The trafficking of retinoids in the retina represents a model to study soluble hormone-binding proteins in a complex system subject to profound evolutionary adaptations. Although a remarkable illustration of convergent evolution, all visual systems detect light in the same way, that is through the photoisomerization of an 11-cis retinoid to a corresponding trans isomer. What is strikingly different between the systems, is the mechanism by which the 11-cis chromophore is reformed and visual pigment regenerated in a process known as the visual cycle. The variations of the cycle address a problem inherent to retinoids themselves. That is, the properties that make these molecules suited for light detection also account for their susceptibility to oxidative and isomeric degradation, and cellular toxicity. The cycle therefore provides an opportunity to examine the role of soluble hormone-binding proteins within an integrative and evolutionary context. The present review focuses on interphotoreceptor retinoid-binding protein (IRBP), a controversial glycolipoprotein that recruits a protein fold common to Cterminal-processing proteases and the crotonase family. This unorthodox retinoid-binding protein is entrapped in the subretinal compartment of those eyes that translocate visual cycle retinoids between the photoreceptors and the retinal pigment epithelium. Recent studies suggest that we should look beyond a strictly carrier function if we are to appreciate the role of IRBP in the visual cycle. Here we draw lessons from other soluble hormone-binding proteins to anticipate avenues of future research likely to provide insight into the structure and function of IRBP in vision.

The mechanism for the development of insulin resistance in normal pregnancy is complex and is associated with serum levels of both progesterone and 17beta-estradiol. However, it remains unclear whether estrogens alone or progestins alone can cause insulin resistance, or whether it is a combination of both which produces this effect. We attempted to determine the role played by progesterone and/or 17beta-estradiol on the phenomena of sensitivity to insulin action that take place during pregnancy in the rat. Ovariectomized rats were treated with different doses of progesterone and/or 17beta-estradiol in order to simulate the plasma levels in normal pregnant rats. A euglycemic/hyperinsulinemic clamp was used to measure insulin sensitivity. At days 6 and 11, vehicle (V)- and progesterone (P)-treated groups were more insulin resistant than 17beta-estradiol (E)- and 17beta-estradiol+progesterone (EP)-treated groups. Nevertheless, at day 16, the V, EP and E groups were more resistant to insulin action than the P group. On the other hand, the V, EP and E groups were more insulin resistant at day 16 than at day 6, whereas the P group was more insulin resistant at day 6 than at day 16. Our results seem to suggest that the absence of female steroid hormones gives rise to a decreased insulin sensitivity. The rise in insulin sensitivity during early pregnancy, when the plasma concentrations of 17beta-estradiol and progesterone are low, could be due to 17beta-estradiol. However, during late pregnancy when the plasma concentrations of 17beta-estradiol and progesterone are high, the role of 17beta-estradiol could be to antagonize the effect of progesterone, diminishing insulin sensitivity.