A vacuolar-type H+-ATPase and a Na+/H+exchanger contribute to intracellular pH regulation in cockroach salivary ducts

PeaTAR1B: Characterization of a Second Type 1 Tyramine Receptor of the American Cockroach, Periplaneta americana

The catecholamines norepinephrine and epinephrine regulate important physiological functions in vertebrates. In insects; these neuroactive substances are functionally replaced by the phenolamines octopamine and tyramine. Phenolamines activate specific guanine nucleotide-binding (G) protein-coupled receptors (GPCRs). Type 1 tyramine receptors are better activated by tyramine than by octopamine. In contrast; type 2 tyramine receptors are almost exclusively activated by tyramine. Functionally; activation of type 1 tyramine receptors leads to a decrease in the intracellular concentration of cAMP ([cAMP]_i) whereas type 2 tyramine receptors can mediate Ca²⁺ signals or both Ca²⁺ signals and effects on [cAMP]_i. Here; we report that the American cockroach (Periplaneta americana) expresses a second type 1 tyramine receptor (PeaTAR1B) in addition to PeaTAR1A (previously called PeaTYR1). When heterologously expressed in flpTM cells; activation of PeaTAR1B by tyramine leads to a concentration-dependent decrease in [cAMP]_i. Its activity can be blocked by a series of established antagonists. The functional characterization of two type 1 tyramine receptors from P. americana; PeaTAR1A and PeaTAR1B; which respond to tyramine by changing cAMP levels; is a major step towards understanding the actions of tyramine in cockroach physiology and behavior; particularly in comparison to the effects of octopamine.

Intracellular pH regulation in unstimulated Calliphora salivary glands is Na+ dependent and requires V-ATPase activity

Salivary gland cells of the blowfly Calliphora vicina have a vacuolar-type H+-ATPase (V-ATPase) that lies in their apical membrane and energizes the secretion of a KCl-rich primary saliva upon stimulation with serotonin (5-hydroxytryptamine). Whether and to what extent V-ATPase contributes to intracellular pH (pHi) regulation in unstimulated gland cells is unknown. We used the fluorescent dye BCECF to study intracellular pHi regulation microfluorometrically and show that: (1) under resting conditions, the application of Na+-free physiological saline induces an intracellular alkalinization attributable to the inhibition of the activity of a Na+-dependent glutamate transporter; (2) the maintenance of resting pHi is Na+, Cl–, concanamycin A and DIDS sensitive; (3) recovery from an intracellular acid load is Na+ sensitive and requires V-ATPase activity; (4) the Na+/H+ antiporter is not involved in pHi recovery after a NH4Cl prepulse; and (5) at least one Na+-dependent transporter and the V-ATPase maintain recovery from an intracellular acid load. Thus, under resting conditions, the V-ATPase and at least one Na+-dependent transporter maintain normal pHi values of pH 7.5. We have also detected the presence of a Na+-dependent glutamate transporter, which seems to act as an acid loader. Despite this not being a common pHi-regulating transporter, its activity affects steady-state pHi in C. vicina salivary gland cells.

Protein kinase A dependent and independent activation of the V-ATPase in Malpighian tubules of Aedes aegypti

Transepithelial ion transport in insect Malpighian tubules is energized by an apical V-ATPase. In hematophagous insects, a blood meal during which the animal ingests huge amounts of salt and water stimulates transepithelial transport processes linked to V-ATPase activation, but how this is accomplished is still unclear. Here we report that membrane-permeant derivatives of cAMP increase the bafilomycin-sensitive ATPase activity in Malpighian tubules of Aedes aegypti twofold and activate ATP-dependent transport processes. In parallel, membraneassociation of the V1 subunits C and D increases, consistent with the assembly of the holoenzyme. The protein kinase A inhibitor H-89 abolishes all cAMP-induced effects, consistent with PKA being involved in V-ATPase activation. Metabolic inhibition induced by KCN, azide and 2,4-dinitrophenol, respectively, also induces assembly of functional V-ATPases at the membrane without protein kinase A involvement, indicating a phosphorylation independent activation mechanism.

