Characteristics of 5'-guanylyl imidodiphosphate-activated adenylate cyclase

δ Opioid Receptor Inverse Agonists and their In Vivo Pharmacological Effects

The discovery of δ opioid receptor inverse agonist activity induced by ICI-174,864, which was previously reported as an δ opioid receptor antagonist, opened the door for the investigation of inverse agonism/constitutive activity of the receptors. Various peptidic or non-peptidic δ opioid receptor inverse agonists have since been developed. Compared with the reports dealing with in vitro inverse agonist activities of novel compounds or known compounds as antagonists, there have been almost no publications describing the in vivo pharmacological effects induced by a δ opioid receptor inverse agonist. After the observation of anorectic effects with the δ opioid receptor antagonism was discussed in the early 2000s, the short-term memory improving effects and antitussive effects have been very recently reported as possible pharmacological effects induced by a δ opioid receptor inverse agonist. In this review, we will survey the developed δ opioid receptor inverse agonists and summarize the possible in vivo pharmacological effects by δ opioid receptor inverse agonists. Moreover, we will discuss important issues involved in the investigation of the in vivo pharmacological effects produced by a δ opioid receptor inverse agonist.

Evidence for stimulation of adenylyl cyclase by an activated G(s) heterotrimer in cell membranes: an experimental method for controlling the G(s) subunit composition of cell membranes

Heterotrimeric (alphabetagamma) G(s) mediates agonist-induced stimulation of adenylyl cyclase (AC). Cholera toxin (CTx) will ADP-ribosylate the alpha-subunit of G(s) (G(s)alpha). G(s)alpha-deficient cyc(-) membranes were "stripped" of Gbeta. When the stripped cyc(-) were incubated with G(s)alpha and/or Gbetagamma, each was incorporated into the membranes independently of the other. Both G(s)alpha and Gbetagamma had to be present in the membranes, and they had to be able to form a heterotrimer in order for CTx to ADP-ribosylate G(s)alpha, indicating that the membrane bound G(s) heterotrimer is a substrate for CTx, but the G(s)alpha subunit by itself is not. When G(s)alpha was completely and irreversibly activated with GTPgammaS and incorporated into stripped cyc(-), it was a poor substrate for CTx and a weak stimulator of AC unless Gbetagamma was also incorporated. Furthermore, the level of AC stimulation corresponded to the amount of G(s) heterotrimer that was formed in the membranes from GTPgammaS-activated G(s)alpha and Gbetagamma. These data suggest that AC is stimulated by an activated G(s) heterotrimer in cell membranes.

Vip-induced cross-talk between G-proteins in membranes from rat anterior pituitary cells

In order to study the activation mechanism of heterotrimeric G-proteins by agonist-liganded receptors, GTP gamma S binding to membranes was measured in rat adenohypophyseal cells after addition of dopamine (DA) or vasoactive intestinal peptide (VIP), which, respectively, inhibit and activate pituitary adenylyl cyclase. G-protein subunit present in anterior pituitary cells was characterized by either ADP-ribosylation catalysed by Bordetella pertussis and cholera toxins or by immunoblot using specific antisera. Binding of GTP gamma S was found to depend upon GTP gamma S and Mg2+ concentrations; it was sensitive to pretreatment of the cells with cholera and Bordetella pertussis toxins (IAP). DA increased binding of the nucleotide. Paradoxically, VIP decreased the rate of GTP gamma S binding; the effect was suppressed by prior treatment of the cells with either cholera toxin or IAP. VIP also increased [33P]ADPribose incorporation in Gi/Go-proteins catalysed by IAP. Forskolin was also able to decrease GTP gamma S binding, thus suggesting that the binding of forskolin with the adenylyl cyclase catalytic unit might activate Gs proteins through an increased interaction between Gs and adenylyl cyclase. Taken together, these results suggest that VIP, as well as forskolin, may both accelerate the activation of Gs and suppress the inhibitory effect of activated Gi/Go-proteins. Interactions between Gs and Gi/Go subunits mediated by beta gamma and/or adenylyl cyclase might thus result in a kinetic coupling of transduction pathways involving distinct G-proteins.

