Opposing interactions of ionophores (valinomycin and monensin) on calcium ion uptake in rat retinal preparations

Taurine: retinal function

The status and potential functions of taurine in the retina have been reviewed. Taurine is present in high concentrations in the retina of all species tested, while the retinal concentrations of the enzymes necessary to synthesize taurine are presumed to vary among those species. The documented low activity of cysteinesulfinic acid decarboxylase, a key enzyme in taurine biosynthesis, in the livers of the cat, monkey and human possibly reflect low activity in their retinas, indicating reliance on the diet as an important source of taurine. Both high- and low-affinity binding proteins and uptake systems have been described for taurine in retinal tissue. Evoked release of taurine by light and other depolarizing stimuli have been well documented. Retinal pathologies including diminished ERGs and morphologic changes have been reported for animals and man deficient in taurine. Possible functions for taurine in the retina include: (1) protection of the photoreceptor - based on the shielding effects of taurine on rod outer segments exposed to light and chemicals; (2), regulation of Ca2+ transport - based on the modulatory effects of taurine on Ca2+ fluxes in the presence and absence of ATP; and (3) regulation of signal transduction - based on the inhibitory effects of taurine on protein phosphorylation.

Taurine analogues as modifiers of the accumulation of 45calcium ions in a rat retinal membrane preparation

The effects of a series of taurine analogues on the accumulation of 45calcium ions in a crude rat retinal membrane preparation are reported. 45Calcium ion accumulation was measured at high (1.4 mM) calcium ion concentration in the presence and absence of ATP. In the absence of ATP, taurine and alpha-sulfo-beta-alanine both inhibit 45calcium ion accumulation. alpha-Sulfo-beta-alanine is the more potent of the two compounds (50% vs. 30% inhibition at 20 mM). The trans aminocycloalkanesulfonic acid analogues of taurine [(+/-) trans-2-aminocyclopentanesulfonic acid and (+/-) trans-2-aminocyclohexanesulfonic acid] are stimulators (3- to 4-fold) of 45calcium ion accumulation. In the presence of ATP (1.2 mM), taurine and the analogues of taurine had no effect. Structure-activity-relationships of these compounds pertinent to their effects on 45calcium ion accumulation at a high calcium ion concentration and in the absence of ATP are discussed. In addition, the inhibitory effects of taurine on 45calcium accumulation in subfractions of the retina (rod outer segments, synaptosomes, and mitochondria) and the effects of the divalent ionophore A23187 on 45calcium accumulation were also investigated.