Dipeptidyl peptidase inhibitors as new drugs for the treatment of type 2 diabetes.

Inhibitors of the regulatory protease dipeptidyl peptidase-IV (DPP-IV) are currently under development in preclinical and clinical studies (several pharmaceutical companies, now in Phase III) as potential drugs for the treatment of type 2 diabetes. Their development is based on the observation that DPP-IV rapidly inactivates the incretin hormone glucagon-like peptide-1 (GLP-1), which is released postprandially from the gut and increases insulin secretion. DPP-IV inhibitors stabilise endogenous GLP-1 at physiological concentrations, and induce insulin secretion in a glucose-dependent manner; therefore, they do not demonstrate any hypoglycaemic effects. Furthermore, they are orally bioavailable. In addition to their ability to protect GLP-1 against degradation, DPP-IV inhibitors also stabilise other incretins, including gastric inhibitory peptide and pituitary adenylate cyclase-activating peptide. They also reduce the antagonistic and desensitising effects of the fragments formed by truncation of the incretins. In clinical studies, when used for the treatment of diabetes over a 1-year period, DPP-IV inhibitors show improved efficacy over time. This finding can be explained by a GLP-1-induced increase in the number of beta cells. Potential risks associated with DPP-IV inhibitors include the prolongation of the action of other peptide hormones, neuropeptides and chemokines cleaved by the protease, and their interaction with DPP-IV-related proteases. Based on their mode of action, DPP-IV inhibitors seem to be of particular value in early forms of type 2 diabetes, either alone or in combination with other types of oral agents.

Restoration of first-phase insulin secretion by the imidazoline compound LY374284 in pancreatic islets of diabetic db/db mice.

The effect of the imidazoline compound LY374284 has been studied in pancreatic islets of db/db mice, a progressive model of diabetes. In perifusion experiments, pancreatic islets of db/db mice showed a progressive deterioration of glucose-induced insulin release with increasing age, whereby the first phase of insulin secretion was almost completely abolished and the second phase was substantially decreased by 15 weeks of age. LY374284 restored the first phase of glucose-induced insulin secretion in islets of 16-week-old db/db mice to 70% of that observed in islets isolated from age-matched nondiabetic db/1 mice. LY374284 did not affect insulin secretion at a low glucose concentration (3.3 mmol/L). A similar restoration of first phase insulin secretion was observed after application of glucagon-like peptide-1, whereas a sulfonylurea agent, tolbutamide, was inactive. LY374284 did not affect cytosolic Ca(2+) concentration or cellular ATP content. Furthermore, LY374284 strongly enhanced insulin secretion in islets of db/db and db/1 mice maximally depolarized by 30 mmol/L K(+) and 250 micromol/L diazoxide. The present data suggest that the imidazoline compound LY374284 restores biphasic insulin secretion in islets of diabetic db/db mice by amplifying glucose-induced insulin secretion at a site distal to Ca(2+)-influx.

Glucose-induced insulin secretion is potentiated by a new imidazoline compound.

Sulfonylureas stimulate insulin secretion independent of the blood glucose concentration. This can lead to hypoglycaemia in type 2 diabetic patients. Over the last years a number of imidazoline derivatives have been identified that stimulate insulin secretion in a more glucose-dependent way. In agreement with this, our aim was to generate imidazoline derivatives with a potential for the treatment of type 2 diabetic patients. We developed the compound 2-[4-(4-chlorophenyl)-3-(2-methoxyethoxy)-2-naphthalenyl]-4,5-dihydro-1-H-imidazole monohydrochloride (LY389382) with an imidazoline moiety and investigated its effects on glucose-dependent insulin secretion in a beta-cell line, isolated rat islets and in vivo. We could demonstrate that LY389382 induces insulin secretion in MIN6 cells and rat islets in a glucose-dependent manner (EC50=1.1 microM and 0.3 microM, respectively). Furthermore during hyperglycaemia LY389382 increased insulin secretion in a dose-dependent manner in healthy rats, whereas the compound had no effect at euglycemia in a tenfold higher dosage. After 7 days of treatment of Zucker Diabetic Fatty [ZDF/ (Gmi/fa)] rats with LY389382 with a dose of 15 mg/kg twice daily the blood glucose concentration was reduced from 22.7 +/- 1.7 mM to 16.6 +/- 2.3 mM. During the same time period the glucose concentration increased from 21.7+/-1.7 mM to 28.9 +/- 1.3 mM in the vehicle-treated group (P<0.05). The drop of the insulin level was also inhibited by LY389382 in ZDF rats. In contrast to other well-characterised imidazolines that have been shown to induce a glucose-dependent insulin secretion only within a limited range of concentrations, LY389382 stimulates insulin secretion over a concentration range of at least two log units in a glucose-dependent manner. These data suggest that this imidazoline compound has a potential for the treatment of type 2 diabetes.