Stimulatory and inhibitory regulation of myocardial adenylate cyclase by 5'-guanylyl-imidodiphosphate

Particulate and soluble rat myocardial adenylate cyclase enzymes were characterized with respect to their stimulatory and inhibitory regulation by Gpp(NH)p. Gpp(NH)p (60 microM) stimulated Mg2+- and Mn2+-dependent adenylate cyclase. High concentrations of Gpp(NH)p (600 microM) attenuated the maximal stimulatory response to Gpp(NH)p but only at low cation concentrations. The attenuating effects of 600 microM Gpp(NH)p resulted predominantly from the introduction of a prolonged lag in the kinetics of activation of adenylate cyclase. Steady-state rates of adenylate cyclase activities were similar with either 60 or 600 microM Gpp(NH)p. At any concentration of Gpp(NH)p, the lag was eliminated by Mg ions or isoproterenol. No antihysteretic property for free Mn ions was evident. Forskolin-sensitive particulate adenylate cyclase was not stimulated further by Gpp(NH)p. A 600 microM concentration of Gpp(NH)p inhibited particulate forskolin-sensitive adenylate cyclase at low Mg ion concentrations. In contrast, Gpp(NH)p at 60 microM consistently activated forskolin-sensitive adenylate cyclase after solubilization. The early transient inhibitory properties of 600 microM Gpp(NH)p which resulted in attenuation of adenylate cyclase by 600 microM Gpp(NH)p were diminished by detergent extraction, resulting in only a minor effect of 600 microM Gpp(NH)p to inhibit solubilized adenylate cyclase. These findings indicate that guanine nucleotides exert both stimulatory and inhibitory control upon the myocardial adenylate cyclase enzyme; that solubilization shifts the balance between the stimulatory and inhibitory properties of Gpp(NH)p to allow more dominant expression of the stimulatory response; and that Mg ions critically modify the nature of the myocardial adenylate cyclase response to Gpp(NH)p.

Guanine-nucleotide-dependent inhibition of adenylate cyclase of rabbit heart by glucagon

The present study demonstrates an inhibitory effect of glucagon on the adenylate cyclase system of rabbit heart. Inhibition was maximal (22-40%) at 0.1-0.01 microM glucagon and required the presence of 0.01-0.1 mM GTP or guanosine 5'-[beta, gamma-imido]triphosphate (GuoPP[NH]P). Reduced or no inhibitor effect of glucagon was observed: (a) after limited proteolysis of plasma membrane proteins by trypsin, (b) in the presence of 1 mM Mn2+, (c) in the absence of Na+, and (d) during the first 10 min of incubation if GuoPP[NH]P was the activating ligand. With GTP as the activating ligand, inhibition of cyclase by glucagon occurred without delay. These data are consistent with a mediation of glucagon inhibition by a guanine-nucleotide-binding protein. In the presence of ethanol (0.2 M) or benzyl alcohol (0.05 M), agents which are known to increase the fluidity of biological membranes, glucagon increased the enzyme activity in a guanine-nucleotide-dependent manner. Activation of cyclase in the presence of alcohols was maximal (30-60%) at 0.1-1.0 microM glucagon and 0.01 mM guanine nucleotides. Data suggest that glucagon receptors can interact with both the activatory and inhibitory guanine-nucleotide-binding proteins and the physical state of membranes may play a role in determining which interaction will be preferential.

