Investigation of the role of sigma1-receptors in inositol 1,4,5-trisphosphate dependent calcium signaling in hepatocytes

In hepatocytes, as in other cell types, Ca(2+) signaling is subject to complex regulations, which result largely from the intrinsic characteristics of the different inositol 1,4,5-trisphosphate receptor (InsP(3)R) isoforms and from their interactions with other proteins. Although sigma1 receptors (Sig-1Rs) are widely expressed in the liver, their involvement in hepatic Ca(2+) signaling remains unknown. We here report that in this cell type Sig-1R interact with type 1 isoforms of the InsP(3) receptors (InsP(3)R-1). These results obtained by immunoprecipitation experiments are confirmed by the observation that Sig-1R proteins and InsP(3)R-1 colocalize in hepatocytes. However, Sig-1R ligands have no effect on InsP(3)-induced Ca(2+) release in hepatocytes. This can be explained by the rather low expression level expression of InsP(3)R-1. In contrast, we find that Sig-1R ligands can inhibit agonist-induced Ca(2+) signaling via an inhibitory effect on InsP(3) synthesis. We show that this inhibition is due to the stimulation of PKC activity by Sig-1R, resulting in the well-known down-regulation of the signaling pathway responsible for the transduction of the extracellular stimulus into InsP(3) synthesis. The PKC sensitive to Sig-1R activity belongs to the family of conventional PKC, but the precise molecular mechanism of this regulation remains to be elucidated.

Ligand-binding affinity of the type 1 and 2 inositol 1,4,5-trisphosphate receptors: effect of the membrane environment

The inositol 1,4,5-trisphosphate (InsP3) receptor is essential for Ca2+ release from intracellular stores. There are three InsP3 receptor types which are targets for several types of regulation. Ca2+, phosphorylation, and protein-protein interactions may contribute to the complex pattern of the Ca2+ signal in stimulated cells. Furthermore, the 3 receptor types could have different affinities for InsP3. We compared the affinities of the type 1 receptor from the cerebellum with the liver type 2 receptor both in their membrane environment and after isolation by immunoprecipitation. Measurements of [3H]InsP3 binding in a cytosol-like medium revealed that the Kd of the liver receptor (45 +/- 5 nM, N = 14) was higher than the Kd of the cerebellar receptor (28 +/- 3 nM, N = 9). Solubilization and immunopurification of the liver InsP3 receptor resulted in a 10-fold increase in its affinity for InsP3. The affinity of the cerebellar receptor did not change under these conditions. Therefore, the extraction of the liver and the cerebellar receptors from their membrane environments induced an inversion of their relative affinities. Treatment of liver membranes with low concentrations of detergents also increased the affinity for InsP3 binding. These data indicate that the type 1 and the type 2 InsP3 receptors have different affinities for InsP3 and that the properties of the type 2 receptor are strongly regulated by hydrophobic interactions within its membrane environment.

Regulation of cerebellar Ins(1,4,5)P3 receptor by interaction between Ins(1,4,5)P3 and Ca2+

We have characterized in detail the Ca(2+)-dependent inhibition of [(3)H]Ins(1,4,5)P(3) ([(3)H]InsP(3)) binding to sheep cerebellar microsomes, over a short duration (3 s), with the use of a perfusion protocol. This procedure prevented artifacts previously identified in studies of this Ca(2+) effect. In a cytosol-like medium at pH 7.1 and 20 degrees C, a maximal inhibition of approx. 50% was measured. Both inhibition and its reversal were complete within 3 s. Ca(2+) decreased the affinity of the receptor for InsP(3) by approx. 50% (K(d) 146+/-24 nM at pCa 9 and 321+/-56 nM at pCa 5.3), without changing the total number of binding sites. Conversely, increasing the [(3)H]InsP(3) concentration from 30 to 400 nM tripled the IC(50) for Ca(2+) and decreased the maximal inhibition by 63%. This is similar to a partial competitive inhibition between InsP(3) binding and inhibitory Ca(2+) binding and is consistent with InsP(3) and Ca(2+) converting InsP(3) receptor into two different states with different affinities for these ligands. Mn(2+) and Sr(2+) also inhibited [(3)H]InsP(3) binding but were respectively only 1/10 and 1/200 as effective as Ca(2+). No inhibition was observed with Ba(2+). This selectivity is the same as that previously reported for the inhibitory Ca(2+) site of InsP(3)-induced Ca(2+) flux, suggesting that the same site is used by Ca(2+) to convert cerebellar InsP(3) receptor to a low-affinity state and to inhibit its channel activity. Our results also suggest a mechanism by which InsP(3) counteracts this Ca(2+)-dependent inhibition.

