Sisto Luciani

Amiloride: Relationship between cardiac effects and inhibition of Na+/Ca2+ exchange

The potassium sparing diuretic amiloride at concentrations ranging between 0.1-0.8 mM inhibited the Na+/Ca2+ exchange in sarcolemmal vesicles isolated from beef heart. The rate of exchange activity was 50% reduced by 0.35 mM amiloride. In spontaneously beating atria isolated from normal and reserpinized guinea-pigs, amiloride produced a concentration-dependent positive inotropic effect and negative chronotropic effect (EC50 = 0.7 mM). Amiloride protected spontaneously beating atria and left atria driven at 1 Hz from digitalis cardiotoxicity assessed in terms of a raised end-diastolic tension. It is suggested that the positive inotropic effect, negative chronotropic effect of amiloride and heart protection against digitalis toxicity are related to the observed inhibition of sarcolemmal Na+/Ca2+ exchange activity.

Mechanisms of chromium toxicity in mammalian cell cultures

Our observations about the cytotoxic and cytogenetic effects of hexavalent and trivalent chromium compounds in mammalian cells cultured in vitro are reviewed. Additional data concerning the induction of chromosomal aberrations and sister chromatid exchanges, the inhibition of nucleic acid and protein synthesis, the interference with nucleotide metabolism, and the modification of membrane-linked enzyme activity are reported. A possible mechanism of chromium action is proposed.

Na/Ca exchange and tension development in vascular smooth muscle: effect of amiloride

1 The potassium‐sparing diuretic, amiloride, has been shown to inhibit the Na/Ca exchange system in various preparations. The effects of this drug have been investigated on the contractions of guinea‐pig aortic strips elicited by reduction of external K, by addition of ouabain and by removal of external Na. 2 Amiloride (5 × 10−6 m‐5 × 10−4 m) inhibited the mechanical responses when it was added before giving the stimulus for contractions, but was not effective in relaxing the contracted strips. The drug shifted to the right the dose‐response curve for Ca in low K solution. 3 The calcium antagonist diltiazem had no effect on the ouabain‐, low K‐ and Na‐free‐induced contractions. 4 Amiloride decreased the rate of relaxation of aortic strips induced by removal of the low K solution. 5 The pattern of amiloride action on ouabain‐, low K‐ and Na‐free‐induced contractions suggests that the drug interferes with Ca influx. The effect of amiloride on the relaxation rate of low K‐ contracted aortic strips is consistent with an interference with Ca efflux. 6 It is suggested that amiloride prevents Ca fluxes through the Na/Ca exchange system of guinea‐pig aortic strips.

Isolation of callipeltins A–C and of two new open-chain derivatives of callipeltin A from the marine sponge Latrunculia sp. A revision of the stereostructure of callipeltins

Two new callipeltin-related acyclic peptides (2 and 3) have been isolated, together with callipeltins A–C from the marine sponge Latrunculia sp. collected at the Vanuatu Islands. The gross structures of new compounds were elucidated by spectroscopic data. The application of the Marfeys analysis on the new derivatives and on callipeltin A (1), allowed us to revise the configuration of two amino acid units in callipeltin A.

COMPETITIVE REVERSAL BY ADENINE NUCLEOTIDES OF ATRACTYLOSIDE EFFECT ON MITOCHONDRIAL ENERGY TRANSFER.

Abstract 1. 1. The atractyloside-induced inhibition of phosphate uptake in tightly coupled rat-liver mitochondria, when incubated under phosphorylative conditions, was competitively removed by increasing concentrations of ADP. 2. 2. ATP competitively removed the inhibitory effect of atractyloside on 2,4-dinitrophenol- -stimulated ATPase, Mg2+-stimulated ATPase and the Pi-ATP exchange reaction. 3. 3. The oligomycin-induced inhibition of oxidative phosphorylation, as well as the partial reaction of the coupling system, were not influenced by increasing the concentration of adenine nucleotides in the incubation medium. 4. 4. It is proposed that the mechanism of the inhibitory effect of atractyloside on the terminal step of the energy-transfer process consists in a competition with adenine nucleotides.

Effects of carboxyatractyloside a structural analogue of atractyloside on mitochondrial oxidative phosphorylation.

Abstract Carboxyatractyloside is more effective than atractyloside in the inhibition of oxidative phosphorylation. Accordingly the “in vivo” toxicity of this compound is ten times higher than that of atractyloside. Similarly to atractyloside it is uneffective on ATPase activity of sonicated mitochondrial particles whereas strongly inhibits translocation of adenine nucleotides across the mitochondrial membrane. In contrast to atractyloside the inhibition of carboxyatractyloside is not reversed by increasing the concentration of adenine nucleotides. The behaviour of carboxyatractyloside on mitochondria resembles that of bonkrekic acid.

Endothelin-1-induced arachidonic acid release by cytosolic phospholipase A2 activation in rat vascular smooth muscle via extracellular signal-regulated kinases pathway.

