Purified NOS from different sources has been reported to have a low half-saturating
l-arginine concentration (EC
50, 10 μM). Since high levels of intracellular
l-arginine ranging from 0.1 1 mM have been measured in many systems,
36 it is expected that endogenous
l-arginine would support the maximum activation of NOS. However, a number of in vivo and in vitro studies indicate that NO production under physiological conditions can be increased by extracellular
l-arginine, despite saturating intracellular
l-arginine concentrations. This has been termed “the arginine paradox.”
37 One explanation could be that intracellular
l-arginine is sequestered in one or more pools that are poorly, if at all, accessible to NOS, whereas extracellular
l-arginine transported into the cells is preferentially delivered to NO biosynthesis.
37 Accordingly, it has been demonstrated that
l-arginine availability controls NMDA-induced NO synthesis in the rat central nervous system.
38 Therefore, it seems likely that to induce the activation of NOS, an obligatory influx of
l-arginine is required. The coordination between NOS activity and
l-arginine uptake has been demonstrated in several systems such as rat brain,
39 and diabetic rat retina.
40 A similar coordination between NO biosynthesis and intracellular
l-arginine availability seems to occur in hypertensive eyes. Recently, it has been demonstrated that activation of NMDA receptors in cultured retinal cells promotes an increase of the intracellular
l-arginine pool available for NO synthesis.
41 This way, the increase in both NOS activity and
l-arginine influx could be triggered by higher levels of synaptic glutamate levels in retinas from eyes injected with HA.