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

Modulation of nicotinic acetylcholine receptor activity in submucous neurons by intracellular messengers

The effects on acetylcholine-induced membrane currents (ACh currents), produced by agents known to modify the activity of intracellular messengers, were studied in the neurons of the guinea-pig ileum submucous plexus (SMP) using a whole-cell patch clamp recording method. The ACh currents were not affected by forskolin, the adenylate cyclase activator, regardless of whether or not ATP and GTP were present in the intracellular solution, and by phorbol 12-myristate 13-acetate, the protein kinase C activator. The ACh currents were strongly suppressed by thapsigargin, the microsomal calcium ATPase inhibitor, and genistein, the tyrosine protein kinase inhibitor. They were also suppressed by 3-isobutyl-1-methylxanthine, the cyclic-AMP phosphodiesterase inhibitor, regardless of the presence of forskolin in the extracellular solution and ATP and GTP in the intracellular solution. In addition, the currents were suppressed by activation of P2 purinoceptors with ATP, which could not be explained by a direct effect of ATP on nicotinic acetylcholine receptors (nAChRs). Reactive blue 2, the P2y purinoceptor antagonist, did not abolish inhibition of the ACh current by ATP. Alpha,beta-Imido-ATP and adenosine caused no membrane current responses and did not influence the ACh currents. These results suggest that the activity of the nAChRs in the SMP neurons is strongly suppressed by raised intracellular Ca2+ level, without involvement of protein kinases A and C, and may involve the participation of tyrosine kinase. The activity of nAChRs is also influenced by the activity of P2 purinoceptors; the mechanisms responsible for this influence are not yet clear. So, the activity of the SMP neuronal nAChRs is relatively independent on the intracellular signaling known to influence many other groups of transmitter-gated receptors of neuronal membrane.

Masking of forskolin-induced long-term potentiation by adenosine accumulation in area CA1 of the rat hippocampus

At hippocampal Schaffer collateral-CA1 synapses, activation of beta-adrenergic receptors and adenylyl cyclase increases transmitter release. However, this effect is transient, which is in contrast to that seen at mossy fiber-CA3 synapses, where activation of cyclic-AMP-dependent protein kinase results in long-lasting facilitation of transmitter release, a phenomenon known as a presynaptic form of long-term potentiation. The present study was aimed at investigating whether forskolin, an adenylyl cyclase activator, could produce long-term effects at the Schaffer collateral-CA1 synapses using extracellular recording techniques. As has been reported previously, forskolin persistently increased the amplitude of evoked population spikes without having a long-term effect on the field excitatory postsynaptic potentials. However, under the conditions where adenosine A1 receptors are inhibited, cyclic-AMP metabolism is disrupted or the transport of cyclic-AMP is blocked, forskolin induces long-term potentiation. Forskolin-induced potentiation is associated with a decrease in paired-pulse facilitation and is blocked by the cyclic-AMP-dependent protein kinase inhibitor Rp-adenosine-3',5'-cyclic monophosphorothioate. Activation of N-methyl-D-aspartate receptors is not required for forskolin-induced long-term potentiation, because pretreatment of slices with the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate did not prevent forskolin-induced potentiation. These results suggest that blockade of adenosine A1 receptors unmasks forskolin-induced long-term potentiation, and activation of cyclic-AMP-dependent protein kinase induces a form of long-term potentiation which is different from that induced by tetanic stimulation.

The in vivo effects of milrinone on the airways of cystic fibrosis mice and human subjects

Previous studies have indicated that milrinone, a specific type III phosphodiesterase inhibitor, may be able to induce chloride secretion in cystic fibrosis (CF) tissues. We have now assessed the effect of this agent in vivo on the nasal epithelium of CF mutant mice and also in the nose and lungs of human subjects with CF. Wild-type mice showed a small hyperpolarization of the nasal potential difference (PD) in response to milrinone (100 microM, 1.6 +/- 0.6 mV, n = 8, P < 0.05). In contrast, CF mice carrying either the most common human mutation of the gene for the CF transmembrane regulator (CFTR), DeltaF508 (protein mislocalized), or the G551D mutation (protein normally localized) failed to demonstrate this response. Milrinone perfused alone had no significant effect on the baseline nasal PD of human subjects without CF (14.7 +/- 4.0 mV preperfusion; 15.3 +/- 4.6 mV postperfusion), but significantly (P < 0.05) augmented the hyperpolarization induced by a subsequently perfused low-chloride solution (with milrinone, 36.8 +/- 3.0 mV, n = 6; without milrinone, 18.1 +/- 2.2 mV, n = 19). In contrast, in human subjects with CF (n = 6), milrinone alone significantly (P < 0. 05) altered the nasal baseline PD (52.2 +/- 3.3 mV preperfusion; 57. 4 +/- 4.2 mV, postperfusion) but not the subsequent responses to the low-chloride solution (with milrinone, 1.1 +/- 2.2 mV, n = 4; without milrinone, 0.6 +/- 0.5 mV, n = 28) or to isoproterenol (100 microM). In a separate study in subjects (n = 6) with the DeltaF508 mutation, nasal coadministration of milrinone with isoproterenol produced no effect in the presence of amiloride and a low-chloride solution (-0.8 +/- 0.5 mV). This was also the case in the nasal epithelium of CF subjects (n = 4) carrying at least one G551D allele (-0.3 +/- 0.8 mV). Similarly, milrinone did not hyperpolarize the PD of either the tracheal (n = 6) or segmental (n = 6) airways of CF subjects (DeltaF508) when applied topically in vivo in the presence of amiloride, isoproterenol, or adenosine triphosphate (all 100 microM) in a low-chloride solution. These data do not support the use of milrinone to induce chloride secretion in CF airways in vivo.

The adenylyl and guanylyl cyclase superfamily

New structures solved in 1997 revealed that the adenylyl cyclase core consists of a pair of catalytic domains arranged in a wreath. Homologous catalytic domains are arranged in diverse adenylyl and guanylyl cyclases as symmetric homodimers or pseudosymmetric heterodimers. The kinship of the adenylyl and guanylyl cyclases has been confirmed by the structure-based interconversion of their nucleotide specificities. Catalysis is activated when two metal-binding aspartate residues on one domain are juxtaposed with a key aspargine-arginine pair on the other. Allosteric activators of mammalian adenylyl cyclase, forskolin and the stimulatory G protein alpha subunit, promote the catalytically optimal juxtaposition of the two domains.

Adenylyl cyclase activity and glucose release from the liver of the European eel, Anguilla anguilla

The properties of adenylyl cyclase (AC) in liver membranes of the European eel (Anguilla anguilla) and the involvement of cAMP in glucose release from isolated hepatocytes in response to catecholamines were studied. Basal enzyme activity seemed essentially unaffected by GTP, while a biphasic response to increasing nucleotide concentrations was obtained in the presence of epinephrine. Eel liver AC was dose-dependently stimulated by guanosine 5'-O-(3-thiotriphosphate) and inhibited by guanosine 5'-O-(2-thiodiphosphate). AC activity, intracellular cAMP levels, and glucose release from isolated hepatocytes were significantly enhanced by NaF, forskolin, epinephrine, and phenylephrine. The rise in cAMP production stimulated by catecholamines was counteracted by propranolol, but not by phentolamine. Catecholamine-induced glucose output was instead partially antagonized by both phentolamine and propranolol. Complete inhibition was obtained only by the simultaneous presence of the two adrenergic antagonists. Glucose release from the cells was induced by dibutyryl cAMP and by the calcium ionophore ionomycin. In summary, these data provide the first characterization of eel liver AC system and suggest a direct role for cAMP in the catecholamine-dependent glucose output. Furthermore, the involvement of calcium ions in this cellular response is hypothesized.

