Evidence for Interdependent Action of Glucagon and Nucleotides on the Hepatic Adenylate Cyclase System

G Protein-Coupled Receptors: A Century of Research and Discovery

GPCRs (G protein-coupled receptors), also known as 7 transmembrane domain receptors, are the largest receptor family in the human genome, with ≈800 members. GPCRs regulate nearly every aspect of human physiology and disease, thus serving as important drug targets in cardiovascular disease. Sharing a conserved structure comprised of 7 transmembrane α-helices, GPCRs couple to heterotrimeric G-proteins, GPCR kinases, and β-arrestins, promoting downstream signaling through second messengers and other intracellular signaling pathways. GPCR drug development has led to important cardiovascular therapies, such as antagonists of β-adrenergic and angiotensin II receptors for heart failure and hypertension, and agonists of the glucagon-like peptide-1 receptor for reducing adverse cardiovascular events and other emerging indications. There continues to be a major interest in GPCR drug development in cardiovascular and cardiometabolic disease, driven by advances in GPCR mechanistic studies and structure-based drug design. This review recounts the rich history of GPCR research, including the current state of clinically used GPCR drugs, and highlights newly discovered aspects of GPCR biology and promising directions for future investigation. As additional mechanisms for regulating GPCR signaling are uncovered, new strategies for targeting these ubiquitous receptors hold tremendous promise for the field of cardiovascular medicine.

Analyzing kinetic signaling data for G-protein-coupled receptors

In classical pharmacology, bioassay data are fit to general equations (e.g. the dose response equation) to determine empirical drug parameters (e.g. EC50 and Emax), which are then used to calculate chemical parameters such as affinity and efficacy. Here we used a similar approach for kinetic, time course signaling data, to allow empirical and chemical definition of signaling by G-protein-coupled receptors in kinetic terms. Experimental data are analyzed using general time course equations (model-free approach) and mechanistic model equations (mechanistic approach) in the commonly-used curve-fitting program, GraphPad Prism. A literature survey indicated signaling time course data usually conform to one of four curve shapes: the straight line, association exponential curve, rise-and-fall to zero curve, and rise-and-fall to steady-state curve. In the model-free approach, the initial rate of signaling is quantified and this is done by curve-fitting to the whole time course, avoiding the need to select the linear part of the curve. It is shown that the four shapes are consistent with a mechanistic model of signaling, based on enzyme kinetics, with the shape defined by the regulation of signaling mechanisms (e.g. receptor desensitization, signal degradation). Signaling efficacy is the initial rate of signaling by agonist-occupied receptor (kτ), simply the rate of signal generation before it becomes affected by regulation mechanisms, measurable using the model-free analysis. Regulation of signaling parameters such as the receptor desensitization rate constant can be estimated if the mechanism is known. This study extends the empirical and mechanistic approach used in classical pharmacology to kinetic signaling data, facilitating optimization of new therapeutics in kinetic terms.

Kinetic operational models of agonism for G-protein-coupled receptors

The application of kinetics to research and therapeutic development of G-protein-coupled receptors has become increasingly valuable. Pharmacological models provide the foundation of pharmacology, providing concepts and measurable parameters such as efficacy and potency that have underlain decades of successful drug discovery. Currently there are few pharmacological models that incorporate kinetic activity in such a way as to yield experimentally-accessible drug parameters. In this study, a kinetic model of pharmacological response was developed that provides a kinetic descriptor of efficacy (the transduction rate constant, k_τ) and allows measurement of receptor-ligand binding kinetics from functional data. The model assumes: (1) receptor interacts with a precursor of the response ("Transduction potential") and converts it to the response. (2) The response can decay. Familiar response vs time plots emerge, depending on whether transduction potential is depleted and/or response decays. These are the straight line, the "association" exponential curve, and the rise-and-fall curve. Convenient, familiar methods are described for measuring the model parameters and files are provided for the curve-fitting program Prism (GraphPad Software) that can be used as a guide. The efficacy parameter k_τ is straightforward to measure and accounts for receptor reserve; all that is required is measurement of response over time at a maximally-stimulating concentration of agonist. The modular nature of the model framework allows it to be extended. Here this is done to incorporate antagonist-receptor binding kinetics and slow agonist-receptor equilibration. In principle, the modular framework can incorporate other cellular processes, such as receptor desensitization. The kinetic response model described here can be applied to measure kinetic pharmacological parameters than can be used to advance the understanding of GPCR pharmacology and optimize new and improved therapeutics.

