Changes in systolic arterial blood pressure in normal and spontaneously hypertensive rats produced by acute administration of inhibitors of prostaglandin biosynthesis

A vasoconstrictor response to COX-1-mediated prostacyclin synthesis in young rat renal arteries that increases in prehypertensive conditions

This study aimed to determine whether prostacyclin (PGI2) functions as an endothelium-derived contracting factor (EDCF) in young rat renal arteries, and, if so, we wanted to examine the underlying mechanism(s) and how it changes in prehypertensive conditions. Vessels from Wistar-Kyoto (WKY) and prehypertensive spontaneously hypertensive rats (SHRs) of 25-28 days of age were isolated for functional and biochemical analyses. Result showed that following NO synthase (NOS) inhibition PGI2 and the thromboxane-prostanoid (TP) receptor agonist U-46619 evoked contractions in young WKY renal arteries that were similar to those in prehypertensive SHRs. Meanwhile, the endothelial muscarinic receptor agonist ACh evoked an endothelium-dependent contraction under NOS-inhibited conditions and a production of the PGI2 metabolite 6-keto-PGF1α; both were sensitive to cyclooxygenase (COX) and/or COX-1 inhibition but higher in prehypertensive SHRs than in young WKYs. Interestingly, in WKY renal arteries PGI2 did not evoke relaxation even after TP receptor antagonism that diminished the contraction evoked by the agonist. Indeed, PGI2 (IP) receptors were not detected in the vessel with Western blot. Moreover, we noted that treatment with the nonselective COX inhibitor indomethacin, which was started at the prehypertensive stage, blunted the elevation of systolic blood pressure and reduced the heart-to-body ratio in SHR within 2 mo of treatment. These results demonstrate that due to scarcity of IP receptors, PGI2, which is derived mainly from COX-1-mediated metabolism, acts as an EDCF in young WKY renal arteries, and it increases in prehypertensive conditions. Also, our data revealed that COX inhibition starting from the prehypertensive stage has an antihypertensive effect in young SHRs.

Inhibition of the Prostaglandin Transporter PGT Lowers Blood Pressure in Hypertensive Rats and Mice

Inhibiting the synthesis of endogenous prostaglandins with nonsteroidal anti-inflammatory drugs exacerbates arterial hypertension. We hypothesized that the converse, i.e., raising the level of endogenous prostaglandins, might have anti-hypertensive effects. To accomplish this, we focused on inhibiting the prostaglandin transporter PGT (SLCO2A1), which is the obligatory first step in the inactivation of several common PGs. We first examined the role of PGT in controlling arterial blood pressure blood pressure using anesthetized rats. The high-affinity PGT inhibitor T26A sensitized the ability of exogenous PGE₂ to lower blood pressure, confirming both inhibition of PGT by T26A and the vasodepressor action of PGE₂ T26A administered alone to anesthetized rats dose-dependently lowered blood pressure, and did so to a greater degree in spontaneously hypertensive rats than in Wistar-Kyoto control rats. In mice, T26A added chronically to the drinking water increased the urinary excretion and plasma concentration of PGE₂ over several days, confirming that T26A is orally active in antagonizing PGT. T26A given orally to hypertensive mice normalized blood pressure. T26A increased urinary sodium excretion in mice and, when added to the medium bathing isolated mouse aortas, T26A increased the net release of PGE₂ induced by arachidonic acid, inhibited serotonin-induced vasoconstriction, and potentiated vasodilation induced by exogenous PGE₂. We conclude that pharmacologically inhibiting PGT-mediated prostaglandin metabolism lowers blood pressure, probably by prostaglandin-induced natriuresis and vasodilation. PGT is a novel therapeutic target for treating hypertension.

Eicosanoids: role in experimental renal disease

Because of their vasodilator and vasoconstrictor properties, vasoactive prostaglandins and thromboxane A2 have been proposed as modulators of the hemodynamic changes that occur in experimental models of renal disease. Increased synthesis of vasodilatory prostaglandins (PGE2) and perhaps prostaglandin I2 (PGI2) play a role in the maintenance of renal blood flow and GFR during states of impaired perfusion. In contrast, thromboxane A2 has been implicated as the vasoconstrictor responsible for the reduction of renal blood flow and GFR in certain animal models of experimental renal disease. These products and other metabolites of arachidonic acid may also participate in the immunological events underlying the onset and/or progression of experimental renal disease. It is evident that the pathophysiologic role of eicosanoids in experimental renal disease is not fully understood. Additional studies and further understanding of the many other potential roles of eicosanoids on immunological events, hemodynamic states, mesangial cell physiology, etc. are needed to comprehend more fully the extent of the participation of eicosanoids in the pathogenesis and pathophysiology of renal disease.

