Extracellular glucose reduces the responsiveness of mesangial cell ion channels to angiotensin II

Abstract

Abnormal cellular ion homeostasis is a well-recognized component of diabetic glomerular disease. In cultured rat glomerular mesangial cells, we have previously shown that insulin regulates Ca(2+)-dependent activation of 4-pS Cl- channels and 27-pS nonselective cation channels (NSCC) by angiotensin II (ANG II). To assess whether extracellular glucose also affects mesangial ion channels, we applied patch-clamp techniques to cells incubated in constant insulin (100 mU/ml) and either "normal" (5 mM) or "high" (30 mM) glucose for 1 wk. In normal glucose, 100 nM ANG II increased Cl- and NSCC activity by > 16-fold and > 60-fold, respectivley. Direct release of intracellular Ca2+ ([Ca2+]i) stores (0.25 microM thapsigargin) mimicked ANG II-induced channel stimulation. In high glucose, Cl- and NSCC stimulation by ANG II was attenuated (< 7-fold), whereas channel activation by thapsigargin was unaffected. Protein kinase C (PKC) inhibition (30-min exposure to 0.5 microM calphostin) or downregulation (24-h exposure to 0.1 microM 4 beta-phorbol 12-myristate 13-acetate), but not aldose reductase inhibition (0.5 mM sorbinil), restored channel responsiveness to ANG II despite high glucose. Channel responsiveness was also restored if mesangial cells were coincubated in both high glucose and 500 microM myo-inositol. Acute exposure to a synthetic diacylglycerol (100 microM 1-oleoyl-2-acetyl glycerol) reestablished channel unresponsiveness to ANG II. We conclude the following in rat mesangial cell cultures: 1) Activation of Ca(2+)-dependent Cl- and NSCCs by ANG II is reduced by high extracellular glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

High glucose level inhibits capacitative Ca 2 + influx in cultured rat mesangial cells by a protein kinase C-dependent mechanism

“…Recent work in human mesangial cells from our laboratory confirmed direct control of Ca 2 + influx by activated PKC, even in the presence of intact Ca 2 + stores [48]. Interestingly, also the electrophysiology studies by Seal et al [46] indicated that PKC inhibition restores channel responsiveness to angiotensin II in the presence of high glucose [46]. This finding points to a general inhibitory activity of PKC on multiple Ca 2 + transport functions, ranging from the above-mentioned release from intracellular stores, to plasma membrane channels [45,46], to Na + /Ca 2 + exchange in our previous studies [49].…”

“…Interestingly, also the electrophysiology studies by Seal et al [46] indicated that PKC inhibition restores channel responsiveness to angiotensin II in the presence of high glucose [46]. This finding points to a general inhibitory activity of PKC on multiple Ca 2 + transport functions, ranging from the above-mentioned release from intracellular stores, to plasma membrane channels [45,46], to Na + /Ca 2 + exchange in our previous studies [49]. Such inhibition may represent an endogenous negative feedback, aimed at limiting the state of activation of the cells in the presence of vasoconstrictors, and preventing [Ca 2 + ] i from rising to uncontrolled levels.…”

“…Other studies have documented inhibition of Ca 2 + influx by insulin-stimulated glucose uptake in VSMC [37]. Recent patch-clamp work has shown that a 27 pS nonselective cation channel is inhibited by growth of rat mesangial cells in 30 mmol/l glucose media [46]. This channel, activated by angiotensin II and thapsigargin, may represent one link between store depletion and Ca 2 + influx in mesangial cells, although through the intermediation of VOC [46,47].…”

“…Recent patch-clamp work has shown that a 27 pS nonselective cation channel is inhibited by growth of rat mesangial cells in 30 mmol/l glucose media [46]. This channel, activated by angiotensin II and thapsigargin, may represent one link between store depletion and Ca 2 + influx in mesangial cells, although through the intermediation of VOC [46,47]. Thus, reduced Ca 2 + entry may be a feature common to several cell types cultured under high glucose conditions, but the mechanism of internalization involved could be different, based on the peculiar features of a given cell population.…”

Thiazolidinedione compounds ameliorate glomerular dysfunction independent of their insulin-sensitizing action in diabetic rats.

“…Finally, reduced contractile responsiveness of mesangial cells to vasoconstrictive peptides, such as angiotensin II, might be related to the development of glomerular hyperfiltration (47). A reduction of both the production of inositol 1,4,5-trisphosphate and the increase in intracellular Ca 2+ in response to angiotensin II was found in mesangial cells cultured under high glucose conditions (48,49); this reduction was shown to be prevented by an inhibition of PKC (49,50). These results strongly suggest that the activation of the PKC-ERK pathway in diabetes is responsible for the development of glomerular hyperfiltration and that troglitazone is able to prevent glomerular hyperfiltration by inhibiting the activation of the PKC-ERK pathway.…”