Vasoactive intestinal peptide potentiates and directly stimulates catecholamine secretion from rat adrenal chromaffin cells

The actions of vasoactive intestinal polypeptide (VIP) on catecholamine secretion and changes in [Ca2+]i in single rat chromaffin cells were studied using amperometry and Indo-1. Application of VIP prior to acetylcholine (ACh) or co-application of VIP and ACh enhanced secretion by 94% and 153% respectively, compared to ACh alone. [Ca2+]i was increased by 17% when VIP was preapplied and by 73% upon co-application. Exposure to VIP before stimulation with 60 mM K+ enhanced secretion by 68%, but not [Ca2+]i. VIP application prior to DMPP and nicotine had no effect on [Ca2+]i, but increased [Ca2+]i signals to muscarine by 18%. VIP co-application potentiated only [Ca2+]i responses to muscarine, by 28%. The effect of VIP on muscarine-induced [Ca2+]i signals was mimicked by 8-Br-cAMP, and both were blocked by H-89, a protein kinase A inhibitor. Long-lasting increases in secretion accompanied by a sustained rise in [Ca2+]i to VIP alone were seen in 55% of cells. Removal of Ca2+ or addition of La3+ inhibited both responses, while L-, N- and P-type Ca2+ channel blockers were ineffective. SK&F 96365 inhibited VIP-induced secretion completely and rises in [Ca2+]i by 75%. Neither 8-Br-cAMP nor 8-Br-cGMP evoked responses similar to VIP alone. Thus in rat chromaffin cells, VIP acts both directly as a neurotransmitter in provoking sustained catecholamine secretion in a cAMP-independent manner, and also by enhancing ACh-induced secretion, via a cAMP-dependent action involving muscarinic receptors.

Inhibition of voltage-activated K+ currents in smooth muscle cells of the guinea pig proximal colon by noradrenergic agonists

1. The effects of noradrenaline and isoprenaline on the Ca2+i-insensitive, voltage-activated K+ current in smooth muscle cells from the circular muscle layer of the guinea pig proximal colon were investigated by using standard whole-cell patch-clamp techniques at room temperature (22-24 degrees C). 2. The Ca2+-activated K+ current was eliminated by bathing cells in tetraethylammonium (TEA;2-5 mM) and a Ca2+-entry blocker (Cd2+, 0.1 mM) or nifedipine, 2-10 microM) and by internally perfusing cells with 3 mM EGTA. 3. Two Ca2+i-insensitive, voltage-activated K+ currents were recorded at potentials positive to -50 mV: (a) a transient K+ current (IKto) that was blocked by 4-aminopyridine (5 mM) and (b) a delayed rectifier-type K+ current (IKdel) that was blocked by TEA (>10 mM). 4. Both noradrenaline (10-50 microM) and isoprenaline (5-50 microM) reduced the amplitudes of IKto and IKdel irreversibly after a slow onset (2-5 min). This reduction was mimicked by forskolin (50-100 microM) and by 8 bromo-c-AMP (500 microM). 5. The voltage of half-maximal availability (V0.5) of IKto (-74.6+/-2.3 mV) was unaffected by isoprenaline (10 microM) (-76.7+/-3.6 mV, n=4), but the background "leak" current (Ileak) was increased from -48+/-9 to -70+/-20 pA. 6. Our data suggest that stimulation of beta-adrenoceptors in the circular muscle layer of the guinea pig proximal colon inhibits voltage-activated Ca2+i-insensitive K+ currents.

Eating induced by perifornical cAMP is behaviorally selective and involves protein kinase activity

It has previously been shown that agents that increase endogenous cAMP elicit robust eating when injected into the perifornical hypothalamus (PFH) but not when injected into surrounding brain sites, suggesting that PFH cAMP may play a role in eating control. We report here that bilateral microinjection of the adenylyl cyclase activator 7-deacetyl-7-O-(N-methylpiperazino)-gamma-butyryl-forskolin dihydrochloride (MPB forskolin; 300 nmol/0.3 microl) into the PFH is sufficient to elicit intense eating (up to 15.7 +/- 2.3 g in 2 h) in satiated rats, without concomitant effects on other behaviors, including gnawing and drinking. In contrast, the inactive analog 1, 9-dideoxyforskolin is ineffective, suggesting that the effects of MPB forskolin are behaviorally selective and pharmacologically specific. We also show that injection of the protein kinase A inhibitor H-89 (100 nmol) into the PFH reduced MPB forskolin-induced eating by up to 50%. Collectively, these results suggest that increased cAMP production in a single brain area may be sufficient to selectively generate a patterned, goal-oriented behavior by activating cAMP-dependent protein kinase.

The adrenomedullary venous vasculature as a target for endothelins: comparison of the porcine and bovine central adrenomedullary veins

The functional properties of the isolated porcine and bovine central adrenomedullary veins were compared, with emphasis on the active tension responses to high K+, endothelin-1 (ET-1) and neuropeptide Y (NPY). In the porcine vein, the contraction evoked by ET-1 was 4--5-fold higher than with high K+, as in the bovine vein. The potencies for ET-1 were similar in ring and strip preparations of the porcine vein, with EC50 values 5--7-fold higher than in the bovine vein. In preparations previously exposed to ET-1 the contractions evoked by high K+ and NPY were potentiated and facilitated, respectively,. However, only in the porcine vein was the ET-1 contraction sustained. This contraction was effectively relaxed by milrinone, indicating a role for cGMP inhibited cyclic nucleotide phosphodiesterase in the sustained contraction. Caffeine and forskolin were also effective relaxants of contractions evoked by ET-1 in both veins, suggesting relaxation by elevated levels of cAMP. The K(+)-contracted porcine, but not bovine, vein was relaxed by acetylcholine (ACh) and vasointestinal polypeptide in a concentration-dependent manner, indicating species differences with respect to signal transduction leading to increases in cyclic nucleotides. In conclusion, these results demonstrate that ET-1 is the main constrictor of the porcine central adrenomedullary vein, with significant species differences in mode of contraction and relaxation. These findings suggest roles for the endogeneously released ET-1 and NPY in regulation of venous contractility within the adrenal gland of mammals.

Influence of aluminum on the regulation of PTH- and 1,25(OH)2D3-dependent pathways in the rat osteosarcoma cell line ROS 17/2.8

The role of hormonal status in the development of aluminum (Al)-dependent renal osteodystrophy, which is characterized by reduced bone matrix deposition, still remains largely unknown. To address this question, we used the osteoblast-like osteosarcoma cell line ROS 17/2.8 to evaluate the role of Al on parathyroid hormone (PTH)- and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3)-dependent activities in these cells. Al (1 microM) caused an inhibition of basal and 1,25(OH)2D3-induced alkaline phosphatase, but only at low doses (< 1 nM) of the steroid. Al partly inhibited basal osteocalcin (OC) secretion in ROS cells (p < 0.001), and the dose-dependent increase in 1,25(OH)2D3-induced OC release by these cells was also reduced by 1 microM Al at low concentrations of the steroid (< or = 1 nM), whereas high doses of 1,25(OH)2D3 (> or = 5 nM) totally prevented the inhibiting effects of Al. Al also had strong inhibitory actions on PTH-dependent cAMP production by ROS cells over the concentration range tested (0.5-50 nM). This inhibitory action of Al was also observed for PTH-related peptide- (PTHrp, 50 nM) but not for Isoproterenol-dependent (100 nM) cAMP formation. To evaluate more fully the mechanism of this inhibition of cAMP formation, we investigated the effect of Al on toxin-modulated, G protein-dependent regulation of cAMP formation and on the activation of adenylate cyclase by Forskolin. Cholera toxin (CT, 10 micrograms/ml), applied to cells for 4 h prior to PTH challenge, enhanced cAMP production about 2-fold above PTH alone (p < 0.001), a process that was further stimulated by Al. Pertussis toxin (PT, 1 microgram/ml, 4 h) did not modify basal PTH-dependent cAMP formation by ROS cells. However, PT treatment prevented the inhibitory effect of Al on cAMP formation by these cells (p < 0.025). The stimulation of adenylate cyclase by Forskolin (0.1 and 1 microM), which bypasses G protein regulation, was not modified by Al, indicating that Al does not affect adenylate cyclase directly. Northern blot analysis of PTH receptor mRNA levels showed that Al did not modify PTH receptor message in ROS cells. Likewise, Western blot analyses of G protein subunits showed that Al did not significantly alter Gs alpha subunit levels, in accordance with the results obtained for cAMP-dependent formation in response to CT. In contrast, Gi alpha-1 and Gi alpha-2 subunits were decreased by Al treatment, consistent with PT-restricted increases in cAMP formation in Al-treated ROS cells. Taken together, these results suggest that Al has multiple actions in osteoblast-like ROS cells. The effects of Al are modulated by hormonal control of the pathways investigated. Al affects 1,25(OH)2D3-regulated functions only when this steroid is low. Al has large inhibitory effects on PTH- and PTHrp-dependent cAMP formation. This last feature is related to the ability of Al to alter the G protein transducing pathway for PTH/PTHrp-dependent formation of cAMP since it does not affect adenylate cyclase activity directly and does not affect the PTH receptor message level. Thus, Al has stronger deleterious effects in osteoblast-like cells with an already compromised 1,25(OH)2D3 status and can modulate specifically PTH/PTHrp-mediated cAMP formation at the postreceptor level.

