Effects of dietary saturated, N-6 and N-3 polyunsaturated fats on blood pressure and prostaglandins outflow from perfused mesenteric vascular bed in rats

Impaired nitric oxide- and prostaglandin-mediated responses to flow in resistance arteries of hypertensive rats

In human and experimental hypertension, flow (shear stress)-induced dilation in large arteries is attenuated and resistant to nitric oxide blockade. We tested the hypothesis that a defect in nitric oxide-and/or prostaglandin-dependent flow-induced dilation might occur in mesenteric resistance arteries from spontaneously hypertensive rats (SHR). We measured resistance mesenteric artery diameter in situ by intravital microscopy and simultaneously measured mesenteric arterial pressure in a collateral artery. The flow-diameter-pressure relationship was established in normotensive Wistar-Kyoto rats (WKY) and in SHR under control conditions and after endothelium removal, inhibition of nitric oxide synthesis with N omega-nitro-L-arginine methyl ester (10 micromol/L), or inhibition of prostaglandin synthesis with indomethacin (10 micromol/L). Production of prostaglandins was determined in the perfusate. Endothelium removal decreased artery diameter by 14 +/- 1.6% in WKY and 5 +/- 0.5% (P<.01 versus WKY) in SHR at a flow rate of 400 microL/min. In WKY, N omega-nitro-L-arginine methyl ester and indomethacin decreased resistance artery diameter by 12 +/- 3% (P<.001) and 5 +/- 2% (P<.01), respectively, at a flow rate of 400 microL/min; neither substance had any significant effect in SHR. In both strains, flow induced the production of 6-keto-prostaglandin F1alpha, the metabolite of prostacyclin; prostaglandin F2alpha; and thromboxane B2, the stable metabolite of thromboxane A2. Production of 6-keto-prostaglandin F1alpha and prostaglandin F2alpha was significantly lower in SHR than WKY, and TxB2 production was significantly higher in SHR than WKY. The present findings suggest that in SHR mesenteric resistance arteries, dilation in response to increases in flow was resistant to nitric oxide and prostaglandin synthesis blockade. A modification of the ratio of vasodilator to vasoconstrictor prostaglandins might be at least partly responsible for the decreased dilator response to flow in SHR.

Effects of long-term treatment with angiotensin-converting enzyme inhibitor on angiotensin II and prostacyclin release from mesenteric arteries in spontaneously hypertensive rats

To evaluate the long-term effects of an angiotensin-converting enzyme inhibitor (ACEI) on vascular angiotensin II (AII), eicosanoid production and vascular reactivities, we treated spontaneously hypertensive rats (SHR) with alacepril for 6 weeks and perfused the isolated mesenteric arterial bed which contains resistance vessels. Alacepril significantly lowered the arterial blood pressure. Changes in perfusion pressure in response to norepinephrine (NE) were attenuated, and isoproterenol-stimulated AII release from the perfused mesenteric arterial beds was inhibited in the alacepril-treated SHR. Vasodilation induced by acetylcholine (Ach) and prostacyclin (PGI2) release was significantly increased in the vasculature of the alacepril-treated SHR. Alacepril exerted no effect on cyclic GMP (cGMP) formation, but increased cAMP formation in the vasculature. These findings suggest that ACEI inhibits AII formation and facilitates PGI2 production in the resistance vessels, which leads to blunting of the pressor response to NE and improvement of endothelial function in SHR. These humoral and mechanical changes in the vasculature may contribute to the depressor and organ protective effects of ACEI.

Effect of diet on hyperthermia-induced cell lethality and prostaglandin release

Hyperthermia-induced cell lethality is thought to be mediated through injury to the cell membrane. Membrane perturbation results in the release of prostaglandins (PG) and leukotrienes (LT). These compounds are potent biological mediators and may modify the tumor microenvironment and therapeutic efficacy. Membrane composition and PG/LT release are influenced by the dietary fatty acids. The relationship between these variables and response to hyperthermia was examined in vitro using murine P388 leukemia cells grown as an ascites in mice provided either saturated fatty acid diet (SFA; 16% beef tallow) or unsaturated fatty acid diet (UFA; 16% safflower oil). Cells were harvested and exposed in vitro to either 37 degrees C or 43.5 degrees C for periods up to 2 hours. Hyperthermic exposure for 2 hours resulted in 40% cell lethality in SFA cells and 55% in UFA cells. The phospholipid and total cholesterol content was higher (33% and 50% respectively) in the UFA versus the SFA cells. Hyperthermia produced a six-fold increase in prostaglandin E2 PGE2 release by SFA cells and a 4.5-fold increase by UFA cells. No LTC4 was detected. Alteration of dietary fat affects cell lethality and PG release following hyperthermic treatment. The increase in phospholipid and cholesterol content of UFA cells may be a response to reduced membrane fluidity.

