Biphasic effects of insulin and ouabain on fluid transport across rabbit corneal endothelium.

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Abstract
Low levels of insulin stimulate transendothelial fluid transport from preswollen stroma to aqueous in rabbit corneal preparations. The rate of stromal thinning at the end of the 1st h averages 30% faster with insulin, 3.5 .times. 10-11 M (4.8 .mu.U/ml), than that of the paired control. This concentration is about the physiological level in rabbit aqueous. The stimulation with insulin is transient. Rates of thinning average higher but not significantly different from control rates by the 2nd h. High levels of insulin between 3.5 .times. 10-9 M (480 .mu.U./ml) and 2.0 .times. 10-6 M (2.75 .times. 105 .mu.U/ml) inhibit fluid transport. The inhibition at the low end of this range of concentrations becomes more pronounced with longer perfusion times but appears not to exceed about 50% of the control rate. Ouabain also induces a biphasic effect on fluid transport which is characteristically different from that with insulin. The maximal stimulation observed at all times occurred with a fixed concentration of 10-10 M. The stimulation is not transient but increases throughout the duration of the perfusion; the average rate is elevated 50% above the control rate by the 3rd h. The transition from a stimulatory to an inhibitory effect occurs consistently at about 10-8 M with ouabain, while a similar transition with insulin occurs at about 10-9 M and appears to shift towards slightly higher concentrations during a 3 h perfusion period. Inhibition of fluid transport with ouabain 3 .times. 10-7 M, is increased from about 50% after l h to more than 70% at the end of the 3rd h of perfusion. The combined presence of stimulatory concentrations of ouabain and insulin affects stromal thinning in a manner resembling the effect of ouabain alone more than that of insulin; additive effects could not be discriminated. Progressively raising the concentration of insulin to a level (10-8 M) that along inhibits stromal thinning, ultimately abolishes the stimulatory effect of ouabain. Based on other evidence and current models of drug/hormone-membrane interaction, these results can be interpreted to indicate a concentration-dependent interaction between receptor complexes of ouabain and insulin with (Na+ + K+)-ATPase.