The pancreatic variant of the sodium bicarbonate cotransporter, pNBC1, mediates basolateral bicarbonate influx in the exocrine pancreas by coupling the transport of bicarbonate to that of sodium, with a 2 HCO3−:1 Na+ stoichiometry. The kidney variant, kNBC1, mediates basolateral bicarbonate efflux in the proximal tubule by coupling the transport of 3 HCO3− to 1 Na+. The molecular basis underlying the different stoichiometries is not known. pNBC1 and kNBC1 are 93 % identical to each other with 41 N‐terminal amino acids of kNBC1 replaced by 85 distinct amino acids in pNBC1. In this study we tested the hypothesis that the differences in stoichiometry are related to the difference between the N‐termini of the two proteins. Mouse renal proximal tubule and collecting duct cells, deficient in both pNBC1‐ and kNBC1‐mediated electrogenic sodium bicarbonate cotransport function were transfected with either pNBC1 or kNBC1. Cells were grown on a permeable support to confluence, mounted in an Ussing chamber and permeabilized apically with amphotericin B. Current through the cotransporter was isolated as the difference current due to the reversible inhibitor dinitrostilbene disulfonate. The stoichiometry was calculated from the reversal potential by measuring the current‐voltage relationships of the cotransporter at different Na+ concentration gradients. Our data indicate that both kNBC1 and pNBC1 can exhibit either a 2:1 or 3:1 stoichiometry depending on the cell type in which each is expressed. In proximal tubule cells, both pNBC1 and kNBC1 exhibit a 3 HCO3−:1 Na+ stoichiometry, whereas in collecting duct cells, they have a 2:1 stoichiometry. These data argue against the hypothesis that the stoichiometric differences are related to the difference between the N‐termini of the two proteins. Moreover, the results suggest that as yet unidentified cellular factor(s) may modify the stoichiometry of these cotransporters.

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In vitro and in vivo data suggest a remarkable plasticity in the differentiated phenotype of intrinsic glomerular cells, which after injury express new structures and functions. We have shown that a protein kinase C (PKC) isoform, beta II, is expressed in diseased but not normal glomeruli. Since intrarenal cytokine synthesis has been implicated in the pathogenesis of progressive glomerular injury, we have hypothesized that these mediators induce a change in isoform profile. To test this hypothesis in vitro, we have determined whether platelet-derived growth factor (PDGF) and interleukin-1 (IL-1) alter the expression or activation of PKC isoforms in cultured mesangial cells (MCs). By immunoblot and ribonuclease (RNase) protection assays, both PDGF and IL-1 induce as early as 2 h de novo synthesis of PKC-beta II. Since MCs constitutively express PKC-alpha, -beta I, and -zeta, we also determined whether IL-1 or PDGF alter the activity of these isoforms. PDGF maximally induced translocation of PKC-alpha (10 min), -beta I (90 min), -epsilon (120 min), and -zeta (120 min) from the cytosolic to the membrane fraction. IL-1, in contrast, did not alter the distribution of alpha, beta I, or epsilon at any time measured but did induce PKC-zeta translocation. These data suggest inflammatory mediators regulate PKC isoform activity in diseased glomeruli both by de novo synthesis of unexpressed isoforms and by activation of constitutively expressed PKC isoforms.

Changes in expression and activity of protein kinase C (PKC) isoforms and early transcription factors may account for alterations in cell behavior seen in diabetes. We studied the expression of PKC-beta(I) in rat glomerular mesangial cells (MCs) cultured in normal or high glucose and compared it with the temporal and spatial expression of dimeric transcription factor (NF-kappaB) p50 and p65. The results show that in unstimulated cells PKC-beta(I) and NF-kappaB p50 are distributed in the cytosol and, on stimulation, their distribution is perinuclear and they are localized to the membrane. Serum-starved MCs cultured in high-glucose medium exhibit a predominantly cytosolic localization of PKC-beta(I) and both p50 and p65 NF-kappaB. However, phorbol 12-myristate 13-acetate (PMA) stimulation of cells grown in the presence of high glucose resulted in membrane translocation of PKC-beta(I) that was associated with nuclear translocation of NF-kappaB p65, but not NF-kappaB p50. Moreover, the translocation to the nucleus for NF-kappaB p65 was significantly higher in MCs exposed to high glucose compared with those exposed to normal glucose. These observations indicate that the NF-kappaB p65, but not NF-kappaB p50, expression and translocation pattern mirrors that of PKC-beta(I), which may be one important pathway by which signaling is enhanced in the high-glucose state.

In the pre- and postnatal period of kidney development, proliferation with subsequent functional maturation of intrinsic glomerular mesangial cells (MCs) continues within the existing framework. Recent work has suggested that PKC beta isoform is responsible for the proliferation observed during maturation. We sought to ascertain whether PKC beta isoform expression is altered during the development of the mesangium. MCs were subcultured from glomerular explants of Sprague-Dawley rat kidneys, days 1, 3, 5, 8, 15 postnatally, and adult. MCs from rat kidneys postnatally days 1-5 proliferated at a significantly greater rate than adult [ > 1.169-fold, P < 0.01] but term day 8 cells did not [ < 1.34-fold, not significant)] as assessed by [3H]thymidine incorporation. Western blot analysis using isoform specific antibodies was performed on confluent neonatal and adult MC. We observed that all neonatal and adult MC express beta I PKC (n = 8 kidneys from separate primaries for each date and adult). However, unlike adult MCs, neonatal MC express beta II in postnatal days 1-5 and none thereafter. Immunofluorescent staining of postnatal kidneys confirmed that PKC beta II is present in neonatal MC up to day 5. By day 8, staining of mesangium with PKC beta II begins to disappear and assumes a parietal epithelial pattern. In adult kidneys, there was only PKC beta II staining of the parietal epithelial cells. Our results demonstrate that differential expression of PKC beta II closely parallels the proliferative behavior of the MCs of the maturing glomerulus. Therefore, PKC beta II expression and activation may play a critical role in development.