Secretion of organic acids and bases by the kidneys of marine teleosts

In the experiments performed on kidneys of 5 species of marine Teleostei, morphological peculiarities in secretion of 8 fluorescent organic acids (uranin, primulin, tripaphlavin, erythrosin etc.) and in 5 organic bases (rhodamin C, auramin etc.) have been studied. At a very low concentration in the incubation medium--about 0.005 mg/ml--the substances mentioned penetrate into the epithelial cell of the canaliculus; its weak fluorescence appears, and soon they begin to be excreted in great amount through the apical part of the plasmolemma and accumulated in the canalicular lumen. All the substances studied accumulate in the cell and only some of them (uranin, primulin, titanic yellow, etc.) are secreted into the canalicular lumen. Penicillinum and probenecide inhibit penetration of the organic acids into the cell through the basal membrane. Uranin secretion into the canalicular lumen is inhibited in the presence of furocemid; amilorid, magnium and sulfate ions do not influence secretion of the organic acids. Secretion of the organic bases does not change when paraaminohippuric acid and furocemid are added to the medium, but it decreases when concentration of magnium ions increases.     

Water and sodium transport: effects of calcium channel blocker and calmodulin antagonists on the apical and basolateral membranes of amphibian epithelia

Ca2+ channel blocker (sensit) and calmodulin antagonists (thioridazine, perphenazine, oxyprothepine) applied to the mucosal side of frog urinary bladder, weakened the response of epithelial cells to vasopressin. Thioridazine (2.7 X 10(-5) mol X l-1) and sensit (1.7 X 10(-4) mol X l-1) applied to the serosal side rapidly increased the permeability of the epithelia for sodium and potassium ions along the concentration gradient (from serosa to mucosa). The same concentrations of these blockers when applied to the mucosal side of frog urinary bladder selectively decreased vasopressin stimulated water permeability and did not influence ionic permeability. Both thioridazine and sensit decreased the short-circuit current across frog skin. The results show that the Ca2+ channel blocker and the calmodulin antagonists tested influenced water and ionic transport across the epithelial cell membranes, and had different effects upon the apical and the basolateral cell membranes.     

Osmotic and ionic homeostasis in insects and vertebrates under extreme influences on water-salt metabolism]

Dehydration which is accompanied by the decrease in the body mass of frogs and pigeons approximately by 10%, results in the corresponding increase in osmolality of the blood serum, as well as in the increase of sodium content of the latter. Under the same conditions, the increase in osmolality and sodium level in rats does not exceed 2% which is associated with the effective renal function. In caterpillars of the insect Vanessa urticae whose haemolymph is rich in organic osmotically active substances and potassium ions, in spite of dehydration and loss of the body mass, osmolality of the haemolymph slightly decreases, while cation concentration in the latter remains essentially constant. Experiments with injections of hyperosmotic solutions to rats and caterpillars, showed that these animals, belonging to different phylogenetic branches, exhibit highly effective physiological mechanisms of ionic and osmotic homeostasis.     

Cisplatin: nephrotoxic action in vertebrates and its prevention

1. The kidney of frog and black sculpin appeared to be much less sensitive to the toxic action of CP in comparison with rat and pigeon. 2. The impairment of renal function after CP administration resulted in increased serum urea in rat, uric acid in pigeon and magnesium in black sculpin. 3. Kidney swelling is important feature of CP nephrotoxicity in rat and pigeon but not in frog and fish. 4. Pretreatment with choline chloride, PAH, furosemide and ethacrynic acid reduced the nephrotoxic action of CP in rat but did not prevent the accumulation of platinum in renal tissue which appeared to be a function of the dose injected to investigated animals.     

Effect of amiloride on the activity of membrane-bound and soluble Na+-,K+-ATPase preparations

Amiloride (8 X 10(-4), an inhibitor of sodium channels of nonexcited membranes, inhibits the activity of Na+,K+-ATPase in the kidney cortex homogenate as well as that of the partially purified membrane-bound and lubrol-soluble Na+,K+-ATPase preparations from the cattle brain. Inhibition of Na+,K+-ATPase from different organs of various animals by amiloride, a blocker of sodium channels, indicates similarity of the molecular organization of the Na+-recognizing component both of sodium channels and sodium centres of Na+,K+-ATPase.     

Modulation of transducin GTPase activity by chimeric RGS16 and RGS9 regulators of G protein signaling and the effector molecule

RGS9, a member of the family of regulators of G protein signaling (RGS), serves as a GTPase-activating protein (GAP) for the transducin alpha-subunit (Gtalpha) in the vertebrate visual transduction cascade. The GAP activity of RGS9 is uniquely potentiated by the gamma-subunit of the effector enzyme, cGMP-phosphodiesterase (Pgamma). In contrast, Pgamma attenuates the GAP effects of several other RGS proteins, including RGS16. We demonstrate here that the Pgamma subunit exerts its effects on the GTPase activity of the Gtalpha-RGS complex via the C-terminal domain, Pgamma-63-87. The structural determinants that control the direction of Pgamma effects on the RGS-Gtalpha system are localized within the RGS domains. The addition of Pgamma caused an increase in the maximal stimulation of Gtalpha GTPase activity by RGS9d without affecting the EC50 value. Modulation of Gtalpha GTPase activity by chimeric RGS16 and RGS9 proteins and Pgamma has been investigated. This analysis suggests that in addition to the differences in primary structures, the overall conformations of the RGS fold in RGS9 and RGS16 are likely to be responsible for the opposite effects of Pgamma on the RGS9 and RGS16 GAP activity. The RGS9 alpha3-alpha5 region constituted the minimal insertion of the RGS9 domain into RGS16 that reversed the inhibitory effect of Pgamma. A model of the RGS9 complex with Gtalpha shows the alpha3-alpha5 helices in RGS9 facing the proximate Pgamma binding site on Gtalpha. Our results and this model demonstrate that the mechanism of potentiation of RGS9 GAP activity by Pgamma involves a more rigid stabilization of the Gtalpha switch regions when Gtalpha is bound to both RGS9 and Pgamma.     

Immunocytochemical localization of vasopressin at its absorption by cells of rat small intestine

Morpho-physiological characteristics of the transport of cyclic nonapeptide arginine vasopressin (AVP) across the rat intestinal epithelium was studied in experiments in vitro. A partial absorption of physiologically active AVP was followed when filling the isolated intestinal lumen by hormone solution. By methods of immunoelectron and immunofluorescence confocal microscopy, using polyclonal anti-AVP antibodies, cytoplasmic localization of AVP label was shown in enterocytes. The AVP label was also observed in the intercellular space in the basal area of epithelium. No label was revealed in the intercellular junctions, and no predominant label accumulation was found in any cytoplasmic structures of the epithelial cells. The obtained results are considered as evidence for the transcellular pathway of partial AVP absorption in rat small intestine.