Kinetics of extracellular ATP in mastoparan 7-activated human erythrocytes

Background

The peptide mastoparan 7 (MST7) stimulated ATP release in human erythrocytes. We explored intra- and extracellular processes governing the time-dependent accumulation of extracellular ATP (i.e., ATPe kinetics).

Methods

Human erythrocytes were treated with MST7 in the presence or absence of two blockers of pannexin 1. ATPe concentration was monitored by luciferin–luciferase based real-time luminometry.

Results

Exposure of human erythrocytes to MST7 led to an acute increase in [ATPe], followed by a slower increase phase. ATPe kinetics reflected a strong activation of ATP efflux and a low rate of ATPe hydrolysis by ectoATPase activity. Enhancement of [ATPe] by MST7 required adhesion of erythrocytes to poly-D-lysin-coated coverslips, and correlated with a 31% increase of cAMP and 10% cell swelling. However, when MST7 was dissolved in a hyperosmotic medium to block cell swelling, ATPe accumulation was inhibited by 49%.Erythrocytes pre-exposure to 10 μM of either carbenoxolone or probenecid, two blockers of pannexin 1, exhibited a partial reduction of ATP efflux.Erythrocytes from pannexin 1 knockout mice exhibited similar ATPe kinetics as those of wild type mice erythrocytes exposed to pannexin 1 blockers.

Conclusions

MST7 induced release of ATP required either cell adhesion or strong activation of cAMP synthesis. Part of this release required cell swelling. Kinetic analysis and a data driven model suggested that ATP efflux is mediated by two ATP conduits displaying different kinetics, with one conduit being fully blocked by pannexin 1 blockers.

General significance

Kinetic analysis of extracellular ATP accumulation from human erythrocytes and potential effects on microcirculation.     

Altering the receptor-effector ratio by transgenic overexpression of type V adenylyl cyclase: enhanced basal catalytic activity and function without increased cardiomyocyte beta-adrenergic signalling

The limiting element in beta-adrenergic receptor (betaAR)-G(s)-adenylyl cyclase (AC) signal transduction in the cardiomyocyte is not known, but it has been proposed that the level of adenylyl cyclase expression constrains betaAR signaling. To alter the above equilibrium, type V AC was overexpressed in a myocyte-specific manner in the hearts of transgenic mice using the alpha-myosin heavy chain promoter. Expression of type V AC was approximately 75% over endogenous levels as quantitated by [(3)H]forskolin binding. Functional activity of the transgene product was evident in cardiac membrane AC studies, where basal (45 +/- 11 vs 19 +/- 5 pmol min(-)(1) mg(-)(1)) and forskolin+Mn(2+) (695 +/- 104 vs 386 +/- 34 pmol min(-)(1) mg(-)(1)) stimulated activities were increased compared to activities in nontransgenic (NTG) littermates. However, while isoproterenol stimulated activities were higher (74 +/- 12 vs 46 +/- 9.8 pmol min(-)(1) mg(-)(1)), the fold stimulation over basal was not increased in ACV overexpressors compared to NTG (line 14.3 = 2.29 +/- 0.44-fold, line 15.1 = 1.70 +/- 0.1-fold, NTG = 2.62 +/- 0.18-fold). Similarly, in whole cell patch-clamp studies, betaAR-mediated opening of L-type Ca(2+) channels was not found to be enhanced in transgenic ACV myocytes (225 +/- 15 vs 216 +/- 10% of basal currents). Basal and isoproterenol stimulated PKA activities were elevated in the ACV mice compared to NTG, but again the extent of stimulation over basal was not enhanced. Phosphorylated phospholamban was approximately 2-fold greater in myocytes from ACV hearts compared to NTG, indicating that distal elements of the contractile cascade are activated by AC overexpression. ACV mice displayed increased heart rates and fractional shortening as assessed by echocardiography. However, in vivo hemodynamic studies revealed that heart rate and contractility responses to agonist infusion were not enhanced in ACV mice compared to NTG. We conclude that at native stoichiometries, the levels of adenylyl cyclase influence basal activities and cardiac function, but do not constrain betaAR signaling in the cardiomyocyte.     

Regulation of Extracellular ATP in Human Erythrocytes Infected with Plasmodium falciparum

In human erythrocytes (h-RBCs) various stimuli induce increases in [cAMP] that trigger ATP release. The resulting pattern of extracellular ATP accumulation (ATPe kinetics) depends on both ATP release and ATPe degradation by ectoATPase activity. In this study we evaluated ATPe kinetics from primary cultures of h-RBCs infected with P. falciparum at various stages of infection (ring, trophozoite and schizont stages). A “3V” mixture containing isoproterenol (β-adrenergic agonist), forskolin (adenylate kinase activator) and papaverine (phosphodiesterase inhibitor) was used to induce cAMP-dependent ATP release. ATPe kinetics of r-RBCs (ring-infected RBCs), t-RBCs (trophozoite-infected RBCs) and s-RBCs (schizont-infected RBCs) showed [ATPe] to peak acutely to a maximum value followed by a slower time dependent decrease. In all intraerythrocytic stages, values of ΔATP₁ (difference between [ATPe] measured 1 min post-stimulus and basal [ATPe]) increased nonlinearly with parasitemia (from 2 to 12.5%). Under 3V exposure, t-RBCs at parasitemia 94% (t94-RBCs) showed 3.8-fold higher ΔATP₁ values than in h-RBCs, indicative of upregulated ATP release. Pre-exposure to either 100 µM carbenoxolone, 100 nM mefloquine or 100 µM NPPB reduced ΔATP₁ to 83–87% for h-RBCs and 63–74% for t94-RBCs. EctoATPase activity, assayed at both low nM concentrations (300–900 nM) and 500 µM exogenous ATPe concentrations increased approx. 400-fold in t94-RBCs, as compared to h-RBCs, while intracellular ATP concentrations of t94-RBCs were 65% that of h-RBCs. In t94-RBCs, production of nitric oxide (NO) was approx. 7-fold higher than in h-RBCs, and was partially inhibited by L-NAME pre-treatment. In media with L-NAME, ΔATP₁ values were 2.7-times higher in h-RBCs and 4.2-times higher in t94-RBCs, than without L-NAME. Results suggest that P. falciparum infection of h-RBCs strongly activates ATP release via Pannexin 1 in these cells. Several processes partially counteracted ATPe accumulation: an upregulated ATPe degradation, an enhanced NO production, and a decreased intracellular ATP concentration.     

Enrichment of the poly (A) sequence and lack of enhancement of total RNA synthesis in cultured Xenopus hepatocytes by estradiol-17 beta

The effect of estradiol-17 beta on RNA synthesis and the amounts of total RNA and polyadenylic acid were determined in primary cultures of Xenopus laevis liver parenchymal cells. Results showed that estradiol did not alter the RNA content significantly; control cells contained 11.9 +/- 0.34 micrograms and estradiol-treated cells 12.4 +/- 0.17 micrograms per 10(6) cells on day 2 of estradiol treatment, and 22.0 +/- 0.61 micrograms and 24.0 +/- 1.09 micrograms on day 5. Hybridization with [3H]poly(U) revealed that estradiol increased the poly(A) content about 1.2-fold more than in the controls on day 2 and 1.6-fold on day 5 of estradiol treatment. The actual rate of RNA synthesis was estimated from analyses of the kinetics of [3H]adenosine incorporation into the ATP pool and into RNA. The initial rate of incorporation of ATP into RNA on day 5 of estradiol treatment was 29.38 pmol/min/10(6) cells and the rate of the controls of 29.35. Subsequent accumulation kinetics of [3H]adenosine into RNA showed no difference between estradiol and the control cells. Thus, estradiol did not alter the rate of total RNA synthesis and the total RNA content significantly, but it did increase the poly(A) content.     

A radiometric method for the determination of NADH in subpicomole amounts

A radiometric method has been devised for the determination of small quantities of NADH formed in preceding dehydrogenase reactions. In a coupled enzymatic reaction, phosphoglycerate kinase (PGK) catalyzes the transfer of [32P]orthophosphate from [gamma-32P]ATP to 3-phosphoglycerate; the intermediate, 1,3-[1-32P]diphosphoglycerate, is dephosphorylated by glyceraldehyde-3-phosphate dehydrogenase (GAP-DH). [32P]Orthophosphate is released proportionally to NADH and can be measured after adsorption of [gamma-32P]ATP to activated charcoal. With this method, 0.2 pmol of NADH are detectable in the presence of a 10(4)-fold excess of NAD over NADH.     

Arg-vasopressin increases proliferation of human osteoblast-like cells and decreases production of interleukin-6 and macrophage colony-stimulating factor

Patients with arginine-vasopressin (AVP) deficiency have been reported to have a decreased bone mass. The mechanism behind this is not known. In this study, the effects of AVP on primary human osteoblast-like (hOB) cells and SaOS-2 cells were investigated. Cell proliferation was measured by [3H]thymidine incorporation or a commercially available kit (EZ4U), and protein synthesis by [3H]proline incorporation. In addition, the production of interleukin-6 (IL-6) and macrophage colony-stimulating factor (M-CSF) in hOB cells was determined. AVP at 10-100 pmol/l increased cell proliferation in hOB and SaOS-2 cells (p < 0.05). Protein synthesis increased in SaOS-2 cells incubated with 10-100 pmol/l AVP (p < 0.01). When hOB and SaOS-2 cells were incubated with AVP together with a vasopressin receptor-1 (V1)-antagonist ([beta-Mercapto-beta,beta-cyclopenta-methylenepropionyl1,O-Me-Tyr2,Arg8]-vasopressin) or a protein kinase C (PKC)-inhibitor (chelerythrine) the increase in cell proliferation in response to AVP was abolished. The production of IL-6 and M-CSF was decreased in hOB-cells incubated with 10 pmol/l AVP (p < 0.01). In addition, by RT-PCR, we found evidence for expression of mRNA for the vasopressin 1a (V1a)-receptor in hOB cells. In conclusion, AVP stimulated proliferation of hOB- and SaOS-2 cells. We suggest that the effect was mediated through the V1-receptor. Additionally, AVP decreased production of IL-6 and M-CSF from the hOB cells. Moreover, the V1a-receptor seems to be expressed in hOB cells.     

Pannexin1 is part of the pore forming unit of the P2X(7) receptor death complex

The purinergic receptor P2X(7) is part of a complex signaling mechanism participating in a variety of physiological and pathological processes. Depending on the activation scheme, P2X(7) receptors in vivo are non-selective cation channels or form large pores that can mediate apoptotic cell death. Expression of P2X(7)R in Xenopus oocytes results exclusively in formation of a non-selective cation channel. However, here we show that co-expression of P2X(7)R with pannexin1 in oocytes leads to the complex response seen in many mammalian cells, including cell death with prolonged ATP application. While the cation channel activity is resistant to carbenoxolone treatment, this gap junction and hemichannel blocking drug suppressed the currents induced by ATP in pannexin1/P2X(7)R co-expressing cells. Thus, pannexin1 appears to be the molecular substrate for the permeabilization pore (or death receptor channel) recruited into the P2X(7)R signaling complex.     

Detection of local ATP release from activated platelets using cell surface-attached firefly luciferase

We have developed a method for measuring the local concentrationof ATP at the extracellular surface of live cells. This method relieson the specific attachment to the cell surface of a chimeric proteinthat consists of the IgG-binding domain ofStaphylococcus aureus protein A fusedin-frame with the complete sequence for firefly luciferase (proA-luc).Expression of proA-luc in Escherichia coli and its one-step affinity purification arestraightforward. Attachment to cells is demonstrated to be specific andantibody dependent using several suspended and adherent cell types.Light production by cell surface-attached luciferase is continuous, linearly related to ATP concentration, and sufficient to provide nanomolar sensitivity. The spatial resolution of this method enables the observation of strictly local changes in extracellular ATP duringits secretion from activated platelets. Furthermore, the activity ofcell-attached luciferase is relatively refractory to the inclusion ofnucleotidases in the medium, arguing for its effectiveness in cellsystems possessing potent ecto-ATPases.

     

Adrenal adenylate cyclase and steroidogenic activities of 63 day old ovine fetuses: in vitro effects of ACTH1-24 and forskolin

This study examines the activity of the adenylate cyclase system and that of some enzymes of the steroidogenic pathway of adrenal cells from 62-63 day old ovine fetuses. Synthetic corticotropin (ACTH1-24), cholera toxin and forskolin stimulated both cAMP and corticoid productions by freshly isolated adrenal cells. The cAMP response to ACTH1-24 was lower than that to forskolin. However, forskolin-induced steroidogenesis was significantly lower than the ACTH1-24-induced steroid output. Freshly isolated cells metabolized quickly [14C]-labeled pregnenolone mainly through the 17-deoxy pathway. The amounts of cortisol and of corticosterone formed, in the presence of exogenous pregnenolone, were roughly 15-fold higher than under maximal stimulation by ACTH1-24. When the cells were cultured for 6 days in the absence or presence of ACTH1-24 (10(-8) M) or forskolin (10(-5) M), a small development of the cAMP response to these factors was observed in the course of the experiment. However, the mechanism of this development appeared different, according to the conditions of culture. The amounts of corticosterone secreted on day 6 by ACTH1-24- or forskolin-treated cells were 2- to 4-fold higher than on day 1, whereas cortisol outputs were much lower on day 6 than on day 1. The response to ACTH1-24 of cells maintained in ACTH-free media decreased dramatically during the culture in terms of both cortisol and of corticosterone. On day 6 of the experiment, the metabolism of [14C]pregnenolone was lower than on day 1 under all 3 conditions of culture. Only the 3 beta-hydroxysteroid dehydrogenase/isomerase activity could be maintained by continuous treatment with forskolin. However, both ACTH1-24 and forskolin enhanced the production of pregnenolone from an endogenous substrate. In conclusion, these results present evidence that: 1) the adenylate cyclase system is not a bottleneck in the steroidogenic response to ACTH1-24 of freshly isolated adrenal cells from 62-63 day old ovine fetuses; 2) the main rate-limiting step for steroidogenesis by these cells is the availability of pregnenolone; 3) neither ACTH1-24 nor forskolin is able to maintain the activity of most enzymes involved in the metabolization of pregnenolone by cultured cells while increasing pregnenolone availability; 4) some inhibiting factors are involved in the loss of adrenal cells responsiveness to ACTH between days 50 and 100 of gestation, and they probably act mainly on the adenylate cyclase system.     

Evidence for a single forskolin-binding site in rat adipocyte membrane. Studies of [14,15-3H]dihydroforskolin binding and adenylate cyclase activation

[14,15-3H] Dihydroforskolin has been used as a tracer in the study of forskolin binding to adipocyte plasma membrane and the subsequent activation of adenylate cyclase (EC 4.6.1.1) of this membrane. The specific binding of radioactivity to the membrane was rapid, temperature-dependent, saturable, and readily reversible. The equilibrium dissociation reaction constant (KD) for the binding was 13 microM, with a maximum binding (Bmax) of 61 pmol of forskolin per mg of membrane protein. The Hill coefficient was 1.0. The bound [14,15-3H] dihydroforskolin was displaced by forskolin with rate constants of 0.07 X 10(6) M-1 min-1 and 1.2 min-1 for the association and dissociation reactions, respectively (30 degrees C). The equilibrium dissociation constant (KD) was approximately the same as the concentration that produced half-maximum activation (EC50) of the adenylate cyclase of rat adipocyte plasma membrane. There was a linear correlation between forskolin binding and adenylate cyclase activation. The results are consistent with the concept of a single class of binding site which binds forskolin. [14,15-3H] Dihydroforskolin appears to be a potentially useful tracer in the study of the mechanism of activation of the catalytic unit of adipocyte adenylate cyclase.     

