Forskolin: a specific stimulator of adenylyl cyclase or a diterpene with multiple sites of action?

Activation of adenylyl cyclase downregulates sodium/calcium exchanger of arterial myocytes

Chronic elevation of adenosine 3',5'-cyclic monophosphate (cAMP) is known to inhibit the proliferation of cultured vascular smooth muscle cells. The present findings show that the activation of adenylyl cyclase with forskolin decreased Na+/Ca2+ exchanger (NCX) mRNA and activity. Fetal bovine serum restored NCX transcript and activity. The changes in NCX transcript preceded the changes in NCX activity. Incubation of low-passage immortalized myocytes with forskolin plus 3-isobutyl-1-methylxanthine (IBMX), which inhibits cAMP phosphodiesterase, decreased NCX mRNA by 60% in 6 h and 80% in 24 h. After a 6-h lag, forskolin plus IBMX decreased NCX activity almost linearly to 20% of control at 40 h. 1,9-Dideoxyforskolin, which does not activate adenylyl cyclase, had no effect on NCX mRNA or activity. Forskolin plus IBMX decreased the c-Myc transcript, an immediate-early gene whose expression correlates with cell proliferation, but had no effect on plasma membrane Ca(2+)-ATPase transcripts. Removal of forskolin plus IBMX and addition of fetal bovine serum increased NCX and c-Myc transcripts seven- to eightfold in 6 h and restored NCX activity in 24 h. Inhibition of protein or RNA synthesis by cycloheximide or actinomycin D, respectively, prevented the increase in NCX mRNA. In contrast to blocking NCX induction, cycloheximide potentiated c-Myc induction by serum. Transcription factors that regulate myocyte growth may mediate the opposing influences of serum and forskolin on NCX mRNA and activity.

Long-term potentiation in mice lacking synapsins

Synapsin I and synapsin II are widely expressed synaptic vesicle phosphoproteins that have been proposed to play an important role in synaptic transmission and synaptic plasticity. To gain further insight into the functional significance of the phosphorylation sites on the synapsins, we have examined a number of synaptic processes thought to be mediated by protein kinases in knockout mice lacking both forms of synapsin (Rosahl et al., 1995). Long-term potentiation (LTP) at both the mossy fiber (MF)-CA3 pyramidal cell synapse and the Schaffer collateral-CA1 pyramidal cell synapse appears normal in hippocampal slices prepared from mice lacking synapsins. Moreover, the effects on synaptic transmission of forskolin at MF synapses and H-7 at synapses on CA1 cells are also normal in the mutant mice. These results indicate that the synapsins are not necessary for: (1) the induction or expression of two different forms of LTP in the hippocampus, (2) the enhancement in transmitter release elicited by activation of the cAMP-dependent protein kinase (PKA) and (3) the depression of synaptic transmission caused by H-7. Although disappointing, these results are important in that they exclude the most abundant family of synaptic phosphoproteins as an essential component of long-term synaptic plasticity.

Dual effect of beta-adrenergic receptors on mitogen-activated protein kinase. Evidence for a beta gamma-dependent activation and a G alpha s-cAMP-mediated inhibition

The enzymatic activity of mitogen-activated protein kinases (MAP kinases) increases in response to agents acting on a variety of cell surface receptors, including receptors linked to heterotrimeric G proteins of the Gi and Gq family. Recently, it has been shown that stimulation of beta-adrenergic receptors, which are typical of those that act through Gs to activate adenylyl cyclases, potently activates MAP kinases in the heart, resulting in the hypertrophy of the cardiac muscle (Lazou, A., Bogoyevitch, M.A., Clerk, A., Fuller, S.J., Marshall, C.J., and Sudgen, P.H. (1994) Circ. Res. 75, 938-941). We have observed that exposure of COS-7 cells to a beta-adrenergic agonist, isoproterenol, raises intracellular levels of cAMP and effectively activates protein kinase A (PKA) and an epitope-tagged MAP kinase. However, MAP kinase stimulation by isoproterenol was neither mimicked by expression of an activated mutant of G alpha s, nor by treatment with PKA-stimulating agents. Moreover, pretreatment of COS-7 with a permeable cAMP analog, 8-Br-cAMP, markedly decreased MAP kinase activation by either isoproterenol or epidermal growth factor. Thus, in COS-7 cells cAMP and PKA do not appear to mediate MAP kinase activation by beta-adrenergic receptors. Signaling from beta-adrenergic receptors to MAP kinase was inhibited by transfection of a chimeric molecule consisting of the CD8 receptor and the carboxyl terminus of the beta-adrenergic receptor kinase, which includes the beta gamma-binding domain. MAP kinase activation by isoproterenol was not affected by depletion of protein kinase C, but it was completely abolished by expression of Ras-inhibiting molecules. We conclude that signaling from beta-adrenergic receptors to MAP kinase involves an activating signal mediated by beta gamma subunits acting on a Ras-dependent pathway and a G alpha s-induced inhibitory signal mediated by cAMP and PKA. The balance between these two opposing mechanisms of regulation would be expected to control the MAP kinase response to beta-adrenergic agonists as well as to other biologically active agents known to act on Gs coupled receptors, including a number of hormones, neurotransmitters, and lipid mediators.

Oxidant stress enhances adenylyl cyclase activation

The generation of oxygen-derived free radicals has been implicated in the disordered vascular regulation of inflammation and reperfusion. In the vasculature, oxygen-derived free radicals are vasodilatory. The mechanisms underlying this effect remain unclear. To examine the cellular processes involved, we studied the effects of hydrogen peroxide (H2O2) on adenylyl cyclase activity in A10 cells, a murine vascular smooth muscle cell line. Pretreatment with H2O2 caused a dose-dependent enhancement of forskolin-stimulated adenylyl cyclase activity (ED50, 44 mumol/L to a maximum of 166% of control activity; n = 4). This enhancement was attenuated by iron chelation with deferoxamine and by the intracellular hydroxyl scavenger dimethylthiourea and mimicked by preincubation with purine/xanthine oxidase either alone or in the presence of superoxide dismutase. The effects of H2O2 were completely blocked by the tyrosine kinase inhibitors genistein and tyrphostin A9 but not by its inactive analogue tyrphostin A1 (H2O2 alone, 149 +/- 13%; H2O2 + tyrphostin A9, 100 +/- 9%; H2O2 + tyrphostin A1, 171 +/- 21%; n = 4). H2O2 comparably enhanced adenylyl cyclase activity stimulated by isoproterenol (166 +/- 17% of control, n = 5) and sodium fluoride (177 +/- 18% of control, n = 5). Thus oxygen-derived free radicals enhance adenylyl cyclase activation, probably via tyrosine kinase-mediated effects on the catalytic subunit of adenylyl cyclase. Sensitization of adenylyl cyclase activation may be an important mechanism by which free radicals modulate hormone-mediated vasodilation.

Quantification of signalling components and amplification in the beta-adrenergic-receptor-adenylate cyclase pathway in isolated adult rat ventricular myocytes

We have investigated the stoichiometric relationship of proteins involved in beta-adrenergic-receptor-mediated signal transduction in isolated rat cardiac myocytes. These cells contain about 2.1 x 10(5) beta-adrenergic receptors per cell, as determined by radio-ligand-binding assays. We have assessed the amount of Gs alpha present in myocyte membranes by immunoblotting using a purified glutathione S-transferase-Gs alpha fusion protein as a standard for quantification. By this method, we determined that cardiac myocytes contain about 35 x 10(6) and 12 x 10(6) molecules per cell of the 45 and 52 kDa forms of Gs alpha, respectively. [3H]Forskolin binding assays were used to assess the formation of high-affinity forskolin binding sites representing Gs alpha-adenylate cyclase complexes occurring in response to Gs alpha activation. Quantification of the adenylate cyclase complexes was facilitated by the permeabilization of cells with saponin. The addition of isoprenaline (isoproterenol) and guanosine 5'-[gamma-thio]trisphosphate to saponin-permeabilized myocytes results in the formation of 6 x 10(5) Gs alpha-adenylate cyclase complexes. Taken together, the data presented here demonstrate that, in a physiologically relevant setting, G-protein is present in large stoichiometric excess relative to both receptor and effector. In addition, we show that, overall, only modest signal amplification occurs between receptor and adenylate cyclase. Thus adenylate cyclase (rather than Gs) is the component distal to receptor that limits agonist-mediated increases in cyclic AMP production. Although limited data are as yet available for other G-protein-regulated effectors, we hypothesize that the stoichiometry of signalling components and the extent of signal amplification described for the beta-adrenergic response pathway will be applicable to other G-protein-coupled hormone receptor systems.

Hypoxic regulation of lactate dehydrogenase A. Interaction between hypoxia-inducible factor 1 and cAMP response elements

The oxygen-regulated control system responsible for the induction of erythropoietin (Epo) by hypoxia is present in most (if not all) cells and operates on other genes, including those involved in energy metabolism. To understand the organization of cis-acting sequences that are responsible for oxygen-regulated gene expression, we have studied the 5' flanking region of the mouse gene encoding the hypoxically inducible enzyme lactate dehydrogenase A (LDH). Deletional and mutational analysis of the function of mouse LDH-reporter fusion gene constructs in transient transfection assays defined three domains, between -41 and -84 base pairs upstream of the transcription initiation site, which were crucial for oxygen-regulated expression. The most important of these, although not capable of driving hypoxic induction in isolation, had the consensus of a hypoxia-inducible factor 1 (HIF-1) site, and cross-competed for the binding of HIF-1 with functionally active Epo and phosphoglycerate kinase-1 sequences. The second domain was positioned close to the HIF-1 site, in an analogous position to one of the critical regions in the Epo 3' hypoxic enhancer. The third domain had the motif of a cAMP response element (CRE). Activation of cAMP by forskolin had no effect on the level of LDH mRNA in normoxia, but produced a magnified response to hypoxia that was dependent upon the integrity of the CRE, indicating an interaction between inducible factors binding the HIF-1 and CRE sites.

