Antagonism of leukotriene B4 receptors does not limit canine myocardial infarct size

Injection of leukotriene (LT)B4 (0.1-3 micrograms/kg i.v.) in normal anesthetized dogs produced dose-related leukopenia that was accompanied by arterial hypotension and tachycardia at higher tested doses. LTD4 (0.1-3 micrograms/kg i.v.), in contrast, increased arterial blood pressure, lowered cardiac rate and produced little change in arterial blood leukocyte count. Continuous infusion of LY255283 [(1-(5-ethyl-2-hydroxy-4-(6-methyl-6-(1H-tetrazol-5-yl)- heptyloxy)phenyl)ethanone] (0.33 mg/kg/min i.v.), a selective LTB4 receptor antagonist, resulted in near complete inhibition of leukopenic, hypotensive and tachycardic responses to LTB4 (3 micrograms/kg i.v.) challenge over a 6-hr test period. Persistent antagonism of canine LTB4 receptors was associated with high circulating levels of LY255283 that were bound extensively to plasma proteins. In subsequent experiments, myocardial infarct size was measured following 1 hr of occlusion of the circumflex coronary artery and 5 hr of reperfusion in control dogs infused with vehicle, and in dogs receiving LY255283 (0.33 mg/kg/min i.v.). Drug and vehicle were infused continuously beginning 15 min before coronary artery occlusion. LY255283 treatment essentially did not alter base-line cardiovascular parameters or myocardial oxygen demand when alterations were compared to time-related changes observed in control dogs. LY255283 infusion also did not alter the degree of myocardial ischemia or the intensity and duration of cardiac arrhythmias associated with coronary artery occlusion and reperfusion. Resultant infarct sizes were 43 +/- 5% of the left ventricle placed at risk in control dogs and 32 +/- 5% in dogs given LY255283; this difference was not statistically significant. The extent of left ventricle placed at risk was similar between groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Primate myocardial and systemic hemodynamic responses to leukotriene D4: antagonism by LY171883

Intravenous infusion of leukotriene D4 (LTD4; 0.1-10 micrograms/kg) produced dose-related depression of myocardial contractility, stroke volume index and mean aortic blood flow index in anesthetized monkeys. These alterations occurred in association with increments in systemic and pulmonary vascular resistances, reductions in calculated left ventricular stroke and minute work indices and relatively little or no dose-dependent change in cardiac rate, mean arterial blood pressure and mean pulmonary arterial blood pressure. Left atrial pressure did not increase in response to LTD4, although cardiac afterload was increased and myocardial contractility and stroke volume index were reduced. LTD4-induced alterations of aortic blood flow index, stroke volume index and systemic and pulmonary vascular resistances were sustained during a 60-min observation period after drug infusion. Pretreatment of monkeys with LY171883 (10 mg/kg i.v.), a selective LTD4-LTE4 receptor antagonist, inhibited the magnitude and duration of hemodynamic responses to LTD4 challenge. For example, LTD4 dose-response curves for depression of mean aortic blood flow index, stroke volume index and left ventricular stroke and minute work indices were shifted to the right approximately 10-fold by LY171883. The plasma concentration of LY171883 effective in this regard was 32 +/- 7 micrograms/ml. Attenuation of LTD4 responses by LY171883 was selective in that antagonist pretreatment did not result in concomitant reduction of vascular and cardiac responses mediated by adrenergic receptors, and LY171883 administration produced little or no change in base-line parameters. The composite data provide evidence of the presence of LT receptors in the primate cardiovascular system and suggest that LY171883 is a selective antagonist of these receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

The new K+ channel opener Aprikalim (RP 52891) reduces experimental infarct size in dogs in the absence of hemodynamic changes.

The objective of the present study was to determine the effect of a novel K+ channel opener, Aprikalim (RP 52891; [trans-(-)-N-methyl-2-(3-pyridyl)-2-tetrahydrothio-pyran carbothiamide-1-oxide]), on myocardial infarct size in barbital-anesthetized dogs subjected to 90 min of left circumflex coronary artery occlusion followed by 5 hr of reperfusion. To determine if RP 52891 is mediating its effects by opening adenosine triphosphate regulated potassium channels (KATP), glibenclamide, a KATP channel antagonist was used. Dogs were pretreated with vehicle, a nonhypotensive dose of RP 52891 (10 micrograms/kg + 0.1 microgram/kg/min i.v.), glibenclamide (1 mg/kg; i.v. bolus) or RP 52891 (10 micrograms/kg and 0.1 microgram/kg/min i.v.) after pretreatment with glibenclamide (1 mg/kg i.v. bolus). At the end of reperfusion, myocardial infarct size was determined by triphenyltetrazolium staining. There were no significant differences in systemic hemodynamics, myocardial oxygen demand, collateral blood flow or ischemic bed size among groups with the exception of an increase in coronary blood flow to the ischemic area at 3 and 5 hr of reperfusion in both RP-treated groups. However, myocardial infarct size, expressed as a percentage of the area at risk, was significantly (P less than .05) reduced (38%) by RP 52891 and significantly increased (38%) by glibenclamide (vehicle, 39 +/- 4%; RP 52891, 24 +/- 2%; and glibenclamide, 54 +/- 5%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacology of LY175326: a potent cardiotonic agent with vasodilator activities

