Gene expression studies have identified that a tissue-based RAS exists
in the adult eye. Renin, angiotensinogen, and ACE mRNA have been
detected in retinal pigment epithelium/choroid and whole neural retina
samples from human eyes.
8 In situ hybridization and
immunohistochemical studies have identified the retinal cell types in
which RAS components are located. In the inner retina of the adult rat,
mouse, rabbits, and humans, the RAS is found in blood vessels, neurons,
and glia.
6 7 9 10 In the present study, RAS components
were located in similar sites from as early as P1 and persisted through
retinal development and into adulthood. The finding of RAS components
in blood vessels of the immature rat retina and Ang II and
AT
1 and AT
2 receptor
protein in hyaloid vessels (a transiently existing network of
capillaries that nourish the developing lens and retina) suggests a
role for the RAS in vascularization of the developing retina. With
regard to ACE, previous studies have reported immunoreactivity in
retinal vessels and in blood vessels in other organs.
46 In
the present study, autoradiographic analysis showed retinal ACE to
increase with retinal maturation, with highest levels in the adult at
P90. In general, Ren-2 rats had slightly lower levels of retinal ACE
than did SD rats. This was most notable at P7 and P90. The reason for
this strain difference is probably the overexpression of the retinal
RAS in Ren-2 rats, which has the effect of suppressing tissue ACE
levels. Our previous studies support this finding, as adult Ren-2 rats
have reduced levels of plasma ACE compared with SD rats.
47 It should be noted that in several species, ACE increases over the
perinatal period in lung, kidneys, heart, and
aorta.
46 48 49 The reason for the increase in ACE with
tissue development is not precisely known, but could relate to the
extent of vascularization. For instance, it has been suggested that the
postnatal increase in pulmonary ACE expression in lambs may partly
reflect the increase in capillary surface area associated with
growth.
50