EXTRACELLULAR GLUCOSE REDUCES THE RESPONSIVENESS OF MESANGIAL CELL ION CHANNELS TO ANGIOTENSIN-II

Abnormal cellular ion homeostasis is a well-recognized component of di abetic glomerular disease. In cultured rat glomerular mesangial cells, we have previously shown that insulin regulates Ca2+-dependent activa tion 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 ''norma l'' (5 mM) or ''high'' (30 mM) glucose for 1 wk. In normal glucose, 10 0 nM ANG II increased Cl- and NSCC activity by > 16-fold and > 60-fold , respectively. Direct release of intracellular Ca2+ ([Ca2+](i)) store s (0.25 mu M 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 mu M calphos tin) or downregulation (24-h exposure to 0.1 mu M 4 beta-phorbol 12-my ristate 13-acetate), but not aldose reductase inhibition (0.5 mM sorbi nil), restored channel responsiveness to ANG II despite high glucose. Channel responsiveness was also restored if mesangial cells were coinc ubated in both high glucose and 500 mu M myo-inositol. Acute exposure to a synthetic diacylglycerol (100 mu M 1-oleoyl-2-acetyl glycerol) re established channel unresponsiveness to ANG II. We conclude the follow ing in rat mesangial cell cultures: 1) Activation of Ca2+-dependent Cl - and NSCCs by ANG II is reduced by high extracellular glucose. 2) The inhibitory effect of high glucose occurs at a step proximal to ANG II -mediated release of [Ca2+](i) stores and involves myo-inositol deplet ion and PKC activation. 3) In diabetic patients, hyperglycemia would b e predicted to decrease the responsiveness of glomerular mesangial cel l ion channels to the vasoactive peptide ANG II.