Compound LY175326 is one of a series of novel cardiovascular agents with both inotropic and vasodilator activities. In cat papillary muscles, LY175326 increased contractility in a concentration-dependent manner; these actions were not blocked by prazosin, propranolol or cimetidine. Inotropic responses were observed in unpaced, perfused guinea-pig hearts and these effects were associated with modest increases in heart rate and coronary flow. An i.v. dose of 0.1 mg/kg of LY175326 caused 54 and 95% increases in contractility in either the anesthetized cat or dog, respectively; corresponding heart rates were increased by less than 10%. Oral administration of 0.5 mg/kg to dogs was associated with an inotropic response that was maximal between 60 and 90 min and lasted in excess of 3 hr. These effects were not accompanied by increases in heart rate, gross behavioral changes or emesis. The pharmacology of LY175326 was evaluated in a propranolol-induced heart failure model using anesthetized beagle dogs. A bolus injection of 0.15 mg/kg of LY175326 followed by an infusion of 0.4 mg/kg/hr reversed the hemodynamic symptoms of heart failure by increasing left ventricular dP/dt60, cardiac output and stroke volume and reducing left atrial filling pressure and vascular resistance; heart rate was unchanged and calculated myocardial oxygen consumption was reduced. This balance of inotropic:vasodilator activities may provide a means of improving cardiac function while maintaining the myocardial oxygen supply:demand.     

Inhibition of leukotriene D4-induced coronary vasoconstriction by leukotriene antagonists in the anesthetized dog

Anesthetized open-chest dogs were instrumented for the measurement of left circumflex coronary artery (LCX) blood flow and aortic blood flow, systemic arterial blood pressure, heart rate, lead II ECG, left ventricular end-diastolic pressure, left ventricular developed pressure and left ventricular positive and negative dP/dt to study the hemodynamic effects of leukotriene D4 (LTD4) and selective LTD4 antagonists on the coronary vasculature. Administration of LTD4 alone into the LCX (0.625-10 micrograms) produced a dose-dependent decrease in LCX blood flow, dP/dt and aortic blood flow and an increase in left ventricular end-diastolic pressure. Systemic arterial blood pressure, left ventricular developed pressure and heart rate were unchanged by LTD4. During i.v. infusions of the LTD4 antagonists, SK&F 102922 or FPL 55712 (1 mg/kg/min), the dose-dependent decreases in LCX flow, dP/dt and aortic blood flow were blocked whereas the increase in left ventricular end-diastolic pressure remained unchanged. The thromboxane A2 antagonist, SK&F 88046 (5 mg/kg + 0.1 mg/kg/min), which has been reported previously to block the coronary blood flow reducing action of LTC4, had no effect on the LCX blood flow responses to intracoronary LTD4. In a separate study, dogs instrumented in a similar manner were given bolus injections of arginine-vasopressin (1 microgram), the thromboxane A2 mimetic, U-46619 (10 micrograms), LTD4 (10 micrograms), angiotensin II (1 microgram) and prostaglandin F2 alpha (100 micrograms) directly into the LCX to provoke coronary vasoconstriction. SK&F 102922 and FPL 55712 selectively blocked the coronary vasoconstriction produced by LTD4, but had no effect on vasoconstriction produced by the other agonists.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of serotonin (5HT2) receptor blockade in myocardial reperfusion injury: effects of LY53857 in a canine model of myocardial infarction.

