Dimethylnitrosamine inhibits the glucagon-stimulated adenylate cyclase activity of rat liver plasma membranes and decreases plasma membrane fluidity

The effect of the hepatocarcinogen dimethylnitrosamine on rat liver plasma membrane adenylate cyclase activity and lipid fluidity was assessed. Glucagon-stimulated adenylate cyclase activity exhibited a complex response to increasing concentrations of dimethylnitrosamine, whereas fluoride-stimulated adenylate cyclase activity was progressively inhibited. Maximal inhibitory effects were observed at a concentration of 15 mM in both cases. The activity of detergent-solubilized adenylate cyclase was unaffected by dimethylnitrosamine. ESR analysis using a fatty acid spin probe showed that dimethylnitrosamine produced a marked, dose-dependent reduction in the fluidity of the plasma membrane with a maximal effect occurring at 20 mM. Dimethylnitrosamine also elevated the temperature at which the lipid phase separation occurred in rat liver plasma membranes, from 28 degrees C to 31 degrees C. The non-carcinogenic but structurally similar compound, dimethylamine hydrochloride neither inhibited adenylate cyclase nor decreased plasma membrane fluidity. It is suggested that the decrease in membrane fluidity, induced by dimethylnitrosamine, via its effects on membrane fluidity, could influence plasma membrane function and cellular regulation.     

Potent and cooperative feedback inhibition of adenylate cyclase activity by calcium in pituitary-derived GH3 cells

Calcium (Ca2+) ion concentrations that are achieved intracellularly upon membrane depolarization or activation of phospholipase C stimulate adenylate cyclase via calmodulin (CaM) in brain tissue. In the present study, this range of Ca2+ concentrations produced unanticipated inhibitory effects on the plasma membrane adenylate cyclase activity of GH3 cells. Ca2+ concentrations ranging from 0.1 to 0.8 microM exerted an increasing inhibition on enzyme activity, which reached a plateau (35-45% inhibition) at around 1 microM. This inhibitory effect was highly cooperative for Ca2+ ions, but was neither enhanced nor dependent upon the addition of CaM (1 microM) to EGTA-washed membranes. The inhibition was greatly enhanced upon stimulation of the enzyme by vasoactive intestinal peptide (VIP) and/or GTP. Prior exposure of cultured cells to pertussis toxin did not affect the inhibition of plasma membrane adenylate cyclase activity by Ca2+, although in these membranes, hormonal (somatostatin) inhibition was significantly attenuated. Maximally effective concentrations of Ca2+ and somatostatin produced additive inhibitory effects on adenylate cyclase. The addition of phosphodiesterase inhibitors demonstrated that inhibitory effects of Ca2+ were not mediated by Ca2(+)-dependent stimulation of a phosphodiesterase activity. These observations provide a mechanism for the feedback inhibition by elevated intracellular Ca2+ levels on cAMP-facilitated Ca2+ entry into GH3 cells, as well as inhibitory crosstalk between Ca2(+)-mobilizing signals and adenylate cyclase activity.     