Ca2+ and cyclic adenosine monophosphate involvement in radiographic contrast medium-induced renal vasoconstriction

PURPOSE

To investigate the role of extracellular Ca2+ and cyclic 3'-5' adenosine monophosphate (cAMP), a known second messenger promoting smooth muscle relaxation, in preventing renal vasoconstriction induced by radiographic contrast medium (RCM).

MATERIALS AND METHODS

Isometric contractions of rabbit renal artery were elicited by potassium chloride and increasing concentrations of meglumine/sodium diatrizoate. To determine the contribution of extracellular Ca2+, nifedipine, a blocker of voltage-dependent L-type Ca2+ channels (VDCC), was applied. The contribution of cAMP was investigated by applying the nonspecific phosphodiesterase (PDE) inhibitors papaverine and theophylline and the specific PDE inhibitor milrinone, all of which prevent degradation of cAMP. Forskolin, an activator of cAMP by stimulating adenylyl cyclase (AC), was also investigated.

RESULTS

RCM elicited contractions that were 24.5% of the KCl control contraction, which was reduced by nifedipine (100 mumol/L) by 34.7%. Papaverine, theophylline, and milrinone inhibited RCM-induced contractions by 69.8%, 64.3%, and 43.7%, respectively. Forskolin reduced the response by 82.2%.

CONCLUSION

Ca2+ influx through VDCC partially contributes to RCM-induced renal artery vasoconstriction. Intracellular cAMP appears to be an important second messenger pathway for prevention of this response. These findings emphasize the role of second messenger systems involved in adverse RCM effects and the potential prevention of these effects.

The effect of nickel chloride on the isolated hemidiaphragm of the rat

1. NiCl2 (cumulative concentrations of 0.56-1.91 mmol 1(-1)) produced concentration-dependent depression of tension developed (Td) and the maximum rate of rise of tension (dT/dt max) of isometric contraction of the isolated rat hemidiaphragm, during direct subtetanic (DST) electrical stimulation, only. EC50 values for NiCl2-induced depression of Td and Dt/dt max were 0.88 +/- 0.06 and 0.83 +/- 0.13 mmol 1(-1), respectively. NiCl2 did not significantly change either parameter of the isometric contraction during direct single-pulse (DSP) electrical stimulation. 2. Maximal depression of Td and dT/dt max, produced by a single concentration of NiCl2 (1 mmol 1(-1)) during DST electrical stimulation was obtained 20 min after addition of the drug in the bathing medium. 3. In the normal Tyrode solution, addition of CaCl2 (final concentration of 5.86 mmol 1(-1)) almost completely antagonized the depressant effect of NiCl2 (1 mmol 1(-1)) on Td and dT/dt max during DST electrical stimulation. In the calcium-free solution, the depression both of Td and dT/dt max produced by NiCl2 (1 mmol 1(-1)) was significantly more pronounced in comparison with the effect of NiCl2 in the normal Tyrode solution. 4. L-calcium channel activator, Bay K 8644 (25 mumol 1(-1)), significantly potentiated both Td and dT/dt max during DST electrical stimulation, but NiCl2 (1 mmol 1(-1)) decreased both parameters of the isometric contraction even in the presence of this concentration of Bay K 8644. On the other hand Bay K 8644 (25 mumol 1(-1)) did not antagonize NiCl2-induced depression of Td and dT/dt max. 5. Verapamil (2.5 mumol 1(-1); 45 min of incubation) and lidocaine (0.10 mmol 1(-1); 30 min of incubation) significantly potentiated the depression of Td and dT/dt max, produced by NiCl2 (1 mmol 1(-1), during DST electrical stimulation. The addition of CaCl2 (final concentration of 7.20 mmol 1(-1)) in the bathing medium only partially antagonized the depressant synergistic action of both verapamil or lidocaine and NiCl2 on Td and dT/dt max. 6. Forskolin (cumulative concentrations of 2.60-44.20 mumol 1(-1)) fully antagonized NiCl2-induced depression of both Td and dT/dt max; propranolol (1 mumol 1(-1)) did not abolish this antagonizing action of forskolin. Also, NiCl2 (cumulative concentrations of 0.56 -1.54 mmol 1(-1)) did not change potentiating effect of forskolin (23.4 mumol 1(-1)).(ABSTRACT TRUNCATED AT 400 WORDS)

[3H]forskolin binding to cardiac adenylate cyclase in guinea pigs chronically infused with isoproterenol

Guinea pigs were infused with the beta-adrenoceptor agonist isoproterenol (400 micrograms/kg/hr, 7 days) and cardiac adenylate cyclase was detected using [3H]forskolin. Two populations of [3H]forskolin binding sites were present in heart, the affinities (KD 2 nM and 280 nM) and densities (Bmax 9 and 900 fmol/mg protein) of which were unchanged by isoproterenol infusion compared with vehicle (1 mM HCl). The autoradiographic localisation of [3H]forskolin binding was also unchanged. The G protein activators NaF 10 mM and 5'-guanylylimidodiphosphate (Gpp(NH)p) 10 microM increased [3H]forskolin binding in heart from vehicle-treated animals by 100% and 80% respectively. NaF-stimulated binding was unchanged in isoproterenol-treated animals, however, Gpp(NH)p-stimulated binding was reduced by 35% which may indicate an increased influence of Gi.

Vasopressin-stimulated electrogenic sodium transport in A6 cells is linked to a Ca(2+)-mobilizing signal mechanism

Vasopressin is known to activate two types of cell surface receptors; V2, coupled to adenylate cyclase, and V1, linked to a Ca(2+)-dependent transduction system. We investigated whether arginine vasopressin (AVP) stimulation of electrogenic sodium transport in A6 cells, derived from Xenopus laevis, is mediated by activation of either one or both types of AVP-specific receptors. AVP caused a rapid increase in electrogenic sodium transport, reflected by the transepithelial potential difference (VT) and equivalent short circuit current (Ieq) measurements. AVP also rapidly increased intracellular Ca2+ (Ca2+i) and total inositol trisphosphate. The increase in Ieq was dependent on the rise in (Ca2+i), because 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) dose-dependently inhibited the Ieq response. There was no evidence, however, that activation of adenylate cyclase mediated AVP-stimulated Ieq; transport was not inhibited after AVP-induced activation of adenylate cyclase was abolished by 2',5'-dideoxyadenosine or when cAMP-dependent protein kinase (PKA) activity was abolished by the specific PKA inhibitor IP20. Further studies showed that although both forskolin and 8-(4-chlorophenylthio)-cAMP stimulated Ieq, this occurred by mechanisms independent of PKA activation. These results indicate that AVP-stimulated Na+ transport is mediated by a V1 receptor and a Ca(2+)-dependent mechanism.

Potentiation of P1075-induced K+ channel opening by stimulation of adenylate cyclase in rat isolated aorta

1. The effects of analogues and stimulators of cyclic AMP on the 86Rb+ efflux-stimulating and binding properties of P1075, an opener of ATP-dependent potassium channels, were studied in rat aortic rings. The increase in 86Rb+ efflux stimulated by P1075 was taken as a qualitative measure of K+ channel opening. 2. Forskolin, a direct activator of adenylate cyclase, isobutylmethylxanthine (IBMX), a phosphodiesterase inhibitor, and dibutyryl-cyclic AMP (db-cyclic AMP), a membrane permeant cyclic AMP-analogue, relaxed rat aortic rings contracted by noradrenaline with EC50 values of 0.06, 2 and 10 microM, respectively. 3. Forskolin, IBMX and db-cyclic AMP produced concentration-dependent increases of the 86Rb+ efflux induced by P1075 (50 nM) by up to twofold with EC50 values of about 0.1, 1.7 and 81 microM. At these concentrations the agents had little effect on the basal rate of 86Rb+ efflux. 4. The 86Rb+ efflux produced by P1075 in the presence of the cyclic AMP stimulators was inhibited by glibenclamide, a blocker of ATP-sensitive potassium channels. 5. IBMX (100 microM) induced a leftward shift of the concentration-86Rb+ efflux curve of P1075 without increasing the maximum. The enhancements of P1075-stimulated 86Rb+ efflux produced by combinations of forskolin and IBMX were either additive or less than additive. 6. The protein kinase A inhibitor, H-89, inhibited P1075-stimulated 86Rb+ efflux in the presence of IBMX significantly more than in the absence of IBMX, suggesting that the effect of increased cyclic AMP levels is mediated by protein kinase A. 7. At high concentrations, forskolin and IBMX slightly increased basal 86Rb+ efflux and inhibited the tracer efflux induced by P1075.8. Binding of [3H]-P1075 to rat aortic rings was either unaffected or inhibited by forskolin, IBMX and db-cyclic AMP.9. This study shows that moderate stimulation of the cyclic AMP system potentiates the K+ channel opening effect of P1075 by activation of protein kinase A. The fact that binding of [3H]-P1075 remains unchanged or is diminished favours the hypothesis that the K'channel openers activate ATP-dependent K+ channels by an indirect mechanism.