The novel imidazoline compound BL11282 potentiates glucose-induced insulin secretion in pancreatic beta-cells in the absence of modulation of K(ATP) channel activity.

The insulinotropic activity of the novel imidazoline compound BL11282 was investigated. Intravenous administration of BL11282 (0.3 mg x kg(-1) x min(-1)) to anesthetized rats did not change blood glucose and insulin levels under basal conditions, but produced a higher increase in blood insulin levels and a faster glucose removal from the blood after glucose infusion. Similarly, in isolated Wistar rat pancreatic islets, 0.1-100 micromol/l BL11282 potently stimulated glucose-induced insulin secretion but did not modulate basal insulin secretion. Unlike previously described imidazolines, BL11282 did not block ATP-dependent K+ channels. Furthermore, the compound stimulated insulin secretion in islets depolarized with high concentrations of KCl or permeabilized with electric shock. Insulinotropic activity of BL11282 was dependent on activity of protein kinases A and C. In pancreatic islets from spontaneously diabetic GK rats, the imidazoline compound restored the impaired insulin response to glucose. In conclusion, the imidazoline BL11282 constitutes a new class of insulinotropic compounds that exerts an exclusive glucose-dependent insulinotropic activity in pancreatic islets by stimulating insulin exocytosis.

Imidazoline RX871024 raises diacylglycerol levels in rat pancreatic islets.

Imidazoline compound RX871024 and carbamylcholine (CCh) stimulate insulin secretion in isolated rat pancreatic islets. Combination of CCh and RX871024 induces a synergetic effect on insulin secretion. RX871024 and CCh produce twofold increases in diacylglycerol (DAG) concentration. The combination of two compounds has an additive effect on DAG concentration. Effects of RX871024 on insulin secretion and DAG concentration are not dependent on the presence of D609, an inhibitor of phosphatidylcholine-specific phospholipase C. It is concluded that as in case with CCh the increase in DAG concentration induced by imidazoline RX871024 contributes to the insulinotropic activity of the compound.

Fused bicyclic Gly-Asp beta-turn mimics with potent affinity for GPIIb-IIIa. Exploration of the arginine isostere.

6-[4-Amidinobenzoyl]amino]-tetralone-2-acetic acid is a potent antagonist of GPIIb-IIIa. Substitution in the meta position of the benzamidine, or replacement with a heteroaryl amidine was tolerated in this series. Use of an acyl-linked 4-alkyl piperidine as an arginine isostere also provided active compounds. Compounds from this series provided substantial systemic exposure in the rat following oral administration.

Fused bicyclic Gly-Asp beta-turn mimics with specific affinity for GPIIb-IIIa.

Disubstituted isoquinolones 2 and 3 have affinity for GPIIb-IIIa and represent leads for further structural evaluation. Structure-activity studies centered on the bicyclic beta-turn mimic contained in these molecules indicated that this moiety could accommodate a variety of modifications. Specifically, monocyclic, 6, 5-bicyclic, and 6,7-bicyclic structures provide compounds with affinity for GPIIb-IIIa. Within the 6,6-series, isoquinoline, tetralin, tetralone, and benzopyran nuclei yield potent antagonists that are specific for GPIIb-IIIa. Attachment of the arginine isostere (benzamidine) to the supporting nucleus can be accomplished with an ether or amide linkage, although the latter enhances activity. Several compounds in this series provided measurable blood levels after oral dosing. Conversion of the acid moiety in these molecules to an ester generally provided compounds which gave greater systemic exposure after oral administration. Absolute bioavailabilities in the rat for the ethyl ester prodrug derivatives of the tetralin, tetralone, and benzopyran analogues of 3 were 28%, 23%, and 24%, respectively.