The second messenger cAMP elicits eating by an anatomically specific action in the perifornical hypothalamus

We have previously shown that a membrane-permeant analog of cAMP, 8-bromo-cAMP (8-br-cAMP), elicits a vigorous eating response when microinjected into the perifornical hypothalamus (PFH) or lateral hypothalamus (LH) of satiated rats, suggesting that increases in cAMP in these areas may be important in the neural control of eating. To determine the locus of this effect, we compared the ability of 8-br-cAMP (1-100 nmol/0.3 microl) to elicit eating after microinjection into the PFH, LH, or the following bracketing areas: the anterior and posterior LH, paraventricular nucleus of the hypothalamus, thalamus, and amygdala. 8-br-cAMP at 50 nmol elicited eating (>/=3.4 gm in 2 hr) exclusively in the PFH and LH. At 100 nmol, 8-br-cAMP elicited a larger response in these areas and elicited a smaller, more variable response in the thalamus. We similarly mapped the feeding-stimulatory effects of compounds that increase endogenous cellular cAMP in naive rats. Combined microinjection of matched doses (300 nmol) of 3-isobutyl-1-methylxanthine and 7-deacetyl-7-O-(N-methylpiperazino)-gamma-butyryl-forskolin was effective exclusively in the PFH, eliciting an average 2 hr food intake of 8.4 +/- 2.0 gm. Collectively, these results suggest that increases in cellular cAMP within a specific brain site, the PFH, may play a role in the neural stimulation of eating.

Decreased expression of natriuretic peptide A receptors and decreased cGMP production in the choroid plexus of spontaneously hypertensive rats

Atrial natriuretic peptide receptor (ANP) subtypes and their signal transduction response were characterized in choroid plexus of spontaneously hypertensive (SHR) and normotensive (WKY) rats. We found two ANP receptor subtypes, guanylate cyclase coupled and uncoupled, in both rat strains. Binding of ANP was lower in SHR choroid plexus when compared to WKY. The lower ANP binding in SHR was the result of a decrease of binding to the guanylate cyclase-coupled receptor subtype A, a decrease that correlated well with the decreased ANP-induced cGMP formation in SHR. Forskolin stimulated cGMP production to the same extent in both strains. In WKY rats, ANP increased basal and forskolin-stimulated cAMP production; conversely, in SHR, ANP did not affect the basal level of cAMP and inhibited the forskolin-stimulated cAMP production. These results demonstrate differences in ANP receptor subtype expression, and ANP signal transduction in choroid plexus of hypertensive and normotensive rats, which is of possible significance to the central mechanisms of blood pressure control.

Long-term potentiation of synaptic transmission in the avian hippocampus

The avian hippocampus plays a pivotal role in memory required for spatial navigation and food storing. Here we have examined synaptic transmission and plasticity within the hippocampal formation of the domestic chicken using an in vitro slice preparation. With the use of sharp microelectrodes we have shown that excitatory synaptic inputs in this structure are glutamatergic and activate both NMDA- and AMPA-type receptors on the postsynaptic membrane. In response to tetanic stimulation, the EPSP displayed a robust long-term potentiation (LTP) lasting >1 hr. This LTP was unaffected by blockade of NMDA receptors or chelation of postsynaptic calcium. Application of forskolin increased the EPSP and reduced paired-pulse facilitation (PPF), indicating an increase in release probability. In contrast, LTP was not associated with a change in the PPF ratio. Induction of LTP did not occlude the effects of forskolin. Thus, in contrast to NMDA receptor-independent LTP in the mammalian brain, LTP in the chicken hippocampus is not attributable to a change in the probability of transmitter release and does not require activation of adenylyl cyclase. These findings indicate that a novel form of synaptic plasticity might underlie learning in the avian hippocampus.

Conversion of forskolin-insensitive to forskolin-sensitive (mouse-type IX) adenylyl cyclase

Forskolin potently activates all cloned mammalian adenylyl cyclases except type IX by interacting with two homologous cytoplasmic domains (C1 and C2) that form the catalytic core. A mutational analysis of the IIC2 protein (C2 domain from type II adenylyl cyclase) and forskolin analogs suggests that Ser942 interacts with the 7-acetyl group of forskolin. The C1/C2 complex has only one forskolin, one ATP, and one binding site for the alpha subunit of the G protein that stimulates adenylyl cyclase (Gsalpha) and its structure may be modeled using the three-dimensional structure of (IIC2/forskolin)2. The Ser942 mutation defines which forskolin in the (IIC2/forskolin)2 structure exists in C1/C2 complex. Thus, the forskolin-binding site is close to the Gsalpha-binding site but distal (15-20A) from the catalytic site. Mutation from Leu912 of IIC2 protein to tyrosine or alanine severely reduces Gsalpha activation and completely prevents forskolin activation. The corresponding residue of Leu912 is Tyr1082 at type IX isoform of adenylyl cyclase. Similar to recombinant type IX enzyme, soluble adenylyl cyclase derived from mouse-type IX adenylyl cyclase is sensitive to Gsalpha activation but not to forskolin. Changing Tyr1082 to leucine makes soluble type IX adenylyl cyclase forskolin-responsive.

Inhibition of forskolin-induced sensitisation of frog skin nociceptors by the cyclic AMP-dependent protein kinase A antagonist H-89

The role of cAMP and protein kinase A (PKA) in nociceptor sensitisation has been studied in frog skin in vitro. Multifibre nerve responses during noxious heating were enhanced by adding forskolin, an agent that elevates cAMP. H-89, a PKA antagonist, blocked the increased responses due to forskolin, but did not affect baseline responses. Thus, cAMP appears to act via PKA when sensitising nociceptors in frog skin.

The phosphoprotein DARPP-32 mediates cAMP-dependent potentiation of striatal N-methyl-D-aspartate responses

The signal transduction pathway underlying the cAMP-dependent modulation of rat striatal N-methyl-D-aspartate (NMDA) responses was investigated by using the two-electrode voltage-clamp technique. In oocytes injected with rat striatal poly(A)+ mRNA, activation of cAMP-dependent protein kinase (PKA) by forskolin potentiated NMDA responses. Inhibition of protein phosphatase 1 (PP1) and/or protein phosphatase 2A (PP2A) by the specific inhibitor calyculin A occluded the PKA-mediated potentiation of striatal NMDA responses, suggesting that the PKA effect was mediated by inhibition of a protein phosphatase. Coinjection of oocytes with striatal mRNA and antisense oligodeoxynucleotides directed against the protein phosphatase inhibitor DARPP-32 dramatically reduced the PKA enhancement of NMDA responses. NMDA responses recorded from oocytes injected with rat hippocampal poly(A)+ mRNA were not affected by stimulation of PKA. When oocytes were coinjected with rat hippocampal poly(A)+ mRNA plus complementary RNA coding for DARPP-32, NMDA responses were potentiated after stimulation of PKA. The results provide evidence that DARPP-32, which is enriched in the striatum, may participate in the signaling between the two major afferent striatal pathways, the glutamatergic and the dopaminergic projections, by the cAMP-dependent regulation of striatal NMDA currents.

The role of plant-derived drugs and herbal medicines in healthcare

Many of our present medicines are derived directly or indirectly from higher plants. While several classic plant drugs have lost much ground to synthetic competitors, others have gained a new investigational or therapeutical status in recent years. In addition, a number of novel plant-derived substances have entered into Western drug markets. Clinical plant-based research has made particularly rewarding progress in the important fields of anticancer (e.g. taxoids and camptothecins) and antimalarial (e.g. artemisinin compounds) therapies. In addition to purified plant-derived drugs, there is an enormous market for crude herbal medicines. Natural product research can often be guided by ethnopharmacological knowledge, and it can make substantial contributions to drug innovation by providing novel chemical structures and/or mechanisms of action. In the end, however, both plant-derived drugs and crude herbal medicines have to take the same pharmacoeconomic hurdle that has become important for new synthetic pharmaceuticals.

