Hsien C. Cheng

NITRIC OXIDE SYNTHESIS IN THE CNS, ENDOTHELIUM AND MACROPHAGES DIFFERS IN ITS SENSITIVITY TO INHIBITION BY ARGININE ANALOGUES

Inhibition of nitric oxide production by arginine analogues was examined in three cell systems; macrophages, CNS tissue and endothelial cells. Nitric oxide production was assessed indirectly using in vitro assays measuring nitrite production (macrophages), cGMP elevation (CNS) and acetylcholine-induced relaxation of aortic ring segments (endothelium). NG-monomethyl-L-arginine and NG-amino-L-arginine possessed similar inhibitory activity in all three assays, while NG-nitro-L-arginine displayed a striking selectivity for inhibition of brain and endothelial cell nitric oxide synthesis, with IC50 values of 0.05 microM in the CNS versus 200 microM in macrophages. These results suggest that distinct enzymes are responsible for nitric oxide synthesis in different cell types, and indicate that it may be possible to selectively modulate nitric oxide production in vivo.

Characterization of the Cardiovascular Activities of a New Cardiotonic Agent, Mdl 17043 (1,3-dihydro-4-methyl-5-[4-(methylthio)-benzoyl]-2h-imidazol-2-one)

Summary The cardiovascular actions of 1,3-dihydro-4-methyl-5[4-(methylthio)-benzoyl]-2H-imidazol-2-one (MDL 17043), a new non-catecholamine, nonglycoside cardiotonic agent, were examined in vivo in anesthetized dogs and in vitro in isolated cat atrial and papillary muscle preparations and guinea pig atria. In the anesthetized dog, intravenous administration of MDL 17043 (0.1 - 1 mg/kg) produced marked increases in cardiac contractile force which were accompanied by minor increases in heart rate and small decreases in blood pressure. These effects were not altered by α-or β-adrenergic receptor blockade, catecholamine depletion produced by reserpine, bilateral vagotomy, or by bilateral carotid sinus denervation. MDL 17043 (105-103 M) produced positive inotropic effects in isolated papillary muscle and left atrial strips of the cat that were much greater than the positive chronotropic effects seen in the spontaneously beating right atrium of the cat. The in vitro inotropic effects of MDL 17043 in guinea pig electrically driven left atrial strips were not modified by adrenergic β-receptor or histamine H1-receptor blockade. The vasodilatory effect of MDL 17043 in the canine isolated pump-perfused hindlimb preparation was not attenuated by surgical sympathectomy, α- or β-adrenergic receptor blockade, cholinergic or histaminergic receptor blockade, or by prostaglandin synthesis inhibition, indicating that MDL 17043 produces direct relaxation of vascular smooth muscle. MDL 17043 was found to be safe and effective when administered acutely in combination with ouabain, hydralazine, nitroglycerin, or furosemide, agents commonly used in the treatment of congestive heart failure.

Pharmacology of enoximone

Enoximone is a new cardiotonic agent, active by both intravenous and oral routes of administration, that is being studied clinically for the treatment of patients with congestive heart failure. The animal pharmacology pertinent to the clinical development of enoximone is reviewed. Direct positive inotropic, positive chronotropic and vasodilator properties have been demonstrated for enoximone in several in vivo and in vitro preparations. However, positive inotropism and vasodilation are the principal effects of this agent with the inotropic effect being the most prominent. In anesthetized dogs, the cardiovascular effects produced by enoximone (0.1 to 1 mg/kg) were not accompanied by significant alterations in myocardial oxygen consumption. Cardiac function was improved by enoximone in anesthetized dogs given myocardial depressant amounts of propranolol. Studies in vivo and in vitro have indicated that the actions of enoximone are direct and not mediated by stimulation of adrenergic receptors, histaminic receptors, cholinergic receptors, Ca++-adenosine triphosphatase, Mg++-adenosine triphosphatase, adenyl cyclase or inhibition of Na+, K+-adenosine triphosphatase. However, enoximone reversed the depressant effects of verapamil in the dog heart-lung preparation; this suggests that its action resulted in the activation of slow calcium channels. Enoximone was found to be potent and highly selective inhibitor of a high affinity cyclic adenosine monophosphate-phosphodiesterase type IV-phosphodiesterase from dog heart, whereas standard inhibitors (e.g., 3-isobutyl-1-methylxanthine and papaverine) inhibit all 3 cardiac phosphodiesterases. Further, enoximone produced an increase in cyclic adenosine monophosphate, but not cyclic guanosine monophosphate, in the isolated, blood perfused dog papillary muscle during the peak inotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular properties of medroxalol, a new antihypertensive drug.