Different modes of action of the imidazoline compound RX871024 in pancreatic beta-cells. Blocking of K+ channels, mobilization of Ca2+ from endoplasmic reticulum, and interaction with exocytotic machinery.

The imidazoline compound RX871024 glucose-dependently potentiates the release of insulin in pancreatic islets and beta-cell lines. This activity of the compound is not related to its action by stimulating alpha 2-adrenoceptors and I1- and I2-imidazoline receptors. There are at least three modes of action of RX871024 in beta-cells: (1) RX871024 blocks the ATP-dependent, Ca(2+)-activated, and delayed rectifier K+ channel activity; (2) RX871024 causes mobilization of Ca2+ from thapsigargin-sensitive intracellular stores, the effect probably controlled by cytochrome P450; and (3) the stimulatory activity of RX871024 on insulin release involves interaction of the compound with the exocytotic machinery, unrelated to the changes in membrane potential and cytoplasmic-free Ca2+ concentration, whereas protein phosphorylation plays an important role in this process. The maximal insulinotropic effect of RX871024 is much higher than that of the sulfonylurea glibenclamide. RX871024 stimulates insulin release and normalizes blood glucose levels in rats in vivo without affecting blood pressure and heart rate.

Influence of different dietary salts on the cardiovascular and renal effects of moxonidine in spontaneously hypertensive rats.

The influence of common salt (NaCl) and a novel potassium-, magnesium-, and L-lysine-enriched mineral salt on the cardiovascular and renal effects of the selective imidazoline I1-receptor agonist moxonidine was examined in spontaneously hypertensive rats (SHR). Common salt was added at the level of 6% of the dry weight of the chow, and mineral salt at a 75% higher level of 10.5% thereof to produce the same NaCl concentration of 6% as in the common salt group. During the control diet an 8-week oral treatment with moxonidine (117 mg/1000 g of the dry weight of the chow producing an approximate daily dose of 10 mg/kg), lowered blood pressure by 13 mmHg. The common salt diet alone raised blood pressure by 27 mmHg. Moxonidine lowered blood pressure by 21 mmHg during the common salt diet, but the blood pressure remained 19 mmHg higher than in the moxonidine-treated SHR receiving the control diet (P<0.05). Unlike common salt, mineral salt alone did not raise blood pressure nor did it interfere with the antihypertensive effect of moxonidine. Moxonidine showed a kidney-protective effect during the control diet measured as decreased urinary protein excretion, but it did not affect the development of left ventricular hypertrophy. Moxonidine increased plasma renin activity during the control diet and it raised the serum aldosterone level both during the control and mineral salt diets. The vascular relaxation responses of the mesenteric arterial rings to both acetylcholine (an indicator of endothelium-dependent vascular relaxation) and nitroprusside and nitroprusside (an indicator of endothelium-independent vascular relaxation) were attenuated by the common salt diet alone but maintained during the moxonidine treatment. Our findings are consistent with the concept that moxonidine is able to improve the excretion of sodium. This effect might explain the maintenance of normal vascular relaxation during a high intake of common salt. These effects may partly account for the antihypertensive effect of moxonidine.

Investigations of the mechanism of the positive inotropic action of BDF 9148: comparison with DPI 201-106 and the enantiomers.