Effects of phosphodiesterase inhibitors on spontaneous electrical activity (slow waves) in the guinea-pig gastric muscle

1. The effects of the phosphodiesterase inhibitors caffeine, theophylline, isobutylmethylxanthine (IBMX) and rolipram on spontaneous electrical activity (slow waves) were studied in the circular muscle of the guinea-pig gastric antrum. 2. All the inhibitors reduced slow wave frequency without changing the membrane potential and the slow wave configuration, but at higher concentrations they blocked the slow waves and caused membrane hyperpolarization. In the presence of the inhibitors a low level of irregular electrical activity could be observed in many preparations. 3. Isoprenaline, forskolin, dibutyryl cAMP and 8-bromo-cAMP all produced effects essentially similar to those of phosphodiesterase inhibitors. K+ (12 mM) and removal of K+ both depolarized the membrane and these were not affected by IBMX (1-3 microM). A decrease in frequency caused by IBMX was also not significantly affected by 12 mM K+ or K+ removal and only partially antagonized by TEA or 4-aminopyridine. 4. These results suggest that an increase in intracellular cAMP inhibits pacemaker activity of slow waves. An increase in K+ conductance does not seem to be a major factor in this inhibition. Slow waves appear to be a compound electrical activity in a group of muscle cells and are likely to be disintegrated by xanthine derivatives.

Detection and analysis of agonist-induced formation of the complex of the stimulatory guanine nucleotide-binding protein with adenylate cyclase in intact wild-type and beta 2-adrenoceptor-expressing NG108-15 cells

Neuroblastoma x glioma hybrid, NG108-15, cells appear to express the alpha-subunit of the guanine nucleotide-binding protein Gs in a substantial molar excess over its effector adenylate cyclase [Kim, Adie and Milligan (1994) Eur. J. Biochem. 219, 135-143]. Addition of the IP prostanoid receptor agonist iloprost to intact NG108-15 cells resulted in a dose-dependent increase in formation of the complex between Gs alpha and adenylate cyclase (GSAC) as measured by specific high-affinity binding of [3H]forskolin. NG108-15 cells transfected to express either relatively high (clone beta N22) or low (clone beta N17) levels of beta 2-adrenoceptor both showed dose-dependent increases in specific [3H]forskolin binding in response to the beta-adrenoceptor agonist isoprenaline, and maximally effective concentrations of isoprenaline resulted in the generation of similar numbers of GSAC complexes in both clones. The dose-effect curve for clone beta N22, however, was some 15-fold to the left of that for clone beta N17, which is similar to that noted for isoprenaline-mediated stimulation of adenylate cyclase activity [Adie and Milligan (1994) Biochem. J. 303, 803-808]. In contrast, dose-effect curves for iloprost stimulation of [3H]forskolin binding were not different in clones beta N22 and beta N17. Basal specific [3H]forskolin binding in the absence of agonist was significantly greater in cells of clone beta N22 than clone beta N17. This was not a reflection of higher immunological levels of adenylate cyclase, indicating that the higher basal formation of GSAC probably reflects empty-receptor activation of Gs. This higher basal specific [3H]forskolin binding was partially reversed by propranolol. The addition of the opioid peptide D-Ala-D-Leu-enkephalin to NG108-15 cells did not reduce iloprost-stimulated [3H]forskolin binding even though this peptide inhibits stimulated adenylate cyclase activity by activation of a delta opioid receptor.

Kinetics and thermodynamics of the binding of forskolin to the galactose-H+ transport protein, GalP, of Escherichia coli

The binding of the transport inhibitor, forskolin, to the galactose-H+ symporter, GalP, of Escherichia coli was evaluated by equilibrium and time-resolved fluorescence measurements. A quench in protein fluorescence of 8-12% was observed upon the binding of forskolin. The overall dissociation constant (Kd) for forskolin determined by fluorescence titration ranged between 1.2 and 2.2 microM, which is similar to that reported from equilibrium dialysis measurements of the binding of [3H]forskolin (Kd = 0.9-1.4 microM). The kinetics of forskolin binding were measured by stopped-flow fluorescence methods. The protein fluorescence was quenched in a biphasic manner; the faster of these two rates was dependent on the concentration of forskolin and was interpreted as the initial binding step from which both the association (kon) and dissociation (koff) rate constants were determined. The association and dissociation rate constants were 5.4-6.2 microM-1.s-1 and 5.1-11.5 s-1 respectively, and the Kd was calculated to be 1.5 microM. The binding of forskolin was inhibited by D-galactose, but not by L-galactose, and displacement by sugar provided an additional method to calculate the dissociation rate constant for forskolin (koff = 12.4-13.0 s-1). The rate of the slow change in protein fluorescence (3-5 s-1) was independent of the forskolin concentration, indicating an isomerization of the transporter between different conformations, possibly outward- and inward-facing forms. These kinetic parameters were determined at a series of temperatures, so that the thermodynamics of forskolin binding and transporter re-orientation could be analysed. The binding process was entropically driven (delta S = 83.7 J.K-1.mol-1; delta H = 8.25 kJ.mol-1), similar to that for cytochalasin B, which is also an inhibitor of GalP. Measurements of the binding of [3H]forskolin by equilibrium dialysis revealed competitive displacement of bound forskolin by cytochalasin B, possibly suggesting that the sugar, forskolin and cytochalasin B binding sites are overlapping; the Kds for forskolin and cytochalasin B were calculated to be 0.85 microM and 4.77 microM respectively, and the concentration of binding sites was 10.2 nmol.mg-1.

Induction of the blood/brain-barrier-associated enzyme alkaline phosphatase in endothelial cells from cerebral capillaries is mediated via cAMP

The mammalian blood/brain barrier is located at the endothelial cells of the cerebral capillaries. Alkaline phosphatase is associated to a very large extent with these cells and has been established as a marker enzyme for a differentiated blood/brain barrier phenotype in vivo and in vitro. Nevertheless cultured brain capillary endothelial cells (BCEC) lose this marker enzyme because of a cessation of de novo synthesis. Since astrocytes have been shown to possess the capability to re-induce the enzymic activity of alkaline phosphatase in BCEC in vitro we were interested in the second messengers involved in the signal-transduction mechanism of this induction in BCEC. For this reason we treated cultured porcine BCEC with a water-soluble and membrane-permeable analogue of cAMP, 8-(4-chlorophenylthio)-cAMP (C1PhS-cAMP) in the absence of astrocytes. By means of enzymic activity assays we were able to show that within three days the activity of alkaline phosphatase increased up to sixfold compared with the controls. The total activity of alkaline phosphatase in C1PhS-cAMP-treated BCEC was comparable to that of freshly isolated cells. Addition of cycloheximide inhibited the alkaline phosphatase activity increase. We conclude that cAMP is one of the second messengers involved in the induction of alkaline phosphatase activity in BCEC in vitro.

Mitogenic and antimitogenic effects of pituitary adenylate cyclase-activating polypeptide (PACAP) in adult rat chromaffin cell cultures

The neurotransmitter, pituitary adenylate cyclase-activating polypeptide (PACAP), is present in the rat adrenal medulla and is a potent stimulus for catecholamine secretion. Previous studies have suggested that neurally derived signals stimulate proliferation of chromaffin cells in adult rats. To determine whether PACAP might be involved in mitogenic signalling, its effects on bromodeoxyuridine incorporation were studied in adrenal medullary cell cultures from adult female rats. Both PACAP 27 and PACAP 38 are able to stimulate proliferation of adult rat chromaffin cells in vitro, either alone or in conjunction with PMA, an activator of protein kinase C. BrdU-labelled nuclei are observed in both epinephrine and norepinephrine cells, and proliferation of both cell types is stimulated by the same concentrations of PACAP that elicit secretion of catecholamines. The mitogenic effects of PACAP are potentiated by indolidan, a phosphodiesterase inhibitor known to cause pheochromocytomas in rats, and are inhibited by H-89, an inhibitor of protein kinase A. Mitogenic concentrations of PACAP inhibit mitogenic effects of nerve growth factor. These findings support the hypothesis that neurally derived signals regulate chromaffin cell proliferation in adult rats. Indolidan and a variety of nongenotoxic agents that cause pheochromocytomas in rats may do so indirectly by increasing neurally mediated chromaffin cell turnover. The antagonism between PACAP and NGF suggests that neurotransmitters may supersede growth factors in regulating chromaffin cell proliferation during development by suppressing or co-opting portions of growth factor signaling pathways.

