Noradrenaline and dopamine infusions modulate arachidonic acid cyclooxygenase and 5-lipoxygenase pathways ex vivo in man

Inhibition of leukotriene synthesis with MK-886 prevents a rise in blood pressure and reduces noradrenaline-evoked contraction in L-NAME-treated rats

(1) Long-term treatment of rats with Nomega-nitro-l-arginine methyl ester (l-NAME) induces hypertension associated with inflammatory and vascular changes. Leukotrienes are proinflammatory vasoactive products that are suspected to be involved in the pathogenesis of hypertension. We investigated, in rats chronically treated with l-NAME, the involvement of leukotrienes in the in vivo regulation of blood pressure and the in vitro contraction elicited by noradrenaline in isolated aorta. (2) Rats were randomly assigned to four groups and orally treated for 3 weeks with l-NAME (1 mg ml-1), l-NAME (1 mg ml-1) plus the leukotriene biosynthesis inhibitor MK-886 (0.1 mg ml-1), MK-886 (0.1 mg ml-1) alone or vehicle (Methocel, 0.1%). All the drugs were added to the drinking fluid. (3) The mean arterial blood pressure (MABP) increased significantly in l-NAME-treated rats (173.3+/-9.4 mmHg (n=25)) vs Methocel-treated rats (110.7+/-4.8 mmHg (n=11), P<0.001). Chronic treatment with MK-886 prevented this rise in MABP. (4) Aortic rings with or without endothelium were suspended in organ baths for recording isometric changes in response to noradrenaline. Pretreatment with either MK-886 (10 microm), the CysLT1 receptor antagonist MK571 (1 microm) or the dual CysLT1/CysLT2 receptor antagonist BAY-u9773 (0.1 microm) reduced (P<0.05) noradrenaline-induced contractions in intact aortic rings from l-NAME-treated rats only. (5) Noradrenaline (0.3 microm) induced a two-fold increase in cysteinyl leukotriene (CysLT) release (measured by enzyme immunoassay) in intact aortic rings from l-NAME-treated rats only. (6) These data suggested (1) a role for the 5-lipoxygenase pathway in the regulation of blood pressure in l-NAME-treated rats and (2) the involvement of endothelial CysLTs in noradrenaline-induced contraction in aorta from l-NAME-treated rats.

Dopamine D(2) receptor-induced COX-2-mediated production of prostaglandin E(2) in D(2)-transfected Chinese hamster ovary cells without simultaneous administration of a Ca(2+)-mobilizing agent

We have earlier demonstrated that dopamine stimulates the liberation of the prostaglandin E(2) (PGE(2)) precursor, arachidonic acid, in Chinese hamster ovary cells transfected with the rat dopamine D(2) receptor (long isoform), also without concomitant administration of a Ca(2+)-releasing agent [Nilsson et al., Br J Pharmacol 1998;124:1651-8]. In the present report, we show that dopamine, under the same conditions, also induces a concentration-dependent increase in the production of PGE(2), with a maximal effect of 235% at approximately 100 microM, and with an EC(50) of 794 nM. The effect was counteracted by the D(2) antagonist eticlopride, pertussis toxin, the inhibitor of intracellular Ca(2+) release TMB-8, incubation in Ca(2+)-free experimental medium, and PKC desensitization obtained by chronic pretreatment with the phorbol ester TPA. It was also antagonized by the non-specific cyclooxygenase (COX) inhibitor, indomethacin, and by the selective COX-2 inhibitor, NS-398, but not by the specific COX-1 inhibitor, valeryl salicylate. Both the non-specific phospholipase A(2) inhibitor, quinacrine, and an inhibitor of cPLA(2) and iPLA(2), AACOF3, counteracted the effect; in contrast, a selective iPLA(2) inhibitor, BEL, and a selective sPLA(2) inhibitor, TAPC, were ineffective. No effects of dopamine were obtained in control cells mock-transfected with the p3C vector only. The results reinforce previous assumptions that dopamine may interact with eicosanoid metabolism by means of D(2) receptor activation, and implicate an involvement of cPLA(2) and COX-2 in this effect. It is suggested that measurement of dopamine-induced PGE(2) production may serve as a convenient way to study D(2) receptor function in vitro.

Treatment with a polyunsaturated fatty acid prevents deleterious effects of Ro4-1284

Ro4-1284 (2-Ethyl-1,3,4,6,7,11b-hexahydro-3-isobutyl-9,10-dimethoxy-2H-benzo[a] quinolizin-2-ol hydrochloride), a benzoquinolizine, is a potent dopamine depletion agent whose acute and chronic administration results in a (1) deterioration of learning in the Morris Water Maze and passive avoidance tasks, (2) decrease in locomotion and rearing, (3) intense hypothermia, and (4) decrease in the percentage of polyunsaturated fatty acids and an increase in the level of cholesterol in neuronal membranes. Pretreatment with a specific mixture of free polyunsaturated fatty acids prevents most of the behavioral, physiological, and biochemical effects of Ro4-1284 except for rearing. We propose that the dopamine-mediated functions tested in this study are dependent on the interaction of intact dopamine D1 and D2 receptors. Rearing, which is controlled only by dopamine D1 receptors, remained, therefore, unaffected. Our hypothesis is that SR-3 exerts its beneficial effects by normalizing the structure and function of the neuronal membrane and by restoring dopamine D2 receptor functions.

Amrinone, a phosphodiesterase III inhibitor, and arachidonic acid metabolism in humans

Amrinone-a phosphodiesterase III inhibitor-is used in the treatment of acute heart failure. In addition to its hemodynamic effects, amrinone has been shown to inhibit thromboxane synthesis in vitro. We investigated the effects of amrinone on thromboxane, prostaglandin, and leukotriene synthesis in humans. Eight healthy male volunteers took part in this single-blind study in which either amrinone (a 1.5-mg/kg bolus in 30 min and after that 10 microg/kg/min for 1 h 30 min) or placebo (0.9% NaCl) were infused. Amrinone infusion increased systolic blood pressure but had no significant effect on diastolic blood pressure or heart rate. Amrinone did not modulate thromboxane B2 synthesis stimulated by either spontaneous clotting or calcium-ionophore A23187 in whole blood. Amrinone had no effects on prostaglandin E2 or leukotriene E4 production in A23187-stimulated whole blood, nor did it affect urinary excretion of 11-dehydrothromboxane B2 or 2,3-dinor-6-keto-prostaglandin F1alpha, the index metabolites of thromboxane A2 and prostacyclin productions, respectively. We conclude that amrinone has no effects on eicosanoid production in humans at the dose level used in this study, and that the hemodynamic effects noticed are not mediated via cyclooxygenase or lipoxygenase products of arachidonic acid metabolism.

Fatty acids and brain peptides

The role of fatty acids (FA) as a mediator and modulator of central nervous system activity in general, and peptides in particular, is only recently becoming understood. This paper reviews numerous findings concerned with the activity of fatty acids, particularly with their interaction with diverse neurochemical systems and their consequences for better understanding neurotransmitters, hormones and peptides. The effects include FA as precursors in the manufacture of neurochemical elements, including enzymes, neurotransmitters, and hormones. Of particular interest is the important changes in neuronal membrane composition that have been attributed to FA. Such changes may account for the changes in thermoregulation, learning, and other functions that accompany dietary manipulation of FA intake. While the total level of FA has been the object of many investigations, this report addresses the need to focus on the ratio of FA, especially alpha-linolenic/linoleic acid, which has been shown to be a critical factor in a number of research studies.

Theophylline infusion modulates prostaglandin and leukotriene production in man

Although theophylline has been used in the treatment of asthma for decades, it is not a first line choice any more. It is a well-known bronchodilator, but was recently discovered also to be an anti-inflammatory, immunomodulatory and bronchoprotective agent. Therefore we wanted to establish the role of theophylline on prostaglandin and leukotriene production, which plays a part in the pathogenesis of asthma. Theophylline was infused (bolus 5 mg/kg in 15 min and infusion 0.4 mg/kg/h for 1 h 45 min) into healthy volunteers. Thromboxane B2, prostaglandin E2 and leukotriene E4 were measured from the A23187-stimulated whole blood samples and stable metabolites of thromboxane A2; prostacyclin and leukotriene E4 were measured from urine. Theophylline increased prostaglandin E2 production and decreased leukotriene E4 production ex vivo in whole blood, thus increasing the prostanoid/leukotriene ratio. It did not change thromboxane B2 production stimulated by either spontaneous clotting or A23187 in the whole blood. Theophylline had hardly any effect on in vivo thromboxane, prostacyclin and leukotriene E4 production measured as urinary metabolites, 11-dehydro-thromboxane B2, 2,3-dinor-6-keto-prostaglandin F1alpha and leukotriene E4, respectively. Serum theophylline concentrations were at the lower level of normal therapeutic range during the infusion. The increase in PGE2 and the decrease in LTE4 synthesis ex vivo may offer a new explanation for the mode of antiasthmatic action of theophylline. It is notable that this phenomenon occurs at low serum theophylline concentrations. These results confirm the idea that theophylline has an anti-inflammatory and bronchoprotective action and support the use of theophylline as a therapeutic agent in asthmatic patients.

Effects of noradrenaline and dopamine infusions on arachidonic acid metabolism in man

We infused noradrenaline (0.025 micrograms/kg/min for 60 min, n=7) and dopamine (3.0 micrograms/kg/min for 60 min, n=6) into healthy male volunteers to study the effects of these catecholamines on in vivo thromboxane A2, prostacyclin and leukotriene E4 production measured as urinary excretions of 11-dehydro-thromboxane (TX) B2, 2,3-dinor-6-keto-prostaglandin (PG) F1alpha and leukotriene (LT) E4, respectively. Plasma noradrenaline and dopamine concentrations were 2.9+/-0.3 and 233+/-17 nmol/l at the endo fo the noradrenaline and dopamine infusions, respectively. Noradrenaline decreased thromboxane production and increased leukotriene production almost two fold. It had hardly any effect on prostacyclin production. Dopamine had no significant effects on any of the variables, however, it had a tendency to increase prostacyclin and leukotriene production. The results indicate that noradrenaline is a more important modulator of arachidonic acid metabolism than dopamine in vivo.