Central cardiovascular and thermal effects of prostacyclin in rats

Prostanoid receptors: structures, properties, and functions

Prostanoids are the cyclooxygenase metabolites of arachidonic acid and include prostaglandin (PG) D(2), PGE(2), PGF(2alpha), PGI(2), and thromboxne A(2). They are synthesized and released upon cell stimulation and act on cells in the vicinity of their synthesis to exert their actions. Receptors mediating the actions of prostanoids were recently identified and cloned. They are G protein-coupled receptors with seven transmembrane domains. There are eight types and subtypes of prostanoid receptors that are encoded by different genes but as a whole constitute a subfamily in the superfamily of the rhodopsin-type receptors. Each of the receptors was expressed in cultured cells, and its ligand-binding properties and signal transduction pathways were characterized. Moreover, domains and amino acid residues conferring the specificities of ligand binding and signal transduction are being clarified. Information also is accumulating as to the distribution of these receptors in the body. It is also becoming clear for some types of receptors how expression of their genes is regulated. Furthermore, the gene for each of the eight types of prostanoid receptor has been disrupted, and mice deficient in each type of receptor are being examined to identify and assess the roles played by each receptor under various physiological and pathophysiological conditions. In this article, we summarize these findings and attempt to give an overview of the current status of research on the prostanoid receptors.

Iloprost, a stable analogue of PGI2, potentiates the hyperthermic effect of PGE2 in rats

Centrally mediated effects of iloprost, a stable analogue of PGI2, on rectal temperature have been investigated in conscious rats. ICV administration of iloprost (100-1,000 ng, ICV) produced a dose-dependent, monophasic hyperthermic response that was not inhibited by indomethacin. When injected into the preoptic anterior hypothalamic (POAH) region, iloprost (2-50 ng/POAH) induced a biphasic increase in rectal temperature. While the first phase was inhibited by AH 6809, an E1-type prostaglandin (EP1) receptor antagonist, the second phase was abolished by indomethacin pretreatment. Iloprost was found not to alter rectal temperature when injected into the ventromedial hypothalamic area. Administration of iloprost into the POAH in a dose that had no effect on rectal temperature significantly potentiated the hyperthermic effect of PGE2 (50 ng, ICV). These findings suggest that the pyrogenic effect of iloprost is partly mediated by EP1 receptors located on the POAH. Regarding the similarities of iloprost and PGI2, it is further proposed that endogenous PGI2 might act to modulate hyperthermic effect of PGE2 released during arachidonic acid- or endogenous pyrogen-induced fever.

Influence of rat prenatal and postnatal exposure to a non-steroidal anti-inflammatory agent (flunoxaprofen) on cardiovascular function in the progeny

The present experiments evaluated in rats the effects of prenatal and postnatal exposure to a non-steroidal antiinflammatory agent, flunoxaprofen (5-10 and 20 mg/kg/day by the oral route), on cardiovascular function in the pups. In both conscious and anaesthetized rats pre- and postnatal flunoxaprofen exposure at the 30th and 60th day of age, significantly (P less than .05) induced a decrease of pressor response to carotid-sinus baroreceptor stimulation and to L-noradrenaline (0.1-1 and 5 micrograms/kg iv), and an increase of the hypotensive responses to L-isoprenaline (0.01-0.1 and 1 microgram/kg iv) and acetylcholine (0.01-0.1 and 1 microgram/kg iv). These effects were not observed in rats on the 90th day of age. Moreover, pre- and postnatal flunoxaprofen exposure did not modify systolic arterial blood pressure of plasma levels of catecholamines and acetylcholinesterases. Our results also show that in normotensive rats flunoxaprofen exposure during pregnancy did not affect the body weight, systolic or diastolic blood pressure or heart rate of pregnant rats. It did not affect the length of gestation, number of pups per litter or pup body weight. No macroscopic teratogenic effects were observed.

Interaction of nonsteroidal anti-inflammatory drugs with antihypertensive and diuretic agents. Control of vascular reactivity by endogenous prostanoids

Indomethacin and some other nonsteroidal anti-inflammatory drugs partially antagonize the blood pressure lowering effect of drugs used to treat hypertension. They can also produce a mild elevation of blood pressure in normotensive individuals. The elevated arterial pressure caused by these agents is associated with increases in the vascular resistance of mainly the renal and splanchnic beds. This may be due to direct inhibition of the synthesis of vasodilator prostanoids, or it may be due to indirect potentiation of the action of the sympathetic nervous system or of angiotensin II. Nonsteroidal anti-inflammatory drugs also cause renal retention of sodium and this probably contributes to their hypertensive effects. In humans, the sodium retention may involve increased reabsorption in the proximal tubule. Although a direct tubular action is possible, these drugs may change proximal sodium reabsorption by their vascular effects. However, the exact mechanism is not understood. These interactions are clinically significant and may complicate the treatment of common diseases.