Reperfusion inhibits elevated splanchnic prostanoid production after hemorrhagic shock

The effect of reperfusion following hemorrhagic shock on splanchnic prostanoid release was studied. Anesthetized male rats were bled to a mean arterial blood pressure of 30 mmHg for 30 minutes and either killed or treated with shed blood for 60 minutes and then killed. The superior mesenteric arterial bed was cannulated and perfused in vitro with oxygenated Krebs. Collected venous effluent (up to 180 minutes) was analyzed for 6-keto-PGF1 alpha (PGI2 metabolite), PGE2, PGF2 alpha, and thromboxane B2 by radioimmunoassay in shock, shock plus reperfusion, and sham groups. The major prostanoid released was 6-keto-PGF1 alpha and was three times higher in the shock group compared to the sham group (p less than 0.05). Reperfusion of shed blood abolished the increase in 6-keto-PGF1 alpha found in the shock group (p less than 0.05). These data show that the attempt of the rat splanchnic bed to compensate for hemorrhagic shock by increasing release of PGI2 (potent vasodilator) was abolished during reperfusion of blood.

The effect of hypoxia on rat splanchnic prostanoid output

The effect of hypoxia on isolated perfused rat mesenteric basal venous prostanoid output was studied. Male rat splanchnic vasculature was removed without (SV) or with its end organ (SV + SI) and perfused with Krebs' buffer with a pO2 of 460 or 60 mm torr. Basal splanchnic venous effluent was assayed for 6-keto-PGF1 alpha, TxB2 and PGE by radioimmunoassay at 30, 60, 120 and 180 min of perfusion. Basal output of SV 6-keto-PGF1 alpha was five and ten fold higher than for PGE and TxB2 respectively and comprised 36% or greater of SV + SI 6-keto-PGF1 alpha output. SV PGE and TxB2 output comprised less than 19 and 12% respectively of SV + SI output. Hypoxia decreased SV + SI PG output, 6-keto-PGF1 alpha being most affected. Hypoxia did not alter SV 6-keto-PGF1 alpha output indicating the SI as the anatomic location most influenced by hypoxia. The relative amounts of distribution of PGE or TxB2 output were not altered by hypoxia. These data suggest that there are two distinct areas of splanchnic prostanoid output, the SV and the SI. Decreased 6-keto-PGF1 alpha output might alter splanchnic blood flow at two levels, the splanchnic vasculature, and/or within the bowel wall.

The prostaglandin outflow from perfused mesenteric vasculature of rats fed different fats

The effects of dietary n-6 polyunsaturated fatty acids and replacement with saturated fat or fish oil on the prostaglandin outflow from perfused mesenteric vasculature in rats were studied. Seventy-two weanling male rats were fed ad libitum a semi-synthetic diet supplemented with 10% by weight of oil, composed wholly of n-6 fatty acid-rich evening primrose oil, or replaced partly or completely (25, 50, 75 or 100%) by n-6 fatty acid-deficient fish oil or hydrogenated coconut oil for 8 weeks. The outflows of 6-keto-PGF1 alpha, thromboxane B2, and prostaglandin E from the perfused mesenteric vasculature were measured at 60 min-time point after starting the perfusion. In general, the release of prostanoids from the mesenteric vasculature was significantly reduced in rats fed a diet in which evening primrose oil was partly or completely replaced by either hydrogenated coconut or fish oil. This was probably due to the insufficient conversion of linoleic acid to arachidonic acid. The extent of reduction was greater in fish oil-fed than in hydrogenated coconut oil-fed rats, while the levels of arachidonic acid in aortic phospholipids were similar between these two groups. This result implies that the greater reduction of prostaglandin synthesis in rats fed fish oil was due to the inhibitory effect of eicosapentaenoic and docosahexaenoic acids in fish oil on the conversion of arachidonate to eicosanoids.

