title: Potential difference measurements of ocular surface Na+ absorption analyzed using an electrokinetic model.
abstract: OBJECTIVE
  Corneal and conjunctival epithelia are capable of transcellular Na+ absorption and Cl- secretion, which drives water movement across these tissues. A recent study demonstrated with a new open-circuit potential difference (PD) technique that Cl- moves across the ocular surface in mice through Ca2+- and cAMP-sensitive Cl- channels, the latter pathway being the cystic fibrosis (CF) transmembrane conductance regulator (CFTR). The purpose of the present study was to identify transporting mechanisms involved in Na+ absorption and to develop a mathematical model of ocular surface ion transport to quantify the relative magnitudes of and electrochemical coupling among transporting processes.
  
  
  METHODS
  PDs across the fluid-bathed ocular surface were measured in anesthetized wild-type and CF mice in response to Na+, Cl-, and K+ ion substitution and transporter agonists, inhibitors, and substrates. An electrokinetic model of the ocular surface epithelium was developed to simulate PD measurements, which involved computation of membrane potentials and cell [Na+], [K+], [Cl-] and volume from transporter activities and extracellular ion concentrations.
  
  
  RESULTS
  Na+ replacement produced a 6 +/- 2-mV depolarization that was blocked by amiloride (K(i) 0.8 microM) and benzamil (Ki 0.2 microM). The Na+-dependent depolarization by amiloride was significantly greater in CF mice (19 +/- 3 mV). In wild-type mice, D-, but not L-glucose produced a phloridzin-sensitive, 4.1-mV hyperpolarization in the presence of Na+ and amiloride, with a Km for D-glucose of 2.5 mM. Glycine and L-arginine also produced Na+-dependent hyperpolarizations. The epithelial transport model accurately reproduced experimental PD measurements.
  
  
  CONCLUSIONS
  PD measurements coupled with model computations defined quantitatively the roles of Na+ and Cl- transport processes in ocular surface ion and fluid secretion, and indicated that CFTR-dependent changes in apparent epithelial Na+ channel (ENaC) activity could be accounted for by electrochemical coupling, without requiring ENaC-CFTR interactions. The data and modeling also predicted significant enhancement of ocular surface fluid secretion by ENaC inhibitors and CFTR activators as possible therapies for dry eye syndromes.
authors.affiliation: Department of Medicine and the Cardiovascular Research Institute, University of California, San Francisco, CA 94143-0521, USA.
authors.forename: Marc H
authors.initials: MH
authors.lastname: Levin
authors.forename: Jung Kyung
authors.initials: JK
authors.lastname: Kim
authors.forename: Jie
authors.initials: J
authors.lastname: Hu
authors.forename: A S
authors.initials: AS
authors.lastname: Verkman
mesh_terms: D000042:Absorption; D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D015220:Calcium Channels / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism; D003228:Conjunctiva / Q000166:cytology / Q000502:physiology*; D003315:Cornea / Q000166:cytology / Q000502:physiology*; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000378:metabolism*; D004594:Electrophysiology; D004847:Epithelial Cells / Q000502:physiology; D053503:Epithelial Sodium Channels; D017136:Ion Transport; D008564:Membrane Potentials / Q000502:physiology; D051379:Mice; D018181:Mice, Inbred CFTR; D008962:Models, Theoretical; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D026941:Sodium Channel Blockers / Q000494:pharmacology; D015222:Sodium Channels / Q000378:metabolism*
chemical_list: D015220:Calcium Channels; D002712:Chlorides; D053503:Epithelial Sodium Channels; D026941:Sodium Channel Blockers; D015222:Sodium Channels; C013407:benzamil; D019005:Cystic Fibrosis Transmembrane Conductance Regulator; D000584:Amiloride; D012964:Sodium; D011188:Potassium