Comparative effects of R 80122, enoximone, and milrinone on left ventricular phosphodiesterase isoenzymes in vitro and on contractility of normal and stunned myocardium in vivo in dogs.

Using different subtypes of cyclic nucleotide phosphodiesterase (PDE) isoenzymes isolated from canine left ventricle, we identified R 80122, a 1,2,3,5-tetrahydro2-oxoimidazo[2,1-b]quinazoline derivative that was a more selective and potent inhibitor of PDE type III than milrinone or enoximone. Such substances improve cardiac contraction and relaxation, elicit vasodilation, and increase cardiac output (CO). To determine the extent to which these compounds affect the contractile force of stunned myocardium, the effects of enoximone, milrinone, and R 80122 on cardiac function were compared in anesthetized dogs subjected to 15-min occlusion of the left anterior descending coronary artery (LAD) followed by reperfusion, and treated beginning 30 min after reperfusion, with the compound being studied. During occlusion, all dogs exhibited passive systolic ventricular wall bulging in the ischemic area. Thirty minutes after reperfusion, systolic wall thickening was significantly decreased in the reperfused LAD segments and remained low (at 36% of baseline) in control animals. After enoximone administration, global left ventricular (LV) function was improved with i.v. doses greater than or equal to 0.64 mg/kg. Systolic wall thickening in the ischemic myocardium was restored less than or equal to 70% of baseline at 1.25 mg/kg i.v., but this dose also induced a marked decrease in arterial pressure and an increase in heart rate (HR). Milrinone and R 80122 significantly increased global LV function and systolic wall thickening in ischemic areas at doses greater than or equal to 0.16 mg/kg i.v. At the highest doses, HR increased slightly with both compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Amplification mechanisms in platelet activation and arterial thrombosis.

Platelets react with different intensities to a large variety of agonists (e.g. 5-hydroxytryptamine, ADP, thrombin, thromboxane A2). In vitro, low concentrations of 'weak' agonists such as 5-hydroxytryptamine markedly enhance the platelet functional response in terms of aggregation, release of intragranular products and arachidonic acid metabolites to otherwise ineffective concentrations of other agonists. This type of amplification takes place at an early, postreceptor stage of the platelet activation cascade, with the turnover of inositol phospholipids and the increase in free cytoplasmic intracellular Ca2+ concentrations. Mutual amplification, rather than the single potency of each, also determines the contribution by platelet agonists to thrombogenesis in vivo. Indeed, in various animal models and, to some extent, in man, a pharmacological blockade of the platelet 5-HT2 serotonergic (e.g. ketanserin) or thromboxane A2/prostaglandin endoperoxide receptor (e.g. ridogrel) and manipulation of arachidonic acid metabolism (e.g. cyclo-oxygenase inhibition with aspirin or thromboxane A2 synthetase inhibition with ridogrel), when applied together, are more effective in preventing extensive platelet activation and arterial thrombotic occlusion and in improving the efficacy of thrombocytic agents (speed up of thrombolysis and prevention of thrombotic re-occlusion after lysis) than when the same compounds are applied separately. These observations suggest that an amplification between 5-hydroxytryptamine and thromboxane A2, for example, as platelet agonists is causally involved in arterial thrombus formation.

Localization of calcium in red blood cells.

The distribution of calcium is demonstrated in human red blood cells (RBC) with a combined phosphate-pyroantimonate technique (PPA). Freshly collected blood and tissue biopsies were initially fixed in potassium phosphate-glutaraldehyde and the complexed calcium was subsequently visualized on Vibratome sections with potassium pyroantimonate. The majority of cells, both in isolated as well as "in situ" preparations, show a fine granular precipitate located at the inner leaflet of the plasma membrane. A minority of cells lack these membrane-associated deposits, exhibiting instead a random distribution of very fine precipitate in their cytoplasm. Capillary endothelial cells and pericytes are devoid of plasma membrane-bound precipitate. When irreversible crenation of RBC is induced by exposure to ionophore A 23187 and calcium, the sphero-echinocytes loose their membrane-bound precipitate, whereas the cells that retain their discocyte shape demonstrate the usual pattern of membrane-bound deposits. Contrarily, cells showing reversible shape changes induced by either A 23187-Ca2+ challenge, by adenosine triphosphate depletion during aging, or contact with lysolecithin, retain or regain the membrane-bound calcium. This cytochemical demonstrable calcium at the inner leaflet of the plasma membrane is probably bound to acidic phospholipids, since it is readily extractable with the nonionic detergent Triton X-100.

5-hydroxytryptamine and platelet aggregation.

5-Hydroxytryptamine (5-HT) activates blood platelets of various species including humans. In contrast to cat, pig, and sheep platelets, human blood platelets respond to 5-HT mainly with a shape change and a reversible aggregation only. However, depending on the concentration and the time interval between its addition and that of another agonist, 5-HT amplifies the human platelet aggregation induced by ADP, collagen, epinephrine, and norepinephrine; the monoamine itself induces strong aggregation of platelets presensitized with norepinephrine, lysolecithin, or Thrombofax. Prolonged exposure of platelets to 5-HT results in transient tachyphylaxis. Pharmacodissection and receptor-binding studies suggest the presence of functional receptors, possibly of the 5-HT2 (S2) type, different from the ones involved in the active uptake of the monoamine by the platelets. As a modulator of platelet reactions, 5-HT may be involved in secondary platelet aggregation, hemostasis, and thrombus formation.