Effect of salt loading on blood pressure and eicosanoid metabolism of spontaneously hypertensive rats fed a fish oil enriched diet

This study investigated the effects of dietary modification of prostaglandin (PG) synthesis on blood pressure regulation in spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats under conditions of normal and elevated salt intake. After an initial period of 4 weeks on either a 2-series PG 'inhibitory' diet of fish oil (maxEPA) or a control diet of saturated fat, half of each group received 1.5% saline for 1 week. Blood pressures were unaffected by diet during the period of normal salt intake, but following salt loading, the maxEPA-fed SHR showed a blood pressure increase (mean = 21 mmHg) relative to the EPA-fed rats on water. Rats on maxEPA showed impaired ability to generate serum thromboxane and diminished excretion of urinary 6-keto-PGF1 alpha and PGE2. SHR on water showed greater serum TXB2 generating capacity than WKY, but diminished urinary PGE2 excretion. Thus, the increased blood pressure observed in the salt-loaded SHR on the maxEPA diet may be explained by reduced renal PG synthesis resulting in either mild sodium retention and/or increased vascular reactivity.

A possible role of gonad and renal prostaglandin E2 on the development of hypertension in spontaneously hypertensive rats

A possible role of prostaglandin E2 (PGE2) in the regulation of blood pressure in spontaneously hypertensive rats (SHR) was investigated. The inhibition of PG synthesis by chronic indomethacin treatment accelerated the elevation of blood pressure with the tendency to decrease renal PGE2. We, therefore, confirmed that PGE2 in SHR may play a role in the antihypertensive mechanism. In this connection, the participation of renal PGE2 in the retardation of the development of hypertension in male SHR induced by orchiectomy was examined. Urinary PGE2 which reflects the renal PGE2 level tended to keep a higher level in the castrated group. Urinary electrolytes excretion also inclined to augment in the castrated group throughout the experiment. These results indicate that renal PGE2 may participate in the gonads-mediated blood pressure regulation system, although the mechanism of the retardation of spontaneous hypertension induced by orchiectomy remains obscure.

The urinary excretion of prostaglandins E2 and F2 alpha in essential hypertension

The 24 h urinary excretions of prostaglandins E2 (E2/d) and F2 alpha (F2 alpha/d) were measured in twenty-five normal subjects and in thirty-five patients with essential hypertension [seventeen with low renin (LRH) and eighteen with normal renin (NRH) hypertension]. E2/d was lower in patients with LRH than in normal subjects (P less than 0.01), whereas no difference was found between patients with NRH and the controls. F2 alpha/d was similar in patients with LRH and in the normal subjects, but was significantly greater in patients with NRH (P less than 0.02). The ratio of prostaglandin E2 to prostaglandin F2 alpha was decreased in hypertensive patients (P less than 0.02), although in the NRH subgroup the difference was not statistically significant. It appears that LRH is associated with impaired production of prostaglandin E2, while a deranged relationship between the two prostaglandins exists in all the patients with essential hypertension. These changes in prostaglandin production could possibly contribute to the pathogenesis of hypertension, by increasing renal vascular resistance and decreasing sodium excretion. Alternatively, they might be a secondary phenomenon, reflecting changes in renal prostaglandin metabolism induced by the hypertensive state.