Multiple mechanisms of regulation of the inositol 1,4,5-trisphosphate receptor by calcium

Ca2+ mobilisation by inositol 1,4,5-trisphosphate (InsP3) is a complex phenomenon which involves positive and negative feedback regulation by cytosolic Ca2+. It has been shown that Ca2+ increased the affinity of [3H]-InsP3 binding to liver membranes and inhibited [3H]-InsP3 binding to cerebellar membranes. We investigated the effects of Ca2+ on the [3H]-InsP3 binding to receptor solubilised and rapidly purified by immunoprecipitation. The InsP3 binding to the purified liver receptor was insensitive to the addition of Ca2+, indicating that Ca2+ did not interact directly with the receptor. The loss of the Ca2+ effect on liver receptor affinity was reproduced by alkaline treatment of liver membranes, which is known to extract the peripheral membrane proteins. This suggests that Ca2+ regulates the liver InsP3 receptor by interacting with a membrane-associated protein. Ca2+ inhibited the binding of [3H]-InsP3 to purified cerebellar receptors as was found with the membrane fraction. The treatment of the purified cerebellar receptor with media of high ionic strength or at alkaline pH did not abolish the effect of Ca2+ on the receptor. This indicates that the inhibitory effect of Ca2+ on [3H]-InsP3 binding to cerebellar membranes occurs either via direct interaction with the receptor or via an integral protein strongly associated with the receptor. In conclusion, the mechanisms of regulation of InsP3-induced Ca2+ release by Ca2+ involve different molecular support in cerebellum and in liver. This may reflect different regulation dependent on the receptor type.

Inositol 1,4,5-trisphosphate slowly converts its receptor to a state of higher affinity in sheep cerebellum membranes

Incubation of cerebellar microsomes with d-myo-inositol 1,4,5-trisphosphate (InsP3) (0.01 1 microM), at 4 or 20 degrees C in a cytosolic-like medium devoid of Ca2+ and Mg2+, followed by InsP3 removal, induced an increase in InsP3 binding determined with 1 nm [3H]InsP3. At 20 degrees C, and pH 7.1, maximal stimulation (1.5 2. 5-fold) was obtained with 1 mum InsP3, and the EC50 was 60 +/- 5 nm. Several lines of evidence suggested that the activating site is identical with the InsP3 binding site: (i) activation and binding exhibited the same inositol phosphate specificity; (ii) addition of decavanadate, a competitive inhibitor of [3H]InsP3 binding, to the preincubation mixture, prevented the activating effect of InsP3; (iii) the concentration of InsP3 giving half-maximal activation was close to that giving half-maximal InsP3 binding. The time course of activation was found to be much slower than that of binding. While a t1/2 less than 0.4 s has been measured recently at neutral pH and 20 degrees C for binding of 0.5 nm [3H]InsP3 (Hannaert-Merah, Z., Coquil, J.-F., Combettes, L., Claret, M., Mauger, J.-P., and Champeil, P. (1994) J. Biol. Chem. 269, 29642-29649), a 20-s preincubation with 1 microM InsP3 was required to half-maximally stimulate binding. Under the present conditions, the InsP3-induced binding increase was only partially reversible. However, this effect was not blocked by antiproteases suggesting that it did not involve proteolysis. Taking advantage of the marked difference in the kinetics of InsP3 binding and InsP3-dependent activation, we performed binding experiments on a short period (3 s) to determine the effect of InsP3 pretreatment on the binding parameters. The data showed that this treatment increased the affinity of the receptor without changing the number of binding sites (control: KD = 107 nm, Bmax = 28 pmol/mg of protein; after preincubation with 1 microM InsP3: KD = 53 nm, Bmax = 32 pmol/mg of protein). The two states of the receptor bound InsP3 with a Hill coefficient close to 1 on a 3-s scale. In agreement with the effect of InsP3 pretreatment, equilibrium binding experiments performed on 10-min incubations revealed an apparent positive cooperative behavior (apparent Hill coefficient = 1.6; apparent KD = 66 nm). These results report a new regulatory process of the InsP3 receptor in cerebellum occurring independently of Ca2+ and on a relatively long time scale.

