Hypoxia‐induced relaxation of porcine retinal arterioles in vitro depends on inducible NO synthase and EP₄ receptor stimulation in the perivascular retina

Abstract

ABSTRACT.Purpose: Hypoxia-induced relaxation of porcine retinal arterioles has been shown to be reduced during inhibition of prostaglandin synthesis and nitric oxide synthase (NOS). The purpose of this study was to identity the specific prostaglandin receptor(s) and source(s) of NO mediating this effect. Methods: Porcine retinal arterioles with preserved perivascular retinal tissue were mounted in a myograph and were exposed to hypoxia in the presence of one of the following: the general NO synthase inhibitor L-NAME, the selective iNOS inhibitor 1400W, the selective nNOS inhibitor 7-nitroindazole, the general cyclooxygenase (COX) inhibitor ibuprofen or an antagonist to the FP-(AL 8810), DP-(BWA868C), EP 1 -(SC-19220), EP 2 -(PF-044189) or EP 4 receptors (GW627368X). The experiments were repeated after removal of the perivascular retinal tissue. Results: Hypoxia induced relaxation of retinal arterioles with preserved perivascular retinal tissue. This relaxation was significantly reduced in the presence of L-NAME, 1400W, ibuprofen and the EP 4 receptor antagonist GW627368X. The simultaneous addition of L-NAME or 1400W in combination with ibuprofen, but not GW627368X, reduced hypoxia-induced vasorelaxation additively as compared to the effect of the compounds individually. Conclusion: Hypoxia-induced vasorelaxation of porcine retinal arterioles is mediated by inducible NOS and stimulation of EP 4 receptors acting through separate pathways, but mechanisms unrelated to the studied prostaglandin receptors and NOS products are also involved.

Preserved Pressure Autoregulation but Disturbed Cyclo-Oxygenase and Nitric Oxide Effects on Retinal Arterioles during Acute Hypoxia in Diabetic Patients without Retinopathy

“…The present findings on humans indicate that flicker-induced vasodilatation during changes in NO synthesis is disturbed in diabetic patients [8]. A recent in vitro study has shown that hypoxia-induced vasodilation depends on inducible NO in the perivascular retina [25], and supports the existence of an interaction between these two signaling pathways in vivo. The diameter responses during simultaneous flicker stimulation and isometric exercise were similar to those observed during flicker stimulation alone, which confirmed that metabolic autoregulation can overrule pressure autoregulation in order to adapt the blood flow to changes in retinal metabolism [28,30].…”

“…It has been shown that hypoxia-induced vasodilatation is influenced by prostaglandins and NO in vitro [4,25]. Therefore, it was a notable finding that the pressure autoregulation during hypoxia in diabetic patients who had not received diclofenac was more reduced than the response induced by hypoxia in normal persons [11].…”

Purinergic Mechanisms and Prostaglandin E Receptors Involved in ATP-Induced Relaxation of Porcine Retinal Arterioles in vitro

“…A similar reduction of relaxation was observed in the presence of the EP 1 receptor antagonist SC 19220 which suggests that the effect had been mediated by prostaglandin E or other prostaglandins acting on this receptor in the vascular wall. Although a study has shown that prostaglandin E may elicit constriction of retinal arterioles [27], the present findings are consistent with several previous studies using the same model on both retinal and ciliary porcine arterioles [5,28,29]. Since ATP-γS-induced relaxation was unaffected by ibuprofen and EP 1 receptor antagonism, it is likely that the effect had been mediated by a degradation product from ATP.…”

“…Additionally, the presence of dual effects of purines on vascular tone with opposite responses at different concentrations and after different time lags [13,24] is supported by a previous study where adenosine was found to contract retinal arterioles at intermediate concentrations mediated by the A 1 and A 3 receptors and relaxation at high concentrations mediated by the A 2A and A 2B receptors [33]. The findings supplement previous studies, which showed that hypoxia-induced relaxation of porcine retinal arterioles and ciliary arteries is mediated by the EP 4 receptor [28,29] and thus adds to our understanding of the complexity of prostaglandin-induced tone regulation in retinal arterioles.…”

Prostaglandin induced changes in the tone of porcine retinal arterioles in vitro involve other factors than calcium activity in perivascular cells

“…However, the addition of Ca 2þ antagonists was unable to increase relaxation beyond the relaxing effect that could be obtained by PGE 2 alone which suggests that the relaxing effects of Ca 2þ antagonists and PGE 2 might be linked and had been saturated during the experiments. PGE 2 -induced relaxation of retinal porcine arteries has been shown to be mediated by EP 1 and EP 2 e receptors (Hansen et al, 2015). A possible link between the relaxing effect of PGE 2 and Ca 2þ antagonists might be that PGE 2 also blocks Ca 2þ channels via cAMP/PKA pathway downstream to the EP 2 receptor (Narayanan et al, 2012;Norel, 2007;Xiong and Sperelakis, 1995).…”