BAX inhibitor-1 enhances cancer metastasis by altering glucose metabolism and activating the sodium-hydrogen exchanger: the alteration of mitochondrial function

The anti-apoptotic protein, BAX inhibitor-1 (BI-1), has a role in cancer/tumor progression. BI-1-overexpressing HT1080 and B16F10 cells produced higher lung weights and tumor volumes after injection into the tail veins of mice. Transfection of BI-1 siRNA into cells before injection blocked lung metastasis. in vitro, the overexpression of BI-1 increased cell mobility and invasiveness, with highly increased glucose consumption and cytosolic accumulation of lactate and pyruvate, but decreased mitochondrial O(2) consumption and ATP production. Glucose metabolism-associated extracellular pH also decreased as cells excreted more H(+), and sodium hydrogen exchanger (NHE) activity increased, probably as a homeostatic mechanism for intracellular pH. These alterations activated MMP 2/9 and cell mobility and invasiveness, which were reversed by the NHE inhibitor, 5-(N-ethyl-N-isopropyl) amiloride (EIPA), suggesting a role for NHE in cancer metastasis. In both in vitro and in vivo experiments, C-terminal deleted (CDeltaBI-1) cells showed similar results to control cells, suggesting that the C-terminal motif is required for BI-1-associated alterations of glucose metabolism, NHE activation and cancer metastasis. These findings strongly suggest that BI-1 reduces extracellular pH and regulates metastasis by altering glucose metabolism and activating NHE, with the C-terminal tail having a pivotal role in these processes.

Source, topography and excitatory effects of GABAergic innervation in cockroach salivary glands

Cockroach salivary glands are innervated by dopaminergic and serotonergic neurons. Both transmitters elicit saliva secretion. We studied the distribution pattern of neurons containing γ-aminobutyric acid (GABA)and their physiological role. Immunofluorescence revealed a GABA-immunoreactive axon that originates within the subesophageal ganglion at the salivary neuron 2 (SN2) and this extends within the salivary duct nerve towards the salivary gland. GABA-positive fibers form a network on most acinar lobules and a dense plexus in the interior of a minor fraction of acinar lobules. Co-staining with anti-synapsin revealed that some putative GABAergic terminals seem to make pre-synaptic contacts with GABA-negative release sites. Many putative GABAergic release sites are at some distance from other synapses and at distance from the acinar tissue. Intracellular recordings from isolated salivary glands have revealed that GABA does not affect the basolateral membrane potential of the acinar cells directly. When applied during salivary duct nerve stimulation, GABA enhances the electrical response of the acinar cells and increases the rates of fluid and protein secretion. The effect on electrical cell responses is mimicked by the GABAB receptor agonists baclofen and SKF97541, and blocked by the GABAB receptor antagonists CGP52432 and CGP54626. These findings indicate that GABA has a modulatory role in the control of salivation, acting presynaptically on serotonergic and/or dopaminergic neurotransmission.

Characterisation of neurotransmitter-induced electrolyte transport in cockroach salivary glands by intracellular Ca2+, Na+ and pH measurements in duct cells

Ion-transporting acinar peripheral cells in cockroach salivary glands are innervated by dopaminergic and serotonergic fibres, but saliva-modifying duct cells are innervated only by dopaminergic fibres. We used microfluorometry to record intracellular Na+, Ca2+ and H+concentrations ([Na+]i, [Ca2+]iand pHi) in duct cells of two types of preparation, viz`lobes' consisting of acini with their duct system and `isolated ducts'without acini, in order to obtain information about the transporters involved in saliva secretion and/or modification. Our results indicate that (1)stimulation of lobes by dopamine (DA) causes a strong drop of pHiand increases in [Na+]i and[Ca2+]i in duct cells; (2) in contrast, DA stimulation of isolated ducts produces only a small pHi drop and no changes in[Na+]i and [Ca2+]i; (3)pHi and [Ca2+]i changes are also induced in duct cells by serotonin (5-HT) stimulation of lobes, but not isolated ducts;(4) in the absence of CO2/HCO3–, the DA-induced pHi drop is strongly reduced by removal of extracellular Cl– or inhibition of the Na+–K+–2Cl– cotransporter(NKCC); (5) in the presence of CO2/HCO3–, the DA-induced pHi drop is not reduced by NKCC inhibition, but rather by inhibition of the Cl–/HCO3–exchanger (AE), Na+/H+ exchanger (NHE) or carbonic anhydrase. We suggest that DA and 5-HT act predominantly on acinar peripheral cells. Their activity (secretion of primary saliva) seems to cause changes in ion concentrations in duct cells. NKCC and/or AE/NHE activities are necessary for pHi changes in duct cells; we consider that these transporters are involved in the secretion of the NaCl-rich primary saliva.