Constitutive release of ATP and evidence for major contribution of ecto-nucleotide pyrophosphatase and nucleoside diphosphokinase to extracellular nucleotide concentrations

Nucleotides are important extracellular signaling molecules. At least five mammalian P2Y receptors exist that are specifically activated by ATP, UTP, ADP, or UDP. Although the existence of ectoenzymes that metabolize extracellular nucleotides is well established, the relative flux of ATP and UTP through their extracellular metabolic products remains undefined. Therefore, we have studied the kinetics of accumulation and metabolism of endogenous ATP in the extracellular medium of four different cell lines. ATP concentrations reached a maximum immediately after change of medium and decreased thereafter with a single exponential decay (t(1/2);1 approximately;230-40 min). ATP levels did not fall to zero but attained a base-line concentration that was independent of the medium volume and of the initial ATP concentration. Although the base-line concentration of ATP remained stable for up to 12 h, [gamma-(32)P]ATP added to resting cells as a radiotracer was completely degraded within 120 min, indicating that steady state reflected a basal rate of ATP release balanced by ATP hydrolysis (20-200 fmol x min(-)(1) x cell(-)(6)). High performance liquid chromatography analysis revealed that the gamma-phosphate of ATP was rapidly, although transiently, transferred during steady state to species subsequently identified as UTP and GTP, indicating the existence of both ecto-nucleoside diphosphokinase activity and the accumulation of endogenous UDP and GDP. Conversely, addition of [gamma-(32)P]UTP to resting cells resulted in transient formation of [gamma-(32)P]ATP, indicating phosphorylation of endogenous ADP by nucleoside diphosphokinase. The final (32)P-products of [gamma-(32)P]ATP metabolism were [(32)P]orthophosphoric acid and a (32)P-labeled species that was further purified and identified as [(32)P]inorganic pyrophosphate. In C6 cells, the formation of [(32)P]pyrophosphate from [gamma-(32)P]ATP at steady state exceeded by 3-fold that of [(32)P]orthophosphate. These results illustrate for the first time a constitutive release of ATP and other nucleotides and reveal the existence of a complex extracellular metabolic pathway for released nucleotides. In addition to the existence of an ecto-ATPase activity, our results suggest a major scavenger role of ecto-ATP pyrophosphatase and a transphosphorylating activity of nucleoside diphosphokinase.     

Binding of [3H]forskolin to solubilized preparations of adenylate cyclase

The binding of [3H]forskolin to proteins solubilized from bovine brain membranes was studied by precipitating proteins with polyethylene glycol and separating [3H]forskolin bound to protein from free [3H]forskolin by rapid filtration. The Kd for [3H]forskolin binding to solubilized proteins was 14 nM which was similar to that for [3H]forskolin binding sites in membranes from rat brain and human platelets. Forskolin analogs competed for [3H]forskolin binding sites with the same rank potency in both brain membranes and in proteins solubilized from brain membranes. [3H]forskolin bound to proteins solubilized from membranes with a Bmax of 38 fmol/mg protein which increased to 94 fmol/mg protein when GppNHp was included in the binding assay. In contrast, GppNHp had no effect on [3H]forskolin binding to proteins solubilized from membranes preactivated with GppNHp. Solubilized adenylate cyclase from non-preactivated membranes had a basal activity of 130 pmol/mg/min which was increased 7-fold by GppNHp. In contrast, adenylate cyclase from preactivated membranes had a basal activity of 850 pmol/mg/min which was not stimulated by GppNHp or forskolin. Thus, the number of high affinity binding sites for [3H]forskolin in solubilized preparations correlated with the activation of adenylate cyclase by GppNHp via the guanine nucleotide binding protein (GS).     