Central blood pressure effects of guanylyl-imido-diphosphate and cyclic guanosine-monophosphate

Injections of guanylyl-imido-diphosphate (250, 500 and 1,000 microgram/kg) into the lateral cerebral ventricle of the anaesthetized cat induced increases in blood pressure and heart rate while the intravenous injections of the same doses were ineffective, thus indicating a central mechanism of action of this compound which activates adenylcyclase at the catalytic subunit. The results support the hypothesis that the activity of cardiovascular centres depends on the prevailing concentration of cAMP. Intracerebroventricular injection of cGMP (125, 250 and 500 microgram/kg) caused hypotension and bradycardia. The effects increased with the dose but were subject to tachyphylaxis. The lack of an effect after intravenous administration indicates a central site of action. This result is in agreement with the Yin Yang hypothesis and suggests that cGMP is a second transmitter in cardiovascular centres which may be involved in central cardiovascular effects in response to stimulation by putative neurotransmitter substances such as acetylcholine.

Association of binding sites for guanine nucleotides with adenylate cyclase activation in rat pancreatic plasma membranes. Interaction of gastrointestinal hormones

1. The activation of rat pancreatic adenylate cyclase by guanosine 5'-(beta-gamma-imido)triphosphate (p[NH]ppG) and GTP, and by the two gastrointestinal hormones pancreozymin (as C-terminal octapeptide) and secretin was correlated with the binding of [8-3H]guanosine 5'-(beta-gamma-imido)triphosphate to rat pancreatic plasma membranes. 2. The low basal adenylate cyclase activity was stimulated 17-fold by p[NH]ppG (after a 2 min lag period), 3,5-fold only by GTP, 21-fold by C-terminal octapeptide of pancreozymin, and 8-fold by secretin. GTP inhibited competitively the activation of adenylate cyclase by p[NH]ppG with a Ki,app almost identical with the Ka,app (0.3 micron). p[NH]ppG and GTP enhanced the stimulation by secretin more markedly than that by the C-terminal octapeptide of pancreozymin, leading to the same maximal activity. Both hormones suppressed the lag period of activation by p[NH]ppG. 3. The binding of [8-3H]p[NH]ppG was dependent on time, temperature and Mg2+ and it was also a saturable and reversible process. Scatchard plots with a concavity upward were linearized after co-addition of ATP, Mg2+ and an ATP-regenerating system that abolished low-affinity sites for p[NH]ppG without saturating higher affinity sites, GTP, ITP and UTP inhibited [8-3H]p[NH]ppG binding to the high-affinity sites in concentration ranges identical with those found for adenylate cyclase activation. Considerable binding of [8-3H]p[NH]ppG was still evident at 20 degrees C, but enzyme activation was not observed any more, except in the presence of hormones.

Catecholamine-stimulated GTPase activity in turkey erythrocyte membranes

Determination of specific GTPase (EC 3.6.1.--) activity in turkey erythrocyte membranes was achieved using low concentration of GTP (0.25 muM), inhibition of nonspecific nucleoside triphosphatases by adenosine 5'(beta,gamma-imino-triphosphate (App(NH)p) and suppression of the transfer of gamma-32P from GTP to ADP with an ATP regeneration system. Under these conditions catacholamines caused a 30--70% increase in GTP hydrolysis. The stimulation of GTPase activity by catecholamines required the presence of Mg2+ or Mn2+. DIfferent batches of membranes revealed the following specific activities (pmol 32Pi/mg protein min): basal GTPase (determined in the absence of catecholamine), 6-- 11; catecholamine-stimulated TTPase, 3--7; and residual non-specific NTPase 3--5. The stimulation of GTPase activity by catecholamines fulfilled the stereospecific requirements of the beta-adrenergic receptor, and was inhibited by propranolol. The concentrations of DL-isoproterenol which half-maximally activated the GTPase and adenylate cyclase were 1 and 1.2 muM, respectively. The following findings indicate that the catecholamine-stimulated GTPase is independent of the catalytic production of cyclic AMP by the adenylate cyclase. Addition of cyclic AMP to the GTPase assay did not change the rate of GTP hydrolysis. Furthermore, treatment of the membrane with N-ethylmaleimide (MalNEt) at 0 degrees C which caused 98% inhibition of the adenylate cyclase, had no effect on the catecholamine-stimulated GTPase. The affinity and specificity for GTP in the GTPase reactions are similar to those previously reported for the stimulation of the adenylate cyclase. The apparent Km for GTP in the basal and the catecholamine-stimulated GTPase reaction was 0.1 muM. These GTPase activities were inhibited by ITP but not by CTP and UTP. It is proposed that a catecholamine-stimulated GTPase is a component of the turkey erythrocyte adenylate cyclase system.

Catecholamine-sensitive adenylate cyclase of human fat cell ghosts. Characteristics of the GMP(PNP)-liganded state

The effects of 5'-guanylyl-imidodiphosphate (GMP(PNP)) on the catecholamine-sensitive adenylate cyclase of human fat cell ghosts were studied. The compound increased basal and epinephrine-stimulated enzyme activity by about 300%; in addition GMP(PNP) increased hormone sensitivity by reducing the epinephrine concentration required, to produce half maximal stimulation. The rate of GMP(PNP)-induced activation was slow in onset and could be enhanced by epinephrine. The GMP(PNP)-activated state was resistant to thermal inactivation and could not be reversed by extensive washing. The application of this compound in clinical studies may be useful because of its stimulating and stabilizing action.

Irreversible activation of adenylate cyclase of toad erythrocyte plasma membrane by 5'-guanylylimidodiphosphate

The irreversible activation of adenylate-cyclase by 5'guanylylimidodiphosphate, a phosphoramidate analog of 5'GTP, has been examined in toad (Bufus marinus) plasma membranes using the technique of preincubating the membranes with the nucleotide under various controlled conditions followed by washing and subsequent assay of enzyme activity. Activation of adenylate cyclase by Gpp(NH)p, but not GTP, is essentially permanent and persists following extensive washing, prolonged incubation at 30 degrees C in the absence of the nucleotide, and after dissolution of the membranes with Lubrol PX. (-)-Isoproterenol increases the activation observed with maximal concentrations of Gpp(NH)p from eight- to 10-fold (in the absence of hormone) to 50- to 100-fold; final activities as high as 10-15 nmoles of cyclic AMP per min per mg protein are achieved. The activated state obtained with isoproterenol and Gpp(NH)p is also permanent and is not inhibited by propranolol. The synergism between Gpp(NH)p and hormone requires the simultaneous presence of these compounds, and the time-dependent enhancement of activation with (-)-isoproterenol may be interrupted by addition of propranolol. The stimulation is slow, and may proceed for as long as 45 min at 30 degrees C in the presence of maximal concentrations of Gpp(NH)p and (-)-isoproterenol. Very little activation occurs at 0 degrees C. The time course of activation at 30 degrees C exhibits an accelerating phase lasting from 5 to 30 min when Gpp(NH)p is added directly during assay of cyclase activity or when the membranes are preincubated for various times and washed prior to assay for a fixed time. The lag period occurs in the presence and absence of (-)-isoproterenol, although the rate of increase in velocity is greater with hormone. The length of the accelerating phase decreases with increasing concentrations of Gpp(NH)p, although it is still evident with maximal levels of Gpp(NH)p and hormone. However, prewarming the membranes at 30 degrees C for 10 min in the absence of Gpp(NH)p or (-)-isoproterenol results in an immediate onset of linear activation at a rate which is achieved in untreated membranes only after about 10 min. The events occurring during prewarming at 30 degrees C are readily reversible since chilling the warmed membranes to 0 degrees C results in a time course of activation identical to that of membranes maintained at 0 degrees C until addition of Gpp(NH)p. Activation is proportional to the concentration of Gpp(NH)p within the range of 10(-8) to 10(-4) mM. The apparent affinity for Gpp(NH)p increases with increasing time of incubation. The primary effect of increasing the concentration of Gpp(NH)p is to decrease the time required to obtain a maximal rate of activation. The possible relevance of these findings to the mechanism of action of Gpp(NH)p, adenylate cyclase and hormones is discussed within the context of current views of biological membranes which recognize the lateral mobility of membrane molecules.