Intracellular calcium stores and inositol 1,4,5-trisphosphate receptor in rat liver cells

The D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] receptor was localized by immunofluorescence experiments in situ in liver cryosections. Two anti-Ins(1,4,5)P3 receptor antibodies (against the 14 C-terminal residues of the type 1 receptor or against the entire cerebellar receptor) weakly decorated the whole cytoplasm, and a more intense labelling was observed at the periphery of the hepatocytes, particularly beneath the canalicular and the sinusoidal domains of the plasma membrane (PM). Antibodies against calreticulin, the Ca2+ pump (SERCA2b) or endoplasmic reticulum (ER) membranes homogeneously labelled the cytoplasm and the subplasmalemmal area. These data indicate that the ER can be divided into at least two specialized subregions: one is located throughout most of the cytoplasm and contains markers of the rough ER (RER), calreticulin, SERCA2b and a low density of Ins(1,4,5)P3 receptor, and the other is confined to the periphery of the cells and contains calreticulin, Ca2+ pump, RER markers and a high density of Ins(1,4,5)P3 receptor. A membrane fraction enriched in Ins(1,4,5)P3 receptor and in markers of the PM was immuno-adsorbed with the antibody against the C-terminal end of the Ins(1,4,5)P3 receptor and pelleted with Sepharose protein A. The immuno-isolated material was enriched in Ins(1,4,5)P3 receptor, but none of the markers of the ER or of the PM could be detected. This suggests that the Ins(1,4,5)P3 receptor is localized on discrete domains of the ER membrane beneath the canalicular and the sinusoidal membranes, where it was found at higher densities than the other markers.

Characterization of the co-agonist effects of strontium and calcium on myo-inositol trisphosphate-dependent ion fluxes in cerebellar microsomes

Using sheep cerebellum microsomes previously loaded with 45Ca2+ or 90Sr2+, we measured the dependence of inositol 1,4,5-trisphosphate (InsP3)-induced efflux of these ions on Ca2+ or Sr2+ on the cytosolic side. At a low InsP3 concentration, Ca2+ in the submicromolar range only poorly activated 45Ca2+ or 90Sr2+ efflux, and higher Ca2+ concentrations were inhibitory. In contrast, Sr2+ in the micromolar range activated release efficiently, while only very high Sr2+ concentrations were inhibitory. Experiments were repeated in the presence of a high InsP3 concentration, which allowed increasing free Ca2+ to micromolar concentrations without inducing complete inhibition of the InsP3-dependent efflux. Under these conditions, micromolar Ca2+ was found to activate efflux to a large extent, similar to that previously found with Sr2+. Optimal activation by Ca2+ of the InsP3-dependent channel occurs at micromolar rather than submicromolar free Ca2+ concentrations, but at too low an InsP3 concentration, Ca(2+)-induced activation is counteracted by Ca(2+)-induced inactivation. Separate measurements of [3H]-InsP3 binding at a low concentration showed that Sr2+ and Ca2+ did not enhance the amount of bound [3H]-InsP3, implying that the activating effect of Sr2+ and Ca2+ in cerebellar microsomes is mediated by an increase in the channel opening probability and not by an increase in the receptor's affinity for InsP3. A similar relationship also holds in the case of the activating effect of nucleotides.