Luciferase inactivation in the luminous marine bacterium Vibrio harveyi.

Luciferase was rapidly inactivated in stationary-phase cultures of the wild type of the luminous marine bacterium Vibrio harveyi, but was stable in stationary-phase cultures of mutants of V. harveyi that are nonluminous without exogenous aldehyde, termed the aldehyde-deficient mutants. The inactivation in the wild type was halted by cell lysis and was slowed or stopped by O2 deprivation or by addition of KCN and NaF or of chloramphenicol. If KCN and NaF or chloramphenicol were added to a culture before the onset of luciferase inactivation, then luciferase inactivation did not occur. However, if these inhibitors were added after the onset of luciferase inactivation, then luciferase inactivation continued for about 2 to 3 h before the inactivation process stopped. The onset of luciferase inactivation in early stationary-phase cultures of wild-type cell coincided with a slight drop in the intracellular adenosine 5'-triphosphate (ATP) level from a relatively constant log-phase value of 20 pmol of ATP per microgram of soluble cell protein. Addition of KCN and NaF to a culture shortly after this drop in ATP caused a rapid decrease in the ATP level to about 4 pmol of ATP per microgram whereas chloramphenicol added at this same time caused a transient increase in ATP level to about 25 pmol/microgram. The aldehyde-deficient mutant (M17) showed a relatively constant log-phase ATP level identical with that of the wild-type cells, but rather than decreasing in early stationary phase, the ATP level increased to a value twice that in log-phase cells. We suggest that the inactivation of luciferase is dependent on the synthesis of some factor which is produced during stationary phase and is itself unstable, and whose synthesis is blocked by chloramphenicol or cyanide plus fluoride.     

Regulation of extracellular ATP of human erythrocytes treated with α-hemolysin. Effects of cell volume,morphology, rheology and hemolysis

Alpha-hemolysin (HlyA) of uropathogenic strains of Escherichia coli irreversibly binds to human erythrocytes (RBCs) and triggers activation of ATP release and metabolic changes ultimately leading to hemolysis.We studied the regulation of extracellular ATP (ATPe) of RBCs exposed to HlyA. Luminometry was used to assess ATP release and ATPe hydrolysis, whereas changes in cell volume and morphology were determined by electrical impedance, ektacytometry and aggregometry.Exposure of RBCs to HlyA induced a strong increase of [ATPe] (3–36-fold) and hemolysis (1–44-fold), partially compensated by [ATPe] hydrolysis by ectoATPases and intracellular ATPases released by dead cells. Carbenoxolone, a pannexin 1 inhibitor, partially inhibited ATP release (43–67%).The un-acylated toxin ProHlyA and the deletion analog HlyA∆914-936 were unable to induce ATP release or hemolysis.For HlyA treated RBCs, a data driven mathematical model showed that simultaneous lytic and non-lytic release mainly governed ATPe kinetics, while ATPe hydrolysis became important after prolonged toxin exposure.HlyA induced a 1.5-fold swelling, while blocking this swelling reduced ATP release by 77%. Blocking ATPe activation of purinergic P2X receptors reduced swelling by 60–80%. HlyA-RBCs showed an acute 1.3–2.2-fold increase of Ca²⁺i, increased crenation and externalization of phosphatidylserine. Perfusion of HlyA-RBCs through adhesion platforms showed strong adhesion to activated HMEC cells, followed by rapid detachment. HlyA exposed RBCs exhibited increased sphericity under osmotic stress, reduced elongation under shear stress, and very low aggregation in viscous media.Overall results showed that HlyA-RBCs displayed activated ATP release, high but weak adhesivity, low deformability and aggregability and high sphericity.     