Cardiac beta-adrenergic receptor function in fetal sheep exposed to long-term high-altitude hypoxemia

In this study, we hypothesized that a reduction in beta-adrenergic receptor number or a decrease in functional coupling of the receptor to the adenylate cyclase system may be responsible for the blunted inotropic response to isoproterenol observed in fetal sheep exposed to high altitude (3,820 m) from 30 to 138-142 days gestation. We measured the contractile response to increasing doses of isoproterenol and forskolin in papillary muscles from both ventricles, estimated beta-adrenergic receptor density (Bmax) and ligand affinity (Kd) using [125I]iodocyanopindolol, and measured adenosine 3',5'-cyclic monophosphate (cAMP) levels before and after maximally stimulating doses of isoproterenol and forskolin. Left ventricular wet weight was unchanged, but right ventricular weight was 20% lower than controls. At the highest concentration of isoproterenol (10 microM), maximum active tension was 32 and 20% lower than controls in hypoxemic left and right ventricles, respectively. The contractile response to forskolin was severely attenuated in both hypoxemic ventricles. Bmax was unchanged in the left ventricle, but increased by 55% in the hypoxemic right ventricle. Kd was not different from controls in either ventricle. Basal cAMP levels were not different from controls, but isoproterenol-stimulated and forskolin-stimulated cAMP levels were 1.4- to 2-fold higher than controls in both hypoxemic ventricles. The results suggest mechanisms downstream from cAMP in the beta-adrenergic receptor pathway are responsible for the attenuated contractile responses to isoproterenol.

Intracellular signal pathways controlling respiratory neurons

Medullary respiratory neurons are influenced by a variety of neuromodulators, but there is a lack of information about the specific intracellular signal pathways involved. In this report we describe the modulatory effects of the cyclic adenosine-triphosphate (cAMP)-dependent protein kinase and of protein kinase C pathways on voltage- and ligand-controlled ionic conductances and demonstrate their functional significance in regulating the excitability of medullary respiratory neurons of the vivo cat. Evidence is presented that PKA and PKC pathways are persistently activated. PKA regulates current flow through persistently activated and GABAB receptor-controlled potassium channels as well as GABAA receptor-controlled chloride channels. PKC also depresses persistent potassium currents but it potentiates excitatory and inhibitory synaptic currents. The clinical significance of these intracellular signal pathways is demonstrated in a case of a child suffering from apneustic breathing, who was successfully treated with a 5HT-1A receptor agonist.

Molecular mechanisms underlying forskolin-mediated up-regulation of human dopamine D2L receptors

1. Human dopamine (DA) D2long (hD2L) receptors, expressed by Ltk- cells, can be up-regulated by treating the cells with forskolin for 16 hr (Johansson and Westlind-Danielsson, 1994). We have examined some of the molecular mechanisms underlying this forskolin-mediated up-regulation. 2. Forskolin (100 microM, 16 hr), but not 1,9-dideoxyforskolin, a forskolin analogue that is unable to activate adenylyl cyclase and raise intracellular cAMP concentrations, up-regulates the hD2L receptor population by 43%. The implication of a cAMP-dependent increase in the receptor up-regulation was further substantiated by treating the cells with 8-bromo-cAMP or prostaglandin E1 (PGE1). The forskolin-mediated rise in receptor number was blocked by cycloheximide or an antisense phosphorothioate oligodeoxynucleotide (ODN) directed toward the hD2L mRNA. KT5720, a specific protein kinase A (PKA) inhibitor, completely blocked the receptor rise, whereas pertussis toxin (PTX) attenuated the increase considerably. Forskolin also produced an increase in the level of the DA hD2short (hD2S) receptor expressed by Ltk- cells. This increase was 2.5-fold higher than that found for the hD2L receptor. 3. The forskolin-mediated hD2L receptor rise is dependent on de novo protein synthesis, a rise in cAMP levels, PKA activation, and, at least partially, PTX-sensitive G proteins. 4. Long-term increases in intracellular cAMP levels may change the sensitivity of a DA receptor expressing cell to DA by increasing D2 receptor density through enhanced cAMP-dependent transcription.

Denervation-induced supersensitivity to forskolin and pinacidil is not related to changes in the adenylate cyclase transduction pathway in the rabbit femoral artery

1. The present study investigates whether chemical sympathectomy compromises the relaxation of the rabbit femoral artery precontracted with serotonin. The vasodilating agents promoted cyclic adenosine monophosphate (cAMP) accumulation or opening of the potassium channels. The effect of denervation on the adenylyl cyclase transduction pathway was also studied. 2. 6-Hydroxydopamine treatment did not impair the relaxation to adenosine, 8-bromoadenosine-cAMP (a membrane-permeable analog of cAMP) and 3-isobutyl-1-methylxanthine (a cAMP phosphodiesterase inhibitor). Moreover, denervation enhanced the relaxation to forskolin (a direct Gs-type protein activator) and pinacidil (a potassium channel opener). 3. Denervation modified neither adenosine diphosphate ribosylation of Gs- and Gi-proteins nor adenylyl cyclase activity.

Structure of the adenylyl cyclase catalytic core

Mammalian adenylyl cyclases contain two conserved regions, C1 and C2, which are responsible for forskolin- and G-protein-stimulated catalysis. The structure of the C2 catalytic region of type II rat adenylyl cyclase has an alpha/beta class fold in a wreath-like dimer, which has a central cleft. Two forskolin molecules bind in hydrophobic pockets at the ends of cleft. The central part of the cleft is lined by charged residues implicated in ATP binding. Forskolin appears to activate adenylyl cyclase by promoting the assembly of the active dimer and by direct interaction within the catalytic cleft. Other adenylyl cyclase regulators act at the dimer interface or on a flexible C-terminal region.

PKC-dependent long-term effect of PMA on protein cell surface expression in Caco-2 cells

Several recent data indicate that protein traffic is under the control of different phosphorylation pathways. In previous works, we have shown that cell surface expression of apical hydrolases and of a basolateral protein, "525" antigen, was impaired in Caco-2 cells treated with forskolin, a potent PKA activator (L. Baricault et al., 1995, J. Cell Sci., 108, 2109-2121). Surprisingly, in these experiments forskolin did not seem to act through PKA activation. These cAMP-independent effects of FK may rely on cross-talk between intracellular phosphorylation pathways as described recently for PKA and PKC pathways. Therefore, we tested the hypothesis that PKC activation may induce effects comparable to those of FK on three brush border hydrolases as well as on 525 antigen cell surface expression in Caco-2 cells. Using enzymatic activity measurements and pulse-chase experiments combined with cell surface biotinylation assays, we show that long-term treatment with phorbol 12-myristate 13-acetate (PMA) impairs the overall expression of neither brush border hydrolases nor that of the 525 antigen but decreases total cell surface expression of these proteins. The apical and basolateral delivery pathways are equally affected. Using confocal laser scanning microscopy we show that the DPP IV and the 525 antigen that were not recovered from the cell surface were sequestrated in Lamp-1-positive lysosomal-related vesicles. PMA stimulates PKC translocation even after a 3-week treatment and induces PKC epsilon redistribution to a vesicular- and membrane-associated compartment also labeled with cytokeratins. These results demonstrate that PMA-dependent PKC activation strongly impairs protein cell surface targeting. They also suggest that these PKC-dependent effects which are similar to those previously obtained with FK are relevant to the described cross-talk between PKA- and PKC-dependent phosphorylation pathways.

cAMP-dependent protein kinase modulates expiratory neurons in vivo

The adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA) second-messenger system influences neuronal excitability by modulating voltage-regulated and transmitter-activated channels. In this study we investigated the influence of the cAMP-PKA system on the excitability of expiratory (E) neurons in the caudal medulla of anesthetized, paralyzed, and artificially ventilated adult cats. We intracellularly injected the PKA inhibitors cAMP-dependent PKA inhibitor 5-22 amide (Walsh inhibitory peptide) and Rp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPS), the PKA activator Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Sp-cAMPS), and the adenylyl cyclase activator forskolin and measured membrane potential, neuronal input resistance, and synaptic membrane currents. Inhibition of cAMP-PKA activity by Walsh inhibitory peptide or Rp-cAMPS injections hyperpolarized neurons, decreased input resistance, and depressed spontaneous bursts of action potentials. Action potential duration was shortened and afterhyperpolarizations were increased. Inhibitory synaptic currents increased significantly. Stimulation of cAMP-PKA activity by Sp-cAMPS or forskolin depolarization neurons and increased input resistance. Spontaneous inhibitory synaptic currents were reduced and excitatory synaptic currents were increased. Rp-cAMPs depressed stimulus-evoked excitatory postsynaptic potentials and currents, whereas Sp-cAMPS increased them. Sp-cAMPS also blocked postsynaptic inhibition of E neurons by 8-hydroxy-dipropylaminotetralin, a serotonin-1A (5-HT-1A) receptor agonist that depresses neuronal cAMP-PKA activity. To determine the predominant effect of G protein-mediated neuromodulation of E neurons, we injected guanosine-5'-O-(3-thiotriphosphate) tetralithium salt (GTP-gamma-S), an activator of both stimulatory and inhibitory G proteins. GTP-gamma-S hyperpolarized E neurons, reduced input resistance, and increased action potential afterhyperpolarization. We conclude that the intracellular cAMP-PKA messenger system play an important role in the activity-dependent modulation of excitability in E neurons of the caudal medulla. In addition, the cAMP-PKA pathway itself is downregulated during activation of 5-HT-1A receptors.

Mechanisms of adaptive supersensitivity: correlation of guinea pig atrial supersensitivity with modifications in adenylyl cyclase activity

The possibility that the cellular mechanism underlying adaptive supersensitivity in right and left atria of the guinea pig may involve either adenylyl cyclase or components of that transduction process was examined in left and right atria obtained from controls or guinea pigs chronically treated with reserpine. Adenylyl cyclase activity and the abundance of alpha-subunits of several G-proteins (i.e. Gs, Gi, and Go) were quantified using standard techniques. Functional concentrations of Gs and Gi were compared in tissues from control and treated animals using pertussis- or cholera toxin-induced protein ribosylation. Chronic treatment with reserpine did not alter basal levels of adenylyl cyclase activity in left or right atrium but did increase significantly the ability of isoproterenol, 5'-guanylylimido diphosphate, and forskolin to activate adenylyl cyclase in the left atrium compared with the control. In contrast, treatment with reserpine increased the ability of only isoproterenol to active adenylyl cyclase in the right atrium. The increases in enzyme activation were not correlated with any detectable change in the concentrations of G-proteins or beta-adrenoceptors. The correlation between the specificity of changes in responsiveness and increased activation of adenylyl cyclase suggests that the cellular mechanism that underlies the development of adaptive supersensitivity in the guinea pig myocardium may involve a modification of adenylyl cyclase. The data also support the idea that the development of enhanced responsiveness in cardiac muscle may not only involve more than one cellular mechanism but may even differ between right and left atrium and ventricles of the same species.

D1/D5 dopamine receptor activation increases the magnitude of early long-term potentiation at CA1 hippocampal synapses

The role of the mesolimbic dopaminergic system in the reinforcement of learning suggests that dopamine should be able to modulate activity-dependent synaptic plasticity. We have examined the effect of D1/D5 agonists on early long-term potentiation (LTP) (40 min) in the CA1 region of hippocampal slices. D1/D5 agonists (+)bromo-APB, 6-chloro-PB, and dihydrexidine increased the magnitude of LTP in a synapse-specific manner (by approximately 10, 15, and 20%, respectively). This D1/D5 effect was mimicked by a low dose (10 microM) of the adenylyl cyclase activator forskolin. The D1/D5 antagonist (+)SCH 23390 reduced early LTP. In catecholamine-depleted slices, LTP was smaller by approximately 20-25% and could not be decreased further by D1/D5 antagonist. Under these conditions, D1/D5 agonist 6-chloro-PB and forskolin produced a larger enhancement of LTP (20-25%), restoring it to the control level. At the same dose, dideoxyforskolin did not affect early LTP. The D1/D5 agonist effect was completely blocked by the D1/D5 antagonist (+)SCH 23390. These results indicate that dopamine produces a synapse-specific enhancement of early LTP through D1/D5 receptors and cAMP.

G-protein function is reduced in hypertension

A functional impairment in vasodilator tone may be important in the pathogenesis and/or maintenance of elevated peripheral vascular resistance in hypertension. Previous studies of hypertensive subjects have demonstrated impaired beta-adrenergic-mediated vasodilation paralleling a reduction in lymphocyte beta-adrenergic-stimulated adenylyl cyclase activity. We have suggested that this impairment is related to a defect in G-protein function. To determine whether this defect alters the coupling between the G-protein complex and adenylyl cyclase, we performed [3H]forskolin binding studies in lymphocytes from hypertensive subjects, older normotensive subjects, and younger normotensive control subjects. Maximal specific [3H]forskolin binding was used as an index of adenylyl cyclase binding sites. Gpp(NH)p-, NaF/AlCl3-, and isoproterenol-stimulated binding were used as indices of G-protein/adenylyl cyclase coupling. In the absence of other stimulators, maximal [3H]forskolin binding was not significantly different among groups. However, both Gpp(NH)p- and isoproterenol-stimulated [3H]forskolin binding were significantly decreased in lymphocytes from hypertensive subjects. Overall, Gpp(NH)p- and isoproterenol-stimulated [3H]forskolin binding were significantly inversely correlated with blood pressure. No differences in NaF/AlCl3-stimulated [3H]forskolin binding were detected between groups. These studies indicate that G-protein/adenylyl cyclase coupling is impaired in lymphocytes from younger hypertensive subjects and may contribute to the blood pressure-related defect in beta-adrenoceptor-stimulated adenylyl cyclase activity.

Interactions of alpha-melanotropin and agouti on B16 melanoma cells: evidence for inverse agonism of agouti

alpha-Melanocyte-stimulating hormone (alpha-MSH, alpha-melanotropin) and agouti control the switch between eumelanin and pheomelanin synthesis in mammalian melanocytes. Here we investigated interactions between alpha-MSH, agouti protein, cAMP elevating agents and phorbol ester on mouse B16 melanoma cells. Agouti (Kd 3.7 nmol/l) and alpha-MSH (Kd 2.3 nmol/l) had similar affinities to the MC1 melanocortin receptor. Both alpha-MSH and agouti induced MC1 receptor down-regulation. Agouti antagonized melanogenesis induced by alpha-MSH, forskolin, cholera toxin (CT), and pertussis toxin (PT). It also reduced the constitutive melanin formation of long-term cultures. Cell proliferation was inhibited by agouti (43% at 100 nM). This effect was reversed by alpha-MSH, forskolin, or CT. B16-G4F cells, a cell variant that lacks the MC1 receptor, did not respond to agouti. From these results we conclude that agouti shows the characteristics of an inverse agonist acting through the MC1 receptor.

Effect of Rp diastereoisomer of adenosine 3',5' cyclic-monophosphothioate on the cAMP-dependent relaxation of smooth muscle

The effect of Rp diastereoisomer of adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS) on relaxation elicited by histamine (1-100 microM), forskolin (1-60 microM), papaverine (1-100 microM), vinpocetine (1-100 microM), rolipram (0.1-1 mM), Sp-cAMPS (10-300 microM), 8-BrcAMP (10 microM - 1 mM) and 8-BrcGMP (3 microM - 1 mM) of the previous vanadate-induced contraction was assayed. The effect of Rp-cAMPS on the relaxing effect produced by forskolin, papaverine, vinpocetine, rolipram, Sp-cAMPS and 8-BrcAMP in KCl-induced tonic contraction was also assayed. Histamine, forskolin, papaverine, rolipram, Sp-cAMPS, 8-BrcAMP and 8-BrcGMP, but not vinpocetine, relaxed the vanadate-induced contractions in rat uterus incubated in medium lacking calcium plus EDTA in a concentration-dependent way. Rp-cAMPS (1-300 microM) had no effect on vanadate contraction. However, it antagonized the relaxation elicited by histamine and papaverine, but not that of forskolin, rolipram, Sp-cAMPS, 8-BrcAMP and 8-BrcGMP. Forskolin, papaverine, vinpocetine, rolipram and 8-BrcAMP, but not Sp-cAMPS, relaxed the KCl-induced contraction. Rp-cAMPS antagonized the relaxation elicited by forskolin, papaverine and vinpocetine, but not that of rolipram and 8-BrcAMP. Our results suggest that: a) Rp-cAMPS is an effective PKA inhibitor that could be used to study the involvement of cAMP on drug-induced response in smooth muscle, and b) the effects of Sp-cAMPS, 8-BrcAMP and rolipram were independent of the activation of protein kinases.

A mathematical model for analysis of pharmacologically induced changes in the kinetics of cardiac muscle

A mathematical model of the isometric contraction of cardiac muscle is developed and utilized to characterize the inotropic and lusitropic effects of cardioactive compounds in isolated guinea pig left atria. In contrast to metrics that are based on minima and maxima of an isometric twitch and its derivative function, the entire time course of the twitch is used to quantify the kinetics of the contraction-relaxation cycle. The model relates observed tension to a time-dependent activation function that describes generation of internal force and a coupling function that determines mechanical response to the activation function. The model is structured so that it is suitable for nonlinear curve fitting to observed data. Results obtained using the model for fitting experimental data from tissues treated with different classes of cardioactive compounds agree with more qualitative results presented by other authors. Experiments using the model to fit data over an extended (90 min) time course revealed differences in the kinetic profiles of milrinone and forskolin. Computer simulations that demonstrate the effect of each model parameter on twitch kinetics are presented, and the relationships between the model and other theoretical and empirical models of cardiac muscle are discussed. The mathematical model is useful to enable a more quantitative understanding of the kinetics of cardiac muscle contraction and relaxation and identify compounds that may be selective for inotropic or lusitropic effects.

Treatment with haloperidol or clozapine causes changes in dopamine receptors but not adenylate cyclase or protein kinase C in the rat forebrain

The effect of treating rats with daily injections of haloperidol (1 mg/kg/day) or clozapine (20 mg/kg/day) for four weeks on second messengers and dopamine receptors was studied. The binding of [3H]forskolin to adenylate cyclase (AC), [3H]phorbol 12,13-dibutyrate (PDBu) to protein kinase C (PKC), [3H]SCH23390 binding to the dopamine D1 (DA-D1) receptor and [3H]spiperone binding to the dopamine D2 (DA-D2) receptor were measured using quantitative autoradiography. The density of AC was greatest in the caudate-putamen, nucleus accumbens and olfactory tubercle, a distribution resembling that of DA-D1 receptor. The distribution of PKC was relatively homogeneous in the forebrain. Neither haloperidol nor clozapine administration significantly altered the levels of AC or PKC in the caudate-putamen. By contrast treatment with haloperidol, but not clozapine, significantly increased the density of DA-D2 receptors in the caudate-putamen without affecting the density of DA-D1 receptors. By contrast, both haloperidol and clozapine increased the density of DA-D1 receptors in the olfactory tubercle.

Adenylate cyclase in striatal cholinergic interneurons regulates acetylcholine release

Fractional [3H]ACH efflux from dissociated rat striata tested whether tonic inhibition prevents stimulation of acetylcholine (ACH) release by adenylate cyclase. Forskolin stimulated release from the dissociated cells (threshold at 300 nM; EC50 > or = 1 MicroM). Release was also stimulated by 3-isobutyl-1-methylxanthine and was additive with forskolin. The 1,9-dideoxy forskolin analog that lacks cyclase-stimulating activity was ineffective. Thus, stimulation of adenylate cyclase within striatal cholinergic interneurons increases ACH secretion but is tonically inhibited by endogenous striatal transmitters. Disinhibition of the excitatory cyclase by denervation of striatal cholinergic interneurons in situ could contribute to supersensitivity without receptor upregulation.

Agonist regulation of adenylate cyclase activity in neuroblastoma x glioma hybrid NG108-15 cells transfected to co-express adenylate cyclase type II and the beta 2-adrenoceptor. Evidence that adenylate cyclase is the limiting component for receptor-mediated stimulation of adenylate cyclase activity

Stable cell lines, derived from NG108-15 cells and transfected to express both the beta 2-adrenoceptor and adenylate cyclase type II, were produced and examined. The absence of adenylate cyclase type II in the parental cells and its presence in these clones was demonstrated by reverse transcriptase-PCR. Total cellular levels of adenylate cyclase were increased in a number of clones between 3- and 8-fold, as assessed by guanine nucleotide-stimulated specific high-affinity binding of [3H]forskolin to cellular membranes. Basal adenylate cyclase activity was markedly elevated compared with a clone expressing similar levels of the beta 2-adrenoceptor in the absence of adenylate cyclase type II. Each of NaF, forskolin and guanosine 5'-[beta, gamma-imido]triphosphate (a poorly hydrolysed analogue of GTP) produced substantially higher levels of adenylate cyclase activity in membranes of the clones positive for expression of adenylate cyclase type II than was achieved with the parental cells. Both isoprenaline, acting at the introduced beta 2-adrenoceptor, and iloprost, acting at the endogenously expressed IP prostanoid receptor, stimulated adenylate cyclase activity to much higher levels in the clones expressing adenylate cyclase type II compared with the clone lacking this adenylate cyclase; however, the concentration-effect curves for adenylate cyclase stimulation by these two agonists were not different between parental cells and clones over-expressing adenylate cyclase type II. A maximally effective concentration of the beta-adrenoceptor partial agonist ephedrine displayed similar intrinsic activity and potency to stimulate adenylate cyclase in membranes of clones both with and without adenylate cyclase type II. Both secretin and 5'-N-ethylcarbox-amidoadenosine (acting at an endogenous A2 adenosine receptor) were also able to produce substantially greater maximal activations of adenylate cyclase in the clones expressing excess adenylate cyclase type II, without alterations in agonist intrinsic activity or potency. These results demonstrate that the maximal output of the stimulatory arm of the adenylate cyclase cascade can be increased by increasing total levels of adenylate cyclase in the genetic background of NG108-15 cells.

Ca2+ and Cl(-)-dependence of the turtle olfactory response to odorants and forskolin

In the turtle olfactory system, large responses to odorants appeared after application of cAMP of forskolin at high concentrations to the isolated olfactory receptor neurons or the olfactory epithelium, suggesting that a cAMP-independent pathway greatly contributes to the generation of odor responses. We measured the effects of the mucosal Ca2+ and Cl- concentrations and a Ca(2+)-activated Cl(-)-channel blocker, 4-acetamide-4'-(isothiocyano) stilbene-2,2'-disulphonic acid (SITS), upon olfactory bulbar responses to explore the contribution of the Ca(2+)-activated Cl(-)-channels to cAMP-dependent and independent pathways. Elimination of mucosal Ca2+ by addition of 2 mM EGTA to the stimulating solution partially inhibited the cAMP-independent responses to 0.1 mM citralva but did not affect those to 0.1 mM lilial or 0.1 mM l-carvone. Substitution of mucosal Cl- with gluconate slightly enhanced the total response to 0.1 mM citralva but practically did not affect the responses to other odorants tested. SITS (2 mM) partially inhibited the responses to 50 microM forskolin and 0.1 mM citralva but did not affect the cAMP-independent response to 0.1 mM citralva in Ca(2+)-free Ringer's solution. These results suggested that the Ca(2+)-activated Cl- channels do not contribute to the generation of the cAMP-independent responses, but that they partially contribute to the generation of the cAMP-dependent responses in the turtle.

Opposite effects of increased intracellular cyclic AMP on the heat and bradykinin responses of canine visceral polymodal receptors in vitro

To clarify the validity of the long standing hypothesis that effects of E series prostaglandin (PG)S are mediated by cyclic AMP (cAMP), we studied the effects of increases in intracellular cAMP on the heat and bradykinin responses of testicular polymodal receptors. Polymodal receptor activities were recorded in vitro from testis-spermatic nerve preparations excised from dogs anesthetized with pentobarbital (30 mg/kg, i.v.). Increases in intracellular cAMP induced by either forskolin (5 or 10 microM), an adenylyl cyclase activator, or a mixture of dibutyryl cAMP (20-100 microM), a membrane permeable cAMP analog, and 3-isobutyl-1-methyl xanthine (20-100 microM), an inhibitor of the cAMP degrading enzyme, significantly augmented the response to heat (42-48 degrees C). In contrast, these substances failed to facilitate the response to bradykinin (0.1 or 1 microM) and instead suppressed it. Dideoxyforskolin (10 microM), an inactive analog of forskolin, had no effects on both the heat and bradykinin responses. These results demonstrate that an increase in intracellular cAMP induces opposite effects on the heat and bradykinin responses. Possible involvement of intracellular cAMP in the facilitatory effects of PGE2 on both responses was discussed in connection with the PGE receptor subtypes involved in the sensitization of the bradykinin and heat responses.

Forskolin carbamates: binding and activation studies with type I adenylyl cyclase

Three series of analogs were regioselectively prepared from a protected forskolin precursor to afford 7-carbamoyl-7-desacetylforskolins (series 1), 6-carbamoyl-7-desacetylforskolins (series 2), and 6-carbamoylforskolins (series 3). The analogs were pharmacologically evaluated for binding (IC50) to and activation (EC50) of type I adenylyl cyclase in membranes from stably transfected Sf9 cell lines expressing a single adenylate cyclase subtype. The following ranges were determined for the IC50's and EC50's of each individual series: series 1, IC50 = 43-1600 nM, EC50 = 0.5-9.6 microM; series 2, IC50 = 65-680 nM, EC50 = 0.63-6.5 microM; series 3, IC50 = 21-271 nM, EC50 = 0.5-8.1 microM (forskolin IC50 = 41 nM and EC50 = 0.5 microM). Activation paralleled binding; however, some analogs exhibited poor binding and good activation whereas others demonstrated good binding but poor activation. Steric bulk tended to diminish binding and activation when at the 6- or 7-position, although bulk was accommodated at the 6-position if the 7-site was reacetylated. Acylation of the 7-position by the carbamoyl linker or acetyl was important for obtaining good binding and activation; however, the effect was more pronounced with binding. For both binding and activation, small, linear, lipophilic substituents (propyl, allyl, isopropyl) are well tolerated at the 7-position but less so in the 6-position, even when the 7-site is reacetylated. Planar aromatic moieties (phenyl and 2-pyridinyl) demonstrated moderate to good potency for binding and activation when located at either the 6- or 7-positions. There is an overall trend toward increasing potency for both binding and activation with polar substituents.

