Cocaine inhibits muscarinic cholinergic receptors in heart and brain.

(-)-Cocaine inhibits M2 muscarinic cholinergic binding measured with [3H]quinuclidinyl benzilate in heart and brain with a Ki of 18.8 microM. The cyclic nucleotide 5'-guanylylimidodiphosphate does not shift the competition curve, suggesting that (-)-cocaine is an antagonist. (-)-Cocaine also reverses the methacholine-induced inhibition of guinea pig atrial contractions at a similar concentration. Although (+)-cocaine is about 8-fold more potent than (-)-cocaine, (+)-cocaine is not present in extracts of the coca plant. Of the many compounds tested, only (-)-cocaine and lidocaine have a higher affinity at M2 muscarinic receptors than at M1 receptors; other compounds such as (+)-cocaine, norcocaine, procaine and dimethocaine are equipotent at the M1 and M2 subtypes. These results indicate that cocaine can act as an antimuscarinic agent, particularly at higher, toxic doses.

Purification and characterization of digitalislike factors from human plasma.

Increased levels of a humoral inhibitor of active sodium transport have been associated with the response to acute and chronic hypervolemia and various forms of experimental as well as human essential hypertension. In this report, we describe the purification of inhibitors of Na+, K+-adenosine triphosphatase (ATPase) from the plasma of volume-expanded individuals. Of the two amphipathic materials obtained, only one of the factors when present in high concentrations showed the slow time-dependent component of inactivation similar to that of the cardiac glycosides. Inhibition was reduced in the presence of plasma proteins and was freely reversible. Both factors inhibited potassium-dependent p-nitrophenylphosphatase activity and specific [3H]ouabain binding in a manner similar to the cardiac glycosides. In contrast to ouabain and vanadate, however, high concentrations of potassium or norepinephrine, respectively, did not affect the magnitude or kinetic characteristics of inhibition. Structural analysis by mass spectroscopy showed a mass of 444 for factor 1, whereas factor 2 was conclusively identified as lysophosphatidylcholine-gamma-palmitoyl. These factors probably inhibit Na+, K+-ATPase by a nonspecific mechanism involving reversible perturbation of lipid-enzyme interactions required for normal catalytic activity. The significance of these factors as modulators of sodium transport may be limited to pathological states associated with abnormalities in plasma protein binding or lipid metabolism. They do not appear to be directly related to the humorally mediated disturbance of cellular sodium transport in hypertension.

Studies aimed at elucidating the mechanism of action of CI-914, a new cardiotonic agent.

CI-914 is a novel positive inotropic agent whose cardiotonic activity is not due to inhibition of Na+, K+-ATPase or to stimulation of cardiac beta-receptors. CI-914 also has no direct effect on sarcoplasmic reticulum, mitochondria or adenylate cyclase activity. CI-914 does, however, exert a potent inhibitory effect on cardiac phosphodiesterase activity. In evaluating the effect of this agent on the different molecular forms of phosphodiesterase present in cardiac muscle, CI-914 was found to selectively inhibit PDE III, which is a low Km, cAMP-specific form of the enzyme (IC50 = 6.1 microM). This inhibitory effect was found to be competitive with respect to the substrate. Papaverine and theophylline on the other hand were found to inhibit all three forms of phosphodiesterase present in cardiac muscle. The role of phosphodiesterase inhibition in mediating the positive inotropic response to CI-914 is supported by the finding that this agent: (i) significantly elevates cyclic AMP levels in ventricular tissue; (ii) shifts the normal concentration-response to the beta-receptor stimulant isoproterenol to the left: and (iii) restores contractility to K+-depolarized papillary muscles.

Relationship between inhibition of cardiac muscle phosphodiesterases, changes in cyclic nucleotide levels, and contractile response for CI-914 and other novel cardiotonics.