Amino acid transport in isolated hepatocytes: effect of glucagon

Amino acid transport was studied in freshly isolated adult rat hepatocytes using non-metabolizable alpha-amino-1-[14C] isobutyric acid and 1-aminocyclopentane-1-[14C] carboxylic acid. In the presence of sodium, hepatocytes concentrated alpha-aminoisobutyric acid; this concentrative component of the transport had properties similar to transport system A. The sodium-independent transport of aminocyclopentane carboxylic acid had properties similar to transport system L (facilitated diffusion). Glucagon stimulated the influx of alpha-aminoisobutyric acid into hepatocytes. The glucagon effect (a) occurred rapidly, but its full expression required two hours of exposure of the cells to hormone; (b) involved new protein (and possibly RNA) synthesis; and (c) occurred at low concentrations of glucagon (50% effect with 0.4 nm). Glucagon stimulated only system A. Cyclic AMP also stimulated the transport of alpha-aminoisobutyric acid. Freshly isolated hepatocytes appear conveniently suited to the investigation of various aspects of the regulation of liver amino acid transport in normal and pathophysiological states.

Guanine nucleotide exchange-independent activation of Gs protein by beta2-adrenoceptor

beta2-adrenoceptor-mediated activation of Gs and adenylyl cyclase or other receptor-mediated G protein activations is believed to occur by receptor-catalyzed replacement of GDP with GTP on the alpha-subunit of the G protein. Here we showed that a beta2-adrenoceptor-Gs system, heterologously expressed in cyc- or human embryonic kidney (HEK)-293 cells, can be activated in the presence of GDP or its phosphorylation-resistant analog, guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS). The potency and maximal ability of GDP to activate Gs and adenylyl cyclase were identical to those of GTP. GDP-mediated activation of adenylyl cyclase, similar to that mediated by GTP, was concentration-dependent, required high magnesium concentrations, was inhibited by inverse agonists, and was correlated with the efficacy of receptor ligands used to stimulate the receptor. UDP did not block the GDP-mediated activation, although it completely blocked the formation of a small amount of GTP ( approximately 5% GDP) from GDP. Moreover, the activation of Gs in the presence of GDP was insensitive to cholera toxin treatment of the cells, whereas that observed in the presence of GTP was amplified by the treatment, which showed that the activation observed in the presence of GDP was not mediated by GTP. Therefore, we concluded that GDP itself could mediate beta-adrenoceptor-induced activation of Gs-adenylyl cyclase system as much as GTP. We discuss the results in the context of the current paradigm of receptor-mediated G protein activation and propose an additional mode of activation for beta2-adrenoceptor-G(s) adenylyl cyclase system where nucleotide exchange is not necessary and GDP and GTP play identical roles in receptor-induced Gs protein activation.

Development of potent glucagon antagonists: structure-activity relationship study of glycine at position 4

We examined the functional role of glycine at position 4 in the potent glucagon antagonist [desHis(1), Glu(9)]glucagon amide, by substituting the L- and D-enantiomers of alanine and leucine for Gly(4) in this antagonist. The methyl and isobutyl side-chain substituents were introduced to evaluate the preference shown by the glucagon receptor, if any, for the orientation of the N-terminal residues. The L-amino acids demonstrated only slightly better receptor recognition than the D-enantiomers. These results suggest that the Gly(4) residue in glucagon antagonists may be exposed to the outside of the receptor. The enhanced binding affinities of analogs 1 and 3 compared with the parent antagonist, [desHis(1), Glu(9)]glucagon amide, may have resulted from the strengthened hydrophobic patch in the N-terminal region and/or the increased propensity for a helical conformation due to the replacement of alanine and leucine for glycine. Thus, as a result of the increased receptor binding affinities, antagonist activities of analogs 1-4 were increased 10-fold compared with the parent antagonist, [desHis(1), Glu(9)]glucagon amide. These potent glucagon antagonists have among the highest pA(2) values of any glucagon analogs reported to date.