VIP-induced relaxation of guinea-pig intestinal smooth muscle cells: sequential involvement of cyclic AMP and nitric oxide

1. A possible interaction between cyclic AMP and nitric oxide (NO) in mediating the relaxant effect of vasoactive intestinal polypeptide (VIP) on intestinal smooth muscle cells has been investigated. The effects of the inhibitor of NO synthesis, NG-nitro-L-arginine methyl ester (L-NAME), have been studied on VIP-, forskolin-, and 8 bromo-cyclic AMP- induced relaxation of cells, dispersed by enzymatic digestion of muscle strips from the circular layer of guinea-pig ileum. 2. VIP alone did not modify the length of isolated muscle cells. By contrast, when the cells were contracted by cholecystokinin octapeptide, CCK8 (10 nM), VIP inhibited this contraction, inducing a concentration-dependent relaxation of the cells. Maximal relaxation was induced by 1 microM VIP (EC50 = 408.2 +/- 16.7 pM). 3. N-ethylmaleimide, inhibitors of adenylate cyclase or somatostatin, abolished the relaxing effect of VIP. (R)-p-cAMPs, an antagonist of cyclic AMP on protein kinase A also inhibited the VIP-induced relaxation by 92.1 +/- 6.3%. Inhibitors of nitric oxide synthase (NOS), L-NAME and L-NMMA, partially inhibited VIP-induced relaxation. The effect of L-NAME was reversed by L-arginine but not by D-arginine. 4. (R)-p-cAMPS and L-NAME also inhibited the cell relaxation induced either by forskolin which directly stimulates adenylate cyclase activity or 8-bromo-cyclic AMP, an analogue of cyclic AMP. 5. When cells were incubated for 30 min with dexamethasone 10 microM, a glucocorticoid known to decrease the synthesis of iNOS, the relaxing effect of a maximal concentration of VIP was decreased by 52 +/- 4% and L-NMMA had no further effect on this residual VIP-induced relaxation. Milrinone, a phosphodiesterase type III inhibitor, potentiated the relaxant effect of VIP. 6. These data demonstrate that the intracellular pathway mediating the relaxant effect of VIP in intestinal smooth muscle cells includes the sequential activation of adenylate cyclase, protein kinase A, activation of NOS and finally production of NO and cyclic GMP. NO could in turn regulate the cyclic AMP-dependent pathway of cell relaxation.

Forskolin's structural analogue 1,9-dideoxyforskolin has Ca2+ channel blocker-like action in rat cerebellar granule cells

Forskolin, routinely used as a specific activator of the cAMP pathway, is also a blocker of various ionic channels in a cAMP-independent way. We investigated, in rat cerebellar granule cells in culture, the effects of forskolin and its structural analogue 1,9-dideoxyforskolin on Ca2+ entry. Changes in cytosolic free Ca2+ concentration ([Ca]i) were monitored using fura-2 microfluorimetry. The increase in [Ca]i observed in response to membrane depolarization by 30 mM KCI was reduced by 20% in the presence of 100 microM forskolin, and by 71% with the same concentration of 1,9-dideoxyforskolin. A dose-response curve for 1,9-dideoxyforskolin gave an estimated IC50 of 54 microM. Additional experiments using the patch-clamp technique showed that 100 microM 1,9-dideoxyforskolin inhibit voltage-activated Ca2+ currents by 63%, although forskolin had no significant effect in the same conditions. This blocking effect of 1,9-dideoxyforskolin is not specific of a given Ca2+ channel type.

Activation of the adenylate cyclase-dependent protein kinase pathway increases high affinity glutamate uptake into rat striatal synaptosomes

This study examined the effects of various agents known to alter protein phosphorylation through protein kinase A or C on high affinity glutamate uptake measured in vitro on rat striatal homogenates. Incubation of synaptosomes with the phosphatase inhibitor, okadaic acid, dramatically increased glutamate uptake indicating that underlying phosphorylation mechanisms may be involved in the regulation of this transport process. The protein kinase C activator, phorbol-12,13-dibutyrate, or inhibitor, staurosporine, did not significantly modify glutamate uptake. In contrast, forskolin, which activates adenylate cyclase, induced a dose-dependent increase in glutamate uptake. Saturation kinetic analysis indicated that forskolin increased the Vmax without modifying the Km of the transport process as compared to control values. The effect of forskolin was mimicked by dibutyryl adenosine monophosphate, an analog of cAMP which activates protein kinase A, and counteracted by high doses of N-[2-(methylamino) ethy1]-5-isoquinoline sulfonamide, a protein kinase A inhibitor. These results suggest that an adenylate cyclase-dependent protein kinase may be involved in the post-translational regulation of glutamate transporters.

Paradoxical facilitation of acetylcholine release from parasympathetic nerves innervating guinea-pig trachea by isoprenaline

1. Previous studies have provided evidence that activation of beta-adrenoceptors on cholinergic nerve terminals can inhibit neurotransmission in the airways. However, in most cases, this conclusion has been based on indirect evidence obtained from mechanical experiments where changes in airways smooth muscle tone were measured. 2. We have assessed whether modulation of cholinergic neurotransmission by beta-adrenoceptor agonists is due to a pre- or post-junctional action by investigating the effect of isoprenaline on contractile responses evoked by exogenous acetylcholine (ACh) and electrical field stimulation (EFS; 4 Hz, 40 V, 0.5 ms pulse width every 15 s), and on EFS-induced ACh release from cholinergic nerves innervating guinea-pig and human trachea. Furthermore, the subtype of beta-adrenoceptor which modulates neurotransmission and the potential role of cyclic AMP in this response were evaluated. 3. In guinea-pig trachea, isoprenaline (1 nM-1 microM) inhibited the contractile response evoked by exogenous ACh (1 microM) to a similar extent to that evoked by EFS (EC50 = 19.9 and 23 nM, respectively). 4. In epithelium-denuded guinea-pig strips treated with indomethacin (10 microM), isoprenaline significantly enhanced EFS-induced ACh release from cholinergic nerve terminals (by 36% at 0.3 microM). This effect was blocked by propranolol and ICI 118, 551 (each 0.1 microM). In contrast, isoprenaline failed to affect EFS-induced ACh release from parasympathetic nerves innervating human trachea. 5. To evaluate the role of cyclic AMP in the beta-adrenoceptor-induced facilitation of cholinergic neurotransmission, the effects of various cyclic AMP elevating drugs on ACh release were studied. Forskolin (10 microM) significantly augmented (by 17%) EFS-induced ACh release, an effect which was not reproduced by 1,9-dideoxyforskolin (10 microM) which does not activate adenylyl cyclase. Similarly, the cyclic AMP analogue, 8-bromo-cyclic AMP (1 mM) and cholera toxin (1 microgram ml-1) facilitated ACh output by 22 and 47% respectively, whereas prostaglandin E2 (PGE2, 0.1 nM-1 microM) inhibited this response (by 67% at 1 microM). 6. Zardaverine (10 microM), a dual inhibitor of the phosphodiesterase (PDE)3 and PDE4 isoenzyme families, did not affect EFS-induced ACh release and failed to facilitate the actions of either isoprenaline or PGE2. Similarly, neither SK&F 94120 (10 microM) nor rolipram (10 microM), selective inhibitors of PDE3 and PDE4 respectively, significantly affected the release of ACh in response to EFS. 7. The result of this study suggests that isoprenaline facilitates cholinergic neurotransmission in guinea-pig, but not human, trachea by activation of pre-junctional beta 2-adrenoceptors, an effect that may be mediated via activation of the cyclic AMP/cyclic AMP-dependent protein kinase cascade. Furthermore, the data presented herein illustrate the need to undertake direct measurements of neurotransmitter release when examining the effect of agents purported to act pre-junctionally.