Multistep regulation of Leydig cell function

LH controls Leydig cell steroidogenesis by interaction with specific membrane receptors initiating membrane coupling events. Stimulation of the androgen pathways occurs mainly through cAMP mediated mechanism including LH induced guanyl nucleotide binding, membrane phosphorylation and adenylate cyclase activation. cAMP dependent kinase activation presumably causes phosphorylation of key proteins of the steroidogenic pathway and consequent increase in testosterone production. The hormone also appears to facilitate the androgen stimulus by a cyclic AMP independent mechanism located at the plasma membrane or intracellular sites. The stimulatory event can be negatively influenced by the action of certain peptide hormones (i.e. angiotensin II) through the guanyl nucleotide inhibitory subunit of adenylate cyclase (Gi). In recent studies we have presented evidence for a Ca2+ sensitive kinase system present in purified cell membranes. Gpp(NH)p, GTP, and phospholipid in presence of nanomolar Ca2+ induce phosphate incorporation into Mr 44,500 substrate with marked inhibition at microM Ca2+. Similarly a biphasic pattern of activation was observed with adenylate cyclase activity. Membrane phosphorylation may be a modifier of LH-stimulated adenylate cyclase activity and possibly other LH induced actions in the activated Leydig cell membrane. Furthermore we have defined the stimulatory effects of forskolin on all Leydig cell cyclic AMP pools and have provided additional evidence of functional compartmentalization and/or cAMP independent facilitory stimulus of steroidogenesis by the trophic hormone. The demonstration of a novel high affinity inhibitory action of forskolin upon adenylate cyclase activity and cyclic AMP generation mediated by the Gi subunit of adenylate cyclase has provided a new approach for direct evaluation of functional inhibitory influence of Gi subunit in the Leydig cell. The cultured fetal Leydig cell system has provided a useful model to elucidate mechanisms involved in the development of gonadotropin induced estradiol mediated desensitization of steroidogenesis. We have isolated from the fetal testis a small population (2-5% of total) of transitional cells with morphological characteristics of cells found in 15 day postnatal testis but functional capabilities of the adult cell. We have also demonstrated after appropriate treatment (i.e. estrogen, and frequent or a high gonadotropin dose) the emergence of a functional adult-like cell type from the fetal Leydig cell population.     

A proper transmembrane Ca2+ gradient is essential for the higher enzymatic activity of adenylate cyclase

Adenylate cyclase from bovine brain cortex was reconstituted into liposomes with (1000 fold) or without transmembrane Ca2+ gradient. The highest enzyme activity (the active center of enzyme exposing outside) was observed in the vesicles with lower Ca2+ concentration outside (approximately 10(-6) M, similar to physiological condition). If the transmembrane Ca2+ gradient was in the inverse direction (i.e. higher Ca2+ concentration outside, 1 mM), a lowest enzyme activity would appear. Such a difference could be diminished following addition of A23187. Obtained results showed that a proper transmembrane Ca2+ gradient is essential for the optimal fluidity of phospholipid bilayer, favouring the formation of suitable conformation of adenylate cyclase with higher enzyme activity.     

Components of purified sarcolemma from porcine skeletal muscle

Sarcolemmal membranes were isolated from porcine skeletal muscle by modifications of a LiBr-extraction technique. Latency determinations of acetylcholinesterase, ouabain-sensitive p-nitrophenylphosphatase, [3H]ouabain binding, and (Na+ + K+)-ATPase activities indicated that 65-76% of the membranes were sealed inside-out vesicles. The preparations were enriched in cholesterol and phospholipid, and demonstrated adenylate cyclase activity and both cAMP and cGMP phosphodiesterase activities. An indication of the purity of this fraction was that the Ca2+-ATPase activity (0.13 mumol Pi mg-1 min-1 at 37 degrees C) was 3.8% of that of porcine skeletal muscle sarcoplasmic reticulum preparations. Pertussis toxin specifically catalyzed the ADP-ribosylation of a Mr 41,000 sarcolemmal protein, indicating the presence of the inhibitory guanine nucleotide regulatory protein of adenylate cyclase, Ni. An endogenous ADP-ribosyltransferase activity, with several membrane protein substrates, was also demonstrated. The addition of exogenous cAMP-dependent protein kinase or calmodulin promoted the phosphorylation of a number of sarcolemmal proteins. The calmodulin-dependent phosphorylation exhibited an approximate K 1/2 for Ca2+ of 0.5 microM, and an approximate K 1/2 for calmodulin of 0.1 microM. 125I-Calmodulin affinity labeling of the sarcolemma, using dithiobis(succinimidyl propionate), demonstrated the presence of Mr 160,000 and 280,000 calmodulin-binding components in these membranes. These results demonstrate that this porcine preparation will be valuable in the study of skeletal muscle sarcolemmal ion transport, protein and hormonal receptors, and protein kinase-catalyzed phosphorylation.     

Acidic phospholipid species inhibit adenylate cyclase activity in rat liver plasma membranes.