Nobel Lecture. Signal transduction: evolution of an idea

“In general there is no set of observations conceivable which can give enough information about the past of a system to give complete information as to its future”: Norbert Wiener. “Think simplicity; then discard it”: Alfred North Whitehead

Effect of insulin and glucagon on the mobility of ESR-probes incorporated in rat liver plasma membranes

1. The effect of different concentrations of insulin (INS) and glucagon (GLU) on the rotational mobility of a membrane-incorporated spin probe 2,2-6,6-tetramethyl-4-capriloyl piperidine-1-oxil (C7) was investigated by electron spin resonance (ESR) technique. 2. Two strong adenylate cyclase effectors guanosine 5'-triphosphate (GTP) and guanylyl 5'-imidodiphosphate [Gpp(NH)p], as well as an antioxidant 4-methyl-2,6-ditretbutilphenol (AO) and a nonprotein hormone prostaglandin E2 (PGE2) were used as reference effectors. 3. Applied effectors reduced by 30-82% the rotation correlation time (TR) of the rat liver plasma membranes spin probe C7. The effect was time-dependent and reached saturation 30-40 min after the effector application. 4. The kinetic- and concentration-dependent changes in TR were described by a simple phenomenological model. The apparent binding constants and the number of the apparent membrane binding sites of the effectors used were calculated using the model.

New glucagon analogues with conformational restrictions and altered amphiphilicity: effects on binding, adenylate cyclase and glycogenolytic activities

In an effort to obtain highly potent glucagon antagonists, we have investigated glucagon (1) structure-function relationships utilizing the following design principles: (1) structural changes known to lead to partial agonist activities; (2) conformational restrictions; (3) changes in the conformational probabilities of the primary sequence; and (4) increased amphiphilicity. In this report we present the total synthesis, purification, receptor binding, adenylate cyclase activity, in vivo glycogenolytic activity and CD spectrum of the following four glucagon analogues: [Ahx17,18]glucagon (2), [D-Phe4,Tyr5, 3,5-diiodo-Tyr10,Arg12,Lys17,18,Glu21]glucagon (3), [Asp9,Lys12,Lys17,18,Glu21]glucagon 4, and [Glu15,Lys17,18]glucagon 5. Compound 2 binds exclusively to the high affinity receptor and compound 3 was a highly potent antagonist with respect to adenylate cyclase activity. Analog 4 showed distinct biphasic binding (IC50 5.6 nM and 630 nM), with only the low affinity binding leading to adenylate cyclase activity. Furthermore in analogue 5 receptor binding and adenylate cyclase activity were dissociated by a factor of 5. The results are consistent with a multistep binding mechanism in which glucagon interacts first nonspecifically with the anisotropic interphase of the cell membrane, followed by a conformational transition which occurs in the sequences 10-14 and 15-18 when the membrane bound peptide binds to its receptor.

Involvement of G-proteins and adenylate cyclase in the action of gonad-stimulating substance on starfish ovarian follicle cells

Gonad-stimulating substance (GSS) secreted from radial nerves induces meiotic maturation of starfish oocytes by stimulating production of 1-methyladenine (1-MeAde) in ovarian follicle cells. We have previously shown that cAMP mediates the action of GSS on 1-MeAde synthesis by starfish ovarian follicle cells. The present study examines the possible involvement of guanine nucleotide-binding regulatory proteins (G-proteins) and adenylate cyclase in the action of GSS on 1-MeAde production by starfish (Asterina pectinifera) follicle cells. GSS slightly stimulated adenylate cyclase activity in crude membrane preparations of follicle cells. GTP markedly enhanced this action of GSS in a dose-dependent manner. Nonhydrolyzable GTP analogs such as guanosine 5'-O-(3-thiotriphosphate) and 5'-guanylylimidodiphosphate, NaF, and forskolin also stimulated adenylate cyclase activity. In addition, chorela toxin (CT) stimulated adenylate cyclase activity in membrane preparations in the presence of NAD and GTP. Unlike adenylate cyclase, phosphodiesterase activity was not influenced by GSS. When crude membranes of follicle cells were incubated with [alpha-32P]NAD in the presence of CT and pertussis toxin, 45-kDa and 41-kDa proteins were ADP-ribosylated, respectively, suggesting the presence of two types (stimulatory and inhibitory) of G-proteins. It is concluded that G-proteins and adenylate cyclase play an important role in the action of GSS on 1-MeAde production by starfish ovarian follicle cells.