Cyclic adenosine-3',5'-monophosphate potentiates the synaptic potential mediated by NMDA receptors in the amygdala

An in vitro slice preparation of rat amygdala was used to study the actions of forskolin and cyclic adenosine-3',5'-monophosphate (cAMP) analogues on the N-methyl-D-aspartate (NMDA) receptor-mediated synaptic potential (EPSPNMDA). Intracellular recordings were made from basolateral amygdala neurons in the presence of 6-cyano-7-nitroquinoxaline-2,3-di-one (CNQX, 10 microM) and picrotoxin (50 microM) to pharmacologically isolate the EPSPNMDA. Application of forskolin (25 microM) markedly and persistently potentiated the EPSPNMDA. In contrast, the inactive forskolin analogue, 1,9-dideoxy-forskolin, failed to affect the EPSPNMDA significantly. Superfusion of dibutyryl-cAMP (dbcAMP, 200 microM) for 15 min caused a transient depression of the amplitude of EPSPNMDA. The EPSPNMDA amplitude was reduced to 68 +/- 3% of control (n = 10) 15 min after the application, restored to its control value within 25 min, and followed by a long-term potentiation (LTP). Pretreating the slices with 8-cyclopentyl-1,3-dipropyl-xanthine (DPCPX, 5 microM), a selective A1 receptor antagonist, blocked the transient depressive phase produced by dbcAMP. This result suggests that the transient depression induced by dbcAMP was likely due to the interaction of dbcAMP or its breakdown products with adenosine A1 receptors. To determine the site of action, we examined the effect of forskolin on the postsynaptic responses to exogenously applied NMDA. Forskolin potentiated the postsynaptic depolarization induced by NMDA, suggesting that the enhancement is mediated, at least in part, by a persistent upregulation of postsynaptic NMDA receptor-operated conductances. Occlusion experiments were performed to examine whether the sustained enhancements of EPSP(NMDA) produced by tetanic stimulation (TS) and forskolin share a common mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of chronic treatment with a nitric oxide donor on fatty streak development and reactivity of the rabbit aorta

1. The influence of chronic treatment with molsidomine, pro-drug of the nitric oxide (NO) donor, 3-morpholino-sydnonimine (SIN-1), on fatty streak development and release of NO and prostacyclin (PGI2) was studied in the aorta of normal and cholesterol-fed rabbits. 2. Groups of 10 rabbits received standard diet (150 g day-1), or diets with 0.3% cholesterol, with 0.02% molsidomine or with the combination of cholesterol and molsidomine for 16 weeks. Lesion area and thickness, maximum change in isometric force (Emax) and sensitivity (-log EC50 or pD2) to constricting and relaxing agonists were assessed in segments of arch, thoracic and abdominal aorta. Bioassay was used to assess NO release. 3. Cholesterol-induced fatty streaks tapered off towards the abdominal aorta. Area, thickness, weight and cholesterylester content of the lesions were augmented by the NO donor, whereas the hypercholesterolaemia remained unchanged. The exacerbation was attributed to co-release of superoxide anion from the sydnonimine. 4. As fatty streaks progressed, amplitude and pD2 of acetylcholine (ACh)-induced relaxations decreased, whereas cyclic GMP and cyclic AMP second messenger systems were not influenced, since Emax and sensitivity to SIN-1 and forskolin remained unchanged. However, extensive lesions apparently trapped some NO, as the pD2 of authentic NO decreased. 5. The fatty streaks curtailed the biosynthesis of PGI2 and the overflow of NO from the perfused thoracic aorta. The latter defect was not restored by L-arginine and appears to be consistent with a functional change of the endothelial muscarinic receptors. 6. The NO donor desensitized the aorta to cyclic GMP-mediated relaxations (ACh, SIN-1 and NO), without affecting cyclic AMP-mediated relaxation to forskolin or constrictor responses to phenylephrine and 5-hydroxytryptamine. 7. The drug also suppressed the ACh-induced overflow of NO, without changing PGI2 release. This selective reduction of endothelial NO release and the desensitization of cyclic GMP-mediated relaxations occurred independently of fatty streak formation. 8. The results indicate that chronic exposure to exogenous NO downregulates endothelial NO release and cyclic GMP-mediated relaxations, and provide evidence for the existence of negative feed-back regulations of the L-arginine NO pathway under in vivo conditions.

Protein kinase inhibitor H-89 reverses forskolin stimulation of cardiac L-type calcium current

Calcium currents (ICa) and barium currents (IBa) were measured in freshly isolated single ferret ventricular myocytes, using the whole cell patch-clamp and perforated patch-clamp techniques with Na and K currents blocked by tetraethylammonium and Cs. The membrane potential (Em) dependence of activation and steady-state inactivation curves were determined using a Boltzmann relation, where E0.5 is the Em at half-maximal conductance. Forskolin (1 microM) increased the rate of ICa inactivation, especially in perforated patch, but slowed IBa inactivation. The acceleration is likely to be due to greater Ca-dependent inactivation of ICa, where the slowing of IBa inactivation may be due to protein kinase A-dependent slowing of Em-dependent inactivation. Forskolin (1-10 microM) also increased ICa amplitude by two- to threefold and shifted the E0.5 for both activation and inactivation to more negative potentials by 7-8 mV. The effect of forskolin on the amplitude of ICa could be reversed by an inhibitor of adenosine 3',5'-cyclic monophosphate-dependent protein kinase, N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89; 1-10 microM). However, H-89 did not reverse the shift of E0.5 induced by forskolin. H-89 application by itself does not decrease basal ICa but does shift the E0.5 of both activation and inactivation to more negative values of Em. It is possible that H-89 reverses the shift induced by regulatory phosphorylation (due to forskolin) but induces a coincidental negative shift itself.

Influence of age on the signal transduction pathway of non-adrenergic non-cholinergic neurotransmitters in the rat gastric fundus

1. The influence of aging on the relaxant response and the change in cyclic nucleotide content induced by vasoactive intestinal polypeptide (VIP), nitric oxide (NO), electrical field stimulation of the non-adrenergic non-cholinergic neurones and substances acting at different levels of the cyclic AMP and cyclic GMP transduction pathways was studied in longitudinal muscle strips of the rat gastric fundus. 2. The relaxant responses to VIP, sustained electrical stimulation, forskolin and 3-isobutyl-1-methylxanthine were reduced with age, while the responses to dibutyryl cyclic AMP were not. The increase in cyclic AMP content induced by sustained electrical stimulation and forskolin was lower in old rats. 3. The relaxant responses to NO and to short train electrical stimulation were similar in the three age groups. The inhibitory effect of NG-nitro-L-arginine methyl ester on relaxations induced by short train electrical stimulation was more pronounced in old rats. The relaxant responses to sodium nitroprusside (SNP), 8-bromo-cyclic GMP and zaprinast were reduced with age. SNP induced a similar elevation of the cyclic GMP content in the three age groups. 4. These results suggest that aging differentially affects the cyclic AMP and cyclic GMP pathway for relaxation by VIP and NO in the rat gastric fundus, as the defect seems to occur at the level of the adenylate cyclase and cyclic GMP-dependent protein kinase respectively.

Effects of mutations in cAMP-dependent protein kinase on chloride efflux in Caco-2 human colonic carcinoma cells

In order to evaluate the importance of cAMP and cAMP-dependent protein kinase (cAMPdPK) in the regulation of chloride efflux via the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, Caco-2, human colonic carcinoma cells were transfected with an expression vector encoding a mutant form of regulatory subunit of cAMPdPK under control of the mouse metallothionein 1 promoter. Four stable transformants were isolated that expressed the mutant subunit in a Zn(2+)-inducible manner and exhibited Zn(2+)-inducible inhibition of cAMPdPK activity. The parental and transformed Caco-2 cells were examined for their abilities to regulate chloride efflux in response to various secretagogues using a radioactive iodide-efflux assay. In the transformants, induction of the protein kinase mutation with ZnSO4 markedly decreased chloride efflux in response to forskolin, the 8-(4-chlorophenylthio) analog of cAMP, vasoactive intestinal polypeptide, prostaglandin E2 and isoproterenol, whereas Zn(2+)-treated parental cells remained responsive to these secretagogues. Treatment with carbachol, calcium ionophores or phorbol ester did not acutely affect chloride efflux. Together, these studies indicate that cAMP and cAMPdPK are essential components of secretagogue-regulated chloride channel activity in the Caco-2 cell line. In whole cell patch clamp recordings, induction of the cAMPdPK mutation inhibited anionic conductances indicative of the CFTR chloride channel, whereas purified catalytic subunit of cAMPdPK, added intracellularly, reversed the inhibition. These latter results demonstrate that the CFTR chloride channels in the protein kinase-defective transformants are normal and that the protein kinase mutation specifically affects their regulation, presumably by direct phosphorylation.

Tonic adenosine A2A receptor activation modulates nicotinic autoreceptor function at the rat neuromuscular junction

The influence of the activation of presynaptic adenosine receptors on nicotinic autofacilitation of electrically evoked [3H]acetylcholine release from rat phrenic motor nerve terminals was investigated. Blocking the adenosine A2A receptor with 3,7-dimethyl-1-propargylxanthine (DMPX, 10 microM) greatly potentiated, whereas the adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 2.5 nM), partially prevented the facilitatory effect of the nicotinic receptor agonist, 1,1-dimethyl-4-phenylpiperazinium (DMPP, 1 microM, 3 min), on evoked [3H]acetylcholine release. The adenosine A2A receptor agonist, 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamideadeno sine (CGS 21680C, 3 nM), but not the adenosine A1 receptor agonist, R-N6-phenylisopropyl adenosine (R-PIA, 300 nM), partially blocked the DMPP (1 microM) facilitation. Forskolin (3 microM) mimicked the attenuation caused by CGS 21680C; inhibition of adenylate cyclase with N-(as-2-phenylcyclopentyl)azacyclo-tridecan-2-imine hydrochloride (MDL 12,330A, 10 microM) markedly enhanced the facilitatory effect of DMPP (1 microM). Prolonged exposure to a high concentration of DMPP (10 microM, 15 min) decreased evoked tritium outflow. The decrease in evoked [3H]acetylcholine release following prolonged exposure to DMPP was augmented by pretreatment with CGS 21680C (3 nM) and forskolin (3 microM), and was abolished by inactivating endogenous adenosine with adenosine deaminase (0.5 U/ml). It is concluded that tonic adenosine A2A receptor activation regulates nicotinic acetylcholine autofacilitation. This action is likely to be mediated through an adenylate cyclase/cyclic AMP-dependent mechanism.

Regulation of expression of cAMP response element-binding protein in the locus coeruleus in vivo and in a locus coeruleus-like cell line in vitro

Expression of the cAMP response element (CRE)-binding protein (CREB) has been thought to be constitutive and not subject to regulation. In the course of investigating effects of chronic morphine on the cAMP pathway in the locus coeruleus, a brain region important for opiate addiction, we found that levels of CREB immunoreactivity and CRE binding were increased by chronic morphine administration. To further investigate possible mechanisms underlying this unexpected finding, we studied the regulation of CREB expression in a cell line (CATH.a) that exhibits many properties of locus coeruleus neurons. Agents that activate the cAMP pathway led to a > 60% decrease in CREB mRNA in this cell line. Moreover, these alterations in CREB mRNA levels were associated with changes in levels of CREB immunoreactivity and CRE-binding activity. In contrast, the same treatments fail to alter CREB expression in PC12 pheochromocytoma cells.

Evidence that the presynaptic A2a-adenosine receptor of the rat motor nerve endings is positively coupled to adenylate cyclase

The action of the A2a-adenosine analogue, CGS 21680C, on electrically evoked [3H]acetylcholine ([3H]-ACh) release, and its interaction with forskolin (an activator of adenylate cyclase), MDL 12,330A (an irreversible inhibitor of adenylate cyclase), rolipram (an inhibitor of cyclic AMP specific phosphodiesterase), dibutyryl- (db-cAMP) and 8-bromo- (8-Br-cAMP) cyclic AMP analogues (substances that mimic intracellular actions of cyclic AMP), were investigated using rat phrenic nerve-hemidiaphragm preparations. CGS 21680C facilitated [3H]-ACh release. Forskolin (but not 1,9-dideoxy forskolin), rolipram, db-cAMP and 8-Br-cAMP also increased evoked neurotransmitter release in a concentration-dependent manner. When the evoked [3H]-ACh release that is dependent on stimulation of the adenylate cyclase/cyclic AMP transduction system was supramaximally stimulated by these compounds, CGS 21680C (3 nmol/l) could not further increase [3H]-ACh release. Phosphodiesterase inhibition with low concentrations (< or = 30 mumol/l) of rolipram significantly potentiated the augmenting effect of CGS 21680C (1 nmol/l) on evoked [3H]-ACh release. MDL 12,330A (an irreversible inhibitor of adenylate cyclase) decreased evoked [3H]-ACh release. The irreversible blocking action of MDL 12,330A on [3H]-ACh release was overcome by by-passing cyclase activation with db-cAMP and 8-Br-cAMP, but could not be overcome with FSK or CGS 21680C. The inhibitory effect of MDL 12,330A on evoked [3H]-ACh release was not mimicked by nifedipine. It is concluded that the increase in [3H]-ACh release caused by CGS 21680C results from activation of an A2a-adenosine receptor positively linked to the adenylate cyclase/cyclic AMP system.

31P NMR investigation of energy metabolism in perifused MMQ cells

The MMQ cell line is a unique prolactin-secreting rat pituitary cell line. MMQ cells entrapped in agarose gel threads are metabolically active, as determined by the uptake and phosphorylation of creatine and the maintenance of high energy phosphates for over 15 h. Forskolin activates the catalytic subunit of adenylyl cyclase and, in MMQ cells, elevates the level of cAMP and stimulates prolactin secretion. 31P NMR spectroscopy was used to investigate the energy metabolism of the MMQ cells during stimulation by forskolin. The ability to measure small changes in the energy status of these cells was enhanced by increasing the PCr levels in the cells. Administration of forskolin to the perifused MMQ cells resulted in acute, reversible, and dose-dependent changes in the 31P NMR spectra of the cells within 12 to 24 min of the beginning of forskolin exposure. Several lines of evidence indicate that the changes observed in the MMQ cells are the composite result of the interaction of forskolin with adenylyl cyclase and the plasma membrane glucose transporter. Also, preincubation of the MMQ cells with the dopamine agonist, bromocriptine, attenuates the forskolin-stimulated decrease in the PCr resonance by approximately 50%. This attenuation indicates that the forskolin-stimulated changes in energy metabolism are probably related to the prolactin secretion process.

Ornithine decarboxylase gene expression is aberrantly regulated via the cAMP signal transduction pathway in malignant H-ras transformed cell lines

We have tested the hypothesis that H-ras transformed cells contain alterations in signal pathways important in controlling the expression of ornithine decarboxylase (ODC), the highly regulated rate-limiting activity in the biosynthesis of polyamines. Mouse 10T1/2 fibroblasts and a series of 10T1/2 H-ras transformed cell lines were treated with stimulators of cAMP synthesis (forskolin and cholera toxin), a biologically stable analogue of cAMP (8-bromo-cAMP), and an inhibitor of cAMP degradation (3-isobutyl-1-methylxanthine). Elevations in ODC gene expression were noted in H-ras transformed cells that were not observed in parental 10T1/2 fibroblasts. The forskolin-mediated effects were not detected with 1,9-dideoxyforskolin, a compound structurally related to forskolin, which does not activate adenyl cyclase. The effects observed with cholera toxin were not detected when cells were treated with the purified subunits of this compound, indicating that the toxin-induced effects were cAMP-specific. Actinomycin D treatment prior to forskolin exposure reduced the elevation observed in ODC gene expression indicating the involvement of the transcriptional process. Furthermore, we observed that cycloheximide treatment of malignant but not benign H-ras transformed cells significantly elevated ODC message level. Treatment of malignant cells with both cycloheximide and forskolin together resulted in a further additive elevation in ODC message, but a similar treatment of benign tumor cells reduced the forskolin-mediated increase in ODC message. In addition, treatment of H-ras transformed cells with the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) led to an elevation in ODC mRNA levels not observed in parental 10T1/2 fibroblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma from patients with seronegative myasthenia gravis inhibit nAChR responses in the TE671/RD cell line

Myasthenia gravis (MG) is an autoimmune disorder in which anti-acetylcholine receptor (AChR) antibodies cause muscle weakness. In 10-15% of MG patients anti-AChR antibodies are undetectable (seronegative MG, SMG), though clinical and experimental evidence points to causative circulating factors. Using whole-cell patch-clamp techniques, we investigated the effects of heat-inactivated plasma from SMG patients (n = 7) on voltage-gated sodium [INa(V)] and ACh-induced nicotinic AChR (nAChR) currents in the human rhabdomyosarcoma cell line TE671/RD, comparing the results to those obtained with plasma from healthy individuals (HC, n = 6), patients with Guillain-Barré syndrome (GBS, n = 3) or those with other neurological diseases (OND, n = 3). None of the plasma samples inhibited INa(V). nAChR currents were rapidly (< 1 min) and significantly (P < 0.01) reduced by a 1:10 dilution of plasma from SMG patients compared with plasma from healthy controls and were not restored by washing. The inhibition appeared in some cases to be calcium dependent since for one of three plasmas it was prevented by 10 mM EGTA in the patch pipette. Currents were also reduced by two of three plasmas obtained from GBS patients at 1:3 dilution, but not by the three plasmas from patients with ONDs. The rapid action of plasma from SMG patients argues against an antibody-induced reduction in nAChR numbers; its calcium dependence in one case suggests action by a second messenger that might involve nAChR phosphorylation.

Characterization of cyclic AMP accumulation in cultured human corpus cavernosum smooth muscle cells

Intracavernous pharmacotherapy relies heavily on the use of vasoactive agents which act by increasing intracellular cAMP levels in human corpus cavernosum smooth muscle. Yet little is known about the cAMP generating system in this tissue, and how it may affect observed patient variability. Thus, the goal of these studies was to better characterize the biochemistry of cAMP formation in human corpus cavernosum smooth muscle, and thus provide more insight into the mechanisms of corporal smooth muscle relaxation in vivo. We studied both receptor and nonreceptor mediated increases in cAMP formation in short-term cultures of human corpus cavernosum smooth muscle cells. Both isoproterenol (ISO) and prostaglandin E1 (PGE1) produced concentration-dependent increases in cAMP, but histamine, serotonin and vasoactive intestinal polypeptide did not. Forskolin, a relatively specific activator of adenylate cyclase, was also a potent stimulant of cAMP formation in these cells. Moreover, there was a direct correlation between the degree of forskolin-induced cAMP accumulation in cultured corporal smooth muscle cells and the magnitude of the forskolin-induced relaxation response of precontracted isolated corporal smooth muscle strips. Prostaglandin E1 and ISO concentration response curves (CRCs) were then assayed in the absence and presence of subthreshold forskolin (0.1 microM.). In the presence of forskolin, the calculated maximal PGE1-induced cAMP accumulation (Emax) was significantly greater than that elicited by PGE1 alone, ISO alone, or ISO + forskolin (p < or = 0.02). In addition, a fixed molar ratio (FMR) (PGE1:ISO) protocol was used to demonstrate that both 80:20 and 70:30 FMRs (but not 95:5 or 90:10), were associated with significantly greater cAMP Emax values than that observed for PGE1 alone (p < or = 0.01). These data provide direct evidence that the degree of cAMP formation in cultured corporal smooth muscle cells is strongly correlated with the magnitude of relaxation of isolated corporal smooth muscle strips. In addition, since simultaneous activation of distinct components of the cAMP generating system produces significant increases in maximal intracellular cAMP accumulation, this suggests that such drug combinations may also augment corporal smooth muscle relaxation in vitro and in vivo.

The effect of GTP on the aluminum fluoride- and forskolin-activated adenylyl cyclase from human embryonic kidney 293 cells

GTP has been shown to inhibit AlF4(-)-stimulated, and to activate forskolin-stimulated adenylyl cyclase activity in the presence of Mg2+ in cell membranes from human embryonic kidney 293 cells. The maximal inhibitory response of AlF4(-)-stimulated adenylyl cyclase activity by GTP was not dependent on the concentration of Mg2+, but was so in the case of forskolin-activated activity at all forskolin concentrations assayed. Mn2+ ions stimulated AlF4(-)- or forskolin-activated adenylyl cyclase activity to a greater extent than Mg2+. The inhibition of AlF4(-)-stimulated cyclase by GTP was still observed with Mn2+, but the activation of forskolin-stimulated cyclase by GTP was not. When assayed together, Mn2+ and Mg2+ showed non-additive behaviours with respect to the amount of cyclic AMP formed after AlF4(-)-stimulation of adenylyl cyclase. The temperature dependence of the activation of adenylyl cyclase by forskolin, AlF4- or under basal conditions was observed to be somehow different in the presence of Mn2+ than in the presence of Mg2+ ions. Cholera toxin treatment produced a markedly increased cyclase activity, specially when assayed with AlF4-. In the case of forskolin-activated adenylyl cyclase, UTP and CTP were unable to reproduce the cyclase activation detected with GTP. However, in the case of AlF4(-)-stimulated adenylyl cyclase, UTP was as good as GTP at inhibiting cyclase activity, and CTP virtually eliminated the activation of the cyclase with AlF4-.

The network organization and the phosphorylation of cytokeratins are concomitantly modified by forskolin in the enterocyte-like differentiated Caco-2 cell line

Confluent Caco-2 cells, originating from a human colon carcinoma, display morphological and functional characteristics of differentiated enterocytes such as the presence of a polarized monolayer covered by an apical brush border that express several hydrolases. The adaptation of these cells to grow in the continuous presence of forskolin, a drug known to stimulate adenylyl cyclase permanently, has been previously shown to result in a decreased apical expression of hydrolases and in morphological alterations including the disappearance of intercellular spaces and shortening of microvilli. In the present work we have analyzed the possibility that cytoskeletal proteins may be the target of forskolin in living Caco-2 cells. We show that forskolin initiates dramatic changes in the spatial organization of the cytokeratin network that correlate with an increased phosphorylation of cytokeratin molecules, whereas microtubules, microfilaments and vimentin remain mainly unaffected. Indirect immunofluorescence studies show that the cytokeratin network is redistributed from the cell periphery to the cytoplasm. Biochemical experiments indicate that forskolin doesn't interfere with the cytokeratin profile, since the three cytokeratins normally found in intestine (CK 8, CK 18, CK 19) are similarly expressed in both control and forskolin-Caco-2 cells. Analysis of 32P-labeled cytokeratin extracted from the two cell populations demonstrates that forskolin quantitatively increases the phosphorylation of type I cytokeratin (CK 18 and CK 19), whereas the phosphorylation of type II cytokeratin (CK 8) is altered both quantitatively and qualitatively with the emergence of a new phosphorylation site. These results provide a new cell system in which it is possible to control the subcellular distribution of cytokeratin by changing their phosphorylation status and therefore to study their potential cellular functions.

Mediation of hippocampal mossy fiber long-term potentiation by cyclic AMP

Repetitive activation of hippocampal mossy fibers evokes a long-term potentiation (LTP) of synaptic responses in pyramidal cells in the CA3 region that is independent of N-methyl-D-aspartate receptor activation. Previous results suggest that the site for both the induction and expression of this form of LTP is presynaptic. Experimental elevation of cyclic adenosine 3',5'-monophosphate (cAMP) both mimics and interferes with tetanus-induced mossy fiber LTP, and blockers of the cAMP cascade block mossy fiber LTP. It is proposed that calcium entry into the presynaptic terminal may activate Ca(2+)-calmodulin-sensitive adenylyl cyclase I which, through protein kinase A, causes a persistent enhancement of evoked glutamate release.

Modulatory mechanisms of cyclic AMP-stimulated steroid content in rat brain cortex

The modulation of cyclic AMP dependent neurosteroidogenesis was studied in minces prepared from the cerebral cortex of adult rat. Forskolin or dibutyryl-cyclic AMP enhanced pregnenolone and progesterone production in a time and dose-dependent manner. The forskolin effect was mimicked by the cyclic AMP phosphodiesterase inhibitor isobutyl-methyl-xanthine, but not by the adenylate cyclase inactive forskolin analogue 1,9,dideoxy-forskolin. 4'-Chloro-diazepam, a high affinity ligand for the mitochondrial diazepam binding inhibitor (DBI) receptor, also elicited a time dependent increase in steroidogenesis. The forskolin and the 4'-chloro-diazepam stimulated pregnenolone increase was prevented by preexposing the rat brain cortical minces to 1-(2-chlorophenyl)-N-methyl-N-(1-methyl-propyl)-3-isoquinoline carboxamide (PK 11195), a high affinity ligand for the mitochondrial DBI receptor endowed with antagonistic properties. The protein synthesis inhibitor cycloheximide prevented the forskolin and 4'-chloro-diazepam stimulation of pregnenolone formation. In brain cortical minces of adrenalectomised/orchiectomised rats dibutyryl-cyclic AMP increased both pregnenolone and progesterone formation, while forskolin only increased progesterone. These data show that cyclic AMP enhances brain steroidogenesis by acting on a labile protein substrate which interacts with the mitochondrial DBI receptor.

Is cyclic AMP involved in excitatory amino acid-evoked adenosine release from rat cortical slices?

Activation of both N-methyl-D-aspartate (NMDA) and non-NMDA receptors releases endogenous adenosine from superfused rat cortical slices. NMDA-evoked adenosine release is Ca(2+)-dependent and results from the extracellular degradation of a released nucleotide, whereas non-NMDA receptor activation releases adenosine per se in a Ca(2+)-independent manner. IBMX selectively inhibits NMDA- but not non-NMDA-evoked adenosine release. Forskolin, but not 1,9-dideoxy-forskolin, produced a slight but significant increase in NMDA-evoked adenosine release, suggesting that the formation of cyclic AMP may somehow be involved. The inhibition of NMDA-evoked adenosine release by IBMX is not accompanied by enhanced cyclic AMP recovery in superfusates, nor is release diminished when cyclic AMP transport is inhibited by probenecid, suggesting that the adenosine is not derived from the extracellular metabolism of released cyclic AMP. It is possible that 5'AMP, derived from the intracellular conversion of cyclic AMP by phosphodiesterase, might be released during NMDA receptor activation. However, more selective inhibitors of the specific phosphodiesterase isozymes known to be located in the cortex failed to diminish NMDA-evoked adenosine release. Therefore, the effects of both forskolin and IBMX on NMDA-evoked adenosine release could be nonspecific, coincidental and unrelated to their actions on cyclic AMP levels in the cortex. However, it is also possible that a novel IBMX-sensitive phosphodiesterase plays a primary role in converting cyclic AMP to 5'AMP intracellularly during NMDA receptor activation; the 5'AMP could then exit the cells and be converted to adenosine extracellularly.

Activation of adenylate cyclase attenuates the hyperpolarization following single action potentials in brain noradrenergic neurons independently of protein kinase A

Afterhyperpolarizations that follow action potentials are a prominent mechanism for the control of neuronal excitability. Such afterhyperpolarizations in many neurons are modulated by a variety of second messenger systems. Here, we examined the regulation of afterhyperpolarizations in noradrenergic locus coeruleus neurons by the adenylate cyclase system. Although superfusion of the adenylate cyclase activator, forskolin, had no effect on hyperpolarizations following trains of action potentials, both forskolin and a membrane permeable analog of cyclic AMP, 8-bromo-cyclic AMP, attenuated the amplitude of afterhyperpolarizations which followed single action potentials of locus coeruleus neurons recorded intracellularly in brain slices. In contrast, superfusion of 1,9-dideoxyforskolin, the forskolin analog that does not activate adenylate cyclase, had no effect on these single action potential afterhyperpolarizations. Co-application of a protein kinase inhibitor (H8, KT5720, staurosporin or Rp-cAMPS) with either forskolin or 8-bromo-cyclic AMP failed to block the reduction of afterhyperpolarization amplitude, but blocked the cyclic AMP-dependent enhancement of opiate responses in the same locus coeruleus neurons. Furthermore, application of a membrane permeable analog of 5'-AMP, 8-bromo-5'-AMP, the cyclic AMP metabolite that does not activate a protein kinase, potently reduced the amplitudes of single action potential afterhyperpolarizations. The afterhyperpolarization amplitude was also reduced in locus coeruleus neurons taken from chronically morphine-treated rats, a treatment known to increase adenylate cyclase activity. These results indicate that elevation of intracellular cyclic AMP or 5'-AMP reduces the single action potential afterhyperpolarization in locus coeruleus neurons. This action may be mediated through a mechanism independent of protein kinase activation.

