Stimulation of prostanoid IP and EP₂ receptors dilates retinal arterioles and increases retinal and choroidal blood flow in rats

Abstract

We examined the effects of vasodilatory prostaglandins (prostacyclin and prostaglandin E₂) and selective agonists for prostanoid EP₂ and EP₄ receptor on the diameters of retinal blood vessels and fundus (retinal/choroidal) blood flow in rats. Male Wistar rats (8- to 10-week-old) were treated with tetrodotoxin (50 μg/kg, i.v.) to eliminate any nerve activity and prevent movement of the eye and infused with a mixture solution of norepinephrine and epinephrine (1:9) to maintain adequate systemic circulation under artificial ventilation. Fundus images were captured with a digital camera that was equipped with the special objective lens for small animals, and the diameters of retinal arterioles and venules were measured on a personal computer. Fundus blood flow was estimated using a laser Doppler flowmetry. Intravenous infusions of prostacyclin and prostaglandin E₂ dilated retinal blood vessels, increased fundus blood flow and decreased systemic blood pressure in a dose-dependent manner. The effects of vasodilatory prostaglandins on retinal arterioles were greater than those on retinal venules. Similarly, a prostanoid EP₂ receptor agonist (ONO-AE1-259-01) dilated retinal blood vessels, and increased fundus blood flow and decreased systemic blood pressure. However, a prostanoid EP₄ receptor agonist (ONO-AE1-329) failed to increase fundus blood flow, despite its comparable depressor response with those to vasodilatory prostaglandins and the prostanoid EP₂ receptor agonist. The responses to forskolin, an activator of adenylyl cyclase, were very similar to those to prostacyclin and the prostanoid EP₂ receptor agonist.

These results suggest that prostacyclin and prostaglandin E₂ act as vasodilators in retinal and choroidal circulation, and prostanoid IP and EP₂ receptors play an important role in the regulation of ocular hemodynamics in rats.

Introduction

Prostaglandins play an important role in the regulation of ocular hemodynamics (Chemtob et al., 1991, Delaey and Van de Voorde, 1998, Delaey and Van De Voorde, 2000, Hardy et al., 1994, Hardy et al., 2005). Among prostaglandins, prostacyclin and prostaglandin E₂ generally relax the isolated blood vessel preparations and act as vasodilators in the vasculatures of many organs (Breyer et al., 2001, Feng et al., 1988, Kaley et al., 1985, Narumiya et al., 1999, Parkington et al., 2004). In the ocular vasculature, prostacyclin consistently produced the vasodilator responses (Abran et al., 1994, Abran et al., 1997b, Beausang-Linder, 1982, Flower et al., 1984, Hata et al., 2000, Nielsen and Nyborg, 1990, Wizemann et al., 1982), whereas the effects of prostaglandin E₂ have been inconsistent across studies (Abran et al., 1994, Abran et al., 1997a, Chemtob et al., 1990, Kosaka, 1995, Nielsen and Nyborg, 1990, Starr, 1971). The inconsistency might be due to species variations and different experimental preparations.

Recently, we found that prostacyclin and prostaglandin E₂ increased fundus blood flow in rats (Mori et al., 2007). The actions of prostacyclin and prostaglandin E₂ are mediated by G protein-coupled prostanoid receptors, IP and EP₁–EP₄ receptors (Coleman et al., 1994, Narumiya et al., 1999, Sugimoto and Narumiya, 2007). The prostanoid IP, EP₂ and EP₄ receptors are coupled to Gs proteins and activate adenylyl cyclase with subsequent increased formation of cAMP. On the other hand, prostanoid EP₁ and EP₃ receptors are coupled to Ca²⁺ mobilization and the inhibition of cAMP formation via Gq/Gi proteins (Coleman et al., 1994, Narumiya et al., 1999, Sugimoto and Narumiya, 2007). Therefore, our previous results suggested that stimulation of prostanoid IP receptor and EP₂ and/or EP₄ receptor on retinal and choroidal vasculature increases fundus blood flow. However, it remains to be determined effects of vasodilatory prostaglandins on rat retinal blood vessels and which prostanoid EP receptor subtypes play an important role in the enhancement of fundus blood flow.

The purpose of this study, therefore, was to determine the effects of prostacyclin and prostaglandin E₂, ONO-AE1-259-01, a prostanoid EP₂ receptor agonist, and ONO-AE1-329, a prostanoid EP₄ receptor agonist (Sugimoto and Narumiya, 2007; Suzawa et al., 2000), on the diameters of retinal blood vessels and fundus blood flow. We also examined the vasodilator effects of the activator of adenylyl cyclase forskolin because vasodilatory prostaglandins could relax vascular smooth muscle through elevation of intracellular cAMP (Coleman et al., 1994, Narumiya et al., 1999). The fundus images were captured with a digital camera that was equipped with the special objective lens and the diameters of retinal blood vessels were measured. Fundus blood flow was measured using a laser Doppler flowmetry.