Neuropeptide Y inhibits 3[H]noradrenaline release in the rat vas deferens independently of cAMP levels

The purpose of the present investigation was to ascertain the functional significance of the reduction in cyclic AMP (cAMP) levels in the inhibitory action of neuropeptide Y (NPY) on [3H]noradrenaline ([3H]NA) release, as well as to further characterize the subtype(s) of NPY receptors involved in the peptide's actions in the rat vas deferens. We studied the effects of NPY, carboxyterminal fragments of this peptide and the NPY analog (Leu31,Pro34)-NPY on three functional responses, namely, the release of [3H]NA and the associated muscle contractions evoked by electrical stimulation, and the accumulation of cAMP stimulated by forskolin. NPY, a known inhibitor of the electrically-evoked [3H]NA release and neurogenic contractions is also a potent inhibitor of the forskolin-stimulated cAMP synthesis in the prostatic portion of the rat vas deferens. However, the ability of NPY to inhibit cAMP accumulation is lost upon tissue denervation, suggesting that this is likely to be a prejunctional effect. Elevation of cAMP levels by the use of the cell permeant analog of cAMP, 8-(p-chlorophenylthio)-cAMP (8pCPTcAMP) increases the electrically-evoked release of [3H]NA. However, the inhibition of [3H]NA release by NPY is not prevented by 8pCPTcAMP. Structure-activity relationship studies reveal that NPY and related peptides inhibit the release of [3H]NA, the muscle contractions and the synthesis of cAMP with a similar pharmacological profile. NPY is the most potent inhibitory agent, whereas [Leu31,Pro34]-NPY and NPY13-36, the respective Y1 and Y2 selective agonists, display similar potencies to inhibit the three responses. It is concluded that NPY inhibits neurotransmission in the rat vas deferens through the activation of a peptide receptor different from the known NPY-Y1 or NPY-Y2 receptor subtypes. NPY receptor activation in the vas deferens is negatively coupled to adenylyl cyclase activity. This intracellular signalling pathway is, however, not likely to mediate the peptide effects on the prejunctional regulation of noradrenaline release.

Consequences of transient focal cerebral ischaemia for second messenger and neurotransmitter binding in the rat: quantitative autoradiographic analysis of forskolin, dopamine D1 receptor binding and cerebral blood flow changes

In order to study the consequences of reperfusion for ischaemic brain injury, quantitative ligand binding autoradiography was carried out in a model of reversible focal cerebral ischaemia. Endothelin-1 applied to the abluminal surface of the middle cerebral artery in anaesthetized Sprague-Dawley rats induced severe focal ischaemia and subsequent reperfusion (assessed by blood flow tracers [99mTc]HMPAO and [14C]iodoantipyrine respectively) by 2 h after insult. Ligand binding autoradiography on consecutive sections demonstrated these blood flow changes to be associated with a significant reduction in forskolin binding throughout the middle cerebral artery territory (e.g. 25% in parietal cortex, 11% in dorsolateral caudate nucleus). The most marked losses in forskolin binding were in areas where ischaemia was severe and reperfusion was poor. However, the same changes in cerebral blood flow had no significant effect on D1 dopamine receptor binding (e.g. < 2% reduction in the caudate nucleus). These data demonstrate that ligand binding characteristics are significantly affected as early as 2 h after insult, with evidence of differential sensitivity for forskolin and D1 dopamine binding. With regard to the consequences of reperfusion, comparison with our previous study of 2 h maintained ischaemia demonstrates reperfusion-related salvage of dopamine and forskolin binding in the caudate nucleus but possible exacerbation of forskolin binding loss in the cortex.

Role of cAMP in the regulation of hepatocytic autophagy

To assess the role of cAMP in the regulation of autophagy, we examined the effects of cAMP analogues and cAMP-elevating agents on freshly isolated rat hepatocytes, using electroinjected [3H]raffinose as an autophagy probe. Glucagon was found to stimulate, inhibit or have no effect on autophagy, depending on the inclusion of metabolites like pyruvate (which caused ATP depletion and autophagy suppression) and amino acids (a complete mixture that antagonized pyruvate) in the incubation medium. Inhibition was also observed with theophylline, a cAMP-elevating inhibitor of cyclic nucleotide phosphodiesterases, and with the adenylyl cyclase activator deacetylforskolin. At low concentrations of deacetylforskolin, the inhibition could be abolished by amino acids. N6,2'-O-Dibutyryladenosine 3',5'-monophosphate (Bt2-cAMP) strongly inhibited both autophagic sequestration of [3H]raffinose and overall autophagic protein degradation; again, amino acids abolished the autophagy-inhibitory effect of low Bt2-cAMP concentrations. Several other cAMP analogues (8-thiomethyl-cAMP, N6-benzoyl-cAMP, (S)-5,6-dichloro-1-D-ribofuranosylbenzimidazole 3',5'-[thio]monophosphate, (S)-8-bromoadenosine 3',5'-[thio]monophosphate) inhibited autophagy as well. The effect of Bt2-cAMP was rapid, dose-dependent, reversible and did not require concomitant protein synthesis. Neither Bt2-cAMP nor deacetylforskolin reduced intracellular ATP levels or cell viability, ruling out inhibition of autophagy by non-specific cytotoxicity. The autophagy-inhibitory effect of Bt2-cAMP could be substantially antagonized (40-50%) by KT-5720, a specific inhibitor of the cAMP-dependent protein kinase A, and by the nonspecific protein kinase inhibitor K-252a. Somewhat surprisingly, KN-62 and KT-5926, allegedly specific inhibitors of Ca2+/calmodulin-dependent protein kinase II and myosin light chain kinase, respectively, were also Bt2-cAMP-antagonistic. These results suggest that cAMP regulates the early, sequestrational step of hepatocytic autophagy by a highly conditional, dual mechanism, inhibition being predominant under most conditions in freshly isolated hepatocytes, whereas stimulation reportedly predominates in vivo. The effect of cAMP is probably mediated by protein kinase A, but other protein kinases would appear to participate in the regulation of autophagic sequestration as well.

Endothelin-1 inhibits L-type Ca currents enhanced by isoproterenol in guinea-pig ventricular myocytes

To investigate the action of endothelin-1 (ET-1) on L-type Ca currents (ICa,L) in guinea-pig ventricular cells, whole-cell currents were recorded at approximately 36-37 degrees C in enzymatically isolated myocytes. ET-1 (> or =10 nM) suppressed the basal ICa,L to 79+/-8% of control at 20 nM. Bath application of isoproterenol (ISO; 10 nM) enhanced ICa,L to 192+/-28% with about a -10-mV shift of its relationship with membrane potential. ET-1 concentration dependently inhibited this ISO-enhanced ICa,L with a half-maximally inhibitory concentration (IC50) of 168 pM. The inhibitory actions of ET-1 were antagonised by BQ-123 (300 nM), cyclo(D-Asp-L-Pro-D-Val-L-Leu-D-Trp), a specific ETA receptor antagonist. Histamine-enhanced ICa,L was also suppressed by ET-1, but ICa, L potentiated by internal adenosine 3',5'-cyclic monophosphate (cAMP) was unaffected. Preincubation of myocytes with pertussis toxin (PTX, at 5 microgram/ml for >60 min at 36 degrees C) completely occluded the ET-1 action. Thus, stimulation of ETA receptors by subnanomolar ET-1 inhibits ICa,L via PTX-sensitive G-proteins.

A wortmannin-sensitive signal transduction pathway is involved in the stimulation of insulin release by vasoactive intestinal polypeptide and pituitary adenylate cyclase-activating polypeptide

Vasoactive intestinal polypeptide (VIP), pituitary adenylate cyclase-activating polypeptide-27 (PACAP-27), and PACAP-38 stimulated insulin release with EC50 values of 0.15, 0.15, and 0.06 nM respectively, as expected for the VIP2/PACAP3 receptor subtype. Secretion was stimulated promptly and peaked at 6-10 min. At 30 min, the secretion rate was still 2-3-fold higher than the control rate. The peptides increased cyclic AMP and [Ca2+]i transiently so that at 30 min they had returned to control values. Therefore, an additional signal is required to explain the prolonged stimulation of release. The prolonged effects, but not the acute effects of VIP and PACAP on insulin release were inhibited by low concentrations of wortmannin, a phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor. While wortmannin inhibited PI 3-kinase activity in cell lysates, no activation by the peptides was seen. Therefore, the wortmannin-sensitive pathway is either dependent on basal PI 3-kinase activity, or another target for wortmanin is responsible for inhibition of the peptide-stimulated secretion. It is concluded that the acute stimulation of insulin release by VIP and PACAP is mediated by increased cyclic AMP and [Ca2+]i, whereas the sustained release is mediated by a novel wortmannin-sensitive pathway.