Renal effects of pinane-thromboxane A2 and indomethacin in saline volume-expanded spontaneously hypertensive rats

In 6-week old spontaneously hypertensive rats (SHR), indomethacin (5 mg/kg i.v.) and pinane-thromboxane A2 (PTA2) (50 micrograms/kg per h i.v.) a thromboxane A2 (TXA2) antagonist as well as a TXA2 synthetase inhibitor, resulted in natriuresis accompanied by an increase in p-aminohippuric acid and inulin clearances. Indomethacin acted as an antidiuretic in 6 week Wistar-Kyoto rats (WKY). PTA2 did not alter renal functions in either 6 week WKY and 18 week SHR. Basal urinary excretion of TXB2 (UTXB2V) was greater in 6 week SHR than in 6 week WKY and 18 week SHR, and that of 6-keto-PGF1 alpha (U6-keto-PGF1 alpha V) did not differ among these strains. Thus, U6-keto-PGF1 alpha V/UTXB2V was lower in the 6 week SHR. Basal urinary excretion of PGE (UPGEV) was much greater in 18 week SHR than in the 2 other groups. In the 6 week SHR, PTA2 decreased UTXB2V and increased U6-keto-PGF1 alpha V without affecting UPGEV, and indomethacin reduced UTXB2V more markedly than did U6-keto-PGF1 alpha V and UPGEV. Thus, both PTA2 and indomethacin increased U6-keto-PGF1 alpha V/UTXB2V in the 6 week SHR. These findings indicate that a disequilibrium in the biosynthesis of vasoconstrictive TXA2 and of vasodilator PGI2 may be involved in water and sodium retention in SHR during the developmental phase of hypertension.

Arachidonic acid metabolism, prostaglandins and the kidney

Renal prostaglandins are gaining increasing recognition as important modulators of hemodynamics and excretory function in the mammalian kidney. Synthesis of these unsaturated fatty acids from arachidonate precursors is closely regulated by intrarenal factors, and circulating angiotensin II, catecholamines, arginine vasopressin and bradykinin. Endogenous prostaglandins exert little influence on renal blood flow and glomerular filtration rate in the basal state, but inhibition of arachidonate metabolism when renal perfusion is impaired causes marked alterations in these parameters. Renal salt and water excretion is modified by the effects of prostaglandins on glomerular filtration rate, proximal tubule fluid reabsorption, medullary solute gradients, and the intrinsic water and ion reabsorptive properties of distal nephron segments. Prostaglandins also mediate renin release under basal conditions and in response to intravascular volume depletion. Abnormalities of renal prostaglandins are evident in various clinical disorders of renal function including hypertension, ureteral obstruction, Bartter syndrome, hypokalemic nephropathy and drug-induced disorders of water metabolism. Appropriate clinical use of nonsteroidal anti-inflammatory agents requires consideration of the potential renal consequences of inhibiting prostaglandin biosynthesis.

Effect of salt on prostaglandin metabolism in hypertension-prone and -resistant Dahl rats

The effect of high salt intake on vascular and renomedullary prostaglandin (PG) synthesis was compared in Sprague-Dawley and salt-sensitive (S) and -resistant (R) Dahl rats. Animals were given a diet containing either 0.6% or 8% NaCl starting at 5 weeks of age, and were sacrificed 6 weeks later. Systolic blood pressure of S rats increased to 220 +/- 7 mm Hg but was unaffected in R and Sprague-Dawley rats. Prostaglandin synthesis was studied in aortic rings and renomedullary microsomes using 14C-arachidonate as substrate. [3H]PGE2 degradation was measured in the renocortical cytosol. In Sprague-Dawley and R rats, aortic PGI2 synthesis was not affected by high salt intake, while a significant increase compared to animals on 0.6% NaCl (from 608 +/- 84 to 992 +/-108 pmoles/60 min, p less than 0.05) was noted in S rats. Enhancement of PGI2 synthesis in S rats may be secondary to the hypertension. Salt-loading consistently stimulated renomedullary PGE2 synthesis in all three animal groups. S rats, however, had the lowest PG synthesis in renal medullas compared to Sprague-Dawley and R rats when placed on either diet. Thus, even after 6 weeks on high salt, S rats did not reach the levels of PGE2 synthesis seen in R or Sprague-Dawley rats on regular diet. The activity of cortical 15-hydroxyprostaglandin dehydrogenase was increased by salt-loading in S and Sprague-Dawley, but not in R rats. R rats had lower dehydrogenase activity than the other two groups when placed on either diet. The observed differences in PG synthesis and catabolism will tend to maintain the net output of renal PGs highest in R and lowest in S rats. These differences correlate with the reported differences in renal papillary flow between these two rat strains and may be relevant to their susceptibility or resistance to hypertension in response to salt.

