REGULATION OF MESANGIAL CELL ION CHANNELS BY INSULIN AND ANGIOTENSIN-II - POSSIBLE ROLE IN DIABETIC GLOMERULAR HYPERFILTRATION

We used patch clamp methodology to investigate how glomerular mesangia l cells (GMC) depolarize, thus stimulating voltage-dependent Ca2+ chan nels and GMC contraction. In rat GMC cultures grown in 100 mU/ml insul in, 12% of cell-attached patches contained a Ca2+-dependent, 4-picosie mens Cl-channel. Basal NP. (number of channels times open probability) was < 0.1 at resting membrane potential. Acute application of 1-100 n M angiotensin II (AII) or 0.25 muM thapsigargin (to release [Ca2+]i st ores) increased NP(o). In GMC grown without insulin, Cl- channels were rare (4%) and unresponsive to AII or thapsigargin in cell-attached pa tches, and less sensitive to [Ca2+]i in excised patches. GMC also cont ained 27-pS nonselective cation channels (NSCC) stimulated by AII, tha psigargin, or [Ca2+]i, but again only when insulin was present. In GMC grown without insulin, 15 min of insulin exposure increased NP(o) (in sulin greater-than-or-equal-to 100 muU/ml) and restored All and [Ca2+] , responsiveness (insulin greater-than-or-equal-to 1 muU/ml) to both C l- and NSCC. GMC All receptor binding studies showed a B. (binding sit es) of 2.44+/-0.58 fmol/mg protein and a K(d) (binding dissociation co nstant) of 3.02+/-2.01 nM in the absence of insulin. B(max) increased by 86% and Kd was unchanged after chronic (days) insulin exposure. In contrast, neither K(d) nor B(max) 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- e fflux and cation influx, respectively, would promote GMC depolarizatio n, leading to voltage-dependent Ca2+ channel activation and GMC contra ction. (c) Responsiveness of Cl- and NSCC to All is dependent on insul in exposure; AII receptor density increases with chronic, but not acut e insulin, and channel sensitivity to [Ca2+]i increases with both acut e and chronic insulin. (d) Decreased GMC contractility may contribute to the glomerular hyperfiltration seen in insulinopenic or insulin-res istant diabetic patients.