In the present study, the inhibitory effects of CI-914, a novel cardiotonic agent, as well as other recently identified positive inotropic agents and reference phosphodiesterase (PDE) inhibitors on the different molecular forms of cardiac PDE were examined, and correlated with changes in tissue levels of cyclic AMP and cyclic GMP, and the contractility of isolated guinea pig left atria produced by these agents. CI-914 was found to be a potent, selective inhibitor of peak III PDE, which is a low Km, cyclic AMP-specific form of the enzyme, with little effect on peak I or peak II PDE, both of which hydrolyze cyclic AMP as well as cyclic GMP. CI-914 also exerts a selective effect on cyclic nucleotides; increasing cyclic AMP levels in a concentration-dependent manner, while having no significant effect on cyclic GMP levels. Increases in contractility produced by CI-914 correlated with changes in the tissue level of cyclic AMP. Non-selective phosphodiesterase inhibitors such as theophylline, and less selective inhibitors such as papaverine, carbazeran, and milrinone increased tissue levels of both cyclic AMP and cyclic GMP. Increases in cyclic GMP, or the cyclic AMP/cyclic GMP ratio, did not appear to have an effect on contractility. These results provide support for the hypothesis that CI-914 increases contractility by selectively inhibiting the activity of the peak III phosphodiesterase. These results also indicate that cyclic AMP and cyclic GMP do not act in an opposing "yin-and-yang" fashion to regulate atrial contractility.

The methylenedioxyindenes, a novel class of "intracellular calcium antagonists": effects on contractility and on processes involved in regulating intracellular calcium homeostasis.

The 2-substituted methylenedioxyindenes are a novel class of "calcium antagonists" which have been characterized as acting intracellularly. In the present study, the effect of 2-propyl-methylenedioxyindene (p-MDI) on the contractility of isolated rabbit papillary muscles and aortic rings as well as on several metabolic processes believed to be important in regulating calcium movement within the cell was examined. p-MDI was found to exert a dose-dependent negative inotropic effect in the range of 1.0 X 10(-5) to 1.0 X 10(-3) M (1.0 X 10(-3) M p-MDI produced a total cessation of contractility). At a concentration of 1.0 X 10(-4) M p-MDI, this depressant effect could be partially reversed by increasing the level of calcium in the tissue bath. p-MDI also relaxed aortic rings previously contracted with 50 mM KCl or 10 microM norepinephrine, although the concentration of p-MDI required to relax norepinephrine-contracted rings was 3 times greater than that required to relax KCl-contracted rings. Such selectivity was also observed when the effects of verapamil on norepinephrine- and KCl-contracted rings were examined, but was not observed with papaverine. To determine whether the effects of p-MDI on cardiac and vascular smooth muscle were due to direct interference with any of the metabolic processes which regulate intracellular calcium homeostasis, the effects of p-MDI on isolated cardiac sarcoplasmic reticulum and mitochondria and on the intracellular calcium-binding protein calmodulin were examined. At concentrations which depressed cardiac contractility, p-MDI had no effect on 1) calcium transport, uptake or ethyleneglycol bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid-induced calcium release by sarcoplasmic reticulum, 2) calcium uptake by mitochondria or 3) calmodulin. p-MDI did, however, exert a biphasic effect of glutamate-supported mitochondrial respiration; stimulating oxygen consumption at concentrations of 1.0 X 10(-5) and 1.0 X 10(-4) M and inhibiting respiration at 1.0 X 10(-3) M. It is therefore concluded that p-MDI has no apparent direct effect on intracellular calcium movement per se and that the calcium antagonist effect of p-MDI may be due in part to calcium channel blockage. Inhibition of mitochondrial respiration may also contribute to the negative inotropic effect of high concentrations of p-MDI.

Adenosine receptors mediating cardiac depression.

Several adenosine analogs were evaluated for their effects on rate and contractility in guinea pig isolated atria. Among adenosine agonists, (-)-N-(1-methyl-2-phenylethyl) adenosine (l-phenylisopropyladenosine; l-PIA) and N-cyclohexyladenosine (CHA), decreased rate and force at nanomolar concentrations, whereas 2-chloroadenosine, N,N-dimethyladenosine (N6-dimethyladenosine) and (+)-N-(1-methyl-2-phenylethyl)adenosine (d-phenylisopropyladenosine; d-PIA) were less potent cardiac depressants. The degree and order of potency of these agonists suggest that the cardiac depressant effects of adenosine are mediated via A1-receptors. The cardiac depressant effects of CHA and l-PIA were antagonized by theophylline and 1,3-diethyl-8-phenylxanthine (DPX).