The potential protective effects of serotonin receptor antagonism during the process of acute myocardial infarction were studied in anesthetized male dogs, which were subjected to a 90-min left circumflex coronary artery occlusion followed by 5 h of reperfusion. Either vehicle (0.9% NaCl) or the serotonin (5HT2) receptor antagonist LY53857 was infused i.v. at a dose of 0.5 mg/kg, followed by a constant infusion of 2 mg/kg/min beginning 5 min before left circumflex coronary artery occlusion and continuing throughout the duration of the ischemia and subsequent reperfusion. Verification of functional 5HT2 receptor antagonism in the circulating blood of the LY53857-treated dogs was monitored throughout the experiments by periodic assessment of ex vivo platelet reactivity to exogenous serotonin. After 5 h of reperfusion, the hearts were excised and analyzed utilizing histochemical staining with triphenyltetrazolium, which demarcates myocardial infarct size and anatomical area of myocardium at risk of infarction. There was not a significant reduction of infarct size with LY53857 treatment: control infarct/area at risk = 38.6 +/- 4.7%, n = 9 LY53857 infarct/area at risk = 33.4 +/- 3.8%, n = 6. Similarly, when myocardial infarct size was analyzed as a function of myocardial collateral blood flow, there were no significant effects of drug treatment on the relationship between collateral blood flow and infarct size. The effects of 5HT on neutrophil activation were determined by measuring the potential ability of 5HT to enhance the chemotactic peptide-induced production of superoxide. 5HT did not activate human neutrophils in vitro and LY53857 had no effect on neutrophil superoxide production.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calcium entry blockade by nitrendipine and alpha adrenergic responsiveness in vivo: comparison of systemic vs. local effects

Calcium entry blockade in vivo preferentially antagonizes systemic pressor responses to alpha-2 adrenergic agonists, whereas relatively sparing alpha-1 mediated vasoconstriction; however, in vitro studies have given results discordant from those obtained in vivo. Because of these discrepancies we have compared the effect of calcium entry blockade by nitrendipine on systemic and local (autoperfused hindquarters) pressor responses to selective adrenergic agonists: cirazoline for alpha-1 and B-HT 920 for alpha-2. Pithed, vagotomized, normotensive Sprague-Dawley rats were used. Confirming previous results, nitrendipine (3.0 and 30.0 micrograms/kg/min X 15 min) selectively antagonized the systemic pressor responses to B-HT 920 without affecting significant responses to the alpha-1 agonist. However, in the isolated, autoperfused hindquarters of pithed rats, these same doses of nitrendipine depressed by 36 and 45% the maximum vasoconstrictor response to cirazoline. Because no significant vasoconstrictor response to B-HT 920 could be demonstrated in this same preparation, we induced supersensitivity to the alpha-2 agonist by reserpine pretreatment (0.3 mg/kg X 3 days). Reserpine increased vascular responsiveness to B-HT 920, without modifying its selective alpha-2 agonistic properties, as assessed by the use of selective alpha-1 (prazosin, 0.5 mg/kg i.v.) and alpha-2 (rauwolscine, 0.5 mg/kg i.v.) antagonists and of phentolamine (5 mg/kg i.v.), a nonspecific alpha adrenergic antagonist. After pretreatment with reserpine, nitrendipine antagonized both the B-HT 920-mediated vasoconstriction (by an average of 40 and 57%, respectively) and the cirazoline alpha-1-mediated vasoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of leukocytes in acute myocardial infarction in anesthetized dogs: relationship to myocardial salvage by anti-inflammatory drugs

The invasion of leukocytes into and around a myocardial infarct was studied in chloralose-anesthetized dogs subjected to 1-hr occlusion of the left anterior descending coronary artery and reperfused for periods up to 5 hr. Polymorphonuclear leukocytes adhering to the endothelium of blood vessels within the ischemic area are evident at the end of the occlusion period. During reperfusion, the leukocytes migrate into the myocardium and large groups of cells can be observed "streaming" toward the irreversibly damaged area after 5 hr reperfusion. Infarcted tissue produces 10 times more 12-hydroxyeicosatetraenoic acid (a metabolite attributed to the invading leukocytes) from arachidonic acid than adjacent "normal" areas of the ventricle. BW755C (10 mg/kg-1 i.v.), which inhibits both the lipoxygenase and cyclooxygenase pathways of arachidonic metabolism, attenuates leukocyte infiltration into the infarcted myocardium, prevents 12-hydroxyeicosatetraenoic acid formation and significantly reduces infarct size (P less than .005). BW755C also significantly diminishes the incidence of cardiac arrhythmias during infarction. In animals where circulating white cells are reduced 60% by treatment with hydroxyurea (20 mg/kg-1 i.v./day for 5 days), there is also a smaller infarct (P less than .01). Indomethacin (5 mg/kg-1 i.v.) and dexamethasone (0.2 mg/kg-1 i.v.), which do not affect leukocyte migration into the ischemic myocardium, do not reduce infarct size. It is proposed that migrating leukocytes contribute to the tissue injury accompanying myocardial ischemia, possibly by the release of proinflammatory mediators such as lipoxygenase products, free radicals (oxygen metabolites) and hydrolytic enzymes. Drugs which reduce the migration and/or activation of leukocytes may be useful in reducing infarct size.     

