SK&F 89124, a potent and selective agonist at prejunctional dopamine receptors.

SK&F 89124 (4-[2-(N,N-di-n-propylamino)ethyl]-7-hydroxy-2(3H) indolone) can be considered as a derivative of N,N-di-n-propyldopamine (DPDA) in which the meta-hydroxyl is replaced by a cyclic amide function. SK&F 89124 is at least one order of magnitude more potent than DPDA as an agonist at peripheral inhibitory prejunctional dopamine receptors (DA2 receptors) in the isolated perfused rabbit ear artery. A potent agonist action of SK&F 89124 at the DA2 receptor can also be demonstrated by inhibition of radioactive overflow from prelabelled canine coronary artery or saphenous vein, and in the anesthetized dog as an inhibition of the tachycardia induced by cardioaccelerator nerve stimulation or the increase in hind-limb perfusion pressure induced by stimulation of the lumbar sympathetic chain. SK&F 89124 is a potent inhibitor of the binding of [3H]spiroperidol to D2 receptors in bovine pituitary homogenates. High concentrations of SK&F 89124 do not activate the adenylate cyclase D1 receptor in rat caudate homogenates, nor produce activation of alpha 2-adrenoceptors or H2-histamine receptors in the guinea pig atrium. Although some alpha 1-adrenoceptor mediated vasoconstriction is produced in the rabbit ear artery and rabbit aorta, the concentrations required are several orders of magnitude higher than those active at the DA2 receptor. From these data it is evident that this structural modification can increase both the potency and selectivity of DPDA as a DA2 receptor agonist. The potency and selectivity of SK&F 89124 make this agent a useful tool for determination of the functional role of the DA2 receptor.

Effects of the thromboxane receptor antagonist SK&F 88046 in the canine, monkey and human coronary vasculature.

U-46619, a stable "functional" thromboxane/endoperoxide receptor agonist, produced potent contractile responses in isolated canine, rhesus monkey and human left circumflex coronary arteries (EC50 = 9.11 x 10(-9)M, 1.98 x 10(-8)M and 3.50 x 10(-9)M, respectively). Canine intrapulmonary veins were also contracted potently by U-46619 (EC50 = 1.22 x 10(-9)M). SK&F 88046, a thromboxane A2 (TxA2) end-organ receptor antagonist, blocked the vasoconstrictor effects of U-46619 in the canine circumflex artery (KB = 1.33 x 10(-8)M), canine intrapulmonary vein (KB = 1.46 x 10(-9)M), monkey circumflex artery (KB = 8.47 x 10(-8)M), and human circumflex artery (KB = 8.49 x 10(-7)M). SK&F 88046 was 10-60 times more potent in the canine and rhesus monkey coronary vasculature than in the human coronary preparations. Intracoronary administration of U-46619 to anesthetized, open chest dogs produced a dose-related decrease in left circumflex coronary artery blood flow which resulted in decreases in left ventricular developed pressure, left ventricular positive and negative dP/dt, ascending aortic blood flow, and an increase in left ventricular end-diastolic pressure. The decrease in coronary blood flow and the hemodynamic changes were either attenuated or completely inhibited by i.v. administration of SK&F 88046 (2.5 mg/kg + 0.05 mg/kg/min or 5.0 mg/kg + 0.1 mg/kg/min). SK&F 88046 was compared to two other TxA2 receptor antagonists in canine isolated intrapulmonary veins. SQ 29,548 was approximately 2-times more potent than SK&F 88046 as an antagonist of U-44619 mediated contractions (KB = 7.0 x 10(-10)M). In contrast, BM 13.177 was 150-fold less potent (KB = 2.19 x 10(-7)M) than SK&F 88046. Thus, the present study demonstrates species variability in response to TxA2 agonists and antagonists and reconfirms the relative importance of species selection in studying these agents.

Effect of the thrombolytic agent, streptokinase, on the responses to endotoxemia in conscious rats.

