Methyl palmitate: a potent vasodilator released in the retina.
OBJECTIVE
To determine whether 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 pharmacologic characteristics of RRF and PAME were compared.


RESULTS
The authors demonstrated that the retina on 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 because 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 degrees C for 1 hour, suggesting that both are heat stable. Exogenous PAME concentration dependently induced aortic relaxation with EC50 of 0.82+/-0.75 pM. The aortic relaxations induced by RRF and exogenous PAME were inhibited by 4-aminopyridine (2 mM) and tetraethylammonium (TEA, 10 mM) but were not affected by TEA at 1 mM or 3 mM, glibenclamide (3 microM), or iberiotoxin (100 nM). The vasodilator activity of Krebs solution containing RRF or exogenous PAME was greatly attenuated after hexane extraction.


CONCLUSIONS
RRF and PAME share similar biochemical properties and react similarly to all pharmacologic 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.
Lee
Yuan-Chieh
YC
Department of Ophthalmology, Buddhist Tzu Chi General Hospital, and Department of Medicine, Graduate Institute of Medical Sciences, Center for Vascular Medicine, College of Life Sciences, Tzu Chi University, Hualien, Taiwan.
Chang
Hsi-Hsien
HH
Liu
Chin-Hung
CH
Chen
Mei-Fang
MF
Chen
Po-Yi
PY
Kuo
Jon-Son
JS
Lee
Tony J-F
TJ
D015761:4-Aminopyridine / Q000494:pharmacology; D000818:Animals; D001011:Aorta / Q000502:physiology*; D001681:Biological Assay; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006586:Hexanes; D007552:Isotonic Solutions / Q000378:metabolism / Q000494:pharmacology; D008297:Male; D008825:Miconazole / Q000494:pharmacology; D019335:Nitroarginine / Q000494:pharmacology; D010168:Palmitates / Q000302:isolation & purification / Q000378:metabolism*; D010455:Peptides / Q000494:pharmacology; D026902:Potassium Channel Blockers / Q000494:pharmacology; D011335:Proadifen / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012160:Retina / Q000378:metabolism*; D012171:Retinal Vessels / Q000502:physiology*; D012997:Solvents; D019789:Tetraethylammonium / Q000494:pharmacology; D014664:Vasodilation / Q000187:drug effects / Q000502:physiology*
D004791:Enzyme Inhibitors; D006586:Hexanes; D007552:Isotonic Solutions; C051865:Krebs-Ringer solution; D010168:Palmitates; D010455:Peptides; D026902:Potassium Channel Blockers; D012997:Solvents; D019335:Nitroarginine; D019789:Tetraethylammonium; C064719:iberiotoxin; D008825:Miconazole; D011335:Proadifen; D015761:4-Aminopyridine; C019012:methyl palmitate; D002118:Calcium