Mc. Okafor et Na. Delamere, The inhibitory influence of endothelin on active sodium-potassium transport in porcine lens, INV OPHTH V, 42(5), 2001, pp. 1018-1023
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.