title: Cyclosporin A inhibits apical secretory K+ channels in rabbit cortical collecting tubule principal cells.
abstract: We used the cell-attached patch clamp configuration to examine the effect of basolateral cyclosporin A (CsA) exposure on low conductance K+ channels found in the principal cell apical membrane of rabbit cortical collecting tubule (CCT) primary cultures. Baseline K+ channel activity, measured as mean NPo (number of channels x open probability), was 2.7 +/- 1.1 (N = 29). NPo fell by 69% (0.84 +/- 0.32; N = 32) in cultures pretreated with 500 ng/ml CsA for 30 minutes prior to patching. Chelation of intracellular [Ca2+]i (10 mM BAPTA/AM; N = 8) or removal of extracellular Ca2+ (N = 9), but not prevention of [Ca2+]i store release (10 microM TMB-8; N = 7), abolished CsA-induced inhibition. This suggested that CsA effects were mediated by an initial rise in [Ca2+]i via Ca2+ influx. Either 25 nM AVP (N = 10) or 0.25 microM thapsigargin (N = 8) (causing IP3-dependent and -independent release of [Ca2+]i stores, respectively) augmented, while 25 pM (N = 6) or 250 pM AVP (N = 8) reversed CSA-induced channel inhibition. Apical membrane protein kinase C (PKC) activation with 0.1 microM phorbol ester, PMA (N = 8) or 10 microM synthetic diacylglycerol, OAG (N = 7), mimicked (mean NPo = 0.99 +/- 0.40) the inhibitory effect of CsA. Apical PKC inhibition by prolonged apical exposure to PMA (N = 10) or 100 microM D-sphingosine (N = 6) blocked CsA's effect. Cyclic AMP increasing maneuvers, 10 microM forskolin (N = 5) or 0.5 mM db-cAMP (N = 8), stimulated basal K+ channel activity in the absence of CsA.
  
  
  CONCLUSIONS
  (1) basolateral exposure to CsA inhibits the activity of apical membrane 13 pS channels responsible for physiologic K+ secretion in rabbit CCT principal cells. (2) The inhibition is mediated by changes in intracellular Ca2+ and activation of apical PKC. (3) Pharmacologic AVP (nM) augments CsA-induced inhibition by releasing intracellular Ca2+ stores; more physiologic AVP (pM) attenuates channel inhibition, probably through cAMP generation. (4) Inhibition of apical secretory K+ channels by CsA likely contributes to decreased kaliuresis and clinical hyperkalemia observed in patients on CsA therapy.
authors.affiliation: Department of Medicine, Emory University School of Medicine, Atlanta, Georgia.
authors.forename: B N
authors.initials: BN
authors.lastname: Ling
authors.forename: D C
authors.initials: DC
authors.lastname: Eaton
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism*; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism; D016572:Cyclosporine / Q000494:pharmacology*; D003600:Cytosol / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D007685:Kidney Tubules, Collecting / Q000166:cytology / Q000378:metabolism*; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000187:drug effects* / Q000378:metabolism*; D011493:Protein Kinase C / Q000502:physiology; D011817:Rabbits
chemical_list: D026902:Potassium Channel Blockers; D015221:Potassium Channels; D016572:Cyclosporine; D000242:Cyclic AMP; D011493:Protein Kinase C; D002118:Calcium