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  • 1
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Vasopressin produces a large increase in the osmotic flow of water across the toad bladder, with little apparent change in the diffusion rate of tritiated water. This discrepancy between osmotic and diffusional net flow is the basis of the pore theory of vasopressin action. The present studies show that there is in fact a large (at least 10-fold) increase in water diffusion subsequent to addition of vasopressin, which is masked by unstirred layers and by the resistance offered to diffusion by the thick layer of connective tissue and muscle supporting the bladder epithelial cells. An even higher diffusion rate would be anticipated with the complete elimination of unstirred layers, and of barriers to diffusion remaining within the epithelial layer itself. An alternative to the pore hypothesis is considered, in which vasopressin acts solely by increasing the diffusion rate of water across the luminal membrane of the epithelial cell.
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  • 2
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Urea and water transport across the toad bladder epithelial cell appears to take place through independent vasopressin-stimulated pathways. Agents such as chromate, for example, when added to the luminal bathing medium, inhibit urea transport without inhibiting osmotic water flow, providing evidence for such independent pathways. In the present study, selective inhibition of urea transport is shown for permanganate and methylene blue, which, like chromate, are oxidizing agents. Permanganate inhibits urea transport irreversibly, while methylene blue acts reversibly. Not all oxidizing agents are inhibitory; perchlorate, peroxide and ferricyanide have no effect on urea transport or water flow. Permanganate and chromate both act at a point beyond the generation of cyclic AMP, since they continue to inhibit urea transport in bladders treated with exogenous cyclic AMP, 8-bromoadenosine 3′,5′-cyclic monophosphate, and a combination of cyclic AMP and theophylline. These findings suggest that selective inhibition of urea transport can be brought about by oxidation of one or more components in its transport pathway, and that, in the case of chromate and permanganate, these components may be in the luminal membrane itself.
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  • 3
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Previous studies with phloretin have shown that the movement of urea and other solutes across the toad bladder can be inhitited with no effect on osmotic water flow, active sodium transport, or the movement of ethanol and ethylene glycol. These findings have suggested that a vasopressin-sensitive carrier is involved in the transport of solutes such as urea across the luminal membrane of the epithelial cell. The present paper describes the effect of two agents other than phloretin: tannic acid and chromate, on water and solute movement across the bladder. The pattern of action of these two agents resembles that of phloretin, and supports our earlier findings of the independence of solute and water movement. The effect of chromate on urea movement is seen only in the presence of vasopressin, and only if chromate is added prior to vasopressin. Chromate also proves to be an irreversible inhibitor of urea movement. The implications of these findings are discussed. In view of the known interactions of both agents with proteins, it is suggested that carrier-mediated transport of urea proceeds across a protein component of the membrane.
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  • 4
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Vasopressin activates a number of transport systems in the toad bladder, including the systems for water, urea, sodium, and other small solutes. Evidence from experiments with selective inhibitors indicates that these transport systems are to a large extent functionally independent. In the present study, we show that the transport systems can be separately activated. Low concentrations of vasopressin (1 mU/ml) activate urea transport with virtually no effect on water transport. This selective effect is due in part to the relatively greater inhibitory action of endogenous prostaglandins on water transport. Low concentrations of 8-bromoadenosine cyclic AMP, on the other hand, activate water, but not urea transport. In additional experiments, we found that varying the ratio of exogenous cyclic AMP to theophylline activated water or urea transport selectively. These studies support the concept of independently controlled systems for water and solute transport, and provide a basis for the study of individual luminal membrane pathways for water and solutes in the accompanying paper.
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  • 5
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Urea and water transport across the toad bladder can be separately activated by low concentrations of vasopressin or 8 Br-cAMP. Employing this method of selective activation, we have determined the reflection coefficient (σ) of urea and other small molecules under circumstances in which the bladder was transporting urea or water. An osmotic method for the determination of σ was used, in which the ability of a given solute to retard water efflux from the bladder was compared to that of raffinose (σ=1.0) or water (σ=0). When urea transport was activated (low concentration of vasopressin), σ for urea and other solutes was low, (σurea,0.08–0.39;σacetamide, 0.55; σethylene glycol, 0.60). When water transport was activated (0.1mm 8 Br-cAMP) σurea approached 1.0 σurea also approached 1.0 at high vasopressin concentrations. In a separate series of studies, σurea was determined in the presence of 2×10−5 m KMnO4 in the luminal bathing medium. Under these conditions, when urea transport is selectively blocked, σurea rose from a value of 0.12 to 0.89. Thus, permanganate appears to “close” the urea transport channel. These findings indicate that the luminal membrane channels for water and solutes differ significantly in their dimensions. The solute channels, limited in number, have relatively large radii. They carry a small fraction (approximately 10%) of total water flow. The water transport channels, on the other hand, have small radii, approximately the size of a water molecule, and exclude solutes as small as urea.
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  • 6
    ISSN: 0730-2312
    Keywords: cathepsin-B ; tissue transglutaminase ; mesangial cell apoptosis ; mRNA expression ; Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Chemistry and Pharmacology , Medicine
    Notes: Mesangial matrix is a dynamic structure which modulates mesangial cell function. Since accumulation of matrix precedes the development of focal glomerulosclerosis, we studied the effect of different matrices on mesangial cell (MC) apoptosis. Suspended mesangial cells became apoptotic in a time dependent manner. Collagen type III did not modulate MC apoptosis when compared to cells grown on plastic. MCs grown on Matrigel, collagen type I and IV showed an increased number of apoptotic cells when compared to MCs grown on plastic. DNA end-labeling further confirmed these observations. MCs grown on Matrigel showed enhanced (P 〈 0.05) mRNA expression for tissue transglutaminase (TTG) and cathepsin-B. Mesangial cells grown on Matrigel also showed enhanced expression of superoxide dismutase (SOD). We conclude that mesangial cells require attachment to the matrix for their survival and alteration of the quality of matrix modulates mesangial cell apoptosis. J. Cell. Biochem. 68:22-30, 1998. © 1998 Wiley-Liss, Inc.
    Additional Material: 6 Ill.
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  • 7
    ISSN: 0091-7419
    Keywords: vasopressin ; nocodazole ; urea transport ; rotenone ; dinitrophenol ; methylene blue ; Life Sciences ; Molecular Cell Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Chemistry and Pharmacology , Medicine
    Notes: Vasopressin increases the permeability of receptor cells to water and, in tissues such as toad bladder, to solutes such as urea. While cyclic AMP appears to play a major role in mediating the effects of vasopressin, there is evidence that activation of the water permeability system and the urea permeability system involves separate pathways. In the present study, we have shown that inhibitors of oxidative metabolism (rotenone, dinitrophenol, and methylene blue) selectively inhibit either vasopressin-stimulated water flow or vasopressin-stimulated urea transport. There was no inhibition, however, when exogenous cyclic AMP was substituted for vasopressin, and little to no inhibition when the potent analogue 8-bromoadenosine 3′,5′-cyclic monophosphate (8-Br-cAMP) was employed. Rotenone had no effect on adenylate cyclase activity or cyclic AMP levels within the cell; dinitrophenol decreased adenylate cyclase activity minimally.Additional studies with vinblastine and nocodazole, inhibitors of microtubule assembly, demonstrated an inhibition of vasopressin and cyclic AMP-stimulated water flow but showed no effect on urea transport.We would conclude that water and urea transport, as examples of hormone-stimulated processes, have different links to cell metabolism, and that in addition to cyclic AMP, a non-nucleotide pathway may be involved in the action of vasopressin.
    Additional Material: 5 Ill.
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