Pharmacological analysis of the tachycardia produced by histamine and specific H1-and H2-receptor agonists in anesthetized dogs

Antithrombotic activity of NSP-513, a novel selective phosphodiesterase 3 inhibitor, on femoral arterial thrombosis induced by physical stenosis and electrical current: comparison of antithrombotic and hemodynamic effects

NSP-513, a novel potent and selective phosphodiesterase 3 (PDE 3) inhibitor, and cilostazol, a previously developed PDE 3 inhibitor, were compared with respect to antiplatelet, antithrombotic, and hemodynamic effects. In the in vitro antiplatelet aggregation studies, NSP-513 and cilostazol inhibited collagen-induced canine platelet aggregation with median inhibitory concentration (IC50) values of 0.093 and 3.1 miccroM, respectively, and inhibited adenosine diphosphate (ADP)-induced canine platelet aggregation with IC50 values of 0.15 and 12 microM, respectively. For ADP-induced platelet aggregation, the presence of prostaglandin E1 (PGE1; 3 and 10 nM) further decreased the IC50 values for NSP-513 to 0.11 and 0.032 microM, respectively. In ex vivo antiplatelet aggregation studies, orally administered NSP-513 (0.03-1 mg/kg) and cilostazol (50 mg/kg) inhibited collagen-induced canine platelet aggregation. In an in vivo canine femoral arterial thrombosis model, intraduodenally administered NSP-513 (0.01-0.03 mg/ kg) dose-dependently prevented thrombus formation without any changes in blood pressure, heart rate, or bleeding time. In conscious dogs, NSP-513 at oral doses of > or =0.3 mg/kg produced hemodynamic changes such as decreased blood pressure and increased heart rate and LVdP/dt(max). Thus the minimal hemodynamically effective dose of NSP-513 was 0.3 mg/kg, and the hemodynamic effects of this dose were comparable to those of 50 mg/kg of cilostazol. In conclusion, these data suggest that NSP-513 has in vivo selectivity for antiplatelet and antithrombotic activities over hemodynamic activity, and that the selectivity of NSP-513 is higher than that of cilostazol in dogs.

Cardiotoxicity of vasodilators and positive inotropic/vasodilating drugs in dogs: an overview

Standard toxicological studies in dogs using high doses of vasodilators and positive inotropic/vasodilating agents give rise to a species-specific cardiotoxicity. The reason may be the extreme sensitivity of the dog to the pharmacological effects of these drugs; exaggerated pharmacodynamic effects and prolonged disturbance of homeostasis mechanisms often are responsible for the observed organ lesions. An assessment of the toxicological relevance and the risk for patients taking the drugs at therapeutic doses cannot be made without taking into account their pathomechanisms and the pathophysiological basis of the exceptional reaction patterns occurring in dogs. A large series of vasodilating and positive inotropic agents are presented, their pharmacological properties are described, and toxicological effects in dogs are compared. In view of the poor correlation between the distinct cardiac lesions induced in dogs and a lack of comparable toxicity in humans, it appears desirable to reassess the adequacy of the standard toxicological approaches for these substances.

Positive inotropic effects of histamine in anaesthetized dogs

1 The cardiovascular effects of histamine were examined in dogs anaesthetized with pentobarbitone 2 The effect of histamine on heart rate, blood pressure, left ventricular pressure, dP/dtmax and dP/dt: IIT (integrated isometric tension) was compared in the presence and absence of autonomic reflexes and blood pressure control. 3 In innervated animals with no attempt to control blood pressure, histamine produced dose-dependent decreases in blood pressure and heart rate and either positive or negative inotropic actions. 4 When autonomic reflexes were abolished, this variability in inotropic response was reduced and histamine produced a slight positive inotropic response. There was a decrease in blood pressure and a positive chronotropic response to histamine. 5 When blood pressure was controlled and the cardiac nerves were intact, histamine produced a decrease in heart rate. However, in the denervated animals, there was a slight increase in heart rate. 6 Inotropic responses to histamine in the blood pressure controlled groups were less variable than when blood pressure was uncontrolled. In all of these animals there was an increase in contractility, the increase being more marked in the denervated group. 7 The H2-receptor agonist impromidine produced a positive inotropic action in intact animals with uncontrolled blood pressure.

Chronotropic cardiac effects of histamine in the conscious dog with chronic atrioventricular block: interactions with the autonomic nervous system

Chronotropic effects of histamine and dimaprit were studied in the conscious dog with chronic atrioventricular block. Histamine at 0.2-5 micrograms/kg and dimaprit at equimolar doses (i.e. 0.25-6.25 micrograms/kg) increased atrial rate dose-relatedly. Blockade of muscarinic receptors reduced these effects and simultaneous blockade of muscarinic and beta-adrenergic receptors abolished them. Histamine and dimaprit moderately increased ventricular rate. Blockade of muscarinic receptors did not modify these effects, but blockade of beta-adrenoceptors with or without simultaneous blockade of muscarinic receptors suppressed them. After blockade of beta-adrenoceptors, histamine and more rarely dimaprit sometimes decreased atrial and ventricular rates. These effects were prevented by additional muscarinic blockade. Histamine and dimaprit lowered mean blood pressure to the same degree before and after each antagonist. The positive chronotropic effects of histamine and dimaprit, at these doses, are probably reflex responses to their hypotensive effects. The negative chronotropic effects of histamine after pindolol are due to muscarinic receptor activation. No evidence was found to implicate histamine-specific receptors in any of the chronotropic effects of histamine and dimaprit.

