Constriction of retinal venules to endothelin-1: Obligatory roles of ET A receptors, extracellular calcium entry, and Rho kinase

Abstract

PURPOSE. Endothelin-1 (ET-1) is a potent vasoconstrictor peptide implicated in retinal venous pathologies such as diabetic retinopathy and retinal vein occlusion. However, underlying mechanisms contributing to venular constriction remain unknown. Thus, we examined the roles of ET-1 receptors, extracellular calcium (Ca 2+ ), L-type voltage-operated calcium channels (L-VOCCs), Rho kinase (ROCK), and protein kinase C (PKC) in ET-1-induced constriction of retinal venules. METHODS. Porcine retinal venules were isolated and pressurized for vasoreactivity study using videomicroscopic techniques. Protein and mRNA were analyzed using molecular tools. RESULTS. Retinal venules developed basal tone and constricted concentration-dependently to ET-1. The ET A receptor (ET A R) antagonist BQ123 abolished venular constriction to ET-1, but ET B receptor (ET B R) antagonist BQ788 had no effect on vasoconstriction. The ET B R agonist sarafotoxin S6c did not elicit vasomotor activity. In the absence of extracellular Ca 2+ , venules lost basal tone and ET-1–induced constriction was nearly abolished. Although L-VOCC inhibitor nifedipine also reduced basal tone and blocked vasoconstriction to L-VOCC activator Bay K8644, constriction of venules to ET-1 remained. The ROCK inhibitor H-1152 but not PKC inhibitor Gö 6983 prevented ET-1-induced vasoconstriction. Protein and mRNA expressions of ET A Rs and ET B Rs, along with ROCK1 and ROCK2 isoforms, were detected in retinal venules. CONCLUSIONS. Extracellular Ca 2+ entry via L-VOCCs is essential for developing and maintaining basal tone of porcine retinal venules. ET-1 causes significant constriction of retinal venules by activating ET A Rs and extracellular Ca 2+ entry independent of L-VOCCs. Activation of ROCK signaling, without involvement of PKC, appears to mediate venular constriction to ET-1 in the porcine retina.