Incubation of rat liver plasma membranes with liposomes of dioleoyl phosphatidic acid (dioleoyl-PA) led to an inhibition of adenylate cyclase activity which was more pronounced when fluoride-stimulated activity was followed than when glucagon-stimulated activity was followed. If Mn2+ (5 mM) replaced low (5 mM) [Mg2+] in adenylate cyclase assays, or if high (20 mM) [Mg2+] were employed, then the perceived inhibitory effect of phosphatidic acid was markedly reduced when the fluoride-stimulated activity was followed but was enhanced for the glucagon-stimulated activity. The inhibition of adenylate cyclase activity observed correlated with the association of dioleoyl-PA with the plasma membranes. Adenylate cyclase activity in dioleoyl-PA-treated membranes, however, responded differently to changes in [Mg2+] than did the enzyme in native liver plasma membranes. Benzyl alcohol, which increases membrane fluidity, had similar stimulatory effects on the fluoride- and glucagon-stimulated adenylate cyclase activities in both native and dioleoyl-PA-treated membranes. Incubation of the plasma membranes with phosphatidylserine also led to similar inhibitory effects on adenylate cyclase and responses to Mg2+. Arrhenius plots of both glucagon- and fluoride-stimulated adenylate cyclase activity were different in dioleoyl-PA-treated plasma membranes, compared with native membranes, with a new 'break' occurring at around 16 degrees C, indicating that dioleoyl-PA had become incorporated into the bilayer. E.s.r. analysis of dioleoyl-PA-treated plasma membranes with a nitroxide-labelled fatty acid spin probe identified a new lipid phase separation occurring at around 16 degrees C with also a lipid phase separation occurring at around 28 degrees C as in native liver plasma membranes. It is suggested that acidic phospholipids inhibit adenylate cyclase by virtue of a direct headgroup specific interaction and that this perturbation may be centred at the level of regulation of this enzyme by the stimulatory guanine nucleotide regulatory protein NS.     

Biphasic Ca2+ response of adenylate cyclase. The role of calmodulin in its activation by Ca2+ ions

The Ca2+-dependent regulation of human platelet membrane adenylate cyclase has been studied. This enzyme exhibited a biphasic response to Ca2+ within a narrow range of Ca2+ concentrations (0.1-1.0 microM). At low Ca2+ (0.08-0.3 microM) adenylate cyclase was stimulated (Ka = 0.10 microM), whereas at higher Ca2+ (greater than 0.3 microM) the enzyme was inhibited to 70-80% control (Ki = 0.8 microM). Membrane fractions, prepared by washing in the presence of LaCl3 to remove endogenous calmodulin (approximately equal to 70-80% depletion), exhibited no stimulation of adenylate cyclase by Ca2+ but did show the inhibitory phase (Ki = 0.4 microM). The activation phase could be restored to La3+-washed membranes by addition of calmodulin (Ka = 3.0 nM). Under these conditions it was apparent that calmodulin reduced the sensitivity of adenylate cyclase to Ca2+ (Ki = 0.8 microM). Prostaglandin E1 (PGE1) did not alter Ki or Ka values for Ca2+. Calmodulin did not alter the EC50 for PGE1 stimulation of adenylate cyclase but increased the Vmax (1.5-fold). The calmodulin antagonist trifluoperazine potently inhibited adenylate cyclase in native membranes (80%) and to a much lesser extent in La3+-washed membranes (15%). This inhibition was due to interaction of trifluoperazine with endogenous calmodulin since trifluoperazine competitively antagonized the stimulatory effect of calmodulin on adenylate cyclase in La3+-washed membranes. We propose that biphasic Ca2+ regulation of platelet adenylate cyclase functions to both dampen (low Ca2+) and facilitate (high Ca2+) the haemostatic function of platelets.     

