Effects of Na-K-2Cl cotransport regulators on outflow facility in calf andhuman eyes in vitro

PURPOSE. Cultured human trabecular meshwork (TM) cells possess substantial Na-K-CL activity, which is involved in the regulation of TM cell volume, Th e hypothesis in the present study was that drugs that affect the cotranspor ter might alter aqueous humor outflow facility (C) in the intact eye. The e ffects of agents and conditions known to modulate Na-K-Cl cotransport activ ity and/or TM cell volume on C in perfused anterior segments were investiga ted. METHODS. Human and calf eyes were dissected and perfused, and C was determi ned according to standard published methods. Perfusates with modified osmol arity were used to cause alterations in TM cell volume. Cl-free perfusate a nd/or bumetanide (10(-5) M) was used to inhibit Na-K-Cl cotransport activit y, and vasopressin (10(-7) M, 10(-8) M) was used to stimulate cotransport a ctivity. RESULTS. In human eyes, hypo-osmotic perfusate decreased C 12%, whereas hyp er-osmotic perfusate increased C 44%. These changes lasted approximately 30 minutes, after which C began to normalize, Inhibition of Na-K-CI cotranspo rt using Cl-free medium or bumetanide resulted in facility increases of 27% and 22%, respectively. There was an additive increase in C with bumetanide plus Cl-free media. Stimulating Na-K-Cl cotransport with 10-8 M and 10(-7) IM vasopressin resulted in 28% and 35% decreases in C, respectively. The r esults were similar in calf eyes: Cl-free medium or bumetanide resulted in 41% and 52% increases in C, whereas 10(-8) M and 10(-7) M vasopressin resul ted in 14% and 19% decreases in C, respectively. CONCLUSIONS. Modulation of Na-K-Cl cotransport results in changes in C that may be mediated in part by cell volume changes.