Citations

Dietary supplementation with cod-liver oil significantly augments endothelium-dependent relaxations in porcine coronary arteries. The present study was designed to examine the effect of dietary administration of omega 3 polyunsaturated fatty acids (mainly eicosapentaenoic acid, the major component of fish oil) on endothelium-dependent relaxations in porcine coronary arteries. Male Yorkshire pigs were maintained 4 wk on a regular diet with or without supplementation with purified eicosapentaenoic acid (3.5 g/day) and docosahexaenoic acid (1.5 g/day). Endothelium-dependent relaxations were examined in vitro. In rings from the treated group, endothelium-dependent relaxations were augmented in response to bradykinin, serotonin, and ADP, but not to the calcium ionophore A23187. These augmentations were not altered by indomethacin but were significantly inhibited by methylene blue, an inhibitor of guanylate cyclase. In the treated group, endothelium-dependent relaxations to aggregating platelets also were significantly augmented; platelet-induced contractions of quiescent rings were inhibited more by the presence of the endothelium than in arteries from the control group. Bioassay experiments demonstrated that the release of endothelium-derived relaxing factor(s) by bradykinin and relaxations of the vascular smooth muscle to the factor(s) were greater in arteries from the treated group. These observations indicate that dietary omega 3 polyunsaturated fatty acids augment receptor-operated endothelium-dependent relaxations, partly due to the augmented release of endothelium-derived relaxing factor(s) and partly due to the augmented relaxation of the vascular smooth muscle to the factor(s).

Experiments were designed to analyze the effects of ouabain on the response of vascular smooth muscle to endothelium-derived relaxing factors released under basal conditions and on stimulation with acetylcholine or bradykinin. Bioassay rings of canine coronary artery (without endothelium) were superfused with perfusate from canine left circumflex coronary arteries with endothelium (donor arteries). During contractions of the bioassay ring evoked by prostaglandin F2 alpha, the relaxations caused by endothelium-derived relaxing factor(s), released under basal conditions or on exposure of the endothelial cells of the donor artery to maximally effective concentrations of acetylcholine, were reduced by incubation of the bioassay ring with ouabain. However, the relaxations evoked by infusion of bradykinin were not altered by incubation of the bioassay rings with ouabain. These experiments demonstrate the release of two endothelium-derived relaxing factors that can be distinguished using ouabain.

Chronic increases in blood flow caused by an arteriovenous fistula augment endothelium-dependent relaxations to acetylcholine. To determine whether endothelial muscarinic receptors are altered, concentration-response curves to acetylcholine were obtained in the presence of pirenzepine in fistula- and sham-operated canine femoral arteries. Pirenzepine inhibited the response to acetylcholine in both arteries. The pA2 (log Kb) for the antagonist was the same. A bioassay system was used to assess release of endothelium-derived relaxing factor. Rings of femoral artery (without endothelium) from unoperated dogs relaxed more when superfused with perfusate derived from endothelium of fistula-operated arteries during acetylcholine stimulation. Rings without endothelium of sham- and fistula-operated arteries relaxed to the same extent when superfused with perfusate derived from the endothelium of unoperated femoral arteries. These results suggest that augmented relaxations to acetylcholine in canine arteries where blood flow is chronically elevated do not result from changes in the subtype of endothelial muscarinic receptors or in the sensitivity of the underlying smooth muscle to endothelium-derived relaxing factor(s). They are likely due to increased release of endothelium-derived relaxing factor(s) on muscarinic activation.

Experiments were designed to determine the role of products of cyclooxygenase in contractions of coronary smooth muscle evoked by serotonin. Rings of canine coronary artery without endothelium were suspended in organ chambers filled with modified Krebs-Ringer bicarbonate solution. Serotonin caused concentration-dependent contractions followed by secondary relaxations at higher doses. Indomethacin and meclofenamate augmented both the contraction and the relaxation. Indomethacin did not affect contractions evoked by increasing concentrations of either phenylephrine, prostaglandin F2 alpha, or potassium chloride. Propranolol did not affect the concentration-response curve to serotonin under control conditions; it prevented the facilitated contraction to the monoamine but not the augmented secondary relaxation caused by the inhibitors of cyclooxygenase. These results suggest that endogenous prostanoids simultaneously inhibit the contractile process and brake relaxations induced by higher concentrations of serotonin. As a consequence, inhibitors of prostanoid formation facilitate the vasospastic component of the response to the monoamine in large coronary arteries. For unknown reasons, propranolol prevents this facilitation.

Experiments were designed to analyze the effects of endothelium-derived relaxing factor(s) (EDRF; released basally or on stimulation with acetylcholine) and nitric oxide (NO) on smooth muscle of coronary arteries of different diameter. During contractions of the bioassay ring evoked with prostaglandin F2 alpha, the relaxations caused by basal EDRF were greater in the distal than in the proximal coronary arteries, whereas there was no difference in response to the EDRF released by acetylcholine. During direct superfusion, NO caused similar relaxations in proximal and distal coronary artery rings. Optimal tension, prostaglandin F2 alpha-induced contractions, and relaxations caused by sodium nitroprusside were comparable in both preparations. In rings of proximal and distal coronary artery studied in organ chambers, acetylcholine caused comparable endothelium-dependent, whereas sodium nitroprusside and NO cause comparable endothelium-independent relaxations. These experiments indicate a difference in response of different-sized coronary arteries to basally released EDRF and suggest that the basally released factor differs from NO.