The potential role of coagulation defects as a pathologic mediator in septic shock is well documented, especially as it relates to increased thromboxane formation, thrombocytopenia, and disseminated intravascular coagulation. The present study was designed to determine the effect of the thrombolytic agent streptokinase on the sequelae of endotoxemia in the rat. Conscious male Sprague-Dawley rats were given a bolus intravenous dose of Salmonella enteritidis endotoxin (20 mg/kg; LD90 dose) 5 min after the intravenous administration of streptokinase (10,000 U/kg bolus + 1,000 U/kg/hr infusion), or heparin (100 U/kg bolus + 30 U/kg/hr infusion). The effects of streptokinase or heparin on blood clotting were determined by measuring ex vivo clot formation 1 hr after the administration of endotoxin. In naive and endotoxemic animals, both agents significantly reduced clot formation (P less than 0.05). In endotoxemic animals, streptokinase or heparin improved survival to 70%, compared to 8% survival in the endotoxin + vehicle group (P less than 0.05). The improvement in survival with streptokinase was dose-dependent. Neither streptokinase nor heparin prevented the thrombocytopenia or hemoconcentration which developed in endotoxemic animals. These results demonstrate the potential utility of streptokinase for improving survival in endotoxemia and further confirm the deleterious contribution of coagulation disorders in endotoxic mortality.

Relationship between alpha adrenoceptor occupancy and response for the alpha-1 adrenoceptor agonist, cirazoline, and the alpha-2 adrenoceptor agonist, B-HT 933, in canine saphenous vein.

The relationship between alpha adrenoceptor occupancy and response was investigated for the alpha-1 adrenoceptor agonist, cirazoline, and the alpha-2 adrenoceptor agonist, B-HT 933, in canine saphenous vein, a tissue known to contain both alpha adrenoceptor subtypes. Both cirazoline and B-HT 933 produced dose-dependent vasoconstrictor responses in canine saphenous vein, with the maximum vasoconstrictor response elicited by B-HT 933 being only 75% of that produced by cirazoline. Dissociation constants were obtained for cirazoline (0.61 microM) and B-HT 933 (4.99 microM) for interaction with postsynaptic vascular alpha-1 and alpha-2 adrenoceptors, respectively, by the method of fractional irreversible receptor inactivation using phenoxybenzamine. Based on this information, alpha-1 and alpha-2 adrenoceptor occupancy-response relationships were constructed. The alpha-1 adrenoceptor occupancy-response relationship obtained for cirazoline was approximately 4-fold more favorable than the alpha-2 adrenoceptor occupancy-response relationship for B-HT 933, although both agonists were associated with a significant receptor reserve. The alpha-1 adrenoceptor occupancy-response relationship of cirazoline and the alpha-2 adrenoceptor occupancy-response relationship of B-HT 933 were both rectangular hyperbolas, suggesting that both compounds have high intrinsic efficacy at their respective alpha adrenoceptor subtypes. The results indicate that a receptor reserve exists for the alpha-1 and alpha-2 adrenoceptor-mediated effects of cirazoline and B-HT 933, respectively, and that the alpha adrenoceptor reserve may be significantly larger for postsynaptic vascular alpha-1 adrenoceptors than for postsynaptic vascular alpha-2 adrenoceptors in canine saphenous vein.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine receptor agonist activity of some 5-(2-aminoethyl)carbostyril derivatives.

The potency of beta-adrenoreceptor agonists, e.g., isoproterenol, is strikingly increased by substitution of the meta catecholic hydroxyl group with the NH group of a carbostyril system. To explore the possibility that comparable potency enhancement might occur upon similar modification of the catechol ring of dopamine, a series of 5-(2-aminoethyl)carbostyril derivatives was prepared and examined for D-1 and D-2 dopamine receptor-stimulating activity. Only the parent compound, 5-(2-aminoethyl)-8-hydroxycarbostyril (2), produced measurable activation of dopamine-sensitive adenylate cyclase (29% at a concentration of 10 microM). Some of the compounds, however, did produce significant activity in tests, namely displacement of [3H]spiroperidol binding from bovine pituitary homogenate and an isolated perfused rabbit ear artery preparation, that measure interaction with D-2 receptors. Potency of the carbostyrils was enhanced by 8-hydroxylation and by appropriate substitution of the amino group of the ethylamine side chain. The most potent member of the series was 8-hydroxy-5-[2-[[2-(4-hydroxyphenyl)ethyl]-n-propylamino]ethyl] carbostyril (16b). This compound was about 3 times more effective than dopamine in the D-2 receptor tests. Clearly, the results of this study indicate that potency of dopamine receptor agonists is not increased by carbostyril replacement of the m-hydroxyl as is noted with the beta-adrenergic receptor agonists.

