Efficacy of an adenosine A1 receptor agonist compared with atropine and pralidoxime in a rat model of organophosphate poisoning

The objective of this study was to evaluate the effects of an adenosine A1 agonist, phenylisopropyl adenosine (PIA), on metamidophos poisoning compared to specific antidotes. Rats were poisoned with metamidophos (30 mg/kg, oral) and observed for 24 hours. One group received sodium chloride (1 mL/kg) and four experimental groups received atropine (5 mg/kg), pralidoxime (PAM, 20 mg/kg), atropine/PAM (5/20 mg/kg) or PIA (1 mg/kg) intraperitoneally. Atropine reduced salivation and prevented respiratory distress when compared to sodium chloride-treated rats. Treatment with PAM did not cause any suppression of cholinergic signs. Atropine and PAM combination prevented salivation, convulsion and respiratory distress. PIA delayed initial time of the salivation, convulsion and time to death. However, PIA was found ineffective against the metamidophos-induced cholinergic symptoms and mortality. All treatments, except PIA, lead to survival of these animals. Acetylcholinesterase (AChE) activity was not normalized by PIA or PAM. PIA prevented metamidophos-induced diaphragmatic muscle necrosis as much as PAM. In conclusion, a single dose of PