Regulation of Mesangial Cell Ion Channels by Insulin and Angiotensin II: Possible Role in Diabetic Glomerular Hyperfiltration

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Abstract

We used patch clamp methodology to investigate how glomerular mesangial cells (GMC) depolarize, thus stimulating voltage-dependent Ca2+ channels and GMC contraction. In rat GMC cultures grown in 100 mU/ml insulin, 12% of cell-attached patches contained a Ca2+-dependent, 4-picosiemens Cl- channel. Basal NPo (number of channels times open probability) was < 0.1 at resting membrane potential. Acute application of 1-100 nM angiotensin II (AII) or 0.25 micromolars thapsigargin (to release (Ca2+)i stores) increased NPo. In GMC grown without insulin, Cl- channels were rare (4%) and unresponsive to AII or thapsigargin in cell-attached patches, and less sensitive to (Ca2+)i in excised patches. GMC also contained 27-pS nonselective cation channels (NSCC) stimulated by AII, thapsigargin, or (Ca2+)i, but again only when insulin was present. In GMC grown without insulin, 15 min of insulin exposure increased NPo (insulin >= 100 microunits/ml) and restored AII and (Ca2+)i responsiveness (insulin >= 1 microunits/ml) to both Cl (-) and NSCC. GMC AII receptor binding studies showed a Bmax (binding sites) of 2.44+/-0.58 fmol/mg protein and a Kd (binding dissociation constant) of 3.02+/-2.01 nM in the absence of insulin. Bmax increased by 86% and Kd was unchanged after chronic (days) insulin exposure. In contrast, neither Kd nor Bmax was significantly affected by acute (15-min) exposure. Therefore, we concluded that: (a) rat GMC cultures contain Ca2+-dependent Cl- and NSCC, both stimulated by AII. (b) Cl- efflux and cation influx, respectively, would promote GMC depolarization, leading to voltage-dependent Ca2+ channel activation and GMC contraction. (c) Responsiveness of Cl- and NSCC to AII is dependent on insulin exposure; AII receptor density increases with chronic, but not acute insulin, and channel sensitivity to (Ca2+)i increases with both acute and chronic insulin. (d) Decreased GMC contractility may contribute to the glomerular hyperfiltration seen in insulinopenic or insulin-resistant diabetic patients. (J. Clin. Invest. 1993. 92:2141-2151.) Key words: patch clamp. Cl- channel. nonselective cation channel. Ca2+ channel. diabetes mellitus

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