Effects of KR-33028, a novel Na+/H+ exchanger-1 inhibitor, on ischemia and reperfusion-induced myocardial infarction in rats and dogs

The present study was performed to evaluate the cardioprotective effects of KR-33028, a novel Na+/H+ exchanger subtype 1 (NHE-1) inhibitor, in rat and dog models of coronary artery occlusion and reperfusion. In anesthetized rats subjected to a 45-min coronary occlusion and a 90-min reperfusion, KR-33028 at 5 min before occlusion (i.v. bolus) dose-dependently reduced myocardial infarct size from 58.0% to 46.6%, 40.3%, 39.7%, 33.1%, and 27.8% for 0.03, 0.1, 0.3, 1.0, and 3.0 mg/kg respectively (P < 0.05). In anesthetized beagle dogs that underwent a 1.0-h occlusion followed by a 3.0-h reperfusion, KR-33028 (3 mg/kg, i.v. bolus) markedly decreased infarct size from 45.6% in vehicle-treated group to 16.4% (P < 0.05), and reduced the reperfusion-induced release in creatine kinase myocardial band isoenzyme (MB), lactate dehydrogenase, troponin-I, glutamic oxaloacetic transaminase, and glutamic pyruvic transaminase. In separate experiments to assess the effects of timing of treatment, KR-33028 (1 mg/kg, i.v. bolus) given 10 min before or at reperfusion in rat models also significantly reduced the myocardial infarct size (46.3% and 44.1% respectively) compared with vehicle-treated group. In all studies, KR-33028 caused no significant changes in any hemodynamic profiles. In an isolated rat heart model of hypothermic cardioplegia, KR-33028 (30 mum), which was added to the heart preservation solution (histidin-tryptophan-ketoglutarate) during hypothermic cardioplegic arrest, significantly improved the recovery of left ventricular developed pressure, heart rate and dP/dt(max) after reperfusion. Taken together, these results indicate that KR-33028 significantly reduced the myocardial infarction induced by ischemia and reperfusion in rats and dogs, without affecting hemodynamic profiles.     

Adrenergic mechanisms underlying cardiac and vascular responses to cocaine in conscious rats.

The contribution of adrenergic receptors to the cardiovascular responses to cocaine (5 mg/kg i.v.) were examined in conscious, free-moving rats instrumented for continuous measurement of arterial pressure, heart rate and blood flows in the mesentery and hindquarters or ascending aorta. Cocaine elicits an immediate (peak) and sustained pressor response with a concomitant reduction in heart rate. Prazosin (0.1 mg/kg i.v.) pretreatment significantly reduced both the peak and sustained pressor responses by attenuating the increases in systemic, mesenteric and hindquarters vascular resistances. Idazoxan pretreatment (1 mg/kg i.v.) attenuated the peak increase in hindquarters vascular resistance. Whereas propranolol pretreatment (1 mg/kg i.v.) attenuated the peak pressor response, the sustained pressor response was enhanced due to increased hindquarters and systemic vascular resistances. Metoprolol pretreatment (1 mg/kg i.v.) enhanced the sustained pressor response to cocaine, in part due to increased heart rate and mesenteric vascular resistances. Upon examination of the cardiac effects of cocaine, a sustained bradycardic response was observed, whereas stroke volume and cardiac output were relatively unaffected. The bradycardic response to cocaine was attenuated by yohimbine (0.1 mg/kg i.v.), prevented by prazosin and converted to a tachycardia after idazoxan (1 mg/kg) pretreatment. After propranolol pretreatment, cocaine substantially decreased cardiac output and stroke volume. Our results demonstrate that cocaine produces a biphasic pressor response in conscious rats and that the mechanisms underlying the dual responses vary in intensity and mode of action in different vascular beds, but are primarily dependent upon alpha-1 adrenergic receptor-mediated vasoconstriction.     