Receptor-effector coupling by G proteins

The primary structure of G proteins as deduced from purified proteins and cloned subunits is presented. When known, their functions are discussed, as are recent data on direct regulation of ionic channels by G proteins. Experiments on expression of alpha subunits, either in bacteria or by in vitro translation of mRNA synthesized from cDNA are presented as tools for definitive assignment of function to a given G protein. The dynamics of G protein-mediated signal transduction are discussed. Key points include the existence of two superimposed regulatory cycles in which upon activation by GTP, G proteins dissociate into alpha and beta gamma and their dissociated alpha subunits hydrolyze GTP. The action of receptors to catalyze rather than regulate by allostery the activation of G proteins by GTP is emphasized, as is the role of subunit dissociation, without which receptors could not act as catalysts. To facilitate the reading of this review, we have presented the various subtopics of this rapidly expanding field in sections 1-1X, each of which is organized as a self-contained sub-chapter that can be read independently of the others.

Effect of pinealectomy and of diabetes on liver insulin and glucagon receptor concentrations in the rat

The studies described in this paper were undertaken to characterize the circulating and hepatic insulin and glucagon receptor concentrations of control (C), diabetic (Db), and pinealectomized-diabetic (Pn + Db) rats. Compared with C rats, an increase in plasma glucose and glucagon levels and a reduction in circulating insulin concentrations in Db animals was observed; these differences were greater in Pn + Db rats. In liver membranes, insulin binding was lower in Db and in Pn + Db than in C rats, and glucagon binding was greater in Db and in Pn + Db than in C rats. The modifications in hormone binding did not reflect changes in the affinity constants. The time courses of hormone association and dissociation from liver membranes were similar in all three experimental groups. The degradation of both hormones and their receptors was similar in all three groups. These findings indicate that in either pinealectomized-diabetic or diabetic rats there were significant changes in the circulating and liver insulin and glucagon receptor concentrations and that the changes in the circulating levels of both pancreatic hormones were more pronounced in pinealectomized-diabetic animals. In addition, the absence in Db and in Pn + Db rats of the insulin and glucagon down-regulation of their own receptors could further modify metabolic activities in these animals.

Effect of pinealectomy on liver insulin and glucagon receptor concentrations in the rat

The studies described here were undertaken to characterize the hepatic insulin and glucagon receptors of control (C), pinealectomized (Pn), and melatonin-treated pinealectomized (Pn + Mel) rats. Compared with C rats, an increase in plasma glucose and glucagon levels and a reduction in circulating concentrations of insulin in Pn animals were observed. Melatonin treatment of Pn rats reverses all three parameters toward the normal values. In liver membranes, insulin binding was lower in Pn than in C rats, and glucagon binding was greater in Pn than in C animals; in Pn + Mel rats both insulin and glucagon binding reverse toward the normal values that were observed in C rats. The modifications in hormone binding reflect changes in the number of receptors but not in the affinity constants. The time courses of hormone association and dissociation from liver membranes were similar in all three experimental groups. The degradation of both hormones by liver membranes was similar in all three groups. Insulin receptor degradation also was similar in the three groups, while glucagon receptor degradation was similar in the liver membranes of C and Pn rats but smaller in Pn + Mel animals. These findings suggest that the pineal gland may modulate the circulating levels and liver receptor concentrations of insulin and glucagon. In addition, our results indicate that insulin and glucagon did not induce a down-regulation of liver receptors in Pn rats.