Soluble adenylyl cyclase mediates bicarbonate-dependent corneal endothelial cell protection

Cyclic AMP produced from membrane receptor complex bound adenylyl cyclases is protective in corneal endothelial cells (CEC). CEC also express soluble adenylyl cyclase (sAC), which is localized throughout the cytoplasm. When activated by HCO(3)(-), cAMP concentration ([cAMP]) increases by ～50%. Here we ask if cAMP produced from sAC is also protective. We examined the effects of HCO(3)(-), pH, phosphodiesterase 4 inhibition by rolipram, sAC inhibition by 2HE (2-hydroxyestradiol), and sAC small interfering RNA (siRNA) knockdown on basal and staurosporine-mediated apoptosis. HCO(3)(-) (40 mM) or 50 μM rolipram raised [cAMP] to similar levels and protected endothelial cells by 50% relative to a HCO(3)(-)-free control, whereas 2HE, which decreased [cAMP] by 40%, and H89 (PKA inhibitor) doubled the apoptotic rate. sAC expression was reduced by two-thirds in the absence of HCO(3)(-) and was reduced to 15% of control by sAC siRNA. Protection by HCO(3)(-) was eliminated in siRNA-treated cells. Similarly, caspase-3 activity and cytochrome c release were reduced by HCO(3)(-) and enhanced by 2HE or siRNA. Analysis of percent annexin V+ cells as a function of [cAMP] revealed an inverse, nonlinear relation, suggesting a protective threshold [cAMP] of 10 pmol/mg protein. Relative levels of phosphorylated cAMP response element binding protein and phosphorylated Bcl-2 were decreased in CEC treated with 2HE or siRNA, suggesting that HCO(3)(-)-dependent endogenous sAC activity can mobilize antiapoptotic signal transduction. Overall, our data suggest a new role for sAC in endogenous cellular protection.     

Evidence that forskolin binds to the glucose transporter of human erythrocytes

Binding of [4-3H]cytochalasin B and [12-3H]forskolin to human erythrocyte membranes was measured by a centrifugation method. Glucose-displaceable binding of cytochalasin B was saturable, with KD = 0.11 microM, and maximum binding approximately 550 pmol/mg of protein. Forskolin inhibited the glucose-displaceable binding of cytochalasin B in an apparently competitive manner, with K1 = 3 microM. Glucose-displaceable binding of [12-3H]forskolin was also saturable, with KD = 2.6 microM and maximum binding approximately equal to 400 pmol/mg of protein. The following compounds inhibited binding of [12-3H]forskolin and [4-3H]cytochalasin B equivalently, with relative potencies parallel to their reported affinities for the glucose transport system: cytochalasins A and D, dihydrocytochalasin B, L-rhamnose, L-glucose, D-galactose, D-mannose, D-glucose, 2-deoxy-D-glucose, 3-O-methyl-D-glucose, phloretin, and phlorizin. A water-soluble derivative of forskolin, 7-hemisuccinyl-7-desacetylforskolin, displaced equivalent amounts of [4-3H]cytochalasin B or [12-3H]forskolin. Rabbit erythrocyte membranes, which are deficient in glucose transporter, did not bind either [4-3H]cytochalasin B or [12-3H]forskolin in a glucose-displaceable manner. These results indicate that forskolin, in concentrations routinely employed for stimulation of adenylate cyclase, binds to the glucose transporter. Endogenous ligands with similar specificities could be important modulators of cellular metabolism.     

Atrial natriuretic peptide receptors and activation of guanylate cyclase in rat cardiac sarcolemma

Two classes of atrial natriuretic peptide (ANP) receptors are present in purified sarcolemmal membrane fractions isolated from rat ventricle. Scatchard analysis using [125I]-ANP reveals high affinity (Kd approximately 10(-11) M) and low affinity (Kd approximately 10(-9) M) binding sites. Basal guanylate cyclase activities associated with these membrane fractions range from 3.2 +/- 1.3 pmol/min/mg protein in the presence of Mg2+ to 129 +/- 17 pmol/min/mg protein in the presence of Mn2+. Millimolar concentrations of adenosine triphosphate (ATP) potentiates Mg2+- but not Mn2+-supported activity. Binding of ANP to the low affinity site but not the high affinity site results in a maximum 2-fold activation of Mn2+- and up to 6-fold activation of Mg2+/ATP supported guanylate cyclase activities.     