Inhibition of hormone-stimulated adenylate cyclase activity after altering turkey erythrocyte phospholipid composition with a nonspecific lipid transfer protein. Phosphatidylinositol uncouples catecholamine binding from adenylate cyclase activation

The nonspecific lipid transfer protein from beef liver was used to modify the phospholipid composition of intact turkey erythrocytes in order to study the dependence of isoproterenol-stimulated adenylate cyclase activity on membrane phospholipid composition. Incorporation of phosphatidylinositol into turkey erythrocytes inhibited isoproterenol-stimulated cyclic AMP accumulation in a linear, concentration-dependent manner. Inhibition was relatively specific for phosphatidylinositol; phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylglycerol and phosphatidic acid were from 3 to 7 times less effective as inhibitors of hormone-stimulated cyclase activity. Inhibition by phosphatidylinositol was not reversible when up to 90% of the incorporated phosphatidylinositol was removed, either by incubation with phosphatidylinositol-specific phospholipase C or a second incubation with transfer protein; possibly adenylate cyclase activity depends on a small pool of phosphatidylinositol that is inaccessible to either phospholipase C hydrolysis or removal by lipid transfer protein. Phosphatidylinositol incorporation inhibits adenylate cyclase activity by uncoupling beta-adrenergic receptors from the remainder of the cyclase complex. Phosphatidylinositol incorporation had no effect on stimulation of cAMP accumulation by either cholera toxin or forskolin, indicating that inhibition occurs only at the level of receptor. Phosphodiesterase activity was not altered in phosphatidylinositol-modified cells. Inhibition of cAMP accumulation was not the result of changes in either membrane fluidity or in cAMP transport out of modified turkey erythrocytes. Phosphatidylinositol inhibition of isoproterenol-stimulated cyclase activity may serve as a useful model system for hormone-induced desensitization.     

Calmodulin regulation of adenylate cyclase activity in human platelet membranes

The mechanism of calmodulin dependent regulation of adenylate cyclase has been studied in human platelet membranes. Calmodulin activated adenylate cyclase exhibited a biphasic response to both Mg2+ and Ca2+. A stimulatory effect of Mg2 on adenylate cyclase was observed at all Mg2+ concentrations employed, although the degree of activation by calmodulin was progressively decreased with increasing concentrations of Mg2+. These results demonstrate that the Vmax of calmodulin dependent platelet adenylate cyclase can be manipulated by varying the relative concentrations of Mg2+ and Ca2+. The activity of calmodulin stimulated adenylate cyclase was always increased 2-fold above respective levels of activity induced by GTP, Gpp(NH)p and/or PGE. The stimulatory influence of calmodulin was not additive but synergistic to the effects of PGE1, GTP and Gpp(NH)p. GDP beta S inhibited GTP-and Gpp(NH)p stimulation of adenylate cyclase but was without effect on calmodulin stimulation. Since the inhibitory effects of GDP beta S have been ascribed to apparent reduction of active N-protein-catalytic unit (C) complex formation, these results suggest that the magnitude of calmodulin dependent adenylate cyclase activity is proportional to the number of N-protein-C complexes, and that calmodulin interacts with preformed N-protein-C complex to increase its catalytic turnover. Our data do not support existence of two isoenzymes of adenylate cyclase (calmodulin sensitive and calmodulin insensitive) in human platelets.     

Calcium-dependent adenylate cyclase of pituitary tumor cells

Effects of Ca2+ and calmodulin on the adenylate cyclase activity of a prolactin and growth hormone-producing pituitary tumor cell strain (GH3) were examined. The adenylate cyclase activity of homogenates was stimulated approx. 60% by submicromolar free Ca2+ concentrations and inhibited by higher (microM range) concentrations of the cation. A 2-3-fold stimulation of the activity in response to Ca2+ was observed at physiologic concentrations of KCl, with both the stimulatory and inhibitory responses occurring at respectively higher free Ca2+ concentrations. Calmodulin in incubations at low KCl concentrations increased the enzyme activity at all Ca2+ concentrations tested. In incubations conducted at physiologic KCl concentrations, both the inhibitory and stimulatory responses to Ca2+ were shifted by calmodulin to lower respective concentrations of the cation, without significant change occurring in the maximal rate of enzymic activity at optimal free Ca2+ X Mg2+ concentrations in the incubation also influenced the Ca2+ concentration dependence of adenylate cyclase; at high Mg2+ more Ca2+ was required to obtain maximal activity. Trifluoperazine inhibited adenylate cyclase of GH3 cells only in the presence of Ca2+; as Ca2+ concentrations in the assay were increased, higher drug concentrations were required to inhibit the enzyme. Ca2+ was also observed to reduce the extent of enzyme destabilization which occurred during pretreatments at warm temperatures. Vasoactive intestinal polypeptide and phorbol myristate acetate, which stimulate prolactin secretion in intact GH3 cells, enhanced enzyme activity 4- and 2.5-fold, respectively, without added Ca2+. Increasing free Ca2+ concentrations reduced the enhancement by VIP and eliminated the stimulation by PMA.     

Ca2+-dependent regulation of calmodulin binding and adenylate cyclase activation in bovine cerebellar membranes

The binding parameters of 125I-labeled calmodulin to bovine cerebellar membranes have been determined and correlated with the activation of adenylate cyclase by calmodulin. In the presence of saturating levels of free Ca2+ calmodulin binds to a finite number of specific membrane sites with a dissociation constant (Kd) of 1.2 nM. Furthermore, Scatchard analysis reveals a second population of binding sites with a 100-fold lower affinity for calmodulin. The Ca2+-dependence of calmodulin binding and of adenylate cyclase activation varies with the amount of calmodulin present, as can be inferred from the model of sequential equilibrium reactions which describes the activation of calmodulin-dependent enzymes. On the basis of this model, a quantitative analysis of the effect of free Ca2+ and of free calmodulin concentration on both binding and activation of adenylate cyclase was carried out. This analysis shows that both processes take place only when calmodulin is complexed with at least three Ca2+ atoms. The concentration of the active calmodulin X Ca2+ species required for half-maximal activation of adenylate cyclase is very similar to the Kd of the high affinity binding sites on brain membranes. A Hill coefficient of approx. 1 was found for both processes indicating an absence of cooperativity. Phenothiazines and thioxanthenes antipsychotic agents inhibit calmodulin binding to membranes and calmodulin-dependent activation of adenylate cyclase with a similar order of potency. These results suggest that the Ca2+-dependent binding of calmodulin to specific high affinity sites on brain membranes regulates the activation of adenylate cyclase by calmodulin.     

Involvement of calcium in the inhibition by insulin of the glucagon-stimulated adenylate-cyclase activity.

1. The inhibitory effect of adenosine on the glucagon-stimulated adenylate cyclase activity of liver plasma membranes, prepared from PVG/c rats, was potentiated by insulin. In the presence of EGTA, such potentiating effect of insulin was lost. 2. Calcium (10 microM) potentiated the inhibitory effects of both adenosine and insulin on the glucagon-stimulated cyclase activity. The synergestic effect of calcium + insulin required the presence of adenosine as judged from the use of adenosine deaminase. 3. Insulin had no significant inhibitory effect on the glucagon-stimulated cyclase activity of liver plasma membranes, prepared from young Wistar rats, unless both adenosine (50 microM) and calcium (10 microM) were added externally. 4. Results demonstrate an interaction of calcium and insulin at membrane level that, in the presence of adenosine, results in the inhibition of the glucagon-stimulated adenylate cyclase activity.     

Calcitonin-sensitive adenylate cyclase in rat renal tubular membranes.

1. Renal tubular membranes from rat kidneys were prepared, and adenylate cyclase activity was measured under basal conditions, after stimulation by NaF or salmon calcitonin. Apparent Km value of the enzyme for hormone-linked receptor was close to 1 x 10(-8) M. 2. The system was sensitive to temperature and pH. pH was found to act both on affinity for salmon calcitonin-linked receptor and maximum stimulation, suggesting an effect of pH on hormone-receptor binding and on a subsequent step. 3. KCl was without effect areas whereas CoCl and CaCl2 above 100 muM and MnCl2 above 1 muM inhibited F- -and salmon calcitonin-sensitive adenylate cyclase activities. The Ca2+ inhibition of the response reflected a fall in maximum stimulation and not a loss of affinity of salmon calcitonin-linked receptor for the enzyme. 4. The measurement of salmon calcitonin-sensitive adenylate cyclase activity as a function of ATP concentration showed that the hormone increases the maximum velocity of the adenylate cyclase. GTP, ITP and XTP at 200 muM did not modify basal, salmon calcitonin- and parathyroid hormone-sensitive adenylate cyclase activities. 5. Basal, salmon calcitonin- and F- -sensitive adenylate cyclase activities decreased at Mg2+ concentrations below 10 mM. High concentrations of Mg2+ (100 mM) led to an inhibition of the F- -stimulated enzyme. 6. Salmon calcitonin-linked receptor had a greater affinity for adenylate cyclase than human or porcine calcitonin-linked receptors. There was no additive effect of these three calcitonin peptides whereas parathyroid hormone added to salmon calcitonin increased adenylate cyclase activity, thus showing that both hormones bound to different membrane receptors. Human calcitonin fragments had no effect on adenylate cyclase activity. 7. Salmon calcitonin-stimulated adenylate cyclase activity decreased with the preincubation time. This was due to progressive degradation of the hormone and not to the rate of binding to membrane receptors.     

