EFFECT OF BETA-ADRENERGIC AGONISTS ON PARACELLULAR WIDTH AND FLUID-FLOW ACROSS OUTFLOW PATHWAY CELLS

PURPOSE. To determine whether the adrenergic agonists epinephrine and isoproterenol regulate fluid flow across endothelial cells cultured fr om the human aqueous outflow pathway and to evaluate associated cellul ar mechanisms. METHODS. Confluent monolayers of human trabecular meshw ork (TM) or Schlemm's canal endothelial (SCE) cells were grown on poro us filter supports. The monolayers were perfused with media while flui d flow, expressed as hydraulic conductivity (HC = mu 1/min/mm Hg/cm(2) ), was continuously measured in preparations treated with isoprotereno l, epinephrine, or control medium. Morphometric ultrastructural method s were used to measure the area occupied by the intercellular space an d by each cell. RESULTS. SCE cells and TM cells exposed to isoproteren ol or epinephrine responded with an increase in transendothelial fluid flow. Dose-response curves for both adrenergic agonists showed that H C increased linearly as a function of the log of the isoproterenol and epinephrine concentration. At 10(-4) M isoproterenol, the HC increase d threefold, and threshold conditions were reached at 10(-9) M. The in crease in HC was apparent after isoproterenol had been applied for 1 h our, reached a peak in 3 to 4 hours, and declined gradually to return to baseline conditions in 10 to 12 hours, Morphometric analyses showed that the area occupied by the intercellular space increased fourfold when isoproterenol was used at 10(-4) M, whereas the cell area decreas ed as a function of the concentration of adrenergic agonist. Epinephri ne's effects on HC and cell morphology were blocked by pretreatment wi th equimolar concentrations of the nonselective beta-blocker, timolol. CONCLUSIONS. Epinephrine and isoproterenol increase flow through the paracellular pathway of SCE and TM cells through a beta-receptor media ted response that widens the intercellular space and reduces cell area . These findings support the hypothesis that epinephrine decreases the intraocular pressure in glaucoma therapy by promoting fluid flow acro ss the SCE and TM cells lining tissues of the major aqueous outflow pa thway.