Characterization of adrenoceptor subtypes in cat cutaneous vasculature

Experiments were undertaken to characterize the relative contribution of adrenoceptor subtypes in mediating vasoconstriction to exogenous agonists in the digital cutaneous vascular bed of the anesthetized cat using laser-Doppler flowmetry. Intra-arterial administration of (-)-epinephrine and (-)-norepinephrine into the brachial artery caused a dose-related vasoconstriction (decreased flow) with ED50 values of 7 and 21 ng, respectively. Blockade of beta adrenoceptors with propranolol did not alter the response to (-)-epinephrine nor did i.a. isoproterenol produce a significant vasodilation. Vasoconstrictor responses elicited by (-)-epinephrine and (-)-norepinephrine were antagonized by treatment with phentolamine (2.5 mg/kg i.v.) and by yohimbine (0.5 mg/kg i.v.) but were only marginally blocked by prazosin (0.1 mg/kg i.v.). A dose-related depression of cutaneous blood flow was also caused by clonidine at doses virtually identical to those of (-)-norepinephrine. Clonidine-induced vasoconstriction was antagonized by rauwolscine (0.5 mg/kg i.v.) but not by prazosin (0.1 mg/kg i.v.). Dose-response curves to a variety of additional adrenoceptor stimulants were constructed with the potency rank order for all agonists being: (-)-epinephrine greater than B-HT 920 = (-)-norepinephrine = clonidine much greater than (-)-phenylephrine much greater than B-HT 933 greater than methoxamine. Treatment with prazosin (0.1 mg/kg i.v.) antagonized methoxamine induced cutaneous vasoconstriction but not the decreased blood flow caused by B-HT 933. In contrast, rauwolscine (0.5 mg/kg i.v.) blocked the responses to B-HT 933 but not methoxamine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Protection from myocardial ischemia/reperfusion injury by a positive allosteric modulator of the A₃ adenosine receptor

Adenosine is increased in ischemic tissues where it serves a protective role by activating adenosine receptors (ARs), including the A? AR subtype. We investigated the effect of N-{2-[(3,4-dichlorophenyl)amino]quinolin-4-yl}cyclohexanecarboxamide (LUF6096), a positive allosteric modulator of the A? AR, on infarct size in a barbital-anesthetized dog model of myocardial ischemia/reperfusion injury. Dogs were subjected to 60 min of coronary artery occlusion and 3 h of reperfusion. Infarct size was assessed by macrohistochemical staining. Three experimental groups were included in the study. Groups I and II received two doses of vehicle or LUF6096 (0.5 mg/kg i.v. bolus), one administered before ischemia and the other immediately before reperfusion. Group III received a single dose of LUF6096 (1 mg/kg i.v. bolus) immediately before reperfusion. In preliminary in vitro studies, LUF6096 was found to exert potent enhancing activity (EC?? 114.3 ± 15.9 nM) with the canine A? AR in a guanosine 5'-[γ-[3?S]thio]triphosphate binding assay. LUF6096 increased the maximal efficacy of the partial A? AR agonist 2-chloro-N?-(3-iodobenzyl)adenosine-5'-N-methylcarboxamide and the native agonist adenosine more than 2-fold while producing a slight decrease in potency. In the dog studies, administration of LUF6096 had no effect on any hemodynamic parameter measured. Pretreatment with LUF6096 before coronary occlusion and during reperfusion in group II dogs produced a marked reduction in infarct size (～50% reduction) compared with group I vehicle-treated dogs. An equivalent reduction in infarct size was observed when LUF6096 was administered immediately before reperfusion in group III dogs. This is the first study to demonstrate efficacy of an A? AR allosteric enhancer in an in vivo model of infarction.     

The bronchopulmonary pharmacology of SK&F 104353 in anesthetized guinea pigs: demonstration of potent and selective antagonism of responses to peptidoleukotrienes

The bronchopulmonary pharmacology of SK&F 104353 [2(S)-hydroxy-3(R)-[2(2-carboxyethyl)thio]-3[2-(8- phenyloctyl)phenyl]-propanoic acid], a potent and selective leukotriene (LT) receptor antagonist in vitro, was assessed in anesthetized, spontaneously breathing guinea pigs. Aerosol administration of SK&F 104353 (5-2000 micrograms/ml x 100 breaths) reduced in a concentration-dependent manner the response to a standard LTD4 challenge (4.33 micrograms/ml x 5 breaths) given 30 min later. Inhalation of a 2000 micrograms/ml solution abolished LTD4-induced bronchoconstriction for at least 2 hr. The i.v. administration of SK&F 104353 10 min before challenge antagonized LTD4-induced bronchoconstriction with an ID50 of 0.55 mumol/kg (0.25 mg/kg). Substantial antagonism of LTD4-induced bronchospasm was observed for at least 60 min after i.v. administration of 5 mumol/kg of SK&F 104353. Infusion of SK&F 104353 at various rates revealed that a steady-state plasma concentration of 0.125 microM (0.06 micrograms/ml) reduced LTD4-induced bronchoconstriction by 60%. In addition to preventing the response to LTD4, i.v. administered SK&F 104353 (10 mumol/kg) rapidly and completely reversed ongoing LTD4-induced bronchoconstriction. SK&F 104353 also was effective when given intraduodenally 1 hr before LTD4 challenge, although the ID50 (32 mumol/kg) was 60-fold greater than the i.v. ID50. Given intragastrically, 100 mumol/kg of SK&F 104353 abolished the response to LTD4 for 1 hr, and reduced the response for 6 hr. SK&F 104353 (20 mumol/kg i.v.) had no effect on the bronchoconstriction induced by aerosolized acetylcholine, histamine or U-44069, but did antagonize the response to LTC4. SK&F 104353 alone did not produce bronchoconstriction when administered by any route or dose.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antagonism of leukotriene C4, leukotriene D4 and leukotriene E4 vasoconstrictor responses in the conscious rat with the peptidoleukotriene receptor antagonist SK&F 104353: evidence for leukotriene D4 receptor heterogeneity

