Methyl Palmitate: A Potent Vasodilator Released in the Retina

Abstract

Purpose: : To determine if palmitic acid methyl ester (PAME) or methyl palmitate is the retina-derived relaxing factor (RRF).

Methods: : A superfusion bioassay cascade technique was used with rat isolated retina as donor tissue and rat aortic ring as detector tissue. The superfusate was analyzed with gas chromatography/mass spectrometry (GC/MS). The biochemical and pharmacological characteristics of RRF and PAME were compared.

Results: : We demonstrated that the retina upon superfusion with Krebs’ solution spontaneously released RRF (indicated by aortic ring relaxation) and PAME (measured by GC/MS). The release of RRF and PAME was calcium-dependent, since the release was abolished when the retinas were superfused with calcium-free Krebs’ solution. Furthermore, aortic relaxations induced by RRF and PAME were not affected after heating their solutions at 70°C for 1 hr, suggesting that both are heat stable. Exogenous PAME concentration-dependently induced aortic relaxation with EC₅₀ of 0.82±0.75 pmol/L. The aortic relaxations induced by RRF and exogenous PAME were inhibited by 4-aminopyridine (4-AP, 2 mmol/L) and tetraethylammonium (TEA, 10 mmol/L), but were not affected by TEA at 1 mmol/L or 3 mmol/L, glibenclamide (3 µmol/L), or iberiotoxin (100 nmol/L). The vasodilator activity of Krebs’ solution containing RRF or exogenous PAME was greatly attenuated following hexane extraction.

Conclusions: : RRF and PAME share similar biochemical properties and react similarly to all pharmacological inhibitors examined. Both act primarily on the voltage-dependent K⁺ (Kv) channel of aortic smooth muscle cells, causing aortic relaxation. These results suggest that PAME is the hydrophobic RRF.

Keywords: retina • ion channels • blood supply