Functional expression of a stably transfected parathyroid hormone/parathyroid hormone related protein receptor complementary DNA in CHO cells

Chinese hamster ovary (CHO) cells were stably transfected with OK-O complementary DNA encoding the parathyroid hormone/parathyroid hormone related protein (PTH/PTHrP) receptor derived from opossum kidney (OK) cells (Jüppner et al., 1991). A subclone of transfected CHO cells, CHO-E2, presented high affinity binding of 125I-labeled [Tyr36]chickenPTHrP(1-36)amide ([125I]chPTHrP(1-36)) (Kd 1.28 +/- 0.10 nM) similar to that of wildtype OK cells (Kd 2.23 +/- 0.16 nM) (P < 0.01). Photoaffinity labeling of the PTH/PTHrP receptors using N-hydroxysuccinimidyl-4-azidobenzoate modified [125I]chPTHrP(1-36) revealed the same specifically labeled 90 kDa protein in CHO-E2 and OK cells. In CHO-cells, chPTHrP(1-36) stimulated cyclic AMP accumulation in dose-dependent fashion (EC50 0.15 +/- 0.04 nM) and raised peak cytosolic free calcium concentration (EC50 2.90 +/- 0.36 nM) independent of extracellular calcium, and stimulated phosphate uptake (EC50 0.21 +/- 0.07 nM). Both, chPTHrP(1-36) and 12-O-tetradecanoylphorbol-13-acetate stimulated phosphate uptake were suppressed by staurosporine. But, Sp-cyclic adenosine-3',5'-monophosphothioate did not affect phosphate uptake in CHO-E2 cells. In conclusion, a PTH/PTHrP receptor stably expressed in CHO cells is linked to stimulation of phosphate uptake. Receptor coupling presumably occurred through the protein kinase C rather than the protein kinase A pathway.

Smooth muscle contractility in prostatic hyperplasia: role of cyclic adenosine monophosphate

The role of cyclic 3'-5' adenosine monophosphate (cAMP) on alpha 1-adrenoceptor (alpha 1-receptor) induced smooth muscle contractions in symptomatic benign prostatic hyperplasia (BPH) was investigated. Application of the selective alpha 1-receptor agonist phenylephrine (PE) induced fully reversible contractions in a dose-dependent fashion. Phosphodiesterase (PDE) inhibitors blocking the degradation of cAMP suppressed the PE induced contractions as follows: theophylline (1 mM), 91.1 +/- 1.4%; papaverine (0.5 mM), 822.8 +/- 3.2%; milrinone (0.5 mM), 68.2 +/- 0.6%. Forskolin (50 microM), which elevates cAMP through direct activation of adenylatecyclase (AC), inhibited the PE induced contractions by 82.4 +/- 3.6%. To further increase the intracellular cAMP concentration ([cAMP]i), the membrane permeable cAMP analogue N6-2'-O-dibutyryladenosine derivative (dBcAMP; 1 mM) was applied and reduced the PE evoked contractions by 69.8 +/- 2.3%. We conclude that elevation of [cAMP]i is an important step in inducing smooth muscle relaxation.

Antagonism by forskolin of the 5-HT3 receptor-mediated current in nodose ganglion neurons is independent of cyclic AMP

The effect of forskolin on the inward current mediated by 5-HT3 receptors (5-HT current) was investigated in rat nodose ganglion neurons. Forskolin inhibited the peak amplitude of the 5-HT current and increased current desensitization in a dose-dependent manner. Dideoxyforskolin, which does not stimulate adenylate cyclase, had a similar inhibitory effect on the 5-HT current. The effect of forskolin was neither mimicked by intracellular application of exogenous cyclic AMP (0.5 to 4 mM) nor occluded by intracellular forskolin (30 microM) or protein kinase inhibitor H-7 (100 microM). Intracellular applications of forskolin, cyclic AMP or H-7 had no effect on 5-HT current. Data suggest that forskolin acted at an extracellular site on 5-HT3 receptors and this effect of forskolin was not mediated by cyclic AMP-dependent processes.

A simple procedure for assaying cAMP

A step-by-step protocol for assaying cAMP is presented. This method is based on the standard binding protein assay that is available commercially. However, using the present procedure, the per tube cost is dramatically reduced. In the current protocol, four different binding proteins are compared for their ability to bind cAMP. The source of all reagents is noted as well as necessary precautions for insuring reliable assays. A simple tissue preparation method is outlined for assaying cAMP in brain. The utility of the assay is illustrated by demonstrating the effect of forskolin on cAMP in mouse striatal tissue.

A large contribution of a cyclic AMP-independent pathway to turtle olfactory transduction

Although multiple pathways are involved in the olfactory transduction mechanism, cAMP-dependent pathway has been considered to contribute mainly to the transduction. We examined the degree of contribution of cAMP-independent pathway to the turtle olfactory response by recording inward currents from isolated cells, nerve impulses from cilia and olfactory bulbar responses. The results obtained by the three recordings were essentially consistent with each other, but detail studies were carried out by recording the bulbar response to obtain quantitative data. Application of an odorant cocktail to the isolated olfactory neuron after injection of 1 mM cAMP from the patch pipette elicited a large inward current. Mean amplitude of inward currents evoked by the cocktail with 1 mM cAMP in the patch pipette was similar to that without cAMP in the pipette. Application of the cocktail after the response to 50 microM forskolin was adapted also induced a large inward current. Application of the odorant cocktail to the olfactory epithelium, after the response to 50 microM forskolin was adapted, brought about an appreciable increase in the impulse frequency. The bulbar response to forskolin alone reached a saturation level around 10 microM. After the response to 50 microM forskolin was adapted, 11 species of odorants were applied to the olfactory epithelium. The magnitudes of responses to the odorants after forskolin were 45-80% of those of the control responses. There was no essential difference in the degree of the suppression by forskolin between cAMP- and IP3-producing odorants classified in the rat, suggesting that certain part of the forskolin-suppressive component was brought about by nonspecific action of forskolin. Application of a membrane permeant cAMP analogue, cpt-cAMP elicited a large response, and 0.1 mM citralva after 3 mM cpt-cAMP elicited 51% of the control response which was close to the response to citralva after 50 microM forskolin. A membrane permeant cGMP analogue, db-cGMP elicited a small response and the response to 0.1 mM citralva was unaffected by db-cGMP. It was concluded that cAMP-independent (probably IP3-independent) pathway greatly contributes to the turtle olfactory transduction.

Forskolin and 3-isobutyl-1-methylxanthine increase basal and sodium nitroprusside-elevated cyclic GMP levels in adult guinea-pig cerebellar slices

In this study, the interaction between 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) in [3H]adenine- or [3H]-guanine-prelabelled adult guinea-pig cerebellar slices was investigated. Basal levels of [3H]cGMP were enhanced by forskolin, although no plateau was reached over the concentration range tested (0.1-100 microM). However, forskolin elicited a concentration-dependent, saturable potentiation of sodium nitroprusside (SNP)-stimulated [3H]cGMP accumulation (forskolin EC50 value of 0.98 +/- 0.23 microM; 10 microM forskolin produced a 1.8 +/- 0.3-fold potentiation of the SNP response at 2.5 min). The forskolin potentiation was observed at all concentrations of SNP tested (0.001-10 mM). Forskolin also elicited a large stimulation of [3H]-cAMP in [3H]adenine-prelabelled guinea-pig cerebellar slices; however, 1,9-dideoxyforskolin failed to elicit either a [3H]cAMP response or a potentiation of the SNP-induced [3H]cGMP response at concentrations up to 100 microM. Pretreatment with oxyhaemoglobin (50 microM) inhibited the response to SNP (1 mM) and forskolin (10 microM), as well as the response evoked by the combination of SNP and forskolin. NG-Nitro-L-arginine (100 microM) inhibited the response to forskolin alone, but did not change the response to SNP or the potentiation induced by forskolin on SNP-induced [3H]cGMP levels. The protein kinase inhibitors 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7; 100 microM), staurosporine (10 microM), polymyxin B (100 microM), and Ro 31-8220 (10 microM) had no effect on the [3H]cGMP response to either SNP or the combination of SNP plus forskolin. N6,2'-Dibutyryl cAMP, at concentrations up to 10 mM, was also without effect on [3H]cGMP levels induced by SNP.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparative study of the role of adenylate cyclase in the release of adrenocorticotropin from the ovine and rat anterior pituitary