The aim of the present study was to investigate the influence of inhibitors of endothelium-dependent relaxing factor on the release of atrial natriuretic factor from isolated rat atria. Electrical stimulation of the tissue produced a frequency-dependent increase in secretion of atriopeptide from basal levels. Saponin (0.3 mg/ml for 45 min) augmented the basal release of the peptide. Methylene blue (10(-5)M), oxyhemoglobin (10(-6)M), or hydroquinone (10(-5)M), agents that inhibit the effects of endothelium-derived relaxing factor by different mechanisms, caused an increase in the basal secretion of atrial natriuretic factor. Blockade of cyclooxygenase with indomethacin (10(-5)M) did not alter the release of atriopeptide, suggesting that it is not controlled by endogenous prostaglandins. These results would be explained if the inhibitors tested were to interfere with a factor released continuously from either endocardial cells or endothelial cells of the atrial blood vessels and having the same nature as endothelium-derived relaxing factor. Such release may exert a tonic negative modulatory influence on the secretion of atrial natriuretic factor.

Nitric oxide may be an endothelium-derived relaxing factor in systemic arteries and pulmonary veins. The endothelium-derived relaxing factor of systemic veins has not been characterized. Experiments were designed to determine whether the endothelium-derived relaxing factor of systemic veins shared chemical properties and mechanisms of action with nitric oxide. Rings of the canine femoral vein with and without endothelium were suspended in organ chambers for the measurement of isometric force. In rings without endothelium, relaxations to nitric oxide were augmented by superoxide dismutase plus catalase and were inhibited by hemoglobin, methylene blue, and LY 83583. The endothelium-dependent relaxations to acetylcholine and A23187 were not augmented by superoxide dismutase plus catalase but were inhibited by hemoglobin and only moderately reduced by either methylene blue or LY 83583. Relaxations to sodium nitroprusside were not inhibited by methylene blue and LY 83583. Relaxations to sodium nitroprusside were inhibited by ouabain and K+-free solution; those to nitric oxide were not. These results indicate that although the endothelium-derived relaxing factor released from canine systemic veins shares some chemical properties with nitric oxide, the mechanism by which relaxations are induced by the two differ. A factor dissimilar to nitric oxide but acting like sodium nitroprusside may be released by the endothelium of canine systemic veins.

The calcium ionophore A23187 causes endothelium-dependent contractions in canine basilar arteries. Removal of the endothelium, or treatment with indomethacin or superoxide dismutase (SOD), prevented the endothelium-dependent excitatory effect of the calcium ionophore. Catalase and deferoxamine were without effect. Superoxide anion generated by xanthine plus xanthine oxidase in the presence of catalase caused contractions of the vascular smooth muscle, which were abolished by SOD or heat inactivation of xanthine oxidase. The A23187-induced production of prostaglandins F2 alpha and E2 and thromboxane B2 was abolished by the removal of endothelium and by treatment with indomethacin but was not affected by the presence of SOD plus catalase. These observations are consistent with the hypothesis that superoxide anion, rather than prostaglandins generated by hydroperoxidase activity of cyclooxygenase, is an endothelium-derived contracting factor in canine cerebral arteries.

In canine systemic veins, in contrast to what is observed in mammalian systemic arteries, endothelium-dependent relaxations to the calcium ionophore A23187 are not diminished by the inhibitor of soluble guanylate cyclase, methylene blue. Therefore, experiments were designed to determine whether these relaxations in the veins are associated with the accumulation of guanosine 3',5'-cyclic monophosphate (cGMP). Rings of canine femoral arteries and veins with and without endothelium were suspended for the measurement of isometric force in organ chambers; cGMP was measured by radioimmunoassay. In arteries and veins contracted with norepinephrine, the tissue content of cGMP was greater in rings with than without endothelium. This difference was decreased by methylene blue (10(-5) M). A23187 (3 X 10(-7) M, for 1 min) increased the accumulation of cGMP, which was temporally related with the onset of relaxation in tissues with endothelium. Methylene blue inhibited the accumulation of cGMP in both blood vessels but inhibited the relaxations only in the arteries. In rings without endothelium, sodium nitroprusside (3 X 10(-7) and 10(-5) M) initiated increases in cGMP, which followed the onset of relaxation. Neither response to sodium nitroprusside was reduced by methylene blue. These results suggest that in canine femoral arteries and veins, relaxation of the smooth muscle to sodium nitroprusside are mediated by a mechanism distinct from changes in cGMP. Likewise, in canine systemic veins, endothelium-derived factor(s) released in response to A23187 also can initiate relaxation of the smooth muscle by a mechanism distinct from changes in cGMP.