Lack of interaction between acid and pepsinogen secretion in isolated gastric glands

Gastric glands isolated from rabbit stomach were employed to study the relation between acid and pepsinogen secretion. The effects of adenosine and guanosine nucleotides were examined for both secretory processes. cAMP, dibutyryl cAMP (DBcAMP), and 8-bromo-cAMP (8BrcAMP) were found to stimulate both acid and pepsinogen secretion with a potency sequence of 8BrcAMP greater than DBcAMP greater than cAMP. Adenosine, ATP, and AMP were ineffective, indicating that the responses to adenosine cyclic nucleotides do not involve an adenosine receptor. 8BrcGMP was found to produce a weak stimulation of both acid and pepsinogen secretions, while GMP, cGMP, and DBcGMP were ineffective. DBcGMP was found to inhibit competitively the stimulation of pepsinogen secretion by cholecystokinin (CCK)-like peptides. No inhibition was found with cGMP or 8BrcGMP. Stimulation of pepsinogen secretion by carbachol or isoproterenol was not inhibited by DBcGMP nor was the stimulation of acid formation by CCK-like peptides. Thiocyanate inhibited acid formation but did not affect pepsinogen secretion stimulated by 8BrcAMP or carbachol, indicating that stimulation of pepsinogen secretion does not require simultaneous acid formation. Costimulation of acid formation by histamine and pepsinogen secretion by isoproterenol showed no interaction between the two secretory processes. The results are interpreted to suggest that correlations between acid and pepsinogen secretion observed in vivo do not result from direct interactions between parietal and chief cells.

Guanine nucleotides reveal differential actions of ergot derivatives at D-2 receptors labelled by [3H]spiperone in striatal homogenates

The specific binding of [3H]spiperone (35 pM), as defined by the D-2 antagonist sulpiride, was potently displaced by ergot derivatives of both the ergoline and ergopeptine type, and by dopamine agonists and antagonists. The potency of the ergot derivatives ranged widely from an IC50 value of 3 nM for bromocriptine to a value of 1000 nM for the partial ergoline, LY-141865. GTP and its stable analogue, guanyl-5'-yl-imidodiphosphate (Gpp(NH)p), did not affect the affinity (100 pM) or density (30 pmol/g wet wt) of [3H]spiperone binding sites, but did decrease the potency of a number of ergoline compounds including pergolide, lergotrile and LY-141865, as well as dopamine agonists to displace [3H]spiperone binding. The affinities of the ergopeptines, bromocriptine and dihydroergocryptine, and of the isolysergic acid ergoline, lisuride, and dopamine antagonists were unaltered by the presence of guanine nucleotides. The effect was specific for guanine nucleotides, with near maximal effects on agonist affinity observed in the presence of a 100 microM concentration of nucleotide. The relative decrease in affinity found in the presence of GTP or Gpp(NH)p varied widely for individual ergot derivatives and dopamine agonists. The largest decrease was seen with dopamine itself, and agonists such as the tetralins and with LY-141865. Many ergolines had shifts in potency between those seen for agonists and antagonists, suggesting a partial agonist action at D-2 receptors. Guanine nucleotide sensitivity may represent a valuable in vitro method for studying the agonist/antagonist properties of dopaminergic drugs, such as the semi-synthetic ergoline compounds.

Inhibitory regulation of adenylyl cyclase in the absence of stimulatory regulation. Requirements and kinetics of guanine nucleotide-induced inhibition of the cyc- S49 adenylyl cyclase

cyc- S49 cell membranes contain an adenylyl cyclase activity which is stimulated by forskolin and inhibited by guanine nucleotides and NaF. These inhibitory effects are mediated by an inhibitory guanine nucleotide-binding regulatory component (Ni) affecting the adenylyl cyclase catalytic unit (Hildebrandt, J. D., Sekura, R. D., Codina, J., Iyengar, R., Manclark, C. R., and Birnbaumer, L. (1983) Nature (Lond.) 302, 706-709). Since cyc- S49 cells do not contain a stimulatory guanine nucleotide-binding regulatory component (Ns), these membranes were used to study the requirements and kinetics of activation of Ni in the absence of Ns. Activation of Ni by guanyl-5'-yl imidodiphosphate was time-dependent (i.e. hysteretic) and pseudo-irreversible. Although GTP and guanosine 5'-(beta-thio)diphosphate could prevent the inhibition caused by guanyl-5'-yl imidodiphosphate if added simultaneously with it, they could not reverse the inhibited state induced by previous exposure to guanyl-5'-yl imidodiphosphate. Activation of Ni had an absolute requirement for Mg2+. Unlike the activation of Ns, however, which requires millimolar concentrations of Mg2+ in the absence of hormonal stimulation, activation of Ni requires only micromolar concentrations of the divalent cation. These results support the contention that hormones which activate Ni or Ns do so by altering different parameters of a similar activation mechanism.

Muscarinic receptors in porcine caudate nucleus. II. Different effects of N-ethylmaleimide on [3H]cis-methyldioxolane binding to heat-labile (guanyl nucleotide-sensitive) sites and heat-stable (guanyl nucleotide-insensitive) sites

Heat treatment of membranes from porcine caudate nucleus (50 degrees for 7 min) caused a marked decrease in [3H]cis-methyldioxolane [( 3H]CD) binding without affecting seriously the binding of [3H]3-quinuclidinyl benzilate [( 3H]QNB). Approximately 20% of the [3H]CD binding at 5 nM [3H]CD remained after the heat treatment. The remaining binding was not affected by 0.1 mM guanylyl-5'-imidodiphosphate (GppNHp) or by nickel or other cations at concentrations below 10 mM. Treatment of the membranes with trypsin (30 micrograms/mg of protein) at 20 degrees for 20 min also caused a marked decrease in [3H]CD binding without affecting seriously the binding of [3H]QNB. About 20% of the original [3H]CD binding remained in the presence of trypsin at a high concentration of protein (90 micrograms/mg). N-Ethylmaleimide (NEM) affected [3H]CD binding in two different ways: (a) preincubation of the membranes with NEM caused a marked reduction in heat- and GppNHp-sensitive [3H]CD binding, and (b) treatment with NEM caused an enhancement of heat-, GppNHp-, and trypsin-insensitive [3H]CD binding. Neither of the NEM effects required the coexistence of agonists. The concentration of NEM required for the first effect was 10 times lower than that for the second effect, indicating the existence of two NEM-binding sites with different affinities for NEM. The equilibrium dissociation constant (Kd) for [3H]CD after NEM treatment was 33 nM and was not affected by GppNHp, Ni2+, or heat treatment; the Kd was only 4 times higher than that (8 nM) without NEM treatment. These findings indicated the existence of two kinds of [3H]CD binding sites with high affinities for agonists: one is sensitive to guanyl nucleotide and is abolished by NEM and the other is induced by NEM and insensitive to guanyl nucleotide.

Calmodulin-sensitive and calmodulin-insensitive components of adenylate cyclase activity in rat striatum have differential responsiveness to guanyl nucleotides

The interaction between the Ca2+-binding protein, calmodulin, and guanyl nucleotides was investigated in a rat striatal particulate fraction. We found that the ability of calmodulin to stimulate adenylate cyclase in the presence of guanyl nucleotides depends upon the type and concentration of the guanyl nucleotide. Adenylate cyclase activity measured in the presence of calmodulin and GTP reflected additivity at every concentration of these reactants. On the contrary, when the activating guanyl nucleotide was the nonhydrolyzable analog of GTP, guanosine-5'-(beta,gamma-imido)triphosphate (GppNHp), calmodulin could further activate adenylate cyclase only at concentrations less than 0.2 microM GppNHp. Kinetic analysis of adenylate cyclase by GppNHp was compatible with a model of two components of adenylate cyclase activity, with over a 100-fold difference in sensitivity for GppNHp. The component with the higher affinity for GppNHp was competitively stimulated by calmodulin. The additivity between calmodulin and GTP in the striatal particulate fraction suggests that they stimulate different components of cyclase activity. The calmodulin-stimulatable component constituted 60% of the total activity. Our two-component model does not delineate, at this point, whether there are two separate catalytic subunits or one catalytic subunit with two GTP-binding proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulatory and inhibitory effects of guanine nucleotides on arginine-vasotocin-sensitive adenylate cyclase in the epithelial cell membranes of the bullfrog bladder

The effects of arginine-vasotocin and nucleotides on the steady-state kinetics of the adenylate cyclase activity in the epithelial cell membranes of the bullfrog (Rana catesbiana) bladder were studied. Arginine-vasotocin stimulated adenylate cyclase more effectively than oxytocin or arginine-vasopressin, with respect to both the maximal hormonal activation ratio relative to basal, and the hormone concentration yielding a half-maximal response (apparent Km). Arginine-vasotocin, GTP and its analogue guanyl-5'-yl imidodiphosphate (Gpp(NH)p) increased the Vmax of the basal adenylate cyclase activity, but showed no effect of the apparent Km of the system for ATP. In addition, Gpp(NH)p enhanced the arginine-vasotocin-stimulated adenylate cyclase activity, further increasing the Vmax, while GTP showed no statistically significant effect. Dual effects of GDP were apparent: it was stimulatory at 1 x 10(-5) mol/l and inhibitory at 1 x 10(-3) mol/l, on both the basal and the arginine-vasotocin-stimulated adenylate cyclase activity. Guanosine 5'-monophosphate, CTP, UTP and ITP showed no apparent effect on the enzyme activity. Sodium fluoride acted in the same manner as GTP on the adenylate cyclase system, increasing only basal activity. Adenylate cyclase activities exhibited pH optima that were less distinct in the presence than in the absence of Gpp(NH)p. The Arrhenius plot of the temperature experiment showed that a high-energy step was involved for activation by Gpp(NH)p or arginine-vasotocin. When the relative activation ratios by arginine-vasotocin at different ATP concentrations were studied, a distinct activation optimum was shown at 2.5 x 10(-4) mol ATP/l, either in the absence or presence of Gpp(NH)p. The possibility that GTP, GDP nd ATP play a regulatory role in the epithelial cells of the bullfrog bladder by adjusting the responsiveness of the system to a natural hormone, arginine-vasotocin, is discussed.

Muscarinic receptors in porcine caudate nucleus. I. Enhancement by nickel and other cations of [3H]cis-methyldioxolane binding to guanyl nucleotide-sensitive sites

Muscarinic acetylcholine receptors in the synaptic membrane fraction of porcine caudate nucleus were characterized by using a radiolabeled agonist, [3H]cis-methyldioxolane [( 3H]CD) and an antagonist, [3H]quinuclidinyl benzilate [( 3H]QNB). Scatchard analysis of the specific binding of [3H]CD gave a single equilibrium dissociation constant of 8.1 nM when a concentration of less than 80 nM [3H]CD was used. The binding capacity was 390 fmoles/mg of protein and corresponded to about 10% of the binding sites of [3H]QNB. Agonist/[3H]CD competition binding experiments indicated that [3H]CD was selectively bound to the sites with a high affinity for agonists. [3H]CD binding was inhibited by Na+, K+, Mg2+, and Ca2+ with the half-maximal effect at 10-50 mM. Nickel ion showed biphasic effects on [3H]CD binding: a 2- to 3-fold enhancement of binding at 0.1-10 mM and inhibition above 10 mM. Other cations, including Co2+, Mn2+, and Zn2+, at 1 mM also increased [3H]CD binding by a factor of 1.5-1.8. Among 18 cations examined, only Cd2+, Hg2+, and Cu2+ caused significant inhibition of [3H]CD binding at 1 mM. [3H]CD binding was decreased to about 20% of the control value in the presence of guanylyl-5'-imidodiphosphate (GppNHp), GTP, and GDP with the half-maximal effect at 1.3, 32, and 45 microM, respectively. [3H]CD binding in the presence of Ni2+ was decreased by GppNHp to a level obtained in the presence of GppNHp alone. The increase caused by Ni2+ in [3H]CD binding was due to the increase in the maximal binding capacity (Bmax) without changes in the affinity for [3H]CD. We conclude that Ni2+ increases the proportion of a muscarinic receptor subclass (or state) that is sensitive to guanyl nucleotide.