Regulation of ciliary adenylate cyclase by Ca2+ in Paramecium.

In the ciliated protozoan Paramecium, Ca2+ and cyclic nucleotides are believed to act as second messengers in the regulation of the ciliary beat. Ciliary adenylate cyclase was activated 20-30-fold (half-maximal at 0.8 microM) and inhibited by higher concentrations (10-20 microM) of free Ca2+ ion. Ca2+ activation was the result of an increase in Vmax., not a change in Km for ATP. The activation by Ca2+ was seen only with Mg2+ATP as substrate; with Mn2+ATP the basal adenylate cyclase activity was 10-20-fold above that with Mg2+ATP, and there was no further activation by Ca2+. The stimulation by Ca2+ of the enzyme in cilia and ciliary membranes was blocked by the calmodulin antagonists calmidazolium (half-inhibition at 5 microM), trifluoperazine (70 microM) and W-7 (50-100 microM). When ciliary membranes (which contained most of the ciliary adenylate cyclase) were prepared in the presence of Ca2+, their adenylate cyclase was insensitive to Ca2+ in the assay. However, the inclusion of EGTA in buffers used for fractionation of cilia resulted in full retention of Ca2+-sensitivity by the ciliary membrane adenylate cyclase. The membrane-active agent saponin specifically suppressed the Ca2+-dependent adenylate cyclase without inhibiting basal activity with Mg2+ATP or Mn2+ATP. The ciliary adenylate cyclase was shown to be distinct from the Ca2+-dependent guanylate cyclase; the two activities had different kinetic parameters and different responses to added calmodulin and calmodulin antagonists. Our results suggest that Ca2+ influx through the voltage-sensitive Ca2+ channels in the ciliary membrane may influence intraciliary cyclic AMP concentrations by regulating adenylate cyclase.     

The effect of divalent cations on bovine spermatozoal adenylate cyclase activity.

The effect of divalent cations on bovine sperm adenylate cyclase activity was studied. Mn2+, Co2+, Cd2+, Zn2+, Mg2+ and Ca2+ were found to satisfy the divalent cation requirement for catalysis of the bovine sperm adenylate cyclase. These divalent cations in excess of the amount necessary for the formation of the metal-ATP substrate complex were found to stimulate the enzyme activity to various degrees. The magnitude of stimulation at saturating concentrations of the divalent cations was strikingly greater with M2+ than with either Ca2+, Mg2+, Zn2+, Cd2+ or Co2+. The apparent Km was lowest for Zm2+ (0.1 - 0.2 mM) than for any of the other divalent cations tested (1.2 - 2.3 mM). The enzyme stimulation by Mn2+ was decreased by the simultaneous addition of Co2+, Cd2+, Ni2+ and particularly Zn2+ and Cu2+. The antagonism between Mn2+ and Cu2+ or Zn2+ appeared to have both competitive and non-competitive features. The inhibitory effect of Cu2+ on Mn2+-stimulated adenylate cyclase activity was prevented by 2,3-dimercaptopropanol, but not by dithiothreitol, L-ergothioneine, EDTA, EGTA or D-penicillamine. Ca2+ at concentrations of 1-5 mM was found to act synergistically with Mg2+, Zn2+, Co2+ and Mn2+ in stimulating sperm adenylate cyclase activity. The Ca2+ augmentation of the stimulatory effect of Zn2+, Co2+, Mg2+ and Mn2+ appeared to be specific.     