The purpose of these experiments was to investigate the effects of the selective peptidoleukotriene receptor antagonist, SK&F 104353, on leukotriene (LT)C4, LTD4 and LTE4 vasopressor responses in conscious, normotensive rats. Steady-state plasma concentrations of SK&F 104353 at infusion rates of 0.2 mg/kg + 1 mg/kg/hr, 1 mg/kg + 3 mg/kg/hr or 2 mg/kg + 10 mg/kg/hr were 0.5, 1.6 and 9.4 micrograms/ml, respectively, indicating that the plasma concentrations of SK&F 104353 were related directly to the infusion rate. LTC4, LTD4 and LTE4 (0.17-170 nmol/kg i.v.) produced dose-dependent increases in mean blood pressure. The ED20 dose (i.e., dose required to increase blood pressure 20 mm Hg) of LTC4, LTD4 or LTE4 was 2.7 +/- 0.4, 2.2 +/- 0.3 and 109 +/- 17 nmol/kg, respectively. SK&F 104353 produced dose-dependent, parallel shifts to the right in the LTC4 dose-response curve. Administration of SK&F 104353 at doses of 0.2 mg/kg + 1 mg/kg/hr, 1 mg/kg + 3 mg/kg/hr or 2 mg/kg + 10 mg/kg/hr produced dose ratios (i.e., ratio of ED20 in presence of SK&F 104353 to that of the vehicle group) of 6, 12 and 26, respectively. Against LTD4 responses, SK&F 104353 at doses of 0.1 mg/kg + 0.3 mg/kg/hr or 0.2 mg/kg + 1 mg/kg/hr produced dose ratios of 3 and 9, respectively. At a dose of 1 mg/kg + 3 mg/kg/hr, there was no further increase in the dose ratio, whereas a dose of 2 mg/kg + 10 mg/kg/hr resulted in a dose ratio of greater than 100.(ABSTRACT TRUNCATED AT 250 WORDS)     

Muscarinic cholinergic and beta-adrenergic contribution to hindquarters vasodilation and cardiac responses to cocaine

Cocaine produces a pressor response associated with an initial hindquarters vasoconstriction followed by a prolonged vasodilation in conscious rats. Propranolol pretreatment prevented the vasodilation and enhanced the pressor response, whereas atropine methylbromide pretreatment reduced the increase in systemic vascular resistance. We studied the role of selective muscarinic and beta-adrenoceptor antagonists on responses to cocaine in rats with an increase in systemic vascular resistance to cocaine (vascular responders). Arterial blood pressure and ascending aortic and distal descending aortic blood flow using pulsed Doppler flowmetry were measured. In conscious rats, cocaine (5 mg/kg i.v.) elicited consistent pressor responses but variable systemic and hindquarters vascular resistance responses that were directly correlated, suggesting that skeletal muscle resistance responses comprise an important component of systemic vascular resistance. ICI 118551 [(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)-amino]-2-butanol] (0.5 mg/kg i.v.) pretreatment prevented the hindquarters vasodilation, enhancing the increase in systemic vascular resistance and the pressor response while further depressing the cardiac output response, similar to the effects of propranolol. Atenolol (1 mg/kg) pretreatment attenuated the stroke volume and cardiac output responses while enhancing the increase in systemic vascular resistance without affecting the hindquarters responses. In contrast, M2 antagonist methoctramine (0.3 mg/kg) pretreatment had similar effects as atropine in reducing the decrease in cardiac output by reducing the increase in systemic vascular resistance, whereas the M1 antagonist pirenzipine (0.02 mg/kg) did not alter responses. Therefore, the cocaine-induced pressor response is ameliorated by beta2-adrenoceptor mediated skeletal muscle vasodilation, whereas the decrease in cardiac output and the increase in systemic vascular resistance are dependent on M2-cholinoceptor activation.     

Cardiac electrophysiologic and hemodynamic activity of pimobendan (UD-CG 115 BS), a new inotropic agent

The hemodynamic and cardiac electrophysiologic properties of pimobendan, a new pyridazinone-benzimidazole type inotropic agent, were studied in urethane-anesthetized dogs. The cumulative i.v. administration of 0.1, 0.3, and 1.0 mg/kg pimobendan caused a dose-dependent decrease in mean arterial pressure and an increase in sinus heart rate. When heart rate was maintained constant by overdrive atrial pacing, hemodynamic changes in response to pimobendan consisted of dose-related increases in right ventricular isometric contractile force (P less than .05 at 0.3 and 1.0 mg/kg), left ventricular +dP/dt (P less than .05 at 0.3 and 1.0 mg/kg), and left circumflex coronary artery blood flow (P less than .05 at 1.0 mg/kg). Increases in each of the aforementioned hemodynamic parameters were sustained for up to 4 hr after the i.v. administration of 1.0 mg/kg pimobendan. The cardiac electrophysiologic changes associated with pimobendan administration included decreases in the atrial (P less than .05 at 1.0 mg/kg), ventricular (P less than .05 at 0.3 and 1.0 mg/kg), and atrioventricular nodal functional (P less than .05 at 0.3 and 1.0 mg/kg) and effective (P less than .05 at 1.0 mg/kg) refractory periods. Atrioventricular conduction velocity was enhanced after pimobendan, as indicated by a shortening of the AH (P less than .05 at 0.3 mg/kg and at 1.0 mg/kg) and PR intervals (P less than 0.05 at 1.0 mg/kg). Pretreatment with propranolol (0.5 mg/kg i.v.) attenuated the pimobendan-induced decrease in the ventricular refractory period and the increase in heart rate, whereas the decrease in arterial pressure was enhanced. These results indicate that the i.v. administration of pimobendan to anesthetized dogs produces a dose-related positive inotropic effect, coronary and peripheral vasodilation, and cardiac electrophysiologic effects that include decreases in atrial, atrioventricular, and ventricular refractoriness as well as a facilitation of atrioventricular conduction. The observed electrophysiologic changes may be mediated, in part, by a baroreceptor-mediated increase in sympathetic nervous system activity.     

Hemodynamic effects of GI 87084B, an ultra-short acting mu-opioid analgesic, in anesthetized dogs.

Hemodynamic effects of GI 87084B, a novel ultra-short acting mu-opioid agonist, were studied in anesthetized dogs. In these studies, GI 87084B (0.3-20 nmol/kg/min i.v.) produced dose-dependent decreases in heart rate, arterial blood pressure, +dP/dt and cardiac output. Alfentanil produced similar effects but was less potent. After termination of the infusions (397 nmol/kg cumulative dose), the hemodynamic effects of GI 87084B dissipated over 30 to 40 min. The effects of alfentanil, however, persisted throughout the 60-min recovery period. Infusion of GI 87084B at lower dose rates (0.001-0.3 nmol/kg/min) in barbiturate-anesthetized dogs showed a threshold dose of 0.1 to 0.3 nmol/kg/min for effects on hemodynamic variables. After infusing 0.3 nmol/kg/min of GI 87084B for 10 min, hemodynamic variables recovered rapidly (10-20 min). Boli of GI 87084B (0.3-100 nmol/kg i.v.) produced dose-dependent decreases in the same hemodynamic variables. The duration of these effects increased from 5 to 20 min at 3 nmol/kg to 15 to 45 min at 100 nmol/kg. Naloxone (0.063 mg/kg/hr) decreased the magnitude and duration of the effects of GI 87084B, suggesting that these effects are mediated through opioid receptors. In summary, GI 87084B produced hemodynamic effects consistent with mu-opioid agonism when administered by infusion or bolus, but these effects were brief in duration compared to other opioids.     

Possible inhibitory role of endogenous gamma-aminobutyric acid in the nonnicotinic functions of the dog cardiac sympathetic ganglia

Participation of endogenous gamma-aminobutyric acid (GABA) in the functions of dog cardiac ganglia was investigated. The ganglionic stimulants as well as agents affecting GABA system were given directly into the cardiac sympathetic ganglia through the right subclavian artery (i.a.), unless otherwise mentioned. Inhibition of endogenous GABA degradation by the GABA-transaminase inhibitor, aminooxyacetic acid (AOAA) administered 10 mg/kg i.v. 2 hr before completion of surgical procedures did not alter the positive chronotropic responses to bethanecol (25 and 50 micrograms) and acetylcholine (25, 50 and 100 micrograms) but reduced markedly those to angiotensin II (1 and 2 micrograms). This reduction was antagonized by picrotoxin (5 mg). Diazepam given 10 mg/kg i.v. also inhibited the ganglionic responses to angiotensin II in both untreated and AOAA-pretreated dogs, this inhibition by diazepam being more marked in the AOAA-pretreated than in the untreated dogs. The same dose of diazepam did not affect the responses to acetylcholine but reduced to a certain degree of responses to bethanechol in the AOAA-pretreated dogs. These inhibitions by diazepam were also reversed by picrotoxin. After i.v. treatment with the glutamic acid decarboxylase inhibitors, 3-mercaptopropionic acid (50 mg/kg) or isoniazid (200 mg/kg), the ganglionic responses to angiotensin II were not altered, but inhibitory effects of diazepam on the responses to angiotensin II were eliminated after 3-mercaptopropionic acid but not after isoniazid. These results suggest that endogenous GABA may play an inhibitory role in the nonnicotinic ganglionic pathways and that diazepam probably exerts a ganglionic action through endogenous GABAergic mechanisms.     