Prostacyclin-induced contraction of isolated aortic strips from normal and spontaneously hypertensive rats (SHR)

Prostacyclin (PGI2) (500-5,000 ng/ml) produced a concentration-dependent increase in contractile tension of isolated thoracic aortic strips (AS) from normotensive (WKY) and spontaneously hypertensive rats (SHR). No significant differences were noted between this response to PGI2 in these two groups. Lower concentrations of PGI2, (10 pg/ml - 100 ng/ml) caused neither contraction nor relaxation of agonist-contracted tissue. PGI2 (500-5,000 ng/ml) did not relax KCl or methoxamine contracted AS. In concentrations above 100 ng/ml, PGI2 caused a further increase in tension in KCl-depolarized preparations. The constrictor effect of PGI2 on AS was attenuated by verapamil pretreatment or removal of extracellular Ca++ from the physiological buffer. This inhibitory effect of Ca++ deficiency on the PGI2 response was significantly greater in AS from SHR compared to WKY tissue. The stable metabolite of PGI2, 6-keto PGF1a, caused a weak constrictor effect (40% of KCl reference contraction) over the concentration range 1,000-5,000 ng/ml. Contraction induced by PGI2 was not prevented by pretreatment with antagonists of adrenergic, histamine, serotonin or cholinergic receptors. The contraction response of the rat AS to PGI2 is similar to that reported for porcine coronary artery and rabbit aortic tissues in vitro.

Central cardiovascular and thermal effects of prostaglandin F2 alpha in rats

Administration of PGF2 ALPHA (0.2--6.4 micrograms) into the lateral cerebral ventricle (i.c.v.) induced dose-dependent increases in blood pressure, heart rate and body temperature in urethane-anaesthetised rats, but had no effect on these parameters when the same dose range was administered intravenously. Peripheral pretreatment with sodium meclofenamate (50 mg/kg s.c.) shifted all the dose-response curves for PGF2 alpha (i.c.v.) to the left, but indomethacin (50 mg/kg s.c.) did not significantly affect those changes. Central pretreatment with sodium meclofenamate or indomethacin (1.25 mg per rat i.c.v.) failed to modify significantly the effects of centrally administered PGF2 alpha. The results support previous suggestions that PGF2 alpha may participate in the central control of the cardiovascular and thermoregulatory systems, and also suggest that there may be differences in the sites and/or modes of action between sodium meclofenamate and indomethacin.

Impaired renal prostaglandin E2 biosynthesis in human hypertensive states

Urinary Prostaglandin E2 (PGE2), a known indicator of renal production, was measured by specific radioimmunoassay in 111 normal volunteers, 85 patients with essential hypertension, 6 with renovascular hypertension, and 23 patients with primary aldosteronism. Women excreted less PGE2 than men in both normotensive and hypertensive groups. When compared to normals, essential hypertensives demonstrated significantly lower PGE2 levels, with one third excreting less than 100 ng/24 hr, values usually seen only in subjects receiving the prostaglandin synthetase inhibitor, indomethacin. Normal PGE2 was seen in patients with renovascular hypertension, and levels were uninfluenced by treatment with the converting enzyme inhibitor SQ14225, Despite normalization of blood pressure and increased plasma renin activity. Normal PGE2 was also encountered in primary aldosteronism. These data indicate that impaired renal PGE2 biosynthesis is specific for human essential hypertension, and is not secondary to the elevated blood pressure. Although PGE2 excretion tends to be lower in low-renin hypertension, a constant relationship between PGE2 and renin is not always apparent.

The effect of an acute or chronic treatment with indomethacin on the blood pressure of DOCA/salt and spontaneously hypertensive rats

The intravenous infusion of indomethacin (5 mg/kg/min) for 10 min did not modify the systolic blood pressure of conscious, unrestrained doca/salt hypertensive, spontaneously hypertensive (SH) and normotensive Wistar (NW) rats. However, this dose of indomethacin was sufficient to prevent completely the hypotensive action of arachidonic acid in these animals. The chronic administration of indomethacin to doca/salt treated- and young SH rats was without any effect on the development of hypertension in these animals. These results suggest a limited role for endogenous PGs during the development of these two types of hypertension.