Rapid kinetics of myo-inositol trisphosphate binding and dissociation in cerebellar microsomes

Using sheep cerebellum microsomes adsorbed on a filter, we measured the kinetics of [3H]inositol 1,4,5-trisphosphate (InsP3) binding and dissociation on the subsecond time scale during rapid perfusion of the filter with [3H]InsP3-containing or InsP3-free media. At 20 degrees C and pH 7.1, in a cytosol-like medium containing MgCl2, the half-time for InsP3 dissociation was as short as 125 ms. The receptor behaved as a simple target for binding of its ligand, with the rate constant for InsP3 binding increasing linearly with InsP3 concentration. Various modulators of InsP3 binding (KCl, NaCl, pH, Mg2+, and Ca2+) were found to affect the receptor's apparent affinity for InsP3 mainly by altering the rate constant for [3H]InsP3 dissociation. ATP (but not InsP3) also accelerated [3H]InsP3 dissociation. In contrast to these modulators, luminal Ca2+ was found to have no effect on the amount of microsome-bound [3H]InsP3.

Rapid filtration studies of the effect of cytosolic Ca2+ on inositol 1,4,5-trisphosphate-induced 45Ca2+ release from cerebellar microsomes

Using microsomal membrane vesicles derived from sheep cerebellum, we measured the rate of inositol 1,4,5-trisphosphate (InsP3)-dependent 45Ca2+ efflux from 45Ca(2+)-loaded compartments during rapid perfusion with a medium containing InsP3 and various concentrations of free 40Ca2+ on the cytosolic side (pH 7.1, 5 mM Mg2+, in the absence of ATP at 20 degrees C). At 0.15 microM InsP3 and pCa 6.5, half-45Ca2+ release was attained within less than 200 ms. At low Ca2+ concentrations, the initial rate of 45Ca2+ release depended smoothly on InsP3 concentration, and InsP3 activated release with moderate positive cooperativity. Preliminary experiments performed at various free 40Ca2+ concentrations were consistent with a bell-shaped 40Ca2+ dependence of 45Ca2+ release. In the range of micromolar or higher free 40Ca2+ concentrations, the apparent inhibition of 45Ca2+ release was dependent on InsP3 concentration, and 45Ca2+ release for intermediate InsP3 concentrations was transient; under selected conditions, a second perfusion period, identical to the first one but separated from it by a short recovery period, was found to allow renewed 45Ca2+ efflux. At high Ca2+ concentration, fast reversible Ca(2+)-dependent desensitization of the channel, and not heterogeneity, was therefore responsible for the termination of InsP3-triggered 45Ca2+ efflux at submaximal concentrations of InsP3. At lower Ca2+ concentrations, a large fraction of the apparent activating effect of submicromolar 40Ca2+ concentrations on 45Ca2+ efflux that we had observed in the preliminary experiments proved to be the artifactual consequence of an inhibitory effect exerted by metal-free Ca2+ chelators on InsP3-dependent efflux at nanomolar 40Ca2+ concentrations. 1,2-Bis(2-aminophenoxy)ethane-N,N,N'-N'-tetraacetic acid, EGTA, and fluo-3 were all effective inhibitors. When this inhibition was taken into account, a rise in free 40Ca2+ concentration from 30 to 300 nM only weakly enhanced 45Ca2+ fluxes in the presence of a low concentration of InsP3. As a result, submicromolar free 40Ca2+ appears to be only a poor activator of InsP3-induced Ca2+ release under these experimental conditions.

