The inhibitory influence of endothelin on active sodium-potassium transport in porcine lens

PURPOSE. Endothelin (ET)-1 is known to inhibit active NaK transport by as m uch as 50% in kidney tubule and other tissues. The presence of low levels o f ET-1 in aqueous humor combined with the potential for release of ET-1 fro m ciliary processes suggests that the lens could be exposed to ET-1 in vivo . in this study, experiments were conducted to examine the influence of ET- 1 on active NaK transport in porcine lens. METHODS. The rate of Na,K-adenosine triphosphatase (Na,K-ATPase) dependent potassium transport was determined by measurements of ouabain-sensitive pot assium (Rb-86) uptake by intact lenses. Lens sodium content was measured by atomic absorption spectrophotometry. Cyclic adenosine monophosphate (cAMP) was measured by radioimmunoassay. Cytoplasmic calcium concentration in cul tured porcine lens epithelium was measured by a fluorescence technique usin g fura-2. RESULTS. In the presence of ET-1 (0.1 nM or higher concentration), the rate of ouabain-sensitive potassium (Rb-86) uptake was diminished. The ET recep tor antagonist PD145065 (2 muM) suppressed the inhibitory effect of ET-1 (1 00 nM) on Rb-86 uptake. Sodium content was detectably increased in lenses e xposed to ET-1 for 24 hours. Forskolin (1 muM) caused an eightfold increase of cAMP in the lens epithelium, but no increase of cAMP was detected in th e epithelium of lenses treated with ET-I. Genistein (150 muM), an inhibitor of tyrosine kinases, abolished the inhibitory effects of ET-I on lens Rb-8 6 uptake. ET-1 caused an increase of cytoplasmic calcium concentration in c ultured porcine lens epithelium. The cytoplasmic calcium response to ET-I w as inhibited by PD145065 and genistein. CONCLUSIONS. The results of the present study suggest that ET-1 causes inhi bition of lens active Na-K transport by a mechanism that involves activatio n of ET receptors. Activation of ET receptors also causes an increase of cy toplasmic calcium concentration in cultured lens epithelial cells. Both res ponses to ET-1 appear to have a tyrosine kinase step, because they could be prevented by genistein. The physiological purpose of an ET-1-induced reduc tion in the rate of active Na-K transport by the lens is unknown at this ti me.