Subclasses of muscarinic receptors in isolated gastric mucosal cells: receptor characterization and parietal cell function

Muscarinic receptors were characterized in isolated intact chief and parietal cell enriched cell populations from canine and guinea-pig gastric mucosa by binding of tritiated N-methylscopolamine ([3H]NMS). Antagonist and agonist binding was studied by displacement of [3H]NMS with non-radioactive atropine, pirenzepine, pilocarpine and carbachol. Model analysis points to the existence of two binding sites in each of the two cell populations. The number of binding sites per cell was 1.7-1.8 times higher in parietal than in chief cell populations. Subclasses of muscarinic receptors as characterized by pirenzepine binding were compatible with the suggested A- and C- (high and low affinity) binding sites. The observation that in canine cells GMPPNP induced a conformational change of the high affinity binding site for pirenzepine could suggest that their proportion might depend on environmental factors. Binding parameters were related to specific parietal cell function as measured by aminopyrine accumulation as index for acid secretion. The carbachol effects depended on the calcium concentration and were competitively inhibited by pirenzepine. The physiological relevance of muscarinic receptor heterogeneity in gastric mucosal cells is unknown although the data support the hypothesis that involvement of muscarinic binding sites in calcium transport mechanisms connected with parietal cell function and possible conformational changes of the binding sites might be regulatory parameters in gastric secretory processes.

Adenosine receptor localization in rat testes: biochemical and autoradiographic evidence for association with spermatocytes

[3H]Cyclohexyladenosine ( [3H]CHA) labels adenosine receptors in rat testes. Testicular adenosine receptors are regulated by guanine nucleotides and divalent cations in a similar fashion to brain adenosine receptors. Endocrine manipulations which selectively decrease sperm cells reduce biochemically determined numbers of [3H]CHA labeled adenosine receptors, whereas adenosine receptor number is not affected by manipulations that primarily influence Leydig cells. Autoradiographic analysis of [3H]CHA binding in the rat testes reveals a localization within seminiferous tubules. Receptor related silver grains occur within tubular epithelium as well as in the lumen of tubules but are absent in interstitial tissue and blood vessels. These data suggest an association of adenosine receptors with spermatocytes within the seminiferous tubule epithelium.

The regulatory effect of ATP and its non-hydrolyzed analogs on heart adenylate cyclase

The effect of ATP on rabbit heart adenylate cyclase was investigated. The activation of adenylate cyclase by isoproterenol, guanyl nucleotides and NaF increases at a rise in ATP concentration. A similar regulatory effect is exerted by nonhydrolyzed analogs of ATP - adenyl-5'-ilimidodiphosphate and adenosine-5'-(alpha, beta-methylene) triphosphate. Consequently the regulatory influence of ATP is not due to chemical modification of the enzyme or to phosphorylation of endogenous GDP. Earlier we demonstrated that regulation of heart adenylate cyclase by ATP is not mediated by the adenosine binding center (Biokhimiya USSR (1982), 47, 455-464). It is assumed that the regulatory effect of ATP is accomplished by a specific binding site for this nucleotide.