The inositol 1,4,5-trisphosphate receptor: kinetic properties and regulation

Inositol 1,4,5-triphosphate (InsP3) is a second messenger responsible for the mobilization of intracellular Ca2+ after receptor-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate. InsP3 binds to a specific receptor located on the membrane of an intracellular compartment and opens a Ca2+ channel causing the cytosolic Ca2+ concentration to increase. Measurement of radiolabelled InsP3 binding and InsP3-induced Ca2+ release in parallel experiments indicated that the liver InsP3 receptor exists in two main states: an active state (A) and an inactive one (I). The "I" form of the receptor is found in the presence of high Ca2+ concentrations (above 1 microM). The binding properties of the "A" and the "I" states of the receptor have been characterized by analysing a membrane fraction enriched in InsP3 receptors. The inactive "I" state displays a high affinity (Kd = 2 nM) and slow rates of association and dissociation. The active state "A" of the receptor displays complex kinetic properties. The rate of association and the rate of dissociation of labelled InsP3 are rapid phenomena probably involving several components. The apparent Kd for the InsP3 binding is about 40 nM in a low Ca2+ medium. The affinity of the "A" state of the receptor is increased by Ca2+ (at concentrations lower than 0.5 microM) and by thiol reagents. The increase of the affinity of the receptor is due to a decrease of the dissociation rate constants. This lowers the threshold such that Ca2+ is released at lower concentrations of InsP3. These data indicate that the binding of InsP3 to its receptor is a complex phenomenon involving the transition among several states.(ABSTRACT TRUNCATED AT 250 WORDS)

Thiol reagents increase the affinity of the inositol 1,4,5-trisphosphate receptor

Thiol reagents have been shown to increase cytosolic Ca2+ in several cell types. In non-muscle cells, these agents induce Ca2+ spikes by increasing the sensitivity of the intracellular Ca2+ stores to D-myo-inositol-1,4,5-trisphosphate (InsP3). We have investigated the effects of thimerosal and oxidized glutathione on the binding properties of the InsP3 receptor in permeabilized hepatocytes and liver and cerebellar membranes. Thimerosal, at the maximal concentration of 100 microM, decreased the KD for the InsP3 binding to permeabilized hepatocytes and cerebellar membranes from 16 to 3 nM and from 25 to 8 nM, respectively, without affecting the maximal binding capacities. On liver membranes, both thimerosal and high Ca2+ concentrations increased the affinity for InsP3 binding. The Ca2+ and the thimerosal effects were differentiated by kinetic experiments. In low Ca2+ media, two kinetic components were identified and thimerosal decreased the rate of dissociation from both these components without affecting the rate of association. In the high Ca2+ medium, a single kinetic component was found with a very slow rate of dissociation. These data suggest that the InsP3 receptor exists in different states. The high-affinity inactive state induced by high Ca2+ concentrations displays slow rates of association and dissociation. The binding properties of the receptor in its active state can be regulated by thiol reagents which increase the affinity by decreasing the dissociation rate constants. At a resting concentration of 100-200 nM, Ca2+ has two effects: it increases the affinity of the active state of the receptor as thiol reagents do and transforms part of the receptors into the inactive high-affinity state.

Adenylate cyclase of platelets from spontaneously hypertensive rats: reduction of the inhibition by GTP

We have compared the effects of Gpp[NH]p on adenylate cyclase activity of platelet membranes in SHR and WKY rats. In the presence of 50 microM forskolin, low concentrations of Gpp[NH]p (0.01 to 0.3 microM) inhibited the enzyme activity in both strains, but the maximal level of inhibition was significantly lower in SHR (- 20%). In the absence of forskolin, 0.1 microM Gpp[NH]p was inhibitory only in WKY and the adenylate cyclase activity was greater in hypertensive rats at this nucleotide concentration. Increasing Gpp[NH]p from 0.1 to 3 microM induced the same increase of enzyme activity in both strains. In SHR, GTP itself induced a lower inhibition of the enzyme stimulated by 50 microM forskolin or 0.1 microM prostaglandin E1. These results suggest that the modulatory effect of the guanine nucleotide inhibitory protein on adenylate cyclase may be reduced in platelets from SHR.

[Anomalies of the adenylate cyclase system in platelets of the SHR rat]

The hormone-sensitive adenylate cyclase system of plasma membrane is composed of at least three types of proteins: hormone receptors, activatory (Gs) and inhibitory (Gi) guanine nucleotide-regulatory proteins and the catalytic unit (C). Abnormal hormonal regulations of platelet adenylate cyclase in both humans and experimental animals have been reported to occur in hypertension. However, little is known about the mechanisms for these alterations. The aim of the present study was to compare the activity of C and the inhibitory capacity of Gi in platelet membranes from spontaneously hypertensive rats (SHR) and their normotensive controls (WKY). Adenylate cyclase activity of 40,000 g membranes was assessed at pH 7.5 with 0.1 mM (alpha-32P) ATP and an appropriate bivalent cation (Mn2+ or Mg2+). Under incubation conditions that uncoupled C from Gs and Gi (25 mM MnCL2, 100 microM forskolin), a significantly lower adenylate cyclase activity was measured in membranes from SHR rats (2.07 +/- 0.12 vs 2.36 +/- 0.1 nmol cAMP/mn/mg of protein, p less than 0.05). This difference between the two strains was also observed in platelet homogenates. In a second kind of experiments, membranes were incubated with 2.1 mM MgCl2 instead of MnCl2. In both strains of rats, low concentrations of Gpp (NH)p (10 to 300 nM) inhibited adenylate cyclase activity when stimulated by 50 microM forskolin. However, the maximal extent of inhibition was significantly reduced in hypertensive rats (49.7 +/- 2.4 vs 60.5 +/- 2.3 p. 100, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Activity of cyclic GMP-dependent protein kinase in aortae from spontaneously hypertensive rats

It has been suggested that various agents induce relaxation of vascular smooth muscles through guanosine 3',5'-cyclic monophosphate (cGMP) and cGMP-dependent protein kinase (cGMP-PK). In this work, the activity of cGMP-PK was studied in the 30,000 g supernatant from aortae of 4, 6, 8 and 12-week-old spontaneously hypertensive (SHR) and age-matched normotensive Wistar-Kyoto (WKY) rats and also of 4 and 12-week-old normotensive Wistar (W) and Sprague Dawley (SD) rats. At 4 weeks of age, both basal and cGMP-stimulated activity were not different in SHR and WKY rats. Nevertheless, a greater basal activity was measured in W (+50%) and SD (+20%) rats than in SHR, while no difference was observed between stimulated activities. In contrast with observations in the three normotensive rat strains, cGMP-PK activity did not decrease in the aortae supernatant of SHR rats aged 4-12 weeks. This resulted in mean increases of 45 and 30% in the basal and the cGMP-stimulated activity, respectively, in the 12-week-old SHR rats. The abnormal evolution of cGMP-PK activity in the hypertensive strain was already detectable at 4-6 weeks of age. In apparent agreement with observations on protein kinase activity, cGMP binding activity attributable to cGMP-PK was 25% greater in 12-week-old hypertensive rats compared with age-matched WKY rats. These results indicate that in aortae of SHR rats, control of cGMP-PK activity is abnormal early in life.

Occurrence of the methylisobutylxanthine-stimulated cyclic GMP binding protein in various rat tissues

A new type of cGMP binding protein, the activity of which is characteristically stimulated by methylisobutylxanthine, has been previously discovered in rat lung and platelets (Hamet, P. and Coquil, J.F. (1978) J. Cyclic Nucleotide Res. 4, 281-290). In the present study, we demonstrate the occurrence of this protein in soluble extracts of a variety of rat tissues fractionated by a DEAE-Sepharose chromatography. In several tissues (spleen, lung and brain) the binding activity of this protein was of the same order of magnitude as that of the cGMP-dependent protein kinase.

Rapid activation of cAMP phosphodiesterase in rat platelets

Incubation of intact platelets with prostaglandins (PGE1 and PGI2) and phosphodiesterase inhibitors (1-methyl-3-isobutylxanthine, indomethacin, dipyridamol) lead to activation of cAMP phosphodiesterase. The activation was rapid (maximal within 30 s) and stable after removal of agents and homogenization of platelets. The activation remained after DEAE-Sepharose chromatography. The effect of the two types of agents on phosphodiesterase activity was more than additive and activation did not alter the nonlinear kinetic behavior of phosphodiesterase. The mechanism of the ex vivo stimulation is unknown at the present time, however, it does not seem to be due to cellular redistribution of the enzyme. The results suggest that activation of a cAMP-dependent protein kinase is an intermediate step. The ex vivo stimulation is regulated by a calcium-dependent process, since addition of Ca2+ ions and ionophore A23187 to Ca2+ depleted platelets abolished the ex vivo stimulation by PGE1 and MIX.

