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In a previous study, measurements were made of facilitated and passive transport of glucose, using [3H]-3-O-methyl-D-glucose and [14C]-L-glucose, respectively, across blood-aqueous and blood-vitreous barriers in long-term streptozotocin-diabetic rats. It was found that passive transport was increased, while facilitated transport was decreased, possibly due to saturation of the transport system. The present study examines the appearance of these changes in glucose transport at various times following streptozotocin (STZ) injection. Passive transport, as indicated by the L-glucose rate constant, began to increase at about 10 days following induction of diabetes, stabilized at the elevated rates at 50-60 days and persisted during the 170-day period of observation. Rate constants for [3H]-3-O-methyl-D-glucose transport decreased within 1 day following induction of diabetes. Prevention of hyperglycemia by insulin treatment upon onset of diabetes prevented the latter changes ruling out a direct effect of STZ. Glucose infusion into normal rats produced a similar decrease in 3-O-methylglucose transport constants suggesting that hyperglycemia was responsible for the early decrease in facilitated transport found in the diabetic rats. It is speculated that increased passive transport of glucose may reflect an early loss in ocular barrier integrity. The later decrease in carrier facilitated transport cannot be explained by hyperglycemia alone and, thus, a loss in carrier function is suggested. Despite a decrease in facilitated transport, absolute glucose entry rates are increased in the diabetic due to elevated plasma glucose, which serves as an inward driving force, due to the significantly increased entry of glucose by the passive route.

The retinal pigment epithelium (RPE) interposed between the vascular system of the choroid and the neural retina performs a variety of functions essential for vision. In order to further elucidate the transport functions of the RPE, apical membranes were isolated from the RPE of the dogfish (Squalus acanthias) by differential precipitation with calcium. Na-K-ATPase, an apical marker enzyme in this tissue, was enriched 15-fold in the final membrane fraction. About 50% of the membranes form right-side-out vesicles in which the membrane has retained its in vivo orientation. Sodium uptake into these vesicles as determined by a rapid filtration method was stimulated 37% by the presence of a proton gradient across the membrane (pHi = 6.1, pHo = 8.1). The stimulation was also observed in membrane vesicles "short-circuited" with valinomycin and K. The pH gradient-dependent sodium uptake but not the uptake in the absence of a pH gradient was completely inhibited by 5 X 10(-4) M amiloride, and 56% inhibition was found at 10(-5) M amiloride. The uptake of 22Na was also strongly decreased in the presence of nonradioactively labelled sodium and lithium; potassium was without effect. pH gradient dependence, amiloride sensitivity, saturability and cation specificity of the sodium flux indicate the presence of a Na/H exchanger in the apical membrane of the retinal pigment epithelium. The presence of the Na/H exchange process might have important implications for the control of pH in the subretinal space, optimum intracellular pH of the RPE and the triggering of other functions of the RPE.

Sections of the ciliary epithelium of adult sharks (Squalus acanthias) were mounted in Ussing-type chambers (area 0.2 cm2) for measurements of transepithelial potential difference (PD), short circuit current (SCC) and calculation of transepithelial resistance (R). In 15 preparations PD was aqueous side negative (-0.51 +/- 0.12 mV; SCC 18.3 +/- 2.5 microA cm-2; R 30.7 +/- 3.1 Ohm cm2). However, in 15 other preparations incubated in identical Ringer's solution PD was aqueous side positive (0.53 +/- 0.09 mV; SCC -19.6 +/- 2.3 microA cm-2; R 27.9 +/- 2.8 Ohm cm2). 10(-5) M ouabain or 10(-4) M furosemide were applied either to the aqueous or blood side of the isolated ciliary epithelium at transepithelial negative or positive PD. When the transepithelial PD was positive on the aqueous side ouabain decreased PD and SCC within 15 to 45 min. When the spontaneous PD was negative both PD and SCC decreased when ouabain was applied to the blood side. When the drug was given to the aqueous side a biphasic response (first stimulation, then inhibition) of PD and SCC was observed. Furosemide when given to the blood side (with aqueous side PD positive) or to the aqueous side (with aqueous side PD negative) decreased PD and SCC. However, a transient stimulation of both electrical parameters was observed when furosemide was applied to either the blood side (with aqueous side PD negative) or to the aqueous side (with aqueous side PD positive). The polarity and magnitude of PD and SCC probably depend on the relative activity of sodium and chloride pumps across the cell membranes of the non-pigmented and/or pigmented cell layer. However, additional transport mechanisms cannot be excluded.

The present study has identified the metabolites of arachidonic acid (AA) formed by the isolated frog cornea and has shown that this tissue is capable of the biosynthesis of both lipoxygenase and cyclo-oxygenase pathway metabolites. The cyclo-oxygenase (CO) metabolites found in greatest abundance were the prostaglandins E2 and F2a (PGE2 and PGF2a). The major lipoxygenase (LO) pathway metabolite found by thin-layer chromatography (TLC) was leukotriene B4 (LTB4), whereas leukotriene C4 (LTC4) biosynthesis was detected by radioimmunoassay. These leukotrienes (LTs) are highly potent modulators of active chloride transport, since incubation with LTC4 (10(-7)-10(-9) M) resulted in a dose-dependent stimulation of both the Cl- originated short-circuit current (SCC) and potential difference (PD). In contrast, LTB4 (10(-7)-10(-9) M) inhibited both of these parameters. Both LTC4 and LTB4 exerted these effects only when applied to the endothelial side. Preincubation with the leukotriene receptor antagonist, FPL 55712 completely blocked the response to LTC4, indicating that the action of LTC4 in the cornea is receptor-mediated. In this report the authors show that LTB4 and LTC4 are formed by the cornea and that they are potent modulators of the SCC and PD in chamber experiments. The possibility exists that LTB4 and LTC4 may function as endogenous regulators of net Cl- transport in the cornea, since the addition of these metabolites resulted in a dose-dependent stimulation (with LTC4), and inhibition (with LTB4), of both the short-circuit current (SCC) and potential difference (PD).