Type-specific regulation of adenylyl cyclase. Selective pharmacological stimulation and inhibition of adenylyl cyclase isoforms

Crystallographic studies have elucidated the binding mechanism of forskolin and P-site inhibitors to adenylyl cyclase. Accordingly, computer-assisted drug design has enabled us to identify isoform-selective regulators of adenylyl cyclase. After examining more than 200 newly synthesized derivatives of forskolin, we found that the modification at the positions of C6 and C7, in general, enhances isoform selectivity. The 6-(3-dimethylaminopropionyl) modification led to an enhanced selectivity for type V, whereas 6-[N-(2-isothiocyanatoethyl) aminocarbonyl] and 6-(4-acrylbutyryl) modification led to an enhanced selectivity for type II. In contrast, 2'-deoxyadenosine 3'-monophosphate, a classical and 3'-phosphate-substituted P-site inhibitor, demonstrated a 27-fold selectivity for inhibiting type V relative to type II, whereas 9-(tetrahydro-2-furyl) adenine, a ribose-substituted P-site ligand, showed a markedly increased, 130-fold selectivity for inhibiting type V. Consequently, on the basis of the pharmacophore analysis of 9-(tetrahydro-2-furyl) adenine and adenylyl cyclase, a novel non-nucleoside inhibitor, 2-amino-7-(2-furanyl)-7,8-dihydro-5(6H)-quinazolinone (NKY80), was identified after virtual screening of more than 850,000 compounds. NKY80 demonstrated a 210-fold selectivity for inhibiting type V relative to type II. More importantly, the combination of a type III-selective forskolin derivative and 9-(tetrahydro-2-furyl) adenine or NKY80 demonstrated a further enhanced selectivity for type III stimulation over other isoforms. Our data suggest the feasibility of adenylyl cyclase isoform-targeted regulation of cyclic AMP signaling by pharmacological reagents, either alone or in combination.     

Selective transport of adenosine into porcine coronary smooth muscle

Adenosine (ADO), an endogenous regulator of coronary vascular tone, enhances vasorelaxation in the presence of nucleoside transport inhibitors such as dipyridamole. We tested the hypothesis that coronary smooth muscle (CSM) contains a high-affinity transporter for ADO. ADO-mediated relaxation of isolated large and small porcine coronary artery rings was enhanced 12-fold and 3.4-fold, respectively, by the transport inhibitor, S-(4-nitrobenzyl)-6-thioinosine (NBTI). Enhanced relaxation was independent of endothelium and was selective for ADO over synthetic analogs. Uptake of [(3)H]ADO into freshly dissociated CSM cells or endothelium-denuded rings was linear and concentration dependent. Kinetic analysis yielded a maximum uptake (V(max)) of 67 +/- 7.0 pmol. mg protein(-1). min(-1) and a Michaelis constant (K(m)) of 10. 5 +/- 5.8 microM in isolated cells and a V(max) of 5.1 +/- 0.5 pmol. min(-1). mg wet wt(-1) and a K(m) of 17.6 +/- 2.6 microM in intact rings. NBTI inhibited transport into small arteries (IC(50) = 42 nM) and cells. Analyses of extracellular space and diffusion kinetics using [(3)H]sucrose indicate the V(max) and K(m) for ADO transport are sufficient to clear a significant amount of extracellular adenosine. These data indicate CSM possess a high-affinity nucleoside transporter and that the activity of this transporter is sufficient to modulate ADO sensitivity of large and small coronary arteries.