Journal - Nephrology, dialysis, transplantation : official publication of the European Dialysis and Transplant Association - European Renal Association (ENGLAND )

Journal - Journal of the American Society of Nephrology : JASN (UNITED STATES )

Abstract :

Phenotype, growth, and functional characteristics of glomerular mesangial "myofibroblasts" are under the control of multiple hormones, vasoactive agents, autacoids, and cytokines. Several parallel signal transduction pathways couple receptor occupancy with functional changes, including phospholipases C, A2, and D breakdown of membrane phospholipids, and adenylate/guanylate cyclase activation. Changes of cytosolic ion concentrations, cyclic nucleotide accumulation, and eicosanoid biosynthesis are currently interpreted as intracellular signals for protein kinase activation. Phosphorylation of multiple substrates by serine/threonine kinases C, A, and G or by tyrosine kinases directly coupled to receptors, is a final step in cell activation. Cross-talk between signal transduction pathways, along with the release of eicosanoids and cytokines acting as intercellular mediators, provides the potential for interactive regulation of glomerular cell functions.

Journal - The Journal of laboratory and clinical medicine (UNITED STATES )

Abstract :

Cultured human glomerular mesangial cells express functional receptors for vasoconstrictor metabolites of arachidonate, such as thromboxane A2. Binding of thromboxane A2 analogs triggers phosphatidylinositol breakdown and a rise of intracellular free Ca2+, followed by complex effects on cell growth. We have evaluated the effects of the thromboxane A2 mimetic, U-46619, on the biosynthesis of human mesangial cell proteins. After 4 to 24 hours of incubation with U-46619, cells were metabolically labeled with tritiated leucine or sulfur 35-methionine and analyzed by one-dimensional and two-dimensional sodium dodecylsulfate polyacrylamide gel electrophoresis and fluorography. U-46619 modestly stimulated total protein synthesis in both quiescent and cycling cells to a maximum of 12% and 23% above control at 1 mumol/L after 24 hours, respectively. The eicosanoid selectively enhanced the labeling of crude membrane and cytosolic proteins of molecular weights of approximately 38 to 53 kd and 125 to 200 kd in the presence or absence of serum, respectively. Cellular tubulin and collagen type IV, but not actin, were markedly stimulated, as determined by immunoblotting of cell-associated proteins. On the contrary, U-46619 potently inhibited labeling of soluble proteins that were released into the media, to a maximum of 49% after 24 hours. Inhibition was confirmed by immunoblotting of collagen type IV. Intraglomerular accumulation of thromboxane A2 during chronic inflammation may modify the functional characteristics of the mesangium by regulation of the biosynthesis of structural proteins.

Journal - Kidney international (UNITED STATES )

Abstract :

Excitable cells express Na+/Ca2+ exchange activity among other mechanisms modulating rapid fluctuations of cytosolic free Ca2+ ([Ca2+]i). We studied functions and regulation of a Na+/Ca2+ exchanger in cultured human glomerular mesangial cells. Fura-2-loaded confluent monolayers reacted to removal of ambient Na+ with an immediate, transient elevation of [Ca2+]i, assessed by single/dual wavelength fluorometry. Peak [Ca2+]i was inversely correlated with the extracellular Na+ concentration. Ca2+ influx was the sole mechanism implicated, as the [Ca2+]i rise was prevented by EGTA. The process was inhibited by 1 mM amiloride, but not by blockers of voltage-operated Ca2+ channels. Re-addition of Na+ resulted in a rapid decrease of [Ca2+]i, indicating bimodal operation of the exchanger. Na(+)-loading the cells with monensin and ouabain enhanced Ca2+ uptake. Prior stimulation of [Ca2+]i with the thromboxane A2 mimetic, U-46619, or angiotensin II also increased Ca2+ uptake upon subsequent Na+ removal, suggesting induction of the exchanger by vasoconstrictors. Moreover, the magnitude of agonist-induced [Ca2+]i transients was amplified by Na+ removal, indicating that the exchanger modulates the effects of vasoconstrictors. These results demonstrate that an inducible Na+/Ca2+ antiporter is operative in resting and stimulated human mesangial cells, further confirming their smooth muscle origin and potential regulatory role on glomerular hemodynamics.