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C Pavoine

Publications and source records attributed to C Pavoine.

32 records · Page 2Linked to original sources
PubMed1990

Activation of Na+/Ca2+ exchange by adenosine in ewe heart sarcolemma is mediated by a pertussis toxin-sensitive G protein.

We studied the effect of adenosine on Na+/Ca2+ exchange activity in ewe heart ventricular sarcolemmal vesicles. Adenosine was found to stimulate Na+/Ca2+ exchange activity in a dose-dependent manner from 0.1 nM to 10 microM, with maximal stimulation (40%) at 0.1 microM adenosine. The Vmax of Na+/Ca2+ exchange was increased, but the Km for Ca2+ was not altered. The effect of adenosine was specific since 1 microM adenine, inosine, and guanosine led to less than 15% stimulation, and adenosine diphosphate had no effect. Caffeine antagonized the activation of Na+/Ca2+ exchange by adenosine, and the order of potency of adenosine analogs was N6-(L-2-phenylisopropyl)adenosine = N6-cyclohexyladenosine = 5'-(N- ethylcarboxamido)adenosine much greater than N6-(D-2-phenylisopropyl)adenosine, indicating the involvement of A1 subclass receptors. The effect of adenosine was mimicked by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and blocked by pertussis toxin treatment. Taken together, these results suggest that A1 subclass receptors coupled to a pertussis toxin-sensitive G protein mediate the activation of Na+/Ca2+ exchange activity by adenosine. We conclude that the negative inotropic effect of adenosine in ventricular muscle, antagonistic toward cyclic AMP, may involve activation of Na+/Ca2+ exchange.

Adenosine↗
PubMed1990

Glucagon-(19-29) exerts a biphasic action on the liver plasma membrane Ca2+ pump which is mediated by G proteins.

We have recently shown that nanomolar concentrations of glucagon-(19-29), which can derive from native glucagon by proteolytic cleavage of the dibasic doublet Arg17-Arg18, inhibit the Ca2+ pump in liver plasma membrane vesicles independently of adenylyl cyclase activation (Mallat, A., Pavoine, C., Dufour, M., Lotersztajn, S., Bataille, D., and Pecker, F. (1987) Nature 325, 620-622). We report here that the regulation of the Ca2+ pump by glucagon-(19-29) is dependent on guanine nucleotides. In the presence of 10 microM guanosine 5'-3-O-(thio) triphosphate (GTP gamma S) or 75 microM GTP, glucagon-(19-29) caused a biphasic regulation of the Ca2+ pump. ATP-dependent Ca2+ transport was inhibited in the presence of 10 pM to 1 nM glucagon-(19-29), while higher concentrations of the peptide (1-100 nM) reversed the inhibition caused by lower ones. GTP gamma S alone, at high concentrations (100 microM), reproduced the inhibitory effect of glucagon-(19-29) and induced a 40% inhibition of the basal activity of the Ca2+ pump which was reversed by low concentrations of glucagon-(19-29) (10 pM to 1 nM). Treatment of rats with cholera toxin resulted in a 70% increase in the basal activity of the Ca2+ pump, a loss of sensitivity to GTP gamma S and to the biphasic regulation by glucagon-(19-29). Treatment with pertussis toxin did not affect the response of the Ca2+ pump to GTP gamma S and glucagon-(19-29). We conclude that glucagon-(19-29) can exert a biphasic effect on the Ca2+ pump which is mediated by G protein(s) sensitive to cholera toxin.

Adenylate Cyclase Toxin↗
PubMed1990

Glucagon stimulates the cardiac Ca2+ current by activation of adenylyl cyclase and inhibition of phosphodiesterase.

Glucagon exerts positive inotropic and chronotropic effects in the heart. Like its glycogenolytic effect in liver cells, the cardiac effects of glucagon are often correlated with adenylyl cyclase stimulation. Therefore, cyclic AMP-dependent phosphorylation of L-type Ca2+ channels might be involved in the inotropic effect of glucagon. There have been no reports, however, of the effects of glucagon on the cardiac Ca2+ current (ICa). Also, the physiological effects of glucagon could involve mechanisms other than stimulation of adenylyl cyclase. Here we show that glucagon enhances ICa in frog and rat ventricular myocytes. The effect of glucagon in rats resulted from a stimulation of adenylyl cyclase. In frogs, however, the effect of glucagon on ICa was smaller and occurred at a concentration tenfold lower than in rats, and adenylyl cyclase was not modified. In addition, cAMP potentiated the effect of glucagon on ICa in frog ventricle, which correlated with the observed inhibition by glucagon of low-Km cAMP phosphodiesterase activity. Therefore, this is an example of a hormone that affects cardiac function in a similar way to a variety of synthetic cardiotonic compounds, such as milrinone and Ro-20-1724. Inhibition of phosphodiesterase activity by glucagon may be essential in animals in which glucagon increases cardiac contractility but does not effectively stimulate adenylyl cyclase.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1990

Inhibition of the calcium pump by human parathyroid hormone-(1-34) and human calcitonin in liver plasma membranes.

