Cytoskeleton elements mediate the inhibition of the (Na++K+)atpase activity by PKC in Rhodnius prolixus malpighian tubules during hyperosmotic shock

Atrial natriuretic peptides and urodilatin modulate proximal tubule Na(+)-ATPase activity through activation of the NPR-A/cGMP/PKG pathway

The signaling pathway mediating modulation of Na(+)-ATPase of proximal tubule cells by atrial natriuretic peptides (ANP) and urodilatin through receptors located in luminal and basolateral membranes (BLM) is investigated. In isolated BLM, 10(-11)M ANP or 10(-11)M urodilatin inhibited the enzyme activity (50%). Immunodetection revealed the presence of NPR-A in BLM and LLC-PK1 cells. Both compounds increased protein kinase G (PKG) activity (80%) and this effect did not occur with 10(-6)M LY83583, a specific inhibitor of guanylyl cyclase. The inhibitory effect of these peptides on Na(+)-ATPase activity did not occur after addition of 10(-6)M KT5823, a specific inhibitor of PKG. LLC-PK1 cells were used to investigate if ANP and urodilatin change the activity of sodium pumps by luminal receptor interaction. ANP and urodilatin inhibited Na(+)-ATPase activity (50%), with maximal effect at 10(-10)M, similar to 10(-7)M db-cGMP, and did not occur with 10(-7)M LY83583, a guanylyl cyclase inhibitor. ANP and urodilatin specifically inhibit Na(+)-ATPase activity by activation of the cGMP/PKG pathway through NPR-A located in luminal membrane and BLM, increasing understanding of the mechanism of natriuretic peptides on renal sodium excretion, with proximal tubule Na(+)-ATPase one possible target.

Rhodnius prolixus Malpighian tubules and control of diuresis by neurohormones

Rhodnius prolixus Malpighian tubules (MTs) are a good model for fluid and ion secretion studies in view of the dramatic postprandial diuresis, which follows its massive blood meals. Ingestion of a blood meal equals to 10-12 times their initial body mass, leads to rapid activation of high output by excretory system, which eliminates 40-50% of the fluid mass. Secretion of ions and water is stimulated 1000-fold by serotonin and diuretic hormone. These hormones cooperate synergistically to activate adenylate cyclase activity from MTs cells, which increase the level of intracellular cAMP. The anti-diuretic hormones have also an important role in the fluid maintenance of Rhodnius prolixus. Several hours after insect feeding occurs a reduction in urine flow, that has been thought to result from a decreased diuretic hormone release or from a novel mechanism of anti-diuresis involving insect cardioacceleratory peptide 2b (CAP2b) and cyclic GMP. In this article it is discussed how the hormone regulation of fluid transport is done in Rhodnius prolixus MTs.

Stimulation of the proximal tubule Na+-ATPase activity by adenosine A(2A) receptor

The aim of this work was to determine the molecular mechanism involved in the stimulation of the pig kidney proximal tubule Na+-ATPase by adenosine (Ado). To study the role of A2 Ado receptors, we added in all experiments 10(-6)M DPCPX, an A1 receptor-selective antagonist, since we have previously shown that Ado inhibits the enzyme activity through this receptor. Ado increased the Na+-ATPase activity with maximal effect observed at 10(-6)M. The presence of both A(2A) and A(2B) receptors were demonstrated by immunoblotting using specific polyclonal antibodies. The stimulatory effect of Ado was completely abolished by 5 x 10(-9)M DMPX, an antagonist of A2 receptor, and 10(-7)M SCH 58261, an A(2A) receptor-selective antagonist. DMPA (10(-7)M), a specific agonist of A(2A) receptor mimicked the stimulatory effect of Ado. Involvement of a Gs protein/adenylate cyclase/PKA pathway was evidenced by: (a) the reversion of Ado-induced effect by GDPbetaS; (b) stimulation of the Na+-ATPase activity in a similar and non-additive manner to Ado by 10(-8)M cholera toxin, 10(-7)M GTPgammaS, 10(-6)M forskolin, 10(-7)M cAMP or 1.25 U catalytic subunit of PKA; (c) the reversion of the stimulatory effect of Ado by 10(-8)M PKA inhibitor peptide; (d) Ado-produced two-fold increase of the PKA activity, which was completely reversed by 10(-6)M DMPX. These are the first evidences showing the modulation of a renal primary active sodium transporter by Ado through A(2A) receptor.

Transport mechanisms of diuresis in Malpighian tubules of insects

We have studied Malpighian tubules of Aedes aegypti using a variety of methods: Ramsay fluid secretion assay, electron probe analysis of secreted fluid, in vitro microperfusion and two-electrode voltage clamp. Collectively, these methods have allowed us to elucidate transepithelial transport mechanisms under control conditions and in the presence of diuretic peptides. Mosquito natriuretic peptide (MNP), a corticotropin-releasing factor (CRF)-like diuretic peptide, selectively increases transepithelial secretion of NaCl and water, meeting the NaCl loads of the blood meal. The intracellular messenger of MNP is cAMP, which increases the Na+ conductance and activates the Na+/K+/2Cl- -cotransporter in the basolateral membrane of principal cells. Leucokinin non-selectively increases transepithelial NaCl and KCl secretion, which may deal with hemolymph volume expansions or reduce the flight pay load upon eclosion from the aquatic habitat. The non-selective NaCl and KCl diuresis stems from the increase in septate junctional Cl- conductance activated by leucokinin using Ca2+ as second messenger. Fundamental to diuretic mechanisms are powerful epithelial transport mechanisms in the distal segment of the Malpighian tubules, where transepithelial secretion rates can exceed the capacity of mammalian glomerular kidneys in the renal turnover of the extracellular fluid compartment. In conjunction with powerful epithelial transport mechanisms driven by the V-type H+-ATPase, diuretic hormones enable hematophagous and probably also phytophagous insects to deal with enormous dietary loads, thereby contributing to the evolutionary success of insects.