Ionic regulation of adenylate cyclase from the cilia of Paramecium tetraurelia.

The kinetics of the ionic regulation of an adenylate cyclase associated with the excitable ciliary membrane from Paramecium tetraurelia was examined. Glycerol (30%, v/v) stabilized the enzyme, and activated by an increase in Vmax. (3-fold) and a decrease in the apparent Km for MgATP (6-fold). Kinetic analysis of Mg2+ effects showed a stimulation via a single metal-binding site separate from the substrate site, with a dissociation constant, Ks, of 0.27 mM. Analysis of Ca2+ effects showed (i) an uncompetitive inhibition with respect to substrate MgATP, and (ii) dependence of the extent of inhibition on the free Mg2+ concentration. Ki values ranged from 4 to 130 microM-Ca2+ in the presence of 0.55-2 mM-Mg2+ respectively. This indicates competition between Mg2+ and Ca2+ at the metal-binding site. The Ca2+ effect was specific; Sr2+ and Ba2+ were almost without effect, and 100 microM-Ba2+ did not interfere with the Ca2+ inhibition. The actions of Ca2+ were readily reversible after addition of EGTA. K+ activated the adenylate cyclase at concentrations around 20 mM. The stimulatory potency of K+ was dependent on the free Mg2+ concentration. At 1 mM free Mg2+, 20 mM-K+ doubled the adenylate cyclase activity. The inhibitory Ca2+ and stimulatory K+ inputs were independent of each other.     

Changes in phospholipid composition and calcium flux in LLC-PK cells cultured at low magnesium concentrations

Monolayers of porcine kidney cells (LLC-PK) were grown in a series of Nu-Serum-supplemented media containing different Mg(2+) concentrations (480, 250, 25, 6.3 or 2.6 microM) to study the effect of Mg(2+) depletion on cellular phospholipid changes and the consequent effect on the membrane permeability to Ca(2+). Cells grown on 6.3 or 2.6 microM Mg(2+) showed a decrease in PE, PS, Sph, PI and an increase of PC. These changes were attributed mainly to the decreased rate of Sph synthesis through the transfer of phosphocholine from PC to ceramide, or due to the increase of PE N-methylation as found in Mg(2+)-deficient cells. The (45)Ca uptake was increased in cells grown on 25.0 microM Mg(2+), while it was decreased in cells grown on 6.3 or 2.6 microM Mg(2+). These changes in Ca(2+) uptake were related to changes of cellular phospholipids and fatty acids which affect adenylate cyclase activity in the membrane, as well as the membrane fluidity.     

Phosphatidic acid inhibition of PGE1-stimulated cAMP accumulation in WI-38 fibroblasts: similarities with carbachol inhibition

To test the hypothesis that phosphatidic acid (PhA) is involved in the carbachol inhibition of hormone stimulated accumulation of cAMP we observed the effects of PhA on PGE1-stimulation of cAMP in WI-38 fibroblasts. PhA inhibited PGE1-stimulated cAMP accumulation of WI-38 fibroblasts; maximum inhibition (approximately 50-80%) occurred at a PhA concentration of 1.0 microM and significant inhibition was observed with a concentration of 0.1 microM. The full effects of PhA were evident within 15 sec after the co-addition of PGE1 and PhA. Addition of PhA to cells which had been pre-stimulated with PGE1 resulted in the rapid decay of cAMP levels to a new steady state level with a t 1/2 of approximately 65 sec. The inhibition produced by PhA did not appear to be simply attributable to a depolarization or increased intracellular Ca2+, since addition of either KCl or the Ca2+ ionophore A23187 did not lower PGE1-stimulated cAMP accumulation. When intact cells were pretreated with PhA then lysed and adenylate cyclase immediately assayed, no detectable changes in broken cell adenylate cyclase activities were observed. Also, PhA added directly to adenylate cyclase assays at concentrations as high as 100 microM produced no detectable inhibition of the membrane fraction adenylate cyclase activities. Nonetheless, our results suggest that adenylate cyclase activity in intact cells may be directly affected by physiological levels of PhA . Further, the similarities of carbachol [Butcher, R. W., Journal of Cyclic Nucleotide Research, 4:411 (1978)] and PhA inhibition support the hypothesis that carbachol (acetylcholine) exerts its effect on adenylate cyclase through alterations of the plasma membrane phospholipid composition.     