Temperature effects on alpha 2-adrenergic receptor-Gi interactions

The effect of temperature on the binding of alpha 2-adrenergic agonists and antagonists to human platelet membranes was studied. Equilibrium binding of the alpha 2 antagonist, [3H]yohimbine, was affected minimally, whereas the rate of dissociation changed 40-fold over a temperature range of 5-35 degrees. The antagonist dissociation rates were characterized by a linear Arrhenius plot and an activation energy of 20.5 kcal/mol. The equilibrium binding of the full alpha 2 agonist, [3H]UK 14,304 [5-bromo-6-N-2-4,5-dihydroimidazolyl)quinoxaline tartrate] showed a 50% decrease in Bmax at 5 degrees as well as a 2-fold decrease in affinity. The kinetics of [3H]UK 14,304 binding were affected more significantly by decreases in temperature. The agonist exhibited fast and slow phases of binding. The fast binding was minimally sensitive to temperature in the range of 0-30 degrees with only a 6-fold change in rate. The slow binding rates changed nearly 100-fold over the same temperature range. Also, the slow rate of agonist binding was characterized by a nonlinear Arrhenius plot with a "break" at approximately 17 degrees, which was found previously to be the phase transition temperature of platelet membrane lipids [Lohse et al., Molec. Pharmac. 29, 228 (1986)]. Despite the reduction of high affinity [3H]UK 14,304 binding at 5 degrees, approximately half of the binding remained sensitive to guanine nucleotides. The data are interpreted in the context of a model in which the fast agonist binding represents a bimolecular interaction of ligand with two pre-existing conformations of the alpha 2 receptor, one coupled to Gi and the other permanently uncoupled. The slow binding of agonist appears to require protein diffusion in the lipid membrane or a protein conformational change which is dependent on the lipid environment.

Guanine nucleotide regulation of the interconversion of the two-state hepatic glucagon receptor system of rat

To investigate whether guanine nucleotides regulate interconversion of the two-state hepatic glucagon receptor we have utilized kinetic assays of glucagon binding to partially purified rat liver plasma membranes. Dissociation of glucagon at 30 degrees C exhibited biexponential character in either the absence or presence of GTP, indicating that the system previously seen in intact hepatocytes is independent of intracellular modulators. In each case the receptors underwent a time-dependent conversion from a low affinity to a high affinity state. However, GTP decreased the fraction of receptors in the high affinity state. The rank order for stabilizing the low affinity state was Gpp(NH)p greater than GTP greater than GDP much greater than GMP = no nucleotides. Data from competition binding assays with increasing concentrations of GTP allow calculation of equilibrium constants which are 3.32 nM for glucagon and receptor in the absence of GTP, 18.6 nM for glucagon and receptor in the presence of GTP, 1.55 microM for the association of receptor and GTP presumably linked to an N protein, and 8.86 microM for the association of the glucagon-receptor complex and GTP again presumably linked to an N protein, Glucagon binding to receptor is noncooperative in both the absence and presence of GTP, distinguishing this system from the beta-adrenergic system. With GTP, binding to the low affinity state is favored because of the relative affinities reported. Therefore, GTP regulates the activation by slowing the conversion of the receptor from a low affinity to high affinity form.

Receptor binding of selectively labeled (Tyr-10) and (Tyr-13)-mono-125I-glucagons and competition by homologous 127I-labeled isomers

Two monoiodinated derivatives of glucagon were prepared by lactoperoxidase catalyzed iodination followed by separation on reverse-phase high-performance liquid chromatography. The purified (Tyr-10) and (Tyr-13)-mono-125I-labeled glucagon isomers were characterized and studied with respect to their binding to the receptors of isolated intact rat hepatocytes. The extent of steady-state binding to cellular receptor sites differed for the two labeled glucagon tracers at 37 degrees C as well as at 15 degrees C with (Tyr-10)-mono-125I-glucagon displaying higher receptor binding. The apparent equilibrium constants, Kd,app at 37 degrees C are 3.6 +/- 0.4 nM (mean +/- S.E. of three independent experiments) for the tyrosine-13-labeled tracer and 5.9 +/- 0.6 nM for the tyrosine-10-labeled glucagon with native glucagon as competitor. Since the observed Kd in the competition assay is a function of the true Kd values of the monoiodinated radioactive glucagon isomers and native glucagon, the dissociation constants were also measured with chemically identical tracer and competitor. Under these conditions, we obtained Kd values of 1.3 +/- 0.2 nM for the tyrosine-10-labeled analog and 2.0 +/- 0.2 nM for the tyrosine-13-labeled glucagon isomers confirming the higher receptor binding affinity of (Try-10)-mono-125I-glucagon. All competition curves fit the mathematical expression for a model of non-cooperative binding to a single class of receptors.

