title: Regulation of mesangial cell ion channels by insulin and angiotensin II. Possible role in diabetic glomerular hyperfiltration.
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 Ca(2+)-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 microM 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 > or = 100 microU/ml) and restored AII and [Ca2+]i responsiveness (insulin > or = 1 microU/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 Ca(2+)-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.
authors.affiliation: Department of Medicine, Emory University School of Medicine, Atlanta, Georgia 30322.
authors.forename: B N
authors.initials: BN
authors.lastname: Ling
authors.forename: E E
authors.initials: EE
authors.lastname: Seal
authors.forename: D C
authors.initials: DC
authors.lastname: Eaton
mesh_terms: D000804:Angiotensin II / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002412:Cations / Q000378:metabolism; D002478:Cells, Cultured; D018118:Chloride Channels / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005920:Glomerular Mesangium / Q000166:cytology / Q000502:physiology*; D007328:Insulin / Q000494:pharmacology*; D007473:Ion Channels / Q000187:drug effects*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D008839:Microelectrodes; D051381:Rats; D017207:Rats, Sprague-Dawley; D011945:Receptors, Angiotensin / Q000378:metabolism*; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
chemical_list: D002412:Cations; D018118:Chloride Channels; D007328:Insulin; D007473:Ion Channels; D011945:Receptors, Angiotensin; D013729:Terpenes; D000804:Angiotensin II; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D002118:Calcium