hand, dopamine causes a decrease in the sodium-binding benzofuran isophthalate tetra-ammonium salt (SBFI) fluorescence and an increase in the Sodium Green fluorescence after 2P excitation. This opposite behavior of both dyes suggests a dopamine-induced [Na+]i rise in the acinar cells, which is supported by the determined 2P-action cross sections of SBFI. The [Na+]i rise is Cl− dependent and inhibited by bumetanide. The Ca2+-ionophore ionomycin also causes a bumetanide-sensitive [Na+]i rise. We propose that a Ca2+-mediated NKCC activity in acinar peripheral cells attributable to dopamine stimulation serves for basolateral Na+ uptake during saliva secretion and that the concomitantly transported Cl− is recycled back to the bath.

Focal adhesion kinase controls pH-dependent epidermal barrier homeostasis by regulating actin-directed Na+/H+ exchanger 1 plasma membrane localization

Ubiquitously expressed focal adhesion kinase (FAK), linked to multiple intracellular signaling pathways, has previously been shown to control cell motility, invasion, proliferation, and survival. Using mice with a keratinocyte-restricted deletion of fak (FAK(K5 KO)), we report here a novel role for FAK: maintenance of adult epidermal permeability barrier homeostasis. Abundant lacunae of unprocessed lipids in stratum corneum (SC) of FAK(K5 KO) mice and delayed barrier recovery pointed to malfunction of pH-dependent enzymes active in extracellular space of SC. Measuring the SC pH gradient showed significantly more neutral pH values in FAK(K5 KO) mice, suggesting the importance of FAK for acidification. Moreover, normal functions were restored when FAK(K5 KO) mice were exposed to a surface pH typical of mouse SC (pH = 5.5). Baseline levels and response to barrier disruption of secretory phospholipase A2 isoforms, enzymes that mediate generation of free fatty acids in epidermis, appeared similar in both FAK(K5 KO) and control littermates. We found that the critical SC acidification regulator Na(+)/H(+) exchanger 1 failed to localize to the plasma membrane in FAK-deficient keratinocytes both in vivo and in vitro. Thus, for plasma membrane localization in terminally differentiated keratinocytes, Na(+)/H(+) exchanger 1 requires an intact actin cytoskeleton, which is impaired in FAK-deficient cells.

Regulation of membrane potential and fluid secretion by Ca2+-activated K+ channels in mouse submandibular glands

We have recently shown that the IK1 and maxi-K channels in parotid salivary gland acinar cells are encoded by the K(Ca)3.1 and K(Ca)1.1 genes, respectively, and in vivo stimulated parotid secretion is severely reduced in double-null mice. The current study tested whether submandibular acinar cell function also relies on these channels. We found that the K(+) currents in submandibular acinar cells have the biophysical and pharmacological footprints of IK1 and maxi-K channels and their molecular identities were confirmed by the loss of these currents in K(Ca)3.1- and K(Ca)1.1-null mice. Unexpectedly, the pilocarpine-stimulated in vivo fluid secretion from submandibular glands was essentially normal in double-null mice. This result and the possibility of side-effects of pilocarpine on the nervous system, led us to develop an ex vivo fluid secretion assay. Fluid secretion from the ex vivo assay was substantially (about 75%) reduced in animals with both K(+) channel genes ablated - strongly suggesting systemic complications with the in vivo assay. Additional experiments focusing on the membrane potential in isolated submandibular acinar cells revealed mechanistic details underlying fluid secretion in K(+) channel-deficient mice. The membrane potential of submandibular acinar cells from wild-type mice remained strongly hyperpolarized (-55 +/- 2 mV) relative to the Cl(-) equilibrium potential (-24 mV) during muscarinic stimulation. Similar hyperpolarizations were observed in K(Ca)3.1- and K(Ca)1.1-null mice (-51 +/- 3 and -48 +/- 3 mV, respectively), consistent with the normal fluid secretion produced ex vivo. In contrast, acinar cells from double K(Ca)3.1/K(Ca)1.1-null mice were only slightly hyperpolarized (-35 +/- 2 mV) also consistent with the ex vivo (but not in vivo) results. Finally, we found that the modest hyperpolarization of cells from the double-null mice was maintained by the electrogenic Na(+),K(+)-ATPase.