L. P. Feigen

Influence of inhibitors of prostaglandin synthesis on renal vascular resistance and on renal vascular responses to vasopressor and vasodilator agents in the cat.

We determined the effects of indomethacin and meclofenamate, two inhibitors of prostaglandin synthesis, on renal vascular resistance and on renal responses to nerve stimulation, pressor and depressor hormones in the in situ feline kidney under conditions of controlled blood flow. Both inhibitors produced a gradual rise in renal vascular resistance which became maximal 15-20 minutes after administration. The increase in renal resistance after indomethacin was not attenuated during intrarenal infusion of either phentolamine or SQ 20881. Pretreat-ment with propranolol, in a dose sufficient to inhibit renin secretion, also did not attenuate the increase in renal resistance produced by indomethacin. However, infusion of [Sar1,Ala8]angiotensin II, an angiotensin II antagonist, did attenuate the indomethacin-induced increase in renal vascular resistance. After indomethacin, the vasoconstrictor response to norepinephrine was enhanced, whereas responses to nerve stimulation and angiotensin were unaffected. Although meclofenamate enhanced renal vascular resistance, its effects on vasoconstrictor responses were inconsistent. After indomethacin, the renal dilator response to bradykinin was enhanced; however, dilator responses to nitroglycerin were unaltered. The present data indicate that the increase in renal vascular resistance after indomethacin does not depend on the adrenergic system but may be dependent on the renin-angiotensin system. The inconsistent effect of the inhibitors of synthesis on renal constrictor responses to nerve stimulation suggests that endogenous prostaglandins do not serve to modulate the effects of the sympathetic nervous system on the feline renal vascular bed. These results also indicate that renal dilator responses to bradykinin are not mediated by prostaglandins in the cat.

Evidence for separate PGD2 and PGF2α receptors in the canine mesenteric vascular bed

Abstract Potential interactions between PGD2 and PGF2α in the mesenteric and renal vascular beds were investigated in the anesthetized dog. Regional blood flows were measured with electromagnetic flow probes. PGD2, PGF2α and Norepinephrine (NE) were injected as a bolus directly into the appropriate artery, and responses to these agents were obtained before, during and after infusion of either PGD2 or PGF2α into the left ventricle. In each case, the infused prostaglandin caused vascular effects of its own. Left ventricular infusion of PGD2 reduced responses to local injections of PGD2 in the intestine, and a similar effect was observed for PGF2α, suggesting significant receptor or receptor-like interactions for each of the prostanoids. However, systemic infusion of prostaglandin F2α (20–100 ng/kg/min) had no effect on renal or mesenteric vascular responses to local injection of prostaglandin D2. Similarly, PGD2 administration (100 ng/kg/min) did not affect responses to PGF2α in the intestine. The present results therefore suggest that these prostaglandins, i.e., D2 and F2α, act through separate receptors in the mesenteric and renal vascular beds. In addition, increased prostaglandin F2α levels produced by infusion of F2α reduced mesenteric but not renal blood flow, suggesting that redistribution of cardiac output might participate in side effects often observed with clinical use of this prostaglandin, such as nausea and abdominal pain.

Inhibition of soybean lipoxygenase by SKF 525-A and metyrapone

To determine whether agents which inhibit cytochrome P-450 enzymes also inhibit lipoxygenase, the effects of metyrapone and SKF 525-A were assessed on soybean lipoxygenase using a spectrophotometric technique which allows for measurement of both the rate and magnitude of product formation. Both SKF 525-A and metyrapone inhibited the rate of product formation and the final amount of product formed in 5 min incubations SKF 525-A was 5 to 5 times more potent than metyrapone, with the IC50 for SKF 525-A 40 microM and for metyrapone between 150 and 200 microM as determined by the total product formation in 5 minutes. Analysis of the reduced product by HPLC confirmed that the substances monitored were those generated by the 15-lipoxygenase enzyme.

Effects of 13, 14-dehydroprostacyclin methyl ester on the feline intestinal vascular bed

The effects of a stable PGI2 analog, 13, 14-dehydro-PGI2 methyl ester and several vasoactive hormones were compared in the feline intestinal vascular bed under conditions of controlled blood flow so that changes in perfusion pressure directly reflect changes in vascular resistance. The PGI2 analog decreased perfusion pressure in a dose-dependent fashion when injected in the range of dose of 0.03-3 microgram and was quite similar to PGE2 whereas isoproterenol was somewhat more potent as a vasodilagor in the feline intestinal vascular bed. The present data show that 13, 14-dehydro-PGI2 methyl ester has potent vasodilator activity in the intestinal vascular bed.

