The influence of ions, ouabain, propranolol and amiloride on the transepithelial potential and resistance of rabbit cornea

Synchronization modulation increases transepithelial potentials in MDCK monolayers through Na/K pumps

Transepithelial potential (TEP) is the voltage across a polarized epithelium. In epithelia that have active transport functions, the force for transmembrane flux of an ion is dictated by the electrochemical gradient in which TEP plays an essential role. In epithelial injury, disruption of the epithelial barrier collapses the TEP at the wound edge, resulting in the establishment of an endogenous wound electric field (∼100 mV/mm) that is directed towards the center of the wound. This endogenous electric field is implicated to enhance wound healing by guiding cell migration. We thus seek techniques to enhance the TEP, which may increase the wound electric fields and enhance wound healing. We report a novel technique, termed synchronization modulation (SM) using a train of electric pulses to synchronize the Na/K pump activity, and then modulating the pumping cycles to increase the efficiency of the Na/K pumps. Kidney epithelial monolayers (MDCK cells) maintain a stable TEP and transepithelial resistance (TER). SM significantly increased TEP over four fold. Either ouabain or digoxin, which block Na/K pump, abolished SM-induced TEP increases. In addition to the pump activity, basolateral distribution of Na/K pumps is essential for an increase in TEP. Our study for the first time developed an electrical approach to significantly increase the TEP. This technique targeting the Na/K pump may be used to modulate TEP, and may have implication in wound healing and in diseases where TEP needs to be modulated.

Intracellular activities of chloride, potassium and sodium ions in rabbit corneal epithelium

The mechanism of ion transport in the epithelium of rabbit cornea was studied by determining the intracellular ion activity of Cl-, Na+ and K+ under various conditions. Ionic activities were measured by means of microelectrodes containing liquid ion-exchangers selective for Cl-, Na+ or K+. The Cl- activity in basal cells of the epithelium in Na+ containing bathing solutions amounts to 28 +/- 2 mM (n = 11). This value is 1.9-times greater than expected on the basis of passive distribution across the tear side membrane. This finding suggests the existence of a Cl- accumulating process. Replacement of Na+ in the aqueous bathing solution by choline or tetraethylammonium results in a reversible decrease in Cl- activity to 22 +/- 1 mM (n = 11, P less than 0.025). The ratio of observed and predicted Cl- activity decreased significantly from 1.9 to 1.4 (P less than 0.05). The decrease in Cl- activity due to Na+ replacement was rather slow. In contrast, after readmittance of Na+ to the aqueous bathing solution, Cl- activity rose to a stable level within 30 min. These results indicate involvement of Na+ in Cl- accumulation into the basal cells of the epithelium. The K+ and Na+ activities of the basal cells of rabbit corneal epithelium in control bathing solutions were 75 +/- 4 mM (n = 13) and 24 +/- 3 mM (n = 12), respectively. The results can be summarized in the following model for Cl- transport across corneal epithelium. Cl- is accumulated in the basal cells across the aqueous side membrane, energized by a favourable Na+ gradient. Cl- will subsequently leak out across the tear side membranes. Na+ is extruded again across the aqueous side membrane of the epithelium by the (Na+ + K+)-ATPase.

Adrenergic regulation of sodium and chloride transport in the isolated cornea of rabbit and man

Isolated corneas were mounted in Ussing-Zerahn-type chambers and short circuit current (SCC) was measured before and after application of drugs (5 X 10(-5) mol X 1(-1)) interfering with adrenergic receptors. Epinephrine increased SCC in the rabbit cornea and decreased SCC in the human cornea. alpha-Adrenergic stimulation or inhibition did not affect SCC. The increase in SCC observed after terbutaline (beta 2-agonist) was similar to the increase after isoproterenol (beta 1- and beta 2-agonist). SCC was not influenced by the beta 1-antagonist atenolol but was modified, although differently in rabbit and man, by the beta 1- and beta 2-antagonist propranolol. Thus, the catecholamine response of the rabbit and human cornea is mainly mediated by beta 2-adrenergic receptors. However, species differences were observed when testing the effect of propranolol on the transcorneal flux of 22Na and 36Cl. In the rabbit cornea the net Cl flux (directed from the aqueous to the tear side) was inhibited by propranolol, whereas net Na flux (from the tear to the aqueous side) was not influenced by the drug. In the human cornea propranolol reduced unidirectional Na flux from the aqueous to the tear side. Thus, the regulatory effect of propranolol on corneal transparency is different in man and the rabbit.

Intracellular ion activities and Cl-transport mechanisms in bullfrog corneal epithelium

Cell membrane potentials, cell membrane resistances, and intracellular ionic activities were measured in bullfrog corneal epithelium. Equivalent circuit analysis was performed by adding adenosine to the apical surface and assuming that only the apical membrane is initially affected. From single-ion substitutions in the apical bathing solution, the apical membrane was found to have a high Cl- permeability, a low K+ permeability, and an unmeasurably small Na+ permeability. Under control conditions intracellular Cl- activity (aCli) was 22 +/- 2 (SE) mM, intracellular Na+ activity (aNai) was 14 +/- 3 mM, and intracellular K+ activity (aKi) was 106 +/- 5 mM. The electrical potential differences across apical and basolateral membranes were about 50 and 67 mV, respectively, both cell negative. aCli and aKi are higher, whereas aNai is much lower than predicted for equilibrium distribution. Inasmuch as Cl- is transported from the basolateral (stromal) to the apical (tear) side, basolateral entry of this anion is uphill and apical exit is downhill. Basolateral entry is Na+ dependent, as evidenced by a fall of aCli to near-equilibrium values after basolateral Na+ removal. The electrochemical gradient for Cl- efflux across the apical membrane is large enough to account for Cl- transport by electrodiffusion only. Na+ removal from the basolateral solution causes a reversible decrease of apical membrane Cl- permeability. The results support the hypothesis that net transepithelial Cl- transport results from coupled NaCl entry (or an equivalent process) at the basolateral membrane and electrodiffusional Cl- exit at the apical membrane.