Guanine nucleotides modulate the binding affinity of the oligopeptide chemoattractant receptor on human polymorphonuclear leukocytes

The oligopeptide chemoattractant receptor on human polymorphonuclear leukocyte (PMN) membranes exists in two affinity states. Since guanine nucleotides regulate the binding affinity and transductional activity of several other types of receptors, we examined the effect of nucleotides on the binding of N-formyl-methionyl peptides to their receptors on human PMN membranes. The addition of guanylylimidodiphosphate (0.1 mM), a nonhydrolyzable derivative of guanosine triphosphate (GTP), to PMN membrane preparations reduced the fraction of high-affinity receptors detected in equilibrium binding studies from 21.3 +/- 0.13 to 11.8 +/- 0.05% (P less than 0.03), without altering the binding affinities. Since the total number of receptors remained unchanged, the effect of guanylylimidodiphosphate was to convert a portion of the receptors from the high-affinity state to the low-affinity state. At the maximal concentration of guanine nucleotide tested, approximately 50% of the high-affinity sites were converted to low-affinity sites. The findings obtained by equilibrium binding were supported by kinetic studies since the dissociation of the radiolabeled oligopeptide chemoattractant N-formyl-methionyl-leucyl-[3H]phenylalanine from PMN membranes was accelerated in the presence of guanine nucleotide. The effect of guanine nucleotides was reversed upon washing, indicating that affinity conversion is bidirectional. The guanine nucleotide effects were greatest with nonhydrolyzable derivatives of GTP followed by GTP then guanosine diphosphate. Neither guanosine monophosphate nor any adenine nucleotide tested had an effect on receptor binding. These data suggest a role for guanine nucleotides in the regulation of stimulus-receptor coupling of chemoattractant receptors on human PMN.

Characterization and visualization of cholecystokinin receptors in rat brain using [3H]pentagastrin

[3H]Pentagastrin binds specifically to an apparent single class of CCK receptors on slide-mounted sections of rat brain (KD = 5.6 nM; Bmax = 36.6 fmol/mg protein). This specific binding is temperature-dependent and regulated by ions and nucleotides. The relative potencies of C-terminal fragments of CCK-8(SO3H), benzotript and proglumide in inhibiting specific [3H]pentagastrin binding to CCK brain receptors reinforce the concept of different brain and pancreas CCK receptors. CCK receptors were visualized by using tritium-sensitive LKB film analyzed by computerized densitometry. CCK receptors are highly concentrated in the cortex, dentate gyrus, granular and external plexiform layers of the olfactory bulb, anterior olfactory nuclei, olfactory tubercle, claustrum, accumbens nucleus, some nuclei of the amygdala, thalamus and hypothalamus.

Detergent-induced distinctions between fluoride- and vanadate-stimulated adenylate cyclases and their responses to guanine nucleotides

The influence of detergents on fluoride- and vanadate-stimulated adenylate cyclases was investigated with enzyme from liver and adipocyte plasma membranes. Stimulation of the adipocyte cyclase by Na3VO4 was maximal (sixfold) at 3 mM, was not additive with fluoride stimulation, and was readily reversed by washing of the membranes. Vanadate stimulation of the hepatic cyclase was specifically blocked by catechol, which had no effect on basal activity or on fluoride- or glucagon-stimulated activities. The hepatic enzyme, stimulated by fluoride ion, guanyl-5'-yl-(beta,gamma-imino)diphosphate (GPP(NH)P), or GPP(NH)P and glucagon, was inhibited by vanadate with 50% inhibition seen with 2 to 6 mM vanadate. The fluoride-activated adipocyte adenylate cyclase was inhibited by guanosine 5'-O-(3-thio-triphosphate) (GTP gamma S) more potently than by GPP(NH)P, with 50% inhibition being seen with 10 nM GTP gamma S or 100 nM GPP(NH)P. These nucleotides also inhibited the vanadate-stimulated enzyme, but with one-third the potency seen with the fluoride-activated cyclase. Dispersion of the adipocyte cyclase by Lubrol-PX into a 30,000g supernatant fraction caused no change in activation of the enzyme by fluoride, but reduced vanadate-stimulated activity 80%. By comparison, this treatment enhanced stimulation by GPP(NH)P twofold and by GTP gamma S threefold. More importantly, perhaps, the treatment with detergent blocked inhibition of the basal enzyme by GTP, blocked inhibition of fluoride- and vanadate-stimulated cyclases by GTP, GPP(NH)P, or GTP gamma S, and rendered vanadate-stimulated activity sensitive to enhancement by guanine nucleotides. The data indicate differences in the actions of vanadate and fluoride, made evident by the influence of guanine nucleotides and detergent treatment. The observations would be consistent with the idea that the effects of vandate may be due to the formation of GDP X V on the enzyme. The data strongly suggest that treatment of adenylate cyclase with Lubrol-PX causes a functional blockade in the guanine nucleotide-dependent inhibitory regulation (mediated by Ni), thereby allowing activation by the stimulatory guanine nucleotide-dependent regulatory component (Ns).

Role of calmodulin in the effect of guanyl nucleotides on rat hippocampal adenylate cyclase: involvement of adenosine and opiates

The adenylate cyclase activity of rat hippocampal plasma membranes can be stimulated by vasoactive intestinal polypeptide (VIP). Low concentrations (10(-9) to 19(-7) M) of 5'-guanylyl-imido diphosphate (GppNHp) evoke a transient inhibition of the enzyme, which is followed by stimulation with increasing GppNHp concentrations (10(-6) to 10(-4) M). Inclusion of Inclusion of ethyleneglycol-bis-(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) during incubation abolishes the GppNHp inhibition while preserving GppNHp activation. The stimulation induced by GppNHp is amplified by VIP, but the inhibition is unaffected. Adenosine analogs and opiates are inhibitory ligands in the presence of GTP, and their effects can be reversed by the appropriate receptor antagonists, 3-isobutyl-1-methylxanthine and naloxone. Treatment of membranes with trypsin abolishes the GppNHp-induced inhibition without affecting the GppNHp stimulation. The inhibition induced by GppNHp is also abolished by EGTA treatment followed by washing, which coincides with a reduction in the adenosine- and opiate-mediated, GTP-dependent inhibition. The GppNHp inhibition can be restored in EGTA-treated but not in trypsin-treated membranes by addition of calcium-calmodulin but not by Ca2+ or Mg2+. Calcium-calmodulin-depleted membranes lack calcium stimulation as well as GppNHp-induced inhibition, whereas untreated membranes and calcium-calmodulin-depleted membranes plus exogenous calcium-calmodulin showed calcium stimulation and GppNHp inhibition. These results suggest that calmodulin is involved in both Ca2+ stimulation and guanine nucleotide-mediated inhibition of rat hippocampal adenylate cyclase.

Effect of guanosine nucleotides on the respiratory burst oxidase from human neutrophils

The respiratory burst is a series of metabolic events which result in the production of microbicidal oxidants by phagocytes. The enzyme responsible for the respiratory burst is a membrane-bound oxidase which catalyzes the production of O-2 by the NADPH-dependent reduction of oxygen. Activity of this oxidase is diminished by a variety of guanosine-containing nucleotides. The decrease in activity caused by GTP, the most potent of these, occurs rapidly and is not solely dependent on hydrolysis of terminal phosphate groups. GTP appears to diminish oxidase activity through both inhibition and inactivation. GTP-mediated inhibition is kinetically mixed with respect to NADPH.

Adenylate cyclase activity in the superior cervical ganglion of the rat

Adenylate cyclase activity in cell-free homogenates of the rat superior cervical ganglion (SCG) was assayed under a variety of experimental conditions. Adenylate cyclase activity was decreased by approximately one-half when 1 mM EGTA was included in the homogenization buffer and assay mixture, indicating the presence of a Ca2+-sensitive adenylate cyclase in the ganglion. In the presence of EGTA, basal adenylate cyclase activity in homogenates of the SCG was 12.9 +/- 0.6 pmol cyclic AMP/ganglion/10 min. Enzyme activity was stimulated three- to fourfold by 10 mM NaF or 10 mM MnCl2. Both GTP and its nonhydrolyzable analog guanylylimidodiphosphate (GppNHp) stimulated adenylate cyclase in a concentration-dependent manner over the range of 0.1-10.0 microM. Stimulation by GppNHp was five to six times greater than that produced by GTP at all concentrations tested. Decentralization of the ganglion had no effect on basal or stimulated adenylate cyclase activity. Receptor-linked stimulation of adenylate cyclase was not obtained with any of the following: isoproterenol, epinephrine, histamine, dopamine, prostaglandin E2, or vasoactive intestinal peptide. Thus the receptor-linked regulation of adenylate cyclase activity appears to be lost in homogenates of the ganglion.

Axonal transport of beta-adrenergic receptors. Antero- and retrogradely transported receptors differ in agonist affinity and nucleotide sensitivity

Beta-receptors were measured in longitudinal sections of ligated rat sciatic nerve by autoradiographic localization of 125I-labeled cyanopindolol binding sites. Receptors accumulated at the ligature, both proximally and distally, in a time-dependent fashion. Receptor transport also occurred in an isolated segment of nerve (i.e., a doubly ligated nerve), suggesting that the movement is by fast transport. Pharmacological analysis of the accumulating binding sites indicates that they are beta 2-adrenergic receptors. In competition studies, agonists were 10-30 times more potent on receptors accumulating proximal to the ligature than on distally accumulating receptors, whereas antagonists were equipotent on both. Guanyl-5'-yl-imidodiphosphate (GppNHp) decreased the potency of agonists at proximal receptors in a dose-dependent fashion. Distal receptors were much less sensitive to GppNHp. Other nucleotides displayed varying abilities to mimic the effect of GppNHp, suggesting the involvement of a guanine nucleotide-binding protein in regulating agonist affinity. Thus, presynaptic beta 2-adrenergic receptors were identified in rat sciatic nerve. A small fraction of them apparently moves by fast transport. The anterogradely transported receptors have binding properties which differ from the retrogradely moving receptor and appear to be in functional association with a nucleotide regulatory protein.

Binding of leukotrienes C4 and D4 to membranes from guinea pig lung: regulation by ions and guanine nucleotides

Tritium-labeled leukotrienes C4 and D4 (LTC4 and LTD4) bind to membranes from guinea pig lung. Binding properties of the two ligands are almost identical. More than 80% of 3H-LTC4 and 3H-LTD4 binding can be blocked by unlabeled LTC4 (IC50 8 nM versus 3H-LTC4 and 8 nM versus 3H-LTD4), LTD4 (12 nM, 16 nM), LTE4 (40 nM, 98 nM), and the leukotriene antagonist FPL 55712 (14 microM, 11 microM). Binding is reversible (50% dissociation at 65 min for both ligands at 25 degrees). Binding of 3H-LTC4 and 3H-LTD4 is enhanced by divalent cations and inhibited by sodium ions, guanine nucleotides, and EDTA. 3H-LTD4 binds in unaltered form, but 3H-LTC4 appears to bind mostly after conversion to 3H-LTD4. The high affinity, reversibility, and regulation by ions and guanine nucleotides of 3H-LTC4 and 3H-LTD4 binding strongly imply that these binding sites are physiological LTD4 receptors.

Two forms of AMP deaminase from chicken liver

Chromatography of chicken liver AMP deaminase on phosphocellulose and DEAE-Sephacel revealed the existence of two separate peaks of enzyme activity. Significant differences have been observed between form I and II of the enzyme in respect to substrate specificity and their kinetic and regulatory properties.

GDP does not support activation of adenylate cyclase nor ADP-ribosylation of a guanine nucleotide binding protein by cholera toxin

The actions of cholera toxin (i.e., activation of adenylate cyclase and ADP-ribosylation of a guanine nucleotide binding protein in purified membranes from rat liver) were GTP dependent. Neither of these actions of cholera toxin was reproduced with GDP. Simultaneous addition of ATP and MgCl2 along with GDP allowed cholera toxin to exert these actions. The role of GDP in adenylate cyclase regulation was discussed.

NMR studies on interactions of ribonuclease Sa with Guo-3'-P

Some features of the interaction of guanyloribonuclease Sa from Streptomyces aureofaciens with its competitive inhibitor Guo-3'-P were investigated by 1H and 31P NMR spectroscopy. The pH dependence of chemical shifts of C(2)-H protons of the histidine residue of the enzyme were analysed, in the absence and presence of Guo-3'-P. This analysis showed that only one of the two histidines of ribonuclease Sa is located in the active site of the enzyme. 31P NMR resonances of the nucleotide and of its complex with the enzyme indicated that this histidine interacts with the phosphate group of the substrate. The possible relationship between the observed perturbation of the NMR titration curve of the active site of histidine and a conformational change in the enzyme molecule at a pH of approximately 7.5 is also discussed.

Secretin: specific binding to rat brain membranes

The binding of [125I]secretin to rat brain membranes was investigated. Radiolabeled secretin bound with high affinity (KD = 0.2 nM) to a single class of noninteracting sites. Binding was specific, saturable, and reversible. Regional distribution studies indicated that the specific binding was greatest in the cerebellum, intermediate in the cortex, thalamus, striatum, hippocampus, and hypothalamus, and lowest in the midbrain and medulla/pons. Pharmacological studies indicated that only secretin, but not other peptides, inhibits binding of [125I]secretin with high affinity. Also, certain guanine nucleotides inhibited high affinity binding. These data indicate that rat brain membranes possess high affinity binding sites specific for secretin and that with the use of [125I] secretin the kinetics, stoichiometry, specificity, and distribution of secretin receptors can be directly investigated.

Guanine nucleotide sensitivity of [125I]-Iodo NTyr somatostatin binding in rat adenohypophysis and cerebral cortex

Specific [125I]-Iodo-NTyr somatostatin binding sites are present in adenohypophyseal and cerebral cortical membranes. Guanine nucleotides reduce the maximal binding capacity of adenohypophyseal binding sites without significantly affecting their apparent affinity. In pituitary as well as in cortex, GTP is the most potent nucleotide followed by GDP and guanylyl imidodiphosphate (GMP-PNP). The effect appears specific of guanine nucleotides since ATP, ADP and AMP are inactive on [125I]-Iodo-NTyr somatostatin binding. These results, showing the nucleotide sensitivity of [125I]-Iodo-NTyr somatostatin binding in pituitary and cerebral cortex, are compatible with a coupling of somatostatin receptors with adenylate cyclase.

alpha 1- and beta 2-adrenergic receptor expression in the Madin-Darby canine kidney epithelial cell line

The Madin-Darby canine kidney (MDCK) cell line, derived from distal tubule/collecting duct, expresses differentiated properties of renal tubule epithelium in culture. We studied the expression of adrenergic receptors in MDCK to examine the role of catecholamines in the regulation of renal function. Radioligand-binding studies demonstrated, on the basis of receptor affinities of subtype-selective adrenergic agonists and antagonists, that MDCK cells have both alpha 1- and beta 2-adrenergic receptors. To determine whether these receptor types were expressed by the same cell, we developed a number of clonal MDCK cell lines. The clonal lines had stable but unique morphologies reflecting heterogeneity in the parent cell line. Some clones expressed only beta 2-adrenergic receptors and were nonmotile, whereas others expressed both alpha 1- and beta 2-receptors and demonstrated motility on the culture substrate at low cell densities. In one clone, alpha- and beta-receptor expression was stable for more than 50 passages. Catecholamine agonists increased phosphatidylinositol turnover by activating alpha-adrenergic receptors and cellular cyclic adenosine monophosphate accumulation by activating beta-adrenergic receptors. Guanine nucleotide decreased the affinity of isoproterenol for the beta 2-receptor but did not alter the affinity of epinephrine for the alpha 1-receptor. These results show that alpha 1- and beta 2-receptors can be expressed by a single renal tubular cell and that the two receptors behave as distinct entities in terms of cellular response and receptor regulation. Heterogeneity of adrenergic receptor expression in MDCK clones may reflect properties of different types of renal tubule cells.