Cytochemical localization of adenylate cyclase and of calcium ion, magnesium ion-activated ATPases in the dense tubular system of human blood platelets.

Cytochemical techniques have been employed to study the localization of adenylate cyclase and (Ca2+ + Mg2+)-stimulated ATPase activities in platelets after fixation. Biochemical analysis of adenylate cyclase demonstrated a 70% reduction in activity in homogenates from fixed cells, but the residual activity could be stimulated 10--20 times by prostaglandin E1 (1 micrometer) under the same incubation conditions as employed in the cytochemical studies (e.g. media containing 2 mM lead nitrate and 10 mM NaF). Adenylate cyclase activity employing 5'-adenylyl-imiodiphosphate (AMP-P(NH)P) as substrate was found to be associated with the dense tubular system (smooth endoplasmic reticulum) in intact fixed platelets, and was apparent only when the cells were incubated with prostaglandin E1. Less activity was found along the membranes of the surface connected open canalicular system and occasionally at the outer cell surface. Enzymatic activity was blocked by the adenylate cyclase inhibitor 9-(tetrahydro-2-furyl) adenine and was not due to AMP-P(NH)P phosphohydrolase activity. The low adenylate cyclase activity in the surface membranes may be due to enzyme inactivation as a result of fixation, since a surface membrane fraction obtained by the glycerol lysis technique from unfixed cells had an adenylate cyclase specific activity equivalent to that in the microsomal membrane fraction. (Ca2+ + Mg2+)-stimulated ATPase activity was found associated with the membranes of the surface connected open canalicular system in unfixed cells. After brief fixation (5--15 min) with glutaradehyde, strong (Ca2+ + Mg2+)ATPase activity became apparent in the dense tubular system. Longer periods of fixation inactivated enzymatic activity. Addition of Ca2+ (1.0 mM) to incubation medium with low Mg2+ (0.2 mM), or increasing Mg2+ to 4.0 mM, in both cases strongly stimulated enzyme activity. The ATPase activity in the platelet membranes was not inhibited by ouabain. It is suggested that the Ca2+-stimulated ATPase and adenylate cyclase activities in the dense tubules may possibly be involved in regulation of intracellular Ca2+ transport.     

Ca2+-Independent Stimulation of Adenylate Cyclase Activity by Muscarinic Receptors in Rat Olfactory Bulb

In rat olfactory bulb homogenate, carbachol stimulated adenylate cyclase activity in a concentration-dependent manner (EC50 = 1.1 microM). The carbachol stimulation occurred fully in membranes that had been prepared in the presence of 1 mM EGTA and incubated in a Ca2(+)-free enzyme reaction medium. Under these conditions, exogenous calmodulin (1 microM) failed to stimulate adenylate cyclase activity. In miniprisms of olfactory bulb, carbachol (1 mM) increased accumulation of inositol phosphates, but this response was markedly reduced in a Ca2(+)-free medium. Moreover, the carbachol stimulation of adenylate cyclase activity was not affected by staurosporine at a concentration (1 microM) that completely blocked the stimulatory effect of phorbol 12-myristate 13-acetate, an activator of Ca2+/phospholipid-dependent protein kinase. Quinacrine, a nonselective phospholipase A2 inhibitor, reduced the carbachol stimulation of adenylate cyclase activity, but this inhibition appeared to be competitive with a Ki of 0.2 microM. Nordihydroguaiaretic acid and indomethacin, two inhibitors of arachidonic acid metabolism, failed to affect the carbachol response. These results indicate that in rat olfactory bulb, muscarinic receptors stimulate adenylate cyclase activity through a mechanism that is independent of Ca2+ and phospholipid hydrolysis.