The electromechanical and biochemical activities of the positive inotropic compounds BDF 9148 and DPI 201-106 were compared in guinea-pig myocardic preparations. Additionally, the properties of the BDF 9148 enantiomers were studied to compare their positive inotropic effects. In guinea pig papillary muscles, BDF 9148 exerted a concentration-dependent increase of force of contraction with a 50% effective concentration (EC50) value of 0.6 microM, compared with 1.3 microM for DPI 201-106. Like that of DPI, the inotropic effect of BDF 9148 was abolished by treatment with tetrodotoxin (TTX) but not affected by treatment with carbachol. Likewise, pretreatment of the papillary muscles with propranolol, cimetidine, and histamine did not affect the contractile effects of BDF 9148. In the left atria, both agents had a positive inotropic effect with an EC50 of 0.2 microM for BDF and 0.8 microM for DPI. Incubation of single concentrations of the respective drugs for a period of 90 min with guinea pig papillary muscles resulted in slightly differing parameters of isometric contraction. In contrast to DPI, BDF 9148 prolonged the contraction time transiently. Time to peak force was not markedly influenced by either drug. The functional refractory period was prolonged by both drugs to a similar extent. At 10 microM, BDF 9148 showed a biphasic effect on the action potential duration (APD) most evident at APD90, whereas DPI prolonged APD90 progressively until the 90 min. The positive inotropic effect of BDF 9148 could be demonstrated by the (S-), whereas the (R+)-enantiomer was without effect. Neither DPI nor BDF 9148 increased myocardial cyclic adenosine monophosphate (cAMP) in isolated rat cardiomyocytes and guinea pig papillary muscles. Additionally, neither BDF 9148 nor DPI showed an inhibitory effect on the guinea pig myocardic Na+/K(+)-ATP'ase activity in the concentration range with a positive inotropic effect in the guinea-pig papillary muscle.

Antiarrhythmic effect of the selective I1-imidazoline receptor modulator moxonidine on ouabain-induced cardiac arrhythmia in guinea pigs.

Moxonidine is a centrally acting antihypertensive agent with potent action on I1-imidazoline receptors. Moxonidine as an SIR modulator elicits a persistent reduction in circulating levels of epinephrine, demonstrating a reduction in sympathetic tone. In the first experiment the threshold dose of ouabain needed to induce ventricular arrhythmia and asystole was determined in guinea pigs, and the influence of moxonidine was tested. In a dose range of 0.1-0.4 mg/kg body weight i.v., moxonidine increased the threshold dose needed to induce ventricular tachycardia, premature ventricular beats, ventricular flutter, ventricular fibrillation, and asystole. The effect was dose-dependent and statistically significant. Clonidine, in a dose range of 0.2-0.8 mg/kg body weight i.v., also increased the threshold dose of ouabain necessary to induce different cardiac rhythm disturbances. Moxonidine was more effective than clonidine. Pretreatment with the alpha 2-receptor and I1-receptor-influencing substances efaroxan, idazoxan, and SKF 86466 attenuated the effect of moxonidine and clonidine. Efaroxan, idazoxan, or SKF 86466 alone reduced the threshold dose of ouabain necessary to induce cardiac arrhythmia as a sign for arrhythmogenic effects. The alpha 1-receptor antagonist prazosin had no influence on ouabain-induced arrhythmia. Pretreatment with prazosin reduced the moxonidine but not the clonidine effect. In the second experiment the influence of moxonidine on aconitine-induced extrasystoles (ES) in the spontaneously beating guinea pig auricle was investigated. Moxonidine in a dose of 10(-7)-10(-8) M reduced the number of ES. A 10-fold higher dose had no influence on ES number. The beta-blocking agent propranolol showed antiarrhythmic effects in both methods. The ouabain-induced cardiac arrhythmia is associated with increased sympathetic tone on central stimulation. The reduced sympathetic tone by centrally acting moxonidine via imidazoline receptors seems responsible for the antiarrhythmic effect of this drug. Clonidine also reduced the sympathetic tone via imidazoline receptor. The selectivity of clonidine to imidazoline receptors is less pronounced than is that of moxonidine. The interaction of moxonidine with imidazoline receptors is not clear. The possible interaction between imidazoline and alpha-adrenoceptors in relation to the antiarrhythmic effect of moxonidine or clonidine is also unknown. Modulation of imidazoline receptors by moxonidine could be an agonistic effect or an antagonism to an endogenous agonistic or antagonistic substance and vice versa.

Why imidazoline receptor modulator in the treatment of hypertension?