Stimulation of sulfated-proteoglycan synthesis by forskolin in monolayer cultures of rabbit articular chondrocytes

Forskolin, a plant cardiotonic diterpene, stimulated proteoglycan biosynthesis by chondrocytes in monolayer culture. The quantitative increase in proteoglycans was dependent on the concentration of forskolin, but was relatively independent of the presence of serum. At forskolin concentrations that stimulated proteoglycan synthesis, a significant stimulation of adenylate cyclase and cAMP was also measured. The quantitative increase in proteoglycans was characterized, qualitatively, by an increased deposition of newly synthesized proteoglycan in the cell-associated fraction. An analysis of the most dense proteoglycans (fraction dA1) in the cell-associated fraction showed that more of the proteoglycans eluted in the void volume of a Sepharose CL-2B column, indicating that an increased amount of proteoglycan aggregate was synthesized in forskolin-treated cultures. The proteoglycan monomer dA1D1 secreted into the culture medium of forskolin-stimulated cultures overlapped in hydrodynamic size with that of control cultures, although cultures stimulated with forskolin and phosphodiesterase inhibitors produced even larger proteoglycans. The hydrodynamic size of 35SO4 and 3H-glucosamine-labelled glycosaminoglycans isolated from the dA1D1 fraction of the culture medium was greater in forskolin-treated chondrocytes, especially from those in which phosphodiesterase inhibitors had been added. These results indicated that forskolin, a direct activator of chondrocyte adenylate cyclase mimicked the effects of cAMP analogues on chondrocyte proteoglycan synthesis previously reported. These results implicate activation of adenylate cyclase as a regulatory event in the biosynthesis of cartilage proteoglycans, and more specifically in the production of hydrodynamically larger glycosaminoglycans.

A comparative study of adenylate cyclase activity and progesterone synthesis in ovine corpora lutea stimulated by different chemical derivatives or natural analogs of ovine LH

The adenylate cyclase activation by ovine native LH, natural analogs (porcine LH, hCG) and chemical derivatives of LH (methylated, ethylated, isopropylated, guanidinated) was studied in purified plasma membranes of ovine corpora lutea, including the regulatory effects of guanyl 5'-yl imidophosphate (Gpp(NH)p) and Mg2+. The Ka app. for native LH (about 15 nM) was independent of Gpp(NH)p and Mg2+. Similar maximal activation of the enzyme was obtained by using ovine LH or natural analogs, but differences were remarked concerning the Ka app. values of these hormones. Porcine LH was equipotent with ovine LH; on the contrary, hCG exhibited a lower Ka app. value (3 nM). All chemical derivatives (Me-LH, Et-LH, Iso-LH and Gu-LH) exhibited Ka app. higher than native (about 2- to 4-fold), but similar maximal activation. No modification was observed in the regulatory effects of Gpp(NH)p and Mg2+ on the adenylate cyclase activation as a consequence of structural modifications of the hormone. A comparison of the steroidogenic activity on intact luteal cells and the adenylate cyclase activation ability on purified plasma membranes of the derivatives mentioned above evidenced some interesting discrepancies. The drop in adenylate cyclase activation potency of Me-LH was not reflected in its steroidogenic activity (Me-LH was equipotent with native LH); on the contrary, the capacity of Gu-LH to stimulate adenylate cyclase was not so much decreased as was its steroidogenic potency which was almost abolished.

Regulation of calcium currents by a GTP analogue: potentiation of (-)-baclofen-mediated inhibition

Voltage-activated calcium currents were recorded in cultured rat dorsal root ganglion neurones using the whole cell clamp technique. The effect of inclusion in the patch pipette of the non-hydrolysable guanine nucleotide analogues, guanosine 5'-O-3-thiotriphosphate (GTP-gamma-S) and guanosine 5'-O-2-thiodiphosphate (GDP-beta-S) was examined both on ICa itself and on the ability of the GABAB agonist baclofen to inhibit ICa. In the absence of either guanine nucleotide, the maximum ICa recorded at between -20 and 0 mV consists of two components, a rapid transient and a non- or slowly inactivating current. Baclofen (50 microM) inhibited peak ICa by 37.5 +/- 4.7%. In the presence of 500 microM GTP-gamma-S the inactivating calcium current was largely absent and the effect of 50 microM baclofen was increased to a 78.1 +/- 2.3% inhibition. GDP-beta-S had opposite effects to GTP-gamma-S. The results suggest that the calcium channels in these neurones are associated with a GTP binding protein which regulates the calcium currents and mediates the inhibitory effect of neurotransmitters.