Summary Medroxalol is a new antihypertensive agent that is presently undergoing clinical trial. Its cardiovascular properties were studied using spontaneously hypertensive rats (SHR), anesthetized dogs, and isolated tissues. Medroxalol produced a long-lasting fall in blood pressure when given by the oral route to SHR. It was more potent than phentolamine in antihypertensive effectiveness. Given intravenously to dogs, medroxalol reduced blood pressure and heart rate at doses that did not greatly reduce cardiac output. The hypotensive effect of medroxalol was reduced but not abolished following α- and β-adrenergic-receptor blockade. Medroxalol inhibited heart rate and blood pressure responses to isoproterenol and phenylephrine in dogs. In vitro medroxalol resembled a competitive antagonist at α-adrenergic receptors in rabbit aortic strips (pA2 6.09) and β-adrenergic receptors in guinea pig atria (pA2 7.73). It was 0.02 as potent as phentolamine at α-receptors and 0.09 as potent as propranolol at β-receptors. It was concluded that the principal action of medroxalol was to produce a fall in blood pressure by decreasing peripheral vascular resistance more than cardiac output. Adrenergic α- and β-receptor blockade alone does not satisfactorily explain the hypotension. A contribution by an active vasodilatory component appears likely.

In vitro and in vivo assessment of the cardiovascular effects of the cardiotonic drug MDL 19205.

Various in vitro and in vivo techniques were used to evaluate the cardiovascular actions of MDL 19205, a new cardiotonic agent. In the anesthetized dog, intravenous administration of MDL 19205 (0.1–1 mg/kg) produced marked increases in cardiac contractile force which were accompanied by small increases in heart rate and minor decreases in blood pressure. These effects were not altered by α- or β-adrenergic receptor blockade, reserpine pretreatment, or bilateral carotid sinus denervation. In isolated cat cardiac tissues, MDL 19205 (10-5-10-3 M) produced a selective inotropic effect relative to isoproterenol and, unlike isoproterenol, was nonarrhythmogenic. Adrenergic β-receptor or histamine H1receptor blockade did not modify the inotropic effects of MDL 19205 in guinea pig atria. The vasodilatory effect of MDL 19205 in the canine isolated pump-perfused hindlimb preparation was not significantly attenuated by surgical sympathectomy, α- or β-adrenergic receptor blockade, cholinergic or histaminergic receptor blockade, or indomethacin pretreatment, indicating that MDL 19205 produced direct relaxation of vascular smooth muscle. MDL 19205 was found to be safe and effective when administered acutely in combination with ouabain, hydralazine, nitroglycerin, or furosemide, agents commonly used in the treatment of congestive heart failure. The pharmacological profile revealed by these and other studies suggests that MDL 19205 should be useful in the clinical treatment of congestive heart failure.

MDL 27,032 relaxes vascular smooth muscle and inhibits protein kinase C.

The smooth muscle relaxant effect of MDL 27,032, 4-propyl-5-(4-pyridinyl)-2(3H)-oxazolone, was studied in vitro using strips of femoral arteries and saphenous veins from dogs and trachea from guinea pigs. MDL 27.032 (10-6–10 -3 M) produced a concentration-dependent relaxation of arterial and venous strips contracted by both phenylephrine and KCI as well as trachea contracted by carhachol. MDL 27,032 also antagonized contractions of arterial and venous strips produced by phorhol 12,13-dihutyrate (PDB), a protein kinase C activator, both in normal-Ca2+ and zero-Ca2+ medium. The compound inhibited protein kinase C in soluble extracts prepared from saphenous veins of dogs, with an IC50 value of 36.6 μM. MDL 27,032 was more effective against the contractions produced by phenylephrine and serotonin than by KCI in arteries, but no such selectivity was noted in veins. MDL 27,032 (10-3 M) also inhibited accumulation of inositol phosphates in femoral artery but not in saphenous vein, and this effect may have contributed to the arterial-relaxant effect. Because the vascular smooth muscle relaxant effect of MDL 27,032 was endothelium independent, did not involve blockade of α-adrenoceptors or inhibition of cyclic nucleotide phosphodiesterases, stimulation of β-adrenergic receptors. stimulation of adenosine A2-receptors, or activation of K+ channels, these data suggest that the relaxant effects of MDL 27,032 primarily involve inhibition of protein kinase C.

Studies on the mechanism of the positive inotropic effect of piroximone in cat papillary muscle.

The mechanism of the inotropic effect of piroximone HC1 [MDL 19205A, 4-ethyl-1,3-dihydro-5-(4-pyridinylcarbonyl)-2H-imidozol-2-one HCl] was studied in the cat papillary muscle paced electrically in vitro. Piroximone produced a concentration-dependent positive inotropic effect accompanied by an increase in rate of contraction and rate of relaxation, but abbreviated time to peak tension and relaxation time. The positive inotropic effect produced by piroximone was antagonized by carbachol, 3 × 10−6 M, whereas that produced by increasing calcium concentration was not affected by carbachol. In potassium chloride (22 mM) depolarized muscle, piroximone restored contractility, which was not affected by propranolol (10−6 M) or by tetrodotoxin (2 × 10−5 M), but was inhibited by nifedipine (10−7 M). Piroximine also elevated tissue cyclic AMP (cAMP) content in the papillary muscle. Although nifedipine inhibited the restoration of contractility, it did so without altering the increase of cAMP produced by piroximone. These results suggest that piroximone causes an increase in calcium influx that is mediated by an increase in cAMP, and the results are consistent with the hypothesis that specific inhibition of the high affinity cAMP phosphodiesterase (PDE III) plays a role in the positive inotropic effect of piroximone.