Properties of the cyclic GMP phosphodiesterase from rat lung. Inhibition by unsaturated fatty acids

Catalytic and regulatory properties of the major form of cyclic GMP phosphodiesterase (3':5'-cyclic-GMP 5'-nucleotidohydrolase, EC 3.1.4.35) from rat lung were studied. The enzyme partially purified by a DEAE-Sepharose chromatography displayed a much higher affinity toward cyclic GMP than toward cyclic AMP, the apparent Km values being 5.7 microM and 482 microM for the guanylic and the adenylic cyclic nucleotide, respectively. In contrast, the V value for cyclic AMP was about 3-times higher than the V value for cyclic GMP. Linear double reciprocal plots of initial velocity were observed with each cyclic nucleotide. From 10(-8) to 3.3 X 10(-6) M, cyclic GMP did not change the hydrolysis of 1 or 10 microM cyclic [3H]AMP, while it became inhibitory at higher concentrations. In contrast with a calmodulin-sensitive phosphodiesterase prepared from rat brain, the lung enzyme was not stimulated by a heat-stable Ca2+-dependent factor from rat lung or by rat brain calmodulin or by lipids including fatty acids and lysophosphatidylcholine. Various unsaturated 18- and 20-carbon fatty acids inhibited at varying degrees the cyclic GMP phosphodiesterase from rat lung. The inhibitory potency increased with the number of double bonds in the hydrocarbon chain. In contrast, the methyl esters of the unsaturated fatty acids and the saturated fatty acids of variable hydrocarbon chain lengths had no appreciable effects. A linear Hill plot of phosphodiesterase inhibition with a slope of unity was obtained with arachidonic acid up to 30 microM, suggesting only one type of inhibitory site. In this range of concentrations the inhibition was entirely reversible. Kinetics analysis demonstrated that up to 30 microM arachidonic acid was a purely competitive inhibitor with an apparent Ki of 20 microM. Over 30 microM, the Hill coefficient increased progressively, indicating the binding to other inhibitory sites, while the reversibility disappeared.

[Urinary excretion of catecholamines in the dog: physiopathologic hypotheses in man]

Renal handling of free catecholamines (dopamine, norepinephrine and epinephrine) was studied in 36 hydropenic anesthetized mongrel dogs in accordance with the clearance technique. There was no statistically significant correlation between plasma concentrations and either systemic (mean blood pressure, or cardiac output) or renal (clearance of PAH or glomerular filtration rate) hemodynamics. Net tubular transport (NTT) was calculated as the difference between filtered load and urinary excretion for any catecholamine. The mean NTTs of free catecholamines were as follows: --2,72 ng/min for dopamine, --3,18 for norepinephrine, and --1,36 for epinephrine, showing that they are mostly reabsorbed. However the use of averages is misleading inasmuch as tubular transport of free catecholamines is a heterogeneous phenomenon: a secretion appears to predominate when plasma concentrations are low and a reabsorption predominates when they are high. Whether such a heterogeneity is due to either a genetic heterogeneity in mongrel dogs, or an age-related difference is discussed.

