Methyl palmitate is a potent vasodilator released in the retina and perivascular adipose tissue

• Main Authors: Yuan-Chieh Lee, 李原傑
• Other Authors: Tony J.-F. Lee
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/22272206532104824181

博士 === 慈濟大學 === 醫學科學研究所 === 100 === Vascular diseases, such as ischemic heart disease, stroke, hypertension, diabetes, etc., are no doubt one of the leading health issues in the whole world. While the regulation of vessels is fundamental to the pathophysiology of these vascular diseases, the discovery of vasodilators or vasoconstrictors is the key to understanding the regulation and also development of new drugs or treatments. By using a similar technique with rat superior cervical ganglion (SCG) as donor tissue and rabbit endothelium-denuded aortic ring as detector tissue, our group reported that a vasodilator, palmitic acid methyl ester (PAME), which is more potent than NO, is released in the SCG upon field electrical stimulation or addition of nicotine. The purpose of this study is to investigate further the importance and physiological role of PAME. Our study is composed of two parts: the first part is to examine if PAME is the retina-derived relaxing factor (RRF); the second part is to examine if PAME is the perivascular adipose tissue-derived relaxing factor (PVATRF) and its function in hypertension. Methyl palmitate is a potent vasodilator released in the retina The retina has been shown to spontaneously release a vasodilating substance. The identity of this retina-derived relaxing factor (RRF) is not known. Using a superfusion bioassay cascade technique with rat isolated retina as donor tissue and rat aortic ring as detector tissue, we determined if palmitic acid methyl ester (PAME) or methyl palmitate is the RRF. 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 almost 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 PAME were inhibited by 4-aminopyridine (4-AP, 2 mmol/L) and tetraethylammonium (TEA, 10 mmol/L), but was not affected by TEA at 1 mmol/L or 3 mmol/L, glibenclamide (3 μmol/L), or iberiotoxin (100 nmol/L). Following hexane extractions, RRF- and PAME-containing Krebs solutions caused no or reduced aortic relaxations, which were further blocked by 4-AP. RRF and PAME share similar biochemical properties and act in parallel to all pharmacological inhibitors examined. Both act primarily on the voltage-dependent K+ (Kv) channel on aortic smooth muscle cells, causing aortic relaxation. These results suggest that PAME is the hydrophobic RRF. Methyl palmitate is a potent vasodilator released in the perivascular adipose tissue Functional significance of perivascular adipose tissue-derived relaxing factor (PVATRF) in regulating vascular tone is recognized. Its chemical nature, however, remains unknown. This part of study aimed to determine if palmitic acid methyl ester (PAME) is the PVATRF. A superfusion bioassay cascade technique was used to estimate PVATRF release. PAME concentration was analyzed with gas chromatography/mass spectrometry. Aortic vasoconstriction was examined using tissue bath myography. Perivascular adipose tissue (PVAT) preparations spontaneously released PVATRF and PAME. The release was calcium-dependent. Both induced aortic relaxations which were inhibited by 4-aminopyridine (4-AP, 2 mmol/L) and tetraethylammonium (TEA) at 5 mmol/L and 10 mmol/L), but were not affected by TEA at 1 or 3 mmol/L, glibenclamide (3 μmol/L) or iberiotoxin (100 nmol/L). Aortic relaxations induced by PVATRF- and PAME-containing Krebs were not affected after heating at 70°C, but was equally attenuated following hexane extraction. The medium of differentiated adipocytes, but not that of fibroblasts, contained significant amount of PAME and caused vasorelaxation. PAME is the likely PVATRF. Both act on voltage-dependent K+ (Kv) channels of aortic smooth muscle cells, causing vasorelaxation. PAME plays a role in hypertension While PVATRF in regulation of vascular tone is gaining weight, its role in hypertension was investigated. Using a superfusion bioassay cascade technique with the PVAT of the spontaneously hypertensive rat (SHR) rat or the PVAT of the Wistar Kyoto (WKY) rat as donor tissue and Spraque-Dawley (SD) rat aortic ring as detector tissue, we determined if difference of release of PAME and PVATRF between PVAT of SHR and that of WKY. In the other way, the response of aortic ring form SHR or WKY to exogenous PAME was examined. In established hypertension, a diminished PAME release and with an increased release of an angiotensin II in the PVAT, and a decreased vasorelaxation response to PAME were noted in SHR. This suggests a new mechanism in pathogenesis of hypertension. Conclusions Methyl palmitate (or PAME) is a newly identified vasodilator substance. Except from superior cervical ganglion, it is also released in the retina and the PVAT, and is likely the RRF and PVATRF. A diminished PAME release and with an increased release of an angiotensin II in the PVAT, and a decreased vasorelaxation response to PAME make a new mechanism in pathogenesis of hypertension. We believe that PAME also plays an important role in the retina; while deficit of its release or response may lead to some vascular diseases of the retina.