Angiotensin, vasopressin, and atrial natriuretic peptide in the rat eye

Regulation of the renin expression in the retinal pigment epithelium by systemic stimuli

The retina expresses a local renin-angiotensin system (RAS). This study aimed to investigate the influence of systemic modulation of renin synthesis on the expression of renin in the retinal pigment epithelium (RPE), which forms part of the blood/retina barrier. Freshly isolated RPE cells showed expression of renin 1A, which is the secreted isoform of renin. Systemic administration of the angiotensin-converting enzyme inhibitor enalapril in mice increased the renin expression in both the kidney and the retina. Systemic infusion of ANG II led to a decrease in the renin expression in the kidney and in the retina and RPE. The ANG II-dependent down-regulation of renin expression in the RPE was prevented by systemic application of the AT(1) receptor blocker losartan. However, water deprivation lead to an increase of the renin expression in the kidney but unexpectedly to a decrease of the renin expression in the retina. In sections of the mouse retina, the ANG II receptor AT(1) was found in the RPE and localized at the blood side of the epithelium. Short-time cultured RPE cells showed increases in intracellular free Ca(2+) in response to stimulation by ANG II that were sensitive to losartan. In summary, we conclude that the renin expression in cells of the blood/retina barrier is influenced by the systemic RAS. ANG II circulating in the plasma is likely a mediator of this influence.

Modulation by brain natriuretic peptide of GABA receptors on rat retinal ON-type bipolar cells

Natriuretic peptides (NPs) may work as neuromodulators through their associated receptors [NP receptors (NPRs)]. By immunocytochemistry, we showed that NPR-A and NPR-B were expressed abundantly on both ON-type and OFF-type bipolar cells (BCs) in rat retina, including the dendrites, somata, and axon terminals. Whole-cell recordings made from isolated ON-type BCs further showed that brain natriuretic peptide (BNP) suppressed GABAA receptor-, but not GABAC receptor-, mediated currents of the BCs, which was blocked by the NPR-A antagonist anantin. The NPR-C agonist c-ANF [des(Gln18, Ser19, Gln20, Leu21, Gly22)ANF(4-23)-NH2] did not suppress GABAA currents. The BNP effect on GABAA currents was abolished with preincubation with the pGC-A/B antagonist HS-142-1 but mimicked by application of 8-bromoguanosine-3',5'-cyclomonophosphate. These results suggest that elevated levels of intracellular cGMP caused by activation of NPR-A may mediate the BNP effect. Internal infusion of the cGMP-dependent protein kinase G (PKG) inhibitor KT5823 essentially blocked the BNP-induced reduction of GABAA currents. Moreover, calcium imaging showed that BNP caused a significant elevation of intracellular calcium that could be caused by increased calcium release from intracellular stores by PKG. The BNP effect was blocked by the ryanodine receptor modulators caffeine, ryanodine, and ruthenium red but not by the IP3 receptor antagonists heparin and xestospongin-C. Furthermore, the BNP effect was abolished after application of the blocker of endoplasmic reticulum Ca2+-ATPase thapsigargin and greatly reduced by the calmodulin inhibitors W-7 and calmidazolium. We therefore conclude that the increased calcium release from ryanodine-sensitive calcium stores by BNP may be responsible for the BNP-caused GABAA response suppression in ON-type BCs through stimulating calmodulin.