Short-term toxicological studies with impromidine (SK&F 92676): a specific histamine H2-receptor agonist

Impromidine is a specific, potent histamine H2-receptor agonist. The present paper describes the results of acute and short-term repeated-dose toxicity studies with impromidine in rodents and dogs. The intravenous LD50 was 9.6 mg kg-1 in male mice, 12.7 mg kg-1 in female mice and 25.5 mg kg-1 in rats. The minimum lethal dose in dogs after 30 min intravenous infusion was 27.7 mg kg-1 during the infusion and 4 mg kg-1 within 14 days. In 12- or 14-day tests in rats by the intravenous (maximum dose 3.24 mg per kg per day) and subcutaneous (maximum dose 20 mg per kg per day) routes, impromidine had no serious toxicological effects. In 12- or 14-day studies in dogs by the i.v. (maximum dose 0.259 mg per kg per day) and intramuscular (maximum dose 0.5 mg per kg per day) routes, impromidine caused vasodilation, tachycardia and vomiting. In a few dogs, at the highest dose levels, there was erosion and irritation of the gastrointestinal tract and myocardial damage. Other minor pathological changes were seen in the liver, kidneys and pancreas. No changes were seen other than those to be expected from the pharmacological actions of impromidine as an H2-receptor agonist. Studies in anaesthetized rats, either spontaneously respiring or sustained by artificial respiration, indicate that, at acute doses, impromidine causes death by respiratory failure.

Epidemiology of adverse reactions in anaesthesia in Germany and the Netherlands

We consider 292 anaphylactoid reactions from the own material (1964-1980) out of two different Departments of anesthesiology to attempt to review the continental epidemiology of this life-threatening adverse drug reaction. We conclude the following: The specialty of this type of adverse reaction is likely to prove a special separating name i.e. "anaphylactoid reaction". The overall frequency can be estimated for 1 : 400-600 in anesthesiology, a preponderance of females may be possible. There is no predictability of the reaction, not even by sophisticated history. But it is possible to avoid the development of severe shock by early diagnosis and immediate full causative therapy. With growing experience in diagnosis and therapy the mortality of anaphylactoid reaction should be irrelevant in the near future.

The effects of an histamine H2-receptor agonist, impromidine (SK & F 92676) on the general and cardiac haemodynamics of the intact dog

1. The effects of impromidine (16 nmol/kg intravenously) were studied in the general and coronary circulations of the intact anaesthetized dog. 2. The drug caused tachycardia, increased cardiac output and coronary sinus flow, and induced minor changes in systemic and pulmonary arterial pressures. 3. It increased left and right ventricular work, and decreased calculated resistance values in the greater and lesser circulations. Coronary vascular resistance fell, cardiac O2 extraction increases. 4. These changes were substantially prevented by treatment with cimetidine. 5. The action of an H2-receptor agonist then is associated with a general and coronary vasodilator response, with maintenance of calculated cardiac efficiency.

Modulation by histamine of peripheral vagal transmission in anesthetized mongrel dogs

Intravenous infusion of histamine (6.0 microgram/kg/min) attenuated bradycardic responses to electrical stimulation of right vagus in anesthetized mongrel dogs. Similarily histamine also reduced reflexly mediated decreases in the heart rate to phenylephrine, while bradycardia induced by exogenous acetylcholine was not altered. This inhibitory action of histamine on responses to vagal stimulation could be antagonized by pretreatment with metiamide, an H2-receptor antagonist; but not by pyrilamine, and H1-receptor blocker. Metiamide significantly potentiated the negative chronotrophic effect of vagal stimulation. These results indicate that histamine could inhibit peripheral vagal transmission via a presynaptic action. The ability of histamine to alter vagal transmission may have physiological significance and play a role in histamine-induced tachycardia.

Gastric and cardiac H-2 receptor effects of dimaprit and N'(Me),5(Me)histamine in conscious dogs

Two new H-2 agonists 5,N'-dimethyl histamine (DMH), an imidazole analog, and dimaprit, a non-imidazole compound, and histamine (H), a mixed H-1, H-2 agonist, were given to four conscious gastric fistula dogs. Gastric acid was stimulated dose responsively. Dimaprit stimulated a 20% greater maximum output of acid, and more pepsin than the other agents, but inhibited pepsin secretion as doses of greater than 0.63 mumol/kg-h. Heart rate was increased dose responsively by all drugs to approximately 200 beats/min with ED50 of 0.17:0.42:0.80 mumol/kg-h (H:DMH:dimaprit). Normalized dose responses showed that histamine was equipotent on acid and heart rate (ED50 0.15 vs. 0.14 respectively) while DMH (ED50 0.21 vs. 0.40) and dimaprit (ED50 0.27 vs. 0.85) stimulated heart rate less effectively than acid. Histamine was more effective at reducing blood pressure, with approximate ED50 of 0.28:0.60:0.96 for H, DMH and dimaprit respectively. The results indicate considerable heterogeneity of histamine responses for different actions mediated by the H-2 receptors, as well as differences for any one action between H-2 agonists.

Influence of prazosin on cardiac reflex function

Prazosin administration significantly decreased arterial blood pressure without affecting heart rate in pentobarbital-anesthetized dogs. Wile pressor and reflex bradycardic responses to angiotensin II were not altered by prazosin, reflex tachycardia produced produced by histamine and acetylcholine were significantly attenuated by prazosin. The failure of prazosin administration to cause an increase in heart rate may be due to the ability of the agent to inhibit reflex tachycardic responses to depressor stimuli, the mechanism of whcih needs to be further investigated.