The interaction between corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) is important in the regulation of adrenocorticotropin (ACTH) release from the anterior pituitary (AP). CRF exerts its effect on the AP by activating the adenylate cyclase (AC) complex whereas AVP increases the turnover of phosphatidylinositol. In the rat and in man, CRF is the most potent ACTH secretagogue whereas AVP alone is only a weak agonist. Since recent studies in the sheep indicate a reversal of this order of potency, these studies were undertaken to test the hypothesis that a functional alteration of the AC in the ovine corticotrope might limit the ability of CRF to release ACTH from these cells. When rat AP cells were incubated with CRF, a dose-dependent increase in AC activity was observed. This effect was potentiated either by AVP or PMA, although neither agent alone altered AC activity. In contrast, CRF alone, or in combination with AVP or PMA, did not increase AC activity in ovine AP cells. Both cholera toxin (CT) and pertussis toxin (PT) caused a dose-dependent release of ACTH from rat and ovine AP cells, but the amount of ACTH released from the ovine AP cells by both agents was relatively reduced. In the ovine cells, however, AVP acted synergistically with CT or PT to markedly increase the release of ACTH to levels which approached those obtained when the rat AP cells were exposed to CT or PT alone. Forskolin increased AC activity in AP cells of both species, but to a much lower extent in ovine cells than in the rat cells. However, when the ovine cells were exposed to AVP, the AC response to forskolin became similar to the response observed in the rat cells when incubated with forskolin alone. Forskolin also released significantly less ACTH from the ovine AP cells, but AVP also acted synergistically with forskolin to greatly enhance the amount of ACTH released from these cells. Finally, 8-bromo-cyclic AMP produced a similar release of ACTH from both ovine and rat AP cells. We conclude that: (1) the decreased ability of CRF to increase ACTH release from the ovine AP reflects a net decrease in AC activity and cannot be ascribed to an ovine corticotropic resistance to cAMP; (2) the decreased activity of the ovine corticotropic AC complex may in turn reflect functional alterations at the level of both the G proteins and the catalytic subunit; (3) since AVP causes protein kinase C substrate phosphorylation in the ovine AP, AVP may increase AC activity in this tissue by phosphorylating the G proteins and/or the catalytic subunit.

Inhibition of protein phosphatases by okadaic acid and calyculin-A differentially modulates hormonal- and forskolin-stimulated formation of cyclic AMP in AtT-20 corticotrophs: effect of pituitary adenylate activating polypeptide and corticotropin-releasing factor

The effect of phosphatase inhibitors okadaic acid and calyculin-A on cAMP formation and adrenocorticotropic hormone (ACTH) secretion in AtT-20 corticotrophs was investigated. Both okadaic acid and calyculin-A inhibited dose-dependently the accumulation of cAMP in cells stimulated with pituitary adenylate cyclase activating factor (PACAP) and corticotropin-relating hormone (CRF). While in the case of okadaic acid the half-maximum inhibiting concentration was similar for both peptides (IC50 = 4 x 10(-7) M), it appeared that calyculin-A was about one order of magnitude more efficient in inhibiting the effect of PACAP than that of CRF (IC50 = 3.8 x 10(-9) M vs 2.0 x 10(-8) M, respectively). Importantly, the inhibitors blocked the activation by cholera toxin (which acts on Gs-like proteins) of cAMP formation, but failed to alter the effect of forskolin (which bypasses the receptor-G protein complex and activates adenylyl cyclase directly). Treatment of cells with calyculin-A significantly dampened adenylyl cyclase activity in cell membrane fraction, though to a lesser extent than it blocked cAMP formation in the whole cell. Both okadaic acid and calyculin-A inhibited CRF- and PACAP-induced secretion of ACTH. Our data hint that in AtT-20 corticotrophs, inhibition of phosphatases by modulating the state of phosphorylation of the receptor-G proteins complexes for CRF and PACAP, regulates cAMP formation and ACTH secretion.

Nicotine-induced antinociception in mice: role of G-proteins and adenylate cyclase

The effects of pertussis toxin, forskolin, and cAMP analogues on the antinociceptive action of nicotine were examined to investigate the possible involvement of adenylate cyclase and G-proteins in nicotine's antinociceptive effect. Intrathecal injection of pertussis toxin (0.25 and 0.50 micrograms) in mice inhibited nicotine-induced antinociception in the tail-flick test. The effect of the toxin was dose and time dependent. Forskolin, a potent adenylate cyclase activator, and 8-(-4-chlorophenylthio) adenosine-3':5' monophosphate, cyclic (8-CPT-cAMP), a cAMP analogue, inhibited the antinociceptive effects of nicotine in a dose-dependent manner. EGTA reversal of 8-CPT-cAMP's inhibitory effects suggests that calcium may to be involved. These data implicate the possible involvement of a G-protein and a second messenger system (activation of a cAMP-dependent protein kinase and increase in cyclic AMP levels) in nicotine-induced analgesia in mice.

Characterization of adenosine receptors in a model of cultured neurons from rat forebrain

The neuromodulator adenosine is acting through specific receptors coupled to adenylate cyclase via G-proteins. The expression of both adenosine receptors A1 and A2 as well as forskolin binding sites was investigated by radioligand binding techniques in 8-day-old neurons isolated from fetal rat forebrain and cultured in chemically-defined medium. Adenosine A1 receptors were specifically labeled with [3H]chloro-N6-cyclopentyladenosine (CCPA), whereas [3H]CGS 21680 was used for the analysis of A2 receptors. Cultured neurons exhibited high affinity binding sites for CCPA (Bmax = 160 fmol/mg protein; Kd = 2.9 nM), and for CGS 21680 (Bmax = 14 fmol/mg protein; Kd = 1.7 nM). These data correlate well with those obtained in crude membranes isolated from the newborn rat forebrain. The incubation of culture membranes in the additional presence of guanylyl-5'-imidodiphosphate (Gpp(NH)p, a GTP analogue) led to significantly increased Kd-values, suggesting the association of adenosine receptors with G-proteins. Finally, cultured neurons also bound specifically [3H]forskolin with characteristics close to those found in the newborn brain, indicating that cultured neurons appear as an appropriate model for studying the neuromodulatory properties of adenosine.

Amylin (islet amyloid polypeptide) inhibition of insulin release in the perfused rat pancreas: implication of the adenylate cyclase/cAMP system

Amylin inhibits glucose-induced insulin secretion in the rat pancreas. To study the mechanism by which amylin acts on the B-cell, we have investigated, in the perfused rat pancreas, the effect of synthetic rat amylin (75 pM) on insulin release elicited by secretagogues acting on the B-cell via the adenylate cyclase/cAMP system, i.e., glucagon (10 nM), gastric inhibitory polypeptide (GIP, 1 nM), forskolin (1 microM) and isobutylmethylxanthine (IBMX, 75 microM). In addition, we examined the effect of amylin on GIP-induced insulin release in pancreata from rats pretreated with pertussis toxin, an agent which inactivates certain Gi proteins coupled to adenylate cyclase. Amylin inhibited the insulin response to glucagon (approx. 70%), GIP (approx. 90%), IBMX (approx. 75%) as well as the early phase of forskolin-induced insulin output (approx. 74%). However, amylin failed to modify GIP-induced insulin release in pancreata obtained from pertussis toxin pretreated rats. These results would indicate that the inhibitory effect of amylin on insulin secretion could be, at least in part, attributed to its interfering with the adenylate cyclase/cAMP system. Furthermore, prevention of the inhibitory effect of amylin on GIP-induced insulin output by pertussis toxin pretreatment, supports the concept that amylin can inhibit insulin release via a pertussis toxin-sensitive Gi protein coupled to the adenylate cyclase system.

Regulation of VLDL secretion in primary culture of rat hepatocytes: involvement of cAMP and cAMP-dependent protein kinases

When hepatocytes were cultured for 24 h in the presence of forskolin (10(-4) mol l-1) or isobutylmethylxanthine (IBMX, 10(-3) mol l-1), the intracellular cAMP concentration peaked (320-380 pmol mg-1 protein) after 10-20 min of culture. This increase was accompanied by a decrease in the secretion of triacylglycerol, cholesterol and apoprotein B associated with VLDL. After 4 h cAMP levels had returned almost to basal values but the inhibition of VLDL secretion persisted. There was a small intracellular accumulation of triacylglycerol but not of apoprotein B. Addition of forskolin and IBMX together led to a further increase in intracellular cAMP and a further suppression of VLDL output. Similar effects on the secretion of VLDL were also observed after addition of Bt2cAMP. Exposure of cell cultures to glucagon (10(-7) mol l-1) for only 10 min raised cellular cAMP levels to > 200 pmol mg-1 protein, and suppressed VLDL secretion during the next 24 h to < 40% of control. All of the substances tested inhibited de novo synthesis of fatty acids but had little or no effect on cholesterol synthesis and did not inhibit oleate esterification to triacylglycerol. The cAMP-dependent protein kinase antagonist Rp-cAMPS prevented suppression of VLDL triacylglycerol secretion induced by glucagon (10(-7) mol l-1) and abolished glucagon-induced ketogenesis. Rp-cAMPS also inhibited Bt2cAMP (7.5 x 10(-6) mol l-1)-induced suppression of VLDL secretion and enhancement of ketogenesis. It is concluded that rat hepatic VLDL metabolism can be regulated by cAMP and cAMP-dependent protein kinases, and that the initial transient rise in cellular cAMP levels induced by glucagon is sufficient to maintain a long-term inhibitory effect on assembly and secretion of VLDL.