Retina derived relaxation is mediated by Kir channels and the inhibition of Ca2+ sensitization in isolated bovine retinal arteries

- Biphasic relaxations to retina in Bay k 8644 and PGF2α contracted retinal arteries.
- Inhibition of Ca2+ influx, Ca2+ storage or removal of Ca2+ unchanged the relaxation.
- Retina derived relaxation is abolished in the presence of an inhibitor of MLCP.
- Kir channel blocker reduced retinal relaxation and removed its biphasic character.
- Rho kinase inhibitor, Y-27632 exerts comparable and additive effects with retina.

Retinal relaxing factor (RRF) has recently been identified as a novel paracrine regulator of retinal circulation acting differently from well known mediators of the endothelium and the retina. Herein, we aimed to characterize the relaxing mechanism of the retina, i.e. RRF, by evaluating the role of Ca+2-dependent and -independent signaling mechanisms as well as inward rectifier K+ (Kir) channels. Retinal relaxation was determined by placing a piece of retinal tissue just on top of the precontracted bovine retinal arteries mounted in a wire myograph. The retina produced a complete relaxation response, which display a biphasic character, in depolarized arteries contracted by L-type Ca2+ channel agonist, Bay k 8644. Blockade of L-type Ca2+ channel by nifedipine, inhibition of sarcoplasmic reticulum Ca2+-ATPase by cyclopiazonic acid or removal of extracellular Ca2+ did not influence the prominent relaxation to the retina. Originally, retinal relaxation was found to be unaffected from the inhibition of myosin light chain kinase by ML7, whereas, completely abolished in the presence of myosin light chain phosphatase (MLCP) inhibitor, Calyculin A. Moreover, the inhibition of Rho kinase by its putative inhibitor, Y-27632 displayed comparable relaxant effects to RRF in retinal arteries precontracted either by prostaglandin F2α or K+, and augmented the moderate response to the retina in K+ precontracted arteries. In addition, retinal relaxation was significantly inhibited and lost its biphasic character in the presence of Kir channel blocker, Ba2+. Our results suggested that inhibition of Ca2+ sensitization through the activation of MLCP, possibly via interfering with Rho kinase, and the opening of Kir channels are likely to be involved in the inhibitory influence of RRF on the retinal arteries.