The effect of human parathyroid hormone-(1-34) (hPTH) and human calcitonin (hCT) on the activity of the Ca2(+)-extrusion pump in liver plasma membranes was studied. Both hormones were found to be potent inhibitors of Ca2+ transport and the related high-affinity (Ca2(+)-Mg2+)-ATPase activity, causing maximal inhibition of 25-30% at concentrations of 100 nM. Half-maximal inhibition was observed with 20 nM-hPTH and with 0.5 nM-hCT. By comparison, salmon calcitonin and intact bovine parathyroid hormone-(1-84) were inhibitory only at 10 microM. The effects of hCT and hPTH on the Ca2+ pump activity were not mimicked by cyclic AMP. Also, 10 microM of either hPTH-(1-34) or hCT did not alter the 45Ca2+ influx rate into isolated hepatocytes. We conclude that inhibition of Ca2+ efflux, rather than the stimulation of Ca2+ influx, may play a functional role in the control of hepatic calcium homeostasis by hPTH-(1-34) and hCT.

Biological Transport, Active↗
PubMed1989

[Glucagon is processed to the (19-29) fragment at the level of the hepatocyte membrane].

Upon incubation with hepatic plasma membranes, glucagon is processed into its (19-29) C-terminal fragment. This suggests that, in physiological conditions, glucagon is processed in a target tissue at the level of its Arg17-Arg18 basic doublet, leading to the production of a fragment which is known to display an original biological specificity, namely the modulation of the calcium pump present in hepatocyte plasma membrane.

Animals↗
PubMed1987

The high affinity (Ca2+-Mg2+)-ATPase in liver plasma membranes is a Ca2+ pump. Reconstitution of the purified enzyme into phospholipid vesicles.

The purified (Ca2+-Mg2+)-ATPase from rat liver plasma membranes (Lotersztajn, S., Hanoune, J., and Pecker, F. (1981) J. Biol. Chem. 256, 11209-11215) was incorporated into soybean phospholipid vesicles, together with its activator. In the presence of millimolar concentrations of Mg2+, the reconstituted proteoliposomes displayed a rapid, saturable, ATP-dependent Ca2+ uptake. Half-maximal Ca2+ uptake activity was observed at 13 +/- 3 nM free Ca2+, and the apparent Km for ATP was 16 +/- 6 microM. Ca2+ accumulated into proteoliposomes (2.8 +/- 0.2 nmol of Ca2+/mg of protein/90 s) was totally released upon addition of the Ca2+ ionophore A-23187. Ca2+ uptake into vesicles reconstituted with enzyme alone was stimulated 2-2.5-fold by the (Ca2+-Mg2+)-ATPase activator, added exogenously. The (Ca2+-Mg2+)-ATPase activity of the reconstituted vesicles, measured using the same assay conditions as for ATP-dependent Ca2+ uptake activity (e.g. in the presence of millimolar concentrations of Mg2+), was maximally activated by 20 nM free Ca2+, half-maximal activation occurring at 13 nM free Ca2+. The stoichiometry of Ca2+ transport versus ATP hydrolysis approximated 0.3. These results provide a direct demonstration that the high affinity (Ca2+-Mg2+)-ATPase identified in liver plasma membranes is responsible for Ca2+ transport.

Adenosine Triphosphate↗
PubMed1987

Cholera toxin blocks glucagon-mediated inhibition of the liver plasma membrane (Ca2+-Mg2+)-ATPase.

We have previously shown that liver plasma membrane (Ca2+-Mg2+)-ATPase activity is inhibited by glucagon. To investigate the possible involvement of a GTP-binding (G) protein in this regulation, we have examined the effects of pertussis toxin and cholera toxin on inhibition of (Ca2+-Mg2+)-ATPase by glucagon. Treatment of liver plasma membranes with pertussis toxin did not affect the sensitivity of (Ca2+-Mg2+)-ATPase to the hormone. In contrast, treatment of plasma membranes or prior injection of animals with cholera toxin prevented inhibition of the (Ca2+-Mg2+)-ATPase by glucagon. Even though adenylate cyclase activity was increased by cholera toxin treatment, addition of cyclic AMP did not mimic the effect of cholera toxin in blocking glucagon-mediated inhibition of (Ca2+-Mg2+)-ATPase activity. These data suggest that a cholera toxin-sensitive protein, perhaps Gs or a Gs-like protein, is involved in the regulation of liver (Ca2+-Mg2+)-ATPase activity. The results emphasize the possible role of Gs-like proteins in regulation of enzymes other than adenylate cyclase and suggest that the study of (Ca2+-Mg2+)-ATPase may provide a useful enzymatic system to examine such regulation.