The effect of PGH2 on blood flow in the canine renal and superior mesenteric vascular beds

Effects of the prostaglandin endoperoxide, PGH2, were investigated in the renal and superior mesenteric vascular beds in anesthetized dogs. Vascular effects of a stable PGH2 analog were also studied in the intestine. Blood flow was measured with electromagnetic flowmeters and vasoactive hormones were administered by close intra-arterial injection. Authentic PGH2 increased blood flow in the kidney and intestine in a dose-related manner. Mesenteric blood flow was reduced by the PGH2 analog in a dose-dependent fashion which was similar to the vasoconstrictor activity of norepinephrine in this organ. PGH2 is biologically unstable and the type and activity of its metabolic products may vary in different regional vascular beds. Most of the known products of PGH2 metabolism in the kidney are vasodilators whereas in the intestine both vasodilator and vasoconstrictor metabolites are formed. It has been suggested that the vascular activity of PGH2 in an organ is dependent on the predominant type and activity of specific terminal enzymes that convert PGH2 to its various vaso-active products.

Inhibition of lipoxygenase enzyme in vitro by the cytochrome P-450 inhibitors metyrapone and SKF-525-A

Abstract It has recently been demonstrated that agents which block lipoxygenase enzymes (i.e. nordihydroguaiaretic and eicosatetraynoic acid) also block cytochrome P-450 in some in vitro preparations. In some cases, therefore, results which were based on these lipoxygenase inhibitors could have been produced by the inhibition of cytochrome P-450. We therefore sought a way to distinguish between effects produced by metabolites of arachidonic acid generated by lipoxygenase enzymes and those produced by metabolites of the cytochrome P-450 system. seemed that a straightforward approach might be to administer drugs that inhibit the cytochrome P-450 system first, and then examine effects of lipoxygenase inhibitors. However, it then became necessary to establish the effect of inhibitors of the cytochrome P-450 system on lipoxygenase enzymes. Initial work was carried out using crystalline soybean lipoxygenase enzyme to avoid the complexities involved in isolating enzymes from biological tissues. Enzyme activity was assessed spectrophotometrically by measuring the increase in absorbance at 240 nm produced by the formation of a conjugated triene (15-HETE) from arachidonic acid. Effects of the two most commonly used cytochrome P-450 antagonists, metyrapone and SKF-525A, were determined by Lineweaver-Burke analysis. One major difficulty was that arachidonic acid is soluble in aqueous media at Ph 8 whereas both SKF-525-A and metyrapone are soluble at pH 7. In order to maintain all components in solution at the same time, experiments were carried out at pH 7.35 in the presence of 5% dimethylsulfoxide. There was no observable difference in the activity of 15-lipoxygenase at pH 9 in the absence of DMSO and that observed at pH 7.35 in the presence of DMSO. Studies were carried out in 2 ml quartz spectrophotometer cuvettes. For each experiment, two cuvettes were prepared containing arachidonic acid, DMSO (0.1 ml), SKF-525-A or metyrapone, and Tris buffer (pH 7.35). The cuvettes were placed in a Beckman DB/G spectrophotometer. One cuvette was used as reference and to the other was added 3ug (0.1 ml) of a solution of crystalline soybean lipoxygenase (Sigma). The change in absorbance at 240 nm was recorded and reflected product formation. Concentrations of arachidonic acid ranged from 10–50 uM, each tube contained either 0–50 uM SKF-525-A or 0–200 uM metyrapone. Results show that both SKF-525-A and metyrapone inhibited soybean lipoxygenase. Lineweaver-Burke analysis indicated that both agents acted in uncompetitive fashion. Of particular importance is that these concentrations of SKF-525-A and metyrapone are similar to those routinely used to block cytochrome P-450. From these results, it appears that experiments in which SKF-525-A and metyrapone were employed may need to be reevaluated. Work is underway using platelet-derived 12-lipoxygenase.

COMPARISON OF VASODEPRESSOR AND VASODILATOR EFFECTS OF PROSTAGLANDINS E1, E2, I2 AND 6-KETO-E1*

This chapter presents a comparison of the vasodepressor and vasodilator effects of prostaglandins (PGs) E 1 , E 2 , I 2 and 6–Keto–E. PGs E 1 , E 2 , and I 2 reduce blood pressure and increase blood flow in most peripheral vascular beds. These substances produce renal and mesenteric vasodilation when administered either intravenously (I.V.) or by direct, close-arterial injection or infusion. A novel metabolite of prostacyclin PGI 2 has been identified, which appears to possess the properties that are similar to those of PGI 2 . This substance, 6–Keto—PGE 1 , inhibits platelet aggregation and lowers blood pressure and renal vascular resistance in the rat. It has been suggested that 6–Keto–PGE 1 may mediate some of the effects of PGI 2 . The aim of a study described in the chapter was to compare the activity of 6–Keto–PGE 1 with that of PGI 2 on blood pressure and renal and mesenteric hemodynamics in the dog—a species in which the effects of 6–Keto–PGE 1 have not been studied.