The influence of the sympathetic nervous system on blood pressure control was impressively demonstrated in 1940 by bilateral excision of sympathetic nerve fibers. Thereafter, the first generation of drugs lowering blood pressure by central modulation of the sympathetic outflow through alpha 2-adrenoceptor for stimulation, such as alpha-methyldopa, guanabenz, clonidine, and guanfacine, were marketed. However, these compounds were often tolerated poorly, because they caused orthostatic hypotension, sedation, tachycardia or bradycardia, dry mouth, and reduced cardiac output. The mode of action of the second generation centrally acting antihypertensive drugs moxonidine and rilmenidine is different from that of the first generation compounds (e.g., clonidine). Contrary to clonidine, the newer drugs bind more selectively to I1-imidazoline receptors rather than to alpha 2-adrenoceptors where first-generation drugs act. The high affinity and selectivity of these two drugs for this recently discovered new receptor class make it possible to discriminate between I1-imidazoline receptor-mediated blood pressure lowering, on the one hand, and alpha 2-adrenoceptor-mediated side effects, on the other. Discrimination of the two effects was substantiated either by studies using moxonidine alone or in interaction experiments with I1-imidazoline receptor or alpha 2-adrenoceptor antagonists. The high selectivity of moxonidine at the I1-imidazoline receptor allows discrimination between alpha 2-adrenoceptors and I1-imidazoline receptors and is reflected in man by the relatively low incidence of adverse drug events during moxonidine treatment. Concentration of endazoline, a specific mediator of I1-imidazoline receptors, is elevated in some patients with essential hypertension. Modulation of I1-imidazoline receptors by moxonidine could be interpreted as antagonism with regard to the endogenous agonistic effect of the endogenous "transmitter" endazoline. On the other hand, moxonidine acted directly as an agonist at the putative I1-imidazoline receptor. Therefore, to clear the ground, characterization as well as physiological function of the mediator for imidazoline receptors seems essential. The therapeutic relevance of using drugs selective for I1-imidazoline receptors for blood pressure reduction in hypertensive patients is substantiated by the finding that in human rostral ventrolateral medulla (RVLM), which is essential in central blood pressure regulation, the relation between alpha 2-adrenoceptors and I1-imidazoline receptors is about one to ten (1:10). Reduction of a long-lasting sympathetic overdrive may avoid the deteriorating effects on the heart and peripheral circulation. These recent findings give a rational explanation for the very low incidence of sedation and the absence of respiratory depression, orthostatic hypotension, and rebound hypertension that banned the former central acting antihypertensive drugs from first-line treatment despite the advantages of central mediated blood pressure control.

Relevance of mediators to cardiac parameters of isolated anaphylactic guinea-pig heart.

The release of histamine, eicosanoids and catecholamines were measured after induction of anaphylaxis in isolated guinea-pig hearts. The concentration-time profile of these mediators was compared with changes of cardiac parameters. The histamine and catecholamine levels of the coronary effluent were determined at 10 s intervals; thromboxane and prostacyclin levels at 60 s intervals. The release of histamine and norepinephrine were maximum between 20 and 30 s after the antigen challenge and decreased rapidly within 60 s. Thromboxane and prostacyclin increased to a maximum after 3 min and declined slowly within 10 min. The rise in histamine release was correlated with tachycardia. The release of thromboxane was correlated with the increase of coronary perfusion pressure. Cimetidine inhibited the tachycardia and clemastine reduced bradyarrhythmia. The inhibition of lipoxygenase and cyclooxygenase also reduced the rise in the perfusion pressure. These data suggest that different mediators are time-dependently involved in anaphylaxis-induced cardiac changes.

Is the antiatherosclerotic potency of HDL modulated by the origin of HDL?