Changes in body temperature after administration of acetylcholine, histamine, morphine, prostaglandins and related agents: II

This survey continues a second series of compilations of data regarding changes in body temperature induced by drugs and related agents. The information listed includes the species used, the route of administration and dose of drug, the environmental temperature at which experiments were performed, the number of tests, the direction and magnitude of change in body temperature and remarks on the presence of special conditions, such as age or brain lesions. Also indicated is the influence of other drugs, such as antagonists, on the response to the primary agent. Most of the papers were published since 1979, but data from many earlier papers are also tabulated.

Role of putative neurotransmitters in the central gastric antisecretory effect of prostaglandin E2 in rats

The role of putative neurotransmitters of the central nervous system in the central gastric antisecretory effect of prostaglandin E2 (PGE2) was investigated in pylorus-ligated rats. Pretreatment of the rats with an intracerebroventricular (i.c.v.) injection of 6-hydroxydopamine (6-OHDA) prevented the antisecretory effect of the i.c.v. administration of PGE2, whereas pretreatment with 5,6-dihydroxytryptamine (5,6-DHT) plus p-chlorophenylalanine (PCPA) had no effect. I.c.v.-administered phentolamine and idazoxan antagonized the inhibition of gastric secretion induced by i.c.v. PGE2, whereas prazosin, propranolol and sulpiride injected via the same route were ineffective. Diphenhydramine, cimetidine, naloxone and theophylline, all administered i.c.v., did not modify the antisecretory effect of i.c.v. PGE2. The results suggest that an activation of alpha 2-adrenoceptors in the brain is involved in the central gastric antisecretory effect of PGE2, whereas neither central 5-hydroxytryptamine receptors, alpha 1- or beta-adrenoceptors, D2-dopamine receptors, histamine or opioid receptors nor adenosine seem to play any role here.

Intracerebroventricular and preoptic injections of leukotrienes C4, D4, and E4 in the rat: lack of febrile effect

Experiments were performed to ascertain the effect on core temperature in the rat of central administration of 3 products of the lipoxygenase pathway of arachidonate metabolism. The agents tested were leukotrienes C4, D4, and E4 (LTC4, LTD4, LTE4). In one series of rats, the leukotrienes were injected into the ventral aspect of the third cerebral ventricle (5 microliter injection volume). Each rat received, in separate experimental sessions, an injection of a control solution, of 1 microgram of prostaglandin E1 (PGE1) and of 1 microgram of LTC4, LTD4, or LTE4. In another series of rats, bilateral 1 microliter injections into the tissue of the preoptic region were made. Each animal received a control solution, 40 ng PGE1 (20 ng/side) and 400 ng LTC4, LTD4, or LTE4 (200 ng/side). Neither the intraventricular nor the preoptic injections of any of the leukotrienes produced a significant increase in colonic temperature. However, PGE1 injected intraventricularly or into the preoptic region produced a large, rapidly developing core temperature rise. The strong febrile response to PGE1 and the results of dye distribution studies indicate that the lack of effect of the leukotrienes was not due to incorrect injection cannula placement. The ineffectiveness of the leukotrienes also cannot be attributed to loss of biological activity of these agents during storage. Near the end of the study, samples of each leukotriene were assayed using the guinea pig tracheal strip method and were found to be highly active. The results suggest that, at least in the rat, these 3 arachidonate metabolites are not likely to be important mediators of fever.