Characteristics of a new binding protein distinct from the kinase for guanosine 3':5'-monophosphate in rat platelets

A new type of cyclic GMP binding protein was recently identified in our laboratory (Hamet, P. and Coquil, J.-F. (1978) J. Cyclic Nucleotide Res. 4, 281--290). The binding, recovered in the supernatant fractions, is highly specific for cyclic GMP and is clearly distinct from the binding to cyclic GMP-dependent protein kinase. Chromatography on DEAE-Sepharose separated the cyclic GMP binding protein from cyclic AMP binding, cyclic AMP-dependent kinase activities, and from guanylate cyclase. The optimal binding occurs at high pH and in the presence of thiol reagents. Several phosphodiesterase inhibitors increase the affinity of binding (Kd was 353 +/- 60 nM in the absence and 13.4 +/- 1.5 nM in the presence of 1-methyl-3-isobutyl-xanthine). The molecular weight of the binding protein was determined to be about 176,000 and the sedimentation coefficient was 6.4 S. While the binding and phosphodiesterase activities co-migrated on DEAE-Sepharose, gel filtration and sucrose gradients, certain treatments (such as increasing the concentrations of salt and heating) were able to influence one activity while having no effect on the other. Hence, the binding activity may be involved in the regulation of the activity of cyclic GMP phosphodiesterase. Since the binding protein appears to be the only 'receptor' for cyclic GMP detectable in platelets, this protein and/or its relation to cyclic GMP phosphodiesterase may play a role in the mechanism of action of cyclic GMP in platelets.

Cyclic nucleotide phosphodiesterase of retinal photoreceptors. Partial purification and some properties of the enzyme

1. A cyclic nucleotide phosphodiesterase (EC 3.1.4.16) has been partially purified from bovine rod outer segments. The enzyme preparation obtained has a very high specific activity towards cyclic GMP and is still able to hydrolyze cyclic AMP. Upon polyacrylamide gel electrophoresis, one major and three minor protein bands are seen, the enzyme activity being associated with the major band. The enzyme eluted from the gels still hydrolyzes both cyclic nucleotides. At all substrate concentrations tested, cyclic GMP was hydrolyzed at a faster rate. The enzyme eluted from the gel columns migrated as a single band upon electrophoresis in 0.1% sodium dodecyl sulfate-polyacrylamide gels corresponding to a molecular weight of 105 000. 2. A complex kinetic pattern was observed for cyclic GMP hydrolysis: the plot of velocity vs substrate concentration was hyperbolic at low and sigmoidal at higher concentrations. By contrast, simple kinetics were observed for cyclic AMP hydrolysis yielding an apparent Km of 0.1 mM. The unusual kinetics may be implicated in the regulation of cyclic GMP levels in rod outer segments. 3. Cyclic AMP stimulated the hydrolysis of cyclic GMP at low and inhibited it at higher concentrations. Addition of Mg2+ appeared to be necessary for optimum activity. The activity measured in the absence of exogenous Mg2+ was abolished by EDTA.

Monoamine oxidase in rat arteries: evidence for different forms and selective localization

1. Two forms of monoamine oxidase activity were differentiated in rat mesenteric and femoral artery by means of substrate and inhibitor specificities: one form deaminated tyramine, 5-hydroxytryptamine and noradrenaline and was highly sensitive to pargyline and clorgyline but resistant towards carbonyl reagents. This form resembled type A monoamine oxidase previously described. The other deaminated tyramine but not 5-hydroxytryptamine or noradrenaline and was inhibited by carbonyl reagents but not by clorgyline or pargyline.2. About one third of the total monoamine oxidase in homogenates of rat mesenteric artery was recovered in a 10(5)g supernatant. Both forms were partially soluble, but relatively less of the type A activity was recovered in the soluble fraction.3. Chemical sympathectomy with 6-hydroxydopamine resulted in a loss of 59% of monoamine oxidase activity in the mesenteric artery. There was a selective loss of type A activity, as revealed by the 70% decrease in 5-hydroxytryptamine deaminating ability and by the marked decrease in clorgyline sensitivity. The second monoamine oxidase species was resistant to 6-hydroxydopamine. The soluble activity was not affected by chemical sympathectomy. Most of the transmitter-specific monoamine oxidase of the arterial wall was localized within the adrenergic nerve endings. Our observations are consistent with the hypothesis that extraneuronal monoamine oxidase plays only a minor role in metabolizing noradrenaline in sympathetically innervated tissues.4. Plasma amine oxidase might originate from the arterial wall since it has similar characteristics to that found in the mesenteric artery.