Different HDL preparations from rabbit blood were injected intravenously (10 mg HDL protein/injection and animal) into cholesterol fed rabbits twice a week for 8 weeks. The composition of the used high density lipoprotein (HDL) was modified by dietary pretreatment. HDL-1 was taken from rabbits after 8 weeks pellet diet, HDL-2 after 8 weeks fish-oil rich diet and HDL-3 after 8 weeks of cholesterol rich diet. The animals treated with HDL-1 and HDL-2 had a significantly smaller area of intima covered with fatty streaks than the control animals (injection of saline instead of HDL). The injection of HDL-3 was without influence. These differences were correlated with changes in the level of free and esterified cholesterol (FC and CE) in kidney, liver and aorta of the rabbits, but not with the level of cholesterol in the serum lipoproteins. We investigated simultaneously the influence of the different HDL preparations on the cholesterol content in rabbit skin fibroblasts (RSF), rabbit smooth muscle cells (SMC) and human hepatoma cell line Hep G2. Additionally the influence on the proliferation of SMC was studied. HDL-1 diminished the level of FC in cholesterol enriched RSF and rabbit SMC, whereas both other HDL preparations had no effect. The level of FC in Hep G2 was not influenced. HDL-1 and HDL-3 stimulated the proliferation of rabbit SMC, whereas HDL-2 had no such influence. The data support the antiatherosclerotic role of HDL injections in cholesterol fed rabbits, but the composition of the used HDL seems to modify this influence, probably by different effects on the biochemical changes induced by the HDL.

Lipoproteins from normolipidemic and dyslipidemic subjects modify the thromboxane A2 generation by platelets in clotting human blood.

The study was performed to investigate the influence of lipoproteins (LP) on the thromboxane (TX) A2 formation capacity of platelets in clotting whole blood in vitro. The different lipoprotein fractions VLDL, LDL, HDL2 and HDL3 were isolated from blood of normo- or dyslipidemic volunteers by ultracentrifugation. These lipoproteins were incubated in blood with different levels of serum total cholesterol (TC) taken from normolipidemics (TC < 200 mg/dl), moderate hypercholesterolemics (TC: 200-250 mg/dl) or subjects with high cholesterol level (TC > 250 mg/dl), respectively. The amount of serum TXA2 formed within 60 min at 37 degrees C was measured by enzyme immunoassay. The results obtained show that the efficacy of separate LP fractions to influence the TXA2 production depends not only on the type of LP fraction but also on the source of plasma used for isolation of LP and on the cholesterol level in the blood for incubation: LDL taken from normolipidemics or moderate hyperlipidemics inhibited the TXA2 formation in blood from normolipidemics (P < 0.02, respectively), but enhanced it in blood from persons with moderate hypercholesterolemia (P < 0.05). LDL from hyperlipidemics enhanced TXA2 production in blood from hyperlipidemics (P < 0.05). The HDL2 fractions inhibited the TXA2 formation in blood from normo- and hypercholesterolemics (P < 0.02, resp.), but there was no effect of HDL2 in clotting blood from persons with moderate hypercholesterolemia. All HDL3 fractions tested inhibited the TXA2 formation in all types of blood used for clotting (P < 0.02, resp.), probably due to their great cholesterol accepting capacity.

Effect of trapidil derivative AR 12456 on intracellular cholesterol homeostasis in human hepatoma cell line Hep G2.

The effect of trapidil derivative AR12456 on intracellular cholesterol metabolism was investigated in human hepatoma cell line HepG2. AR12456 enhanced the uptake and degradation of 125I-LDL in a dose-dependent manner. The drug inhibited cholesterol synthesis and esterification without affecting cellular cholesterol content and bile acid synthesis; cholesterol efflux was slightly increased. These results show that the inhibition of cholesterol synthesis together with the enhanced expression of LDL receptors may partially explain the hypocholesterolemic activity of compound AR12456.

Does a HDL injection reduce the development of serum hyperlipidemia and progression of fatty streaks in cholesterol fed rabbits?