The present study investigates whether endothelin-1 (ET-1), like noradrenaline (NA), stimulates the release of arachidonic acid (AA) via cytosolic phospholipase A2 (cPLA2) in rat tail artery. In tail artery segments labelled with [3H]AA, ET-1-induced AA release in a concentration-dependent manner with an EC50 of 1.3 nM. The effect of ET-1 was inhibited by bosentan and was insensitive to BQ788, suggesting the involvement of ETA receptor. The stimulation of AA release induced by ET-1 was prevented by arachydonyl trifluoromethyl ketone (AACOCF3), a selective inhibitor of cPLA2 and not by RHC80267, a diacylglycerol lipase inhibitor. Furthermore, PD98059, inhibitor of mitogen-activated protein kinase kinase (MEK) cascade and calphostin C, a protein kinase C (PKC) inhibitor, prevented the stimulation of AA release induced by ET-1 and NA. Immunoblotting of the cytosolic fraction of rat tail arteries stimulated with ET-1 or NA showed an increase in extracellular signal-regulated kinases (ERKs) phosphorylation and this effect was abolished by calphostin C treatment. These findings show that in rat tail artery ET-1 and NA induce a sequential activation of protein kinase C and extracellular signal-regulated kinases that results in stimulation of AA release via cPLA2 activation. This may represent a general pathway by which G-proteins coupled receptors stimulate AA release and its metabolites in vascular smooth muscle.

Pharmacological properties of tetrahydropapaveroline.

Tetrahydropapaveroline (thp) exerts β‐sympathomimetic effects similar to those of isoprenaline. On guinea‐pig isolated atria, thp elicits positive inotropic and chronotropic activities which are not abolished by previous reserpinization of the animals; on isolated mammalian heart these effects are associated with an increase in coronary flow. In the dog, thp increases myocardial contractile force and rate, elicits a hypotensive effect and stimulates respiratory activity in normal and reserpinized animals; when injected intra‐arterially the drug causes vasodilatation. All the effects are prevented by the β‐adrenergic blocking agents propranolol, dichloroisoprenaline and pronethalol. Structure‐activity relationships between tetrahydroisoquinoline derivatives and their open‐ring phenylethylamine congeners, which are closely related to sympathomimetic drugs, are discussed.

Vasorelaxant properties of norbormide, a selective vasoconstrictor agent for the rat microvasculature.

1 The effects of norbormide on the contractility of endothelium‐deprived rat, guinea‐pig, mouse, and human artery rings, and of freshly isolated smooth muscle cells of rat caudal artery were investigated. In addition, the effect of norbormide on intracellular calcium levels of A7r5 cells was evaluated. 2 In resting rat mesenteric, renal, and caudal arteries, norbormide (0.5–50 μm) induced a concentration‐dependent contractile effect. In rat caudal artery, the contraction was very slowly reversible on washing, completely abolished in the absence of extracellular calcium, and antagonized by high concentrations (10–800 μm) of verapamil. The norbormide effect persisted upon removal of either extracellular Na+ or K+. The contractile effect of norbormide was observed also in single, freshly isolated smooth muscle cells from rat caudal artery. 3 In resting rat and guinea‐pig aortae, guinea‐pig mesenteric artery, mouse caudal artery, and human subcutaneous resistance arteries, norbormide did not induce contraction. When these vessels were contracted by 80 mM KCl, norbormide (10–100 μm) caused relaxation. Norbormide inhibited the response to Ca2+ of rat aorta incubated in 80 mM KCl/Ca2+‐free medium. Norbormide (up to 100 μm) was ineffective in phenylephrine‐contracted guinea‐pig and rat aorta. 4 In A7r5 cells, a cell line from rat aorta, norbormide prevented high K+‐but not 5‐hydro‐xytryptamine‐induced intracellular calcium transients. 5 These findings indicate that in vitro, norbormide induces a myogenic contraction, selective for the rat small vessels, by promoting calcium entry in smooth muscle cells, presumably through calcium channels. In rat aorta and arteries from other mammals, norbormide behaves like a calcium channel entry blocker.

Toxic effect of chromium on cellular metabolism

The cytotoxic action of Cr(VI) in intact mammalian cells is strictly related to its carrier-mediated transport across the plasma membrane and its reduction to Cr(III) inside the cell. A marked decrease in the ratio of reduced/oxidized glutathione in rat thymocytes treated with dichromate indicates an involvement of glutathione in the reduction of Cr(VI) to Cr(III) in these cells. Intracellular chromium is shown to interfere with specific steps of cellular energy metabolism in that it produces severe unbalance of purine ribonucleotide pools in hamster fibroblast cultures and inhibition of mitochondrial oxygen consumption in rat thymocytes at micromolar concentrations. To what extent the effects caused by Cr(VI) are directly related to the reduction process or to the subsequent binding of Cr(III), and possibly of Cr(V), to biological molecules playing a critical role in cell physiology, remains to be elucidated.