The stimulation by calcium and its inhibition by ADP of cholesterol side-chain cleavage activity in adrenal mitochondria

Cholesterol side-chain cleavage activity (cholesterol SCC) in a mitochondrial preparation is increased by calcium in a sigmoidal manner. A 5-10-fold increase is obtained and an effect may be seen at 20 microM CaCl2. ADP inhibits the stimulation by calcium with a shift of the sigmoid curve to the right and 10 microM ADP results in a 4-fold increase in the amount of CaCl2 required to obtain one-half the maximal stimulation value. The inhibition is specific for ADP and inhibition by ATP is due to the formation of ADP. The characteristics of the calcium-ADP modulation are such that a suitable ADP-inhibited level of cholesterol sidechain cleavage activity will be stimulated by a given increment of calcium to a greater extent than in the absence of the added ADP. Steroid 11 beta-hydroxylation is also stimulated in a sigmoidal manner by calcium and this stimulation is inhibited by ADP. The 11 beta-hydroxylation, however, is less sensitive to calcium and ADP so that changes in cholesterol SCC are obtained at concentration of calcium and ADP where minimal effects on 11 beta-hydroxylation are found. Calmodulin-like activity is present in the mitochondrial preparation. No evidence, however, for a role for calmodulin in the calcium-ADP effects could be obtained, but the possibility of its involvement cannot be excluded. The calcium-ADP modulations are of a magnitude and take place at sufficiently low concentrations to suggest a physiological role in the regulation of mitochondrial steroidogenesis.

Pharmacological investigation of ritodrine hydrochloride, a beta 2-adrenoceptor stimulant

Ritodrine hydrochloride (ritodrine) has been effectively prescribed for the prevention of premature labor. The present study was carried out to investigate the mode of action of ritodrine on the uterus and heart in comparison with those of isoxsuprine and isoproterenol. 1) Ritodrine (10(-8)-10(-6) M) suppressed the spontaneous motility of pregnant rat uterus and showed positive chronotropic action at the doses of 10(-6)-10(-4) M in guinea-pig atria. 2) In the Ca2+-free, K+-rich Tyrode solution, ritodrine suppressed the Ca2+ induced contracture of pregnant rat uterus, while it potentiated the carbachol induced contraction. 3) Ritodrine increased the amount of cyclic AMP in the uterus but not in heart. This action of ritodrine was suppressed by pretreatment with propranolol (10(-6) M). 4) These results suggest that ritodrine causes actions through activation of cyclic AMP production, as in the case of isoproterenol, and it acts more selectively on beta 2-adrenoceptors than on beta 1-adrenoceptors.

Characterization of forskolin binding sites in rat brain membranes using [14,15-3H]14,15-dihydroforskolin as a ligand

[14,15-3H]14,15-Dihydroforskolin [( 3H]DHF) has been used as a radioactive ligand to identify forskolin binding sites in rat brain membranes. The binding was saturable and reversible. The binding sites showed positive cooperative properties as evident from an upward convex Scatchard plot and a Hill coefficient of 1.6. The equilibrium dissociation constants (KD) were in the range between 10 microM and 10 nM as estimated from the limiting slopes of the curved Scatchard plot. Half-maximal saturation of the binding sites was observed at a ligand concentration of 225 nM. The binding kinetics were very rapid: Binding equilibrium was reached in less than 2 min and a large excess of cold forskolin displaced 80% of the radioligand within 2 min. The dissociation reaction was not first order, characterized by a decreasing dissociation rate constant. Bound [3H]DHF could be displaced with forskolin (IC50 0.3 microM), 14,15-dihydroforskolin (IC50 0.8 microM) and 7-desacetylforskolin (IC50 3 microM). However, nucleotides (ATP, GTP) and other receptor ligands (adenosine, isoproterenol) had no effect on the binding. Although the density of the forskolin binding sites (3.2 pmole/mg protein) is similar to those of other adenylate cyclase linked receptors, discrepancies between the KD and the ED50 obtained in adenylate cyclase studies and the finding that activation of the enzyme by forskolin is negative cooperative makes it difficult to clearly relate the binding sites to adenylate cyclase.