Pharmacological characterization of LY233053: a structurally novel tetrazole-substituted competitive N-methyl-D-aspartic acid antagonist with a short duration of action

This study reports the activity of a structurally novel excitatory amino acid receptor antagonist, LY233053 [cis-(+-)-4-[(2H-tetrazol-5-yl)methyl]piperidine-2-carboxylic acid], the first tetrazole-containing competitive N-methyl-D-aspartic acid (NMDA) antagonist. LY233053 potently inhibited NMDA receptor binding to rat brain membranes as shown by the in vitro displacement of [3H] CGS19755 (IC50 = 107 +/- 7 nM). No appreciable affinity in [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) or [3H]kainate binding assays was observed (IC50 values greater than 10,000 nM). In vitro NMDA receptor antagonist activity was further demonstrated by selective inhibition of NMDA-induced depolarization in cortical wedges (IC50 = 4.2 +/- 0.4 microM vs. 40 microM NMDA). LY233053 was effective after in vivo systemic administration in a number of animal models. In neonatal rats, LY233053 selectively blocked NMDA-induced convulsions (ED50 = 14.5 mg/kg i.p.) with a relatively short duration of action (2-4 hr). In pigeons, LY233053 potently antagonized (ED50 = 1.3 mg/kg i.m.) the behavioral suppressant effects of 10 mg/kg of NMDA. However, a dose of 160 mg/kg, i.m., was required to produce phencyclidine-like catalepsy in pigeons. In mice, LY233053 protected against maximal electroshock-induced seizures at lower doses (ED50 = 19.9 mg/kg i.p.) than those that impaired horizontal screen performance (ED50 = 40.9 mg/kg i.p.). Cholinergic and GABAergic neuronal degenerations after striatal infusion of NMDA were prevented by single or multiple i.p. doses of LY233053. In summary, the antagonist activity of LY233053 after systemic administration demonstrates potential therapeutic value in conditions of neuronal cell loss due to NMDA receptor excitotoxicity. The relatively short duration of action of LY233053 may make this compound particularly advantageous as a neuroprotective agent in the treatment of acute conditions such as cerebral ischemia.     

Beneficial effects of two specific bradycardic agents AQ-A39 (falipamil) and AQ-AH 208 on reversible myocardial reperfusion damage in anesthetized dogs

The effects of 2 specific bradycardic agents, AQ-A39 [5,6-dimethoxy-2-[3-([alpha-(3,4-dimethoxy)phenylethyl]methyl-amino) propyl]phthalimidine hydrochloride] and AQ-AH 208 [3,4-dihydro-6,7-dimethoxy-2-]3-[(2-[3, 4-dimethoxy-phenyl)-ethyl]-aminoethyl]propyl[-1(2H)isoquinone] were evaluated for their effects on subendocardial segment shortening (% SS) as measured by sonomicrometry and regional myocardial blood flow (radioactive microspheres) in anesthetized dogs subjected to a 15 min left anterior descending coronary occlusion followed by 3 hr of reperfusion. AQ-A39 (2.5-mg/kg bolus + 100 micrograms/kg/min i.v.) and AQ-AH 208 (0.5-mg/kg bolus + 25 micrograms/kg/min i.v.) were administered 15 min before coronary occlusion, during occlusion and throughout reperfusion. Both agents produced equivalent reductions in heart rate (24%) and the heart rate-systolic pressure product (27%) without any other significant hemodynamic changes. Collateral blood flow to the ischemic area was not different between the drug-treated and control groups. During coronary occlusion and throughout reperfusion, however, % SS was significantly (P less than .05) improved by both agents in the ischemic-reperfused area as compared to a control group. Thus, the beneficial actions of AQ-A39 and AQ-AH 208 on improving the recovery of subendocardial contractile function (% SS) may be explained partially by a reduction in myocardial oxygen requirements as a result of bradycardia. These results suggest that specific bradycardic agents may have potential for treatment of certain types of myocardial ischemia.