Adenylate Cyclase Toxin↗
PubMed1985

The inhibitor of liver plasma membrane (Ca2+-Mg2+)-ATPase. Purification and identification as a mediator of glucagon action.

Rat liver plasma membranes contain (Ca2+-Mg2+)-ATPase sensitive to inhibition by both glucagon and Mg2+. We have previously shown that Mg2+ inhibition is mediated by a 30,000-dalton inhibitor, originally identified as a membrane-bound protein. In fact, this inhibitor is also present in the 100,000 X g supernatant of the total liver homogenate. Its purification was achieved from this fraction by a combination of ammonium sulfate washing, gel filtration, and cationic exchange chromatography. N-Ethylmaleimide (NEM) treatment caused the inactivation of the purified inhibitor, which suggested that this protein possesses at least one NEM-sensitive sulfhydryl group essential for its activity. Treatment of the liver plasma membranes with NEM resulted in a 2- and 5-fold decrease in the affinity of the (Ca2+-Mg2+)-ATPase for glucagon and Mg2+, respectively, while the basal enzyme activity remained unchanged. This effect of NEM was concentration-, pH-, and time-dependent, optimal conditions being obtained by a 60-min treatment of plasma membranes with 50 mM NEM, at pH 7 and at 4 degrees C. The presence of 0.5 mM Mg2+ during NEM treatment of the plasma membranes prevented NEM inactivation. Reconstitution experiments showed that addition of the purified inhibitor to NEM-treated plasma membranes restored the inhibitions of the (Ca2+-Mg2+)-ATPase by both magnesium and glucagon. It is proposed that the (Ca2+-Mg2+)-ATPase inhibitor not only confers its sensitivity of the liver (Ca2+-Mg2+)-ATPase to Mg2+, but also mediates the inhibition of this system by glucagon.

Animals↗
PubMedDate not supplied

The liver plasma membrane Ca2+ pump: hormonal sensitivity.

The liver plasma membrane Ca2+ pump is supposed to extrude cytosolic calcium out of the cell. This system has now been well defined on the basis of its plasma membrane origin, its high affinity Ca2+ -stimulated ATPase activity, its Ca2+ transport activity, its phosphorylated intermediate. The liver calcium pump appears to be a target of hormonal action since it has been shown that glucagon and calcium mobilizing hormones namely alpha 1-adrenergic agonists, vasopressin, angiotensin II inhibit this system. The present review details the mechanism of calcium pump inhibition by glucagon and points out its difference from the inhibition process induced by calcium mobilizing hormones. We conclude that the inhibitory action of the Ca2+ mobilizing hormones and glucagon on the liver plasma membrane Ca2+ pump might play a key role in the actions of these hormones by prolonging the elevation in cytosolic free Ca2+.

Adrenergic alpha-Agonists↗
PubMedDate not supplied

A glucagon fragment is responsible for the inhibition of the liver Ca2+ pump by glucagon.

Glucagon specifically inhibits the Ca2+ pump in liver plasma membranes independently of adenylate cyclase activation. However, this inhibition is only observed at high concentrations of glucagon (Ki = 0.7 microM). Moreover, in the presence of bacitracin, an inhibitor of glucagon degradation, the Ca2+ pump is no longer sensitive to glucagon. These findings suggest that a fragment of glucagon might be the true effector of the liver Ca2+ pump. Pairs of basic amino acids are recognized as potential cleavage sites in post-translational processing of peptide hormones. The glucagon molecule includes a dibasic doublet (Arg 17-Arg 18). Therefore, we have examined the action of glucagon(19-29) on the liver Ca2+ pump. This peptide was obtained from glucagon by tryptic cleavage and separated by reverse-phase high-performance liquid chromatography. We found that glucagon(19-29), which is totally ineffective in activating adenylate cyclase, inhibited both the Ca2+-activated and Mg2+-dependent ATPase activity [Ca2+-Mg2+) ATPase) and Ca2+ transport in liver plasma membranes with an efficiency 1,000-fold higher than that of glucagon. Glucagon(1-21) was completely inactive; glucagon(18-29) and glucagon(22-29) acted only as partial agonists of glucagon(19-29). These results indicate that glucagon(19-29), obtained by proteolytic cleavage of glucagon, is likely to be the active peptide involved in the inhibition of the liver Ca2+ pump. We suggest that glucagon may be a precursor of at least one biologically active peptide.

Ca(2+) Mg(2+)-ATPase↗