Dietary N-6 and N-3 fatty acids and salt-induced hypertension in the borderline hypertensive rat

This study examined the effects of salt-loading on blood pressure (BP) development in the borderline hypertensive rat (BHR) and its modification by dietary n-3 and n-6 fatty acids. In experiment 1, 4 groups (n = 10/group) of male BHR receiving 1% NaCl as a replacement for tap water were placed on chow enriched with either olive oil (OL), sunflower oil (SUN), evening primrose oil (EPO) or fish oil for 6 weeks. BP, heart rate, body weight, water, Na+ and K+ intake and urinary output were measured weekly. SUN and fish oil reduced the pressor response to salt seen vs the OL group by 50%, and EPO abolished the pressor response, reducing BP below control levels. The BP response was unrelated to either food intake or water and electrolyte intake and excretion. In experiment 2, male BHR received water +/- 18:3n-6 (0.04 mg/hr in OL via ip pump) or 1% NaCl +/- 18:3n-6 (n = 12/group) for 12 weeks, followed by 2 weeks recovery on tap water. Salt increased BP, and 18:3n-6 decreased this response, but had no effect on animals receiving tap water. In experiment 3, effects of 3 doses of 18:3n-6 (0.04, 0.08, and 0.12 mg/hr) on the pressor response to 1% NaCl were examined. All doses reduced the BP response to salt vs controls with no dose-response. These data suggest that the BHR is genetically salt-sensitive, and that dietary n-3 and n-6 fatty acids can attenuate the cardiovascular response to salt in this model.

Prostaglandin I2 and thromboxane A2 production in relation to alpha 1 and alpha 2-adrenoreceptor activation in the normotensive and hypertensive rat

1. The perfused mesenteric arterial bed (MAB) from normotensive (WKY) and hypertensive (SHR) rats was used as a model to investigate the relationship between alpha 1 and alpha 2-adrenoreceptor activation and prostanoid release. 2. Prostaglandin I2 (PGI2) and small amounts of thromboxane A2 (TXA2) are released from this preparation and can be monitored in the perfusate under basal and stimulated conditions. 3. Noradrenaline (NA) caused a significant increase in the release of PGI2 from the MAB of both WKY and SHR. Both basal and NA-stimulated PGI2 release were lower in the SHR, whereas pressor responses to NA were higher. Noradrenaline caused a very small increase in TXA2 release in the WKY only and the basal release of this prostanoid was again lower in the SHR. 4. Prazosin, an alpha 1-selective antagonist (10(-10), 10(-9), 10(-8) M) was more effective in blocking pressor responses to NA in the SHR than in the WKY, did not affect the NA-induced release of PGI2 in either group and abolished the small increase of TXA2 release in WKY. 5. Rauwolscine, an alpha 2-selective antagonist (10(-8), 10(-7), 10(-6) M) was more effective in reducing pressor responses to NA in the SHR than in the WKY. It abolished, from 10(-7) M upwards, the NA-induced release of PGI2 in the WKY and reduced it in the SHR. The NA-induced release of TXA2 was abolished in WKY. 6. The observed differences in the effects of rauwolscine and prazosin do not correlate with the effects of these antagonists on pressor responses since both reduce the latter but only rauwolscine abolishes the stimulated PGI2 release. 7. These results indicate that in the SHR, the mechanisms mediating the release of PGI2 from the MAB may be similar but not identical to those in the WKY. The observation that an increase in adrenergically induced pressor responses is not followed by an increase in PGI2 release, which is in fact reduced in the SHR, suggests a more complex regulation of this relationship which may have pathophysiological implications.

Effects of low magnesium diet on the vascular prostaglandin and fatty acid metabolism in rats

Deficiency of magnesium with cardiovascular effects is thought to be related to alterations in the biosynthesis of prostaglandins (PGs) in the vasculature. Measurements were made of the PGE2, 6-keto-PGF1 alpha and thromboxane B2 (TxB2) outflow from the perfused isolated mesenteric arterial bed and the fatty acid composition of the tissue in rats maintained for 14 weeks on a low magnesium (LMg) diet. The serum Mg levels were significantly decreased and the serum Ca levels were significantly increased in the LMg group as compared to the controls. The arachidonic acid concentration in the triacylglyceride fraction was significantly increased in the LMg group. Long chain polyunsaturates such as 22:4n6 and 22:6n3 were consistently increased in the LMg rats as compared to the controls in both the phospholipid and triglyceride fractions as previously reported in other tissues. The PGE2, 6-keto-PGF1 alpha and TxB2 outflows were significantly increased in the LMg group as compared to the controls. These findings suggest that the biosynthesis of eicosanoids, mainly PGI2, is stimulated in Mg deficiency, and this may provide protection against intracellular Mg depletion and Ca accumulation, so as to counteract to the constricted and hyperreactive state of the vasculature in such a condition.