The influences of homologous (rabbit) or heterologous (human) high density lipoprotein (HDL) on the development of serum hyperlipidemia and progression of fatty streaks were studied in cholesterol fed rabbits. Three groups of New Zealand rabbits were fed a 0.5% cholesterol rich diet for 8 weeks. Additionally into these animals the following solutions were injected intravenously two times per week: group 1 (control): saline; group 2: human HDL dissolved in saline; group 3: rabbit HDL dissolved in saline. The animals of group 2 had lower serum cholesterol levels during the dietary period than rabbits of group 1 (p < 0.05) but the surface of intima covered with fatty streaks was the same as in group 1. On the other hand, the serum cholesterol level in rabbits of group 3 was the same as in group 1 during the whole experimental period, but the surface of aorta covered with fatty streaks was significantly lower (p < 0.05) in group 3 than in group 1. The results of this study support the hypothesis of an antiatherogenic action of HDL, which seems to be independent of the influence of HDL on the serum lipids but depends on the source of HDL.

Interactions of prostanoids with the platelet activating factors.

The platelet-activating factor (PAF) induced a marked increase of the thromboxane (TX) B2-formation in the incubation medium of isolated myocardium and tissue from other organs. The content of the 6-oxo-prostaglandin (PG)F1 alpha, the inactive metabolite of PGI2, remained uninfluenced or showed a small decrease. PAF, given in a concentration of 2.10(-9) mol/l or a single dose of 100 ng, significantly reduced the contraction force and the coronary flow of isolated guinea-pig hearts. This effect was connected with a high efflux of TXA2. The PAF-antagonist, WEB 2086, nearly abolished the cardiac effects of PAF, and iloprost or a pretreatment with indomethacin markedly reduced the PAF-influence on the heart. The TXA2-antagonist BM 13177 was ineffective. The results indicate a close interaction between the myocardial PAF-effect and the TXA2-formation of the heart tissue, but gave no suggestion for a mediation of the PAF-effect by TXA2. The PAF-antagonistic action of WEB 2086, iloprost and indomethacin could be of some interest in the therapy of cardiovasculatory diseases.

Relevance of vasoactive mediators for the blood pressure effects of intravenous arachidonic acid injection in rats.

Vasopressor response and release of eicosanoids following intravenous injection of arachidonic acid (AA) were examined in normotensive rats. AA administration caused a rapid initial fall of arterial pressure followed by a brief rise and a subsequent prolonged fall in anesthetized rats. Immediately after AA injection the blood levels of TXB2 and 6-keto-PGF1 alpha, the stable metabolites of TXA2 and prostacyclin, rose, from 1.52 +/- 0.23 ng/ml to 176.4 +/- 42.6 ng/ml and from 4.05 +/- 0.67 ng/ml to 171.4 +/- 31.2 ng/ml, respectively. Blood pressure behaviour and eicosanoid blood level were influenced by different inhibitors and antagonists of vasoactive mediators. The cyclooxygenase inhibitor acetylsalicylic acid completely eliminated the second blood pressure depression after AA injection and simultaneously diminished TXB2 and 6-keto-PGF1 alpha formation in murine blood, whereas the TXA2 receptor antagonist BM 13.177 prevented the return of the blood pressure to preinjection level after the initial brief fall in arterial pressure. Although the TXA2 synthase inhibitor HOE 944 markedly inhibited TXB2 formation, no influence on AA-induced blood pressure changes could be registered. The receptor antagonist of platelet activating factor BN 52021 and the serotonin and histamine receptor antagonist cyproheptadine also reduced TXB2 amounts, in murine blood without any effects on blood pressure behaviour.

Electrophysiological responses of the cochlea to transient asphyxia are influenced by arachidonate metabolites.

The influence of a transient asphyxia on cochlear potentials, i.e. endolymphatic potential (EP), summating potential (SP) and cochlear microphonics (CM) was investigated in guinea pigs when pretreating the animals with various substances interacting with the arachidonic acid (AA) cascade at the level of thromboxane. The controls showed the well-known decline of the EP, CM and the SP increase. When infusing a thromboxane synthetase inhibitor (dazoxiben) or two different thromboxane receptor blockers (daltroban or sulotraban) before the 3-minutes' period of asphyxia was started, the electrophysiological responses of the inner ear (cochlea) could significantly be influenced. The results indicate that a shift of the thromboxane (TXA2)/prostacyclin (PGI2)-balance in favour of the last improve the metabolic conditions for a survival of the cochlea when it is challenged.