Ouabain-insensitive Na+-ATPase of proximal tubules is an effector for urodilatin and atrial natriuretic peptide

In the present paper we studied the effect of urodilatin and atrial natriuretic peptide (ANP) on the proximal tubule Na+-ATPase and (Na+K+)ATPase activities. Urodilatin and ANP inhibit the Na+-ATPase activity but not the (Na+K+)ATPase activity. Maximal effect was observed at a concentration of 10(-11) M for both peptides. In this condition, the enzyme activity decreases from 10.8 +/- 1.6 (control) to 5.7 +/- 0.9 or 6.1 +/- 0.7 nmol Pi mg(-1) min(-1) in the presence of urodilatin or ANP, respectively. This effect was completely reversed by 10(-6) M LY83583, a guanylyl cyclase inhibitor, and mimicked by 10 nM cGMP. Furthermore, both ANP and urodilatin increase cGMP production by 33% and 49%, respectively. This is the first demonstration that it was shown that urodilatin and ANP directly modulate primary active sodium transport in the proximal tubule. The data obtained indicate that this effect is mediated by the activation of the NPR-A/guanylate cyclase/cGMP pathway.

Inhibition of the SAPK/JNK pathway blocks the stimulatory effects of glutamine on fluid secretion by the Malpighian tubules of Rhodnius prolixus

Physiological levels of amino acids such as glutamine, glutamate, aspartate and proline increase the rates of fluid secretion and ion transport by serotonin-stimulated Malpighian tubules (MTs) of Rhodnius prolixus. Here, we examine the proposal that the effects of glutamine are mediated through activation of specific kinases to produce the observed increases in fluid secretion. The glutamine-dependent increase in MT fluid secretion rate was blocked by two chemically unrelated inhibitors of the stress activated protein kinase (SAPK) pathway, SP600125 and dicumarol. Inhibitors of phosphatidyl inositol-3 kinase, p38 mitogen activated protein kinase (MAPK), extracellular-signal regulated kinases and MAPK kinase did not block glutamine's effects on fluid secretion rate when applied at commonly used concentrations. Inhibitors of protein kinase A or C reduced fluid secretion rates of serotonin-stimulated MTs, but did not block the response to glutamine. The glutamine-dependent increase in fluid secretion was also insensitive to cytoskeletal disrupting agents and protein synthesis inhibitors. Results of this study are the first to suggest a role for the SAPK pathway in the control of fluid secretion rates by insect MTs.

Expression and function of classical protein kinase C isoenzymes in gastric cancer cell line and its drug-resistant sublines

AIM: To investigate the expression and function of classical protein kinase C (PKC) isoenzymes in inducing MDR phenotype in gastric cancer cells.

METHODS: Two cell lines were used in the study: gastric cancer cell SGC7901 and its drug-resistant cell SGC7901/VCR stepwise-selected by vincristine 0.3, 0.7 and 1.0 mg·L^-1, respectively. The expression of classical PKC (cPKC) isoenzymes in SGC7901 cells and SGC7901/VCR cells were detected using immunofluorescent cytochemistry, laser confocal scanning microscope and Western blot. The effects of anti-PKC isoenzymes antibody on adriamycin accumulation in SGC7901/VCR cells were determined using flow cytometric analysis.

RESULTS: (1) SGC7901 cells exhibited positive staining of PKC-α. SGC7901/VCR cells exhibited stronger staining of PKC-α than SGC7901 cells. The higher dosage vincristine selected, the much stronger staining of PKC-α was observed on SGC7901/VCR cells. (2) Both SGC7901 and SGC7901/VCR cells exhibited positive staining of PKC-β I and PKC-β II with no significant difference. (3) Compared with SGC7901, SGC7901/VCR cells had decreased adriamycin accumulation and retention. Accumulation of adriamycin in SGC7901 was 5.21 ± 2.56 mg·L^-1, in SGC7901/VCR 0.3 was 0.85 ± 0.29 mg·L^-1, in SGC7901/VCR 0.7 was 0.81 ± 0.32 mg·L^-1, and in SGC7901/VCR 1.0 was 0.80 ± 0.33 mg·L^-1; Retention of adriamycin in SGC7901 was 2.51 ± 1.23 mg·L^-1, in SGC7901/VCR 0.3 was 0.47 ± 0.14 mg·L^-1, in SGC7901/VCR 0.7 was 0.44 ± 0.15 mg·L^-1, and in SGC7901/VCR 1.0 was 0.41 ± 0.11 mg·L^-1. (4) Fluorescence intensity presented adriamycin accumulation in SGC7901/VCR cells was increased from 1.14 ± 0.36 to 2.71 ± 0.94 when cells were co-incubated with anti-PKC-α but not with anti-PKC-β I, PKC-α II and PKCγ antibodies.

CONCLUSION: PKC-α, but not PKC-β I, PKC-β II or PKCγ, may play a role in multidrug resistance of gastric cancer cells SGC7901/VCR.