Relationship between alpha 2-adrenoceptor occupancy and response for B-HT 933 in canine saphenous vein.

The relationship between alpha 2-adrenoceptor occupancy and contractile response for B-HT 933 was investigated in canine saphenous vein. B-HT 933 produced a concentration-dependent, alpha 2-adrenoceptor mediated contractile response in canine saphenous vein with an ED50 of 0.65 microM. The dissociation constant (KA) of B-HT 933 was calculated to be 5 microM. The relationship between alpha 2-adrenoceptor occupancy and contractile response for B-HT 933 in canine saphenous vein was hyperbolic, typical of full agonists or agonists with high intrinsic efficacies. B-HT 933 produced a half-maximal response in canine saphenous vein at only 11% alpha 2-adrenoceptor occupancy, with a maximal response being obtained with 40-60% alpha 2-adrenoceptor occupancy. We conclude that the selective alpha 2-adrenoceptor agonist, B-HT 933, has high efficacy at alpha 2-adrenoceptors, such that a significant alpha 2-adrenoceptor reserve, or spare alpha 2-adrenoceptors, exists for this compound in canine saphenous vein.

Dopaminergic and cholinergic muscarinic receptor effects of L-alphacetylmethadol (LAAM) and its metabolites.

In isolated rat hearts L-alphacetylmethadol (LAAM) produced a concentration-dependent decrease in the spontaneous beating rate. This effect was completely prevented by 1.0 microM atropine. Chronic treatment of rats with LAAM increased the number of striatal dopamine receptors measured by [3H]spiroperidol binding. The affinity of these binding sites for [3H]spiroperidol was unchanged by LAAM treatment. There were no significant changes in the number or affinity of binding sites for the labeled muscarinic antagonist [3H]quinuclidinyl benzilate ([3H]QNB) with chronic LAAM treatment. The ability of LAAM, nor-LAAM, or dinor-LAAM to antagonize the binding of [3H]spiroperidol (40 pM) or [3H]QNB (125 pM) to striatal membrane fragments was tested. The measured affinity constants for LAAM and metabolites were 100-3000 times higher than the affinity constants of unlabeled spiroperidol at [3H]spiroperidol binding sites. The affinity constants of LAAM and metabolites at muscarinic binding sites were 10-20 times higher than pilocarpine and 5000-8000 times higher than atropine. These results suggest that LAAM can produce some of its effects by acting as a weak agonist at muscarinic receptor sites.

Cardiac and autonomic nervous system effects of L-alphacetylmethadol (LAAM).

The effects of 1-alphacetylmethadol (LAAM) on heart rate and force of contraction of isolated guinea-pig hearts and on release of tritium from sympathetic nerves were investigated. In vitro perfusion with LAAM depressed resting heart rate and right ventricular pressure. Nerve stimulation-induced tritium overflow was inhibited in a concentration related manner by LAAM. Increasing the calcium concentration in the perfusate partially inhibited the negative chronotropy and completely prevented the negative inotropic effect. One micromolar atropine prevented the negative chronotropy, partially inhibited the negative inotropic effect of LAAM and the depression of tritium release. Ten micromolar naltrexone failed to antagonize the effects of LAAM. Our results suggest that LAAM: 1) has a direct myocardial depressant effect which may be due to an inhibition of calcium influx; 2) produces a negative chronotropy through stimulation of atrial muscarinic receptors; and 3) interacts with presynaptic muscarinic receptors which modulate nerve stimulation-induced release of noradrenaline.