Changes of mean arterial pressure of the rat resulting from intracerebroventricular injections of prostacyclin or 6-keto-prostaglandin F1 alpha

Repeated intracerebroventricular injections of 1 microgram prostacyclin reduce mean arterial pressure of conscious normotensive rats and reverse the elevation of blood pressure of conscious rats resulting from the intraventricular injection of angiotensin II. The reduction of blood pressure of normotensive rats by prostacyclin is enhanced by pretreatment with probenecid, an inhibitor of fatty-acid transport across biological membranes. Although probenecid does not completely inhibit the transport of fatty acids from the brain to the periphery, the greater effectiveness of prostacyclin in probenecid-treated animals suggests that centrally injected prostacyclin need not leak into the periphery to reduce blood pressure. When the dose of prostacyclin is reduced to 100 ng repeated each minute for 10 min, no change of blood pressure of normotensive rats is observed. The failure of the lower dose of prostacyclin to reduce blood pressure may be due to its rapid degradation. Ventricular injections of 6-keto-prostaglandin F1a, a major product of prostacyclin metabolism, causes an increase of blood pressure which may counteract the action of prostacyclin itself.

Inhibition of adenosine uptake into rat brain synaptosomes by prostaglandins, benzodiazepines and other centrally active compounds

A number of compounds have been tested for their abilities to inhibit the rapid uptake of adenosine by rat cerebral cortical synaptosomes. Prostaglandins PGI2, PGA2, and PGE1 and PGE2 were potent inhibitors of adenosine uptake with IC20 values in the 10(-7) M-10(-6) M range. PGA1, PGD2 and PGF2 alpha also inhibited uptake but were less active. The benzodiazepine antagonist Ro 15-1788 inhibited adenosine uptake and failed to antagonize the effects of diazepam. Another antagonist, ethyl-beta-carboline-3-carboxylate, was a weak inhibitor of adenosine uptake. Ro 5-4864, the so-called peripheral benzodiazepine ligand, inhibited adenosine uptake. Hydroxyzine and tracazolate, two anxiolytic agents, inhibited uptake as did flunarizine, a coronary vasodilator. Two calmodulin antagonists, W7 and R 24571, were effective inhibitors of adenosine uptake. Their IC50 values were comparable to those at which they have been demonstrated to inhibit calmodulin-mediated reactions in other systems. These observations suggest that adenosine uptake may be a calmodulin-regulated process.

Central cardiovascular and thermal effects of prostaglandin E2 in rats

Prostaglandin E2 (PGE2) increased the blood pressure, heart rate and body temperature, when administered at the doses of 0.001-10 micrograms into the lateral cerebral ventricle (i.c.v.) of the urethane-anesthetised rat. The highest dose of 10 micrograms/rat induced a strong initial hypotensive effect. Intravenously (i.v.), PGE2 at the doses of 0.01-10 micrograms/rat caused a biphasic blood pressure response with dose-related initial decreases followed by slight increases in blood pressure. The heart rate and body temperature were slightly increased by i.v. administrations of PGE2. The highest i.v. dose of 10 micrograms/rat initially decreased also the heart rate. Central pretreatment with indomethacin (1 mg/rat i.c.v.) partly antagonised all of the recorded central effects of PGE2, while sodium meclofenamate (1 mg/rat i.c.v.) abolished the hypertensive response to i.c.v. administered PGE2 but failed to significantly affect the PGE2-induced rises of heart rate and body temperature. The results support the previous suggestions that PGE2 may participate in the central cardiovascular and thermoregulatory control. The results also suggest that indomethacin and sodium meclofenamate antagonize the effects of exogenous prostaglandins. Since sodium meclofenamate, unlike indomethacin, affected preferentially the hypertensive response to centrally administered PGE2, there may be differences in the sites and/or modes of action between these drugs.

Central cardiovascular and thermal effects of prostaglandin D2 in rats

Prostaglandin D2 (PGD2) is the most common prostaglandin type of the rat brain. Recently a neuromodulator role for PGD2 has been suggested. In the present work the central cardiovascular and thermal effects of PGD2 were studied in urethane-anaesthetised rats. When administered at the doses of 0.001-10 micrograms/rat into the lateral cerebral ventricle (i.c.v.), PGD2 slightly increased the blood pressure, heart rate and body temperature. The highest dose caused also an initial hypotensive effect. Upon intravenous injections PGD2 (0.1-10 micrograms/rat) initially decreased and then weakly increased the blood pressure but had only negligible effects on heart rate and body temperature. Central pretreatment with sodium meclofenamate or indomethacin (1 mg/rat i.c.v.) antagonised effectively all the recorded central effects of PGD2. The central cardiovascular and thermal effects of PGD2 were much weaker than those obtained earlier with other prostaglandins, such as PGF2 alpha and PGE2. Therefore, in spite of its abundance in the brain PGD2 may not be very important for the central cardiovascular and thermal regulation in the rat.