Catecholamine and divalent cation effects on frog liver adenylate cyclase

Frog liver adenylate cyclase was characterized with respect to divalent cation interaction and hormonally stimulated activities. The enzyme catalyzed the synthesis of cyclic [32P]3',5'-AMP from alpha-32P-labeled ATP. The activity of the enzyme was linear with time and protein concentration. The Km for ATP was 0.5 mM, in the presence or absence of stimulators. The temperature optimum was 25 degrees. GTP (10(-4) M) increased the stimulation of adenylate cyclase by epinephrine. Similar activities were obtained using 5 mM Mg2+ or Mn2+. At higher concentrations, both ions inhibited epinephrine-stimulated, but not basal or fluoride-stimulated activities. Approximately equivalent hormonal stimulation was obtained with maximal stimulating concentrations of epinephrine, isoproterenol, glucagon, and prostaglandin E1. Norepinephrine was less stimulatory. Only catecholamine-stimulated activities were inhibited by propranolol (10(-5) M). The data suggest that catecholamines stimulate frog liver adenylate cyclase through interactions with beta adrenergic receptors. The adenylate cyclase in frog liver differs from its mammalian counterpart in its response to temperature and maximally stimulatory concentrations of hormones.

Noncooperative receptor interactions of glucagon and eleven analogues: inhibition of adenylate cyclase

Glucagon and 11 glucagon derivatives were characterized and compared with respect to the cooperativity of their receptor interactions and their ability to elicit a biphasic (activation-inhibition) response from the adenylate cyclase system of rat liver plasma membranes. Slope factors were evaluated from two sets of experimental data, binding to hepatocyte receptors and activation of adenylate cyclase. The results are consistent with noncooperative binding to a single affinity state of the glucagon receptor for all derivatives, irrespective of the modification and the agonist properties of the derivatives. High-dose inhibition of adenylate cyclase activity was observed for native glucagon and all of the derivatives which were examined at high concentrations (greater than 10(-5) M). Partial agonism of some low-affinity glucagon derivatives is not caused by high-dose inhibition. Several mechanisms which might give rise to high-dose inhibition such as receptor cross-linking or multivalent receptor binding are discussed in relationship to the glucagon-receptor interaction. These phenomena indicate that significant differences exist between the glucagon system and the beta-adrenergic system.

Characteristics of the calcium-mediated mechanism activating adenylate cyclase in Trypanosoma brucei

1. The adenylate cyclase of bloodstream forms of Trypanosoma brucei is regulated by Ca2+. 2. The Ca2+-stimulated activity is not inhibited by trifluperazine, tentatively suggesting that the stimulation may not be mediated by calmodulin. 3. The Ca2+-binding site of the receptor regulating the adenylate cyclase is buried within the hydrophobic region of the plasma membrane and is not exposed to either the inner or the outer aqueous-phase bounding the membrane in unstimulated cells. 4. The response of the adenylate cyclase to stimulation by Ca2+ within the membrane is a transient increase in activity followed by a period of 'down regulation'. 5. The 'down regulation' of the adenylate cyclase also occurs by a Ca2+-dependent mechanism.

Regulatory states of adenylate cyclase in RL-PR-C cloned rat hepatocytes

The adenylate cyclase of cloned differentiated rat hepatocytes (RL-PR-C) is regulated by cholera toxin, guanine nucleotides and fluoride. The activation of hepatic adenylate cyclase by cholera toxin was additive with that by GTP and synergistic with that by epinephrine. In contrast, when membranes were exposed to cholera toxin in the presence of Gpp(NH)p or fluoride, the response was the same as to these agents in the absence of cholera toxin. Cholera toxin-activated membranes were responsiveness only to epinephrine and GTP, while fluoride-activated membranes responded somewhat to all other agents, and Gpp(NH)p-activated membranes responded to no other agents. These data suggest that responsiveness of hepatic adenylate cyclase to cholera toxin, fluoride and Gpp(NH)p cannot be expressed simultaneously. A model is presented to explain these observations which invokes multiple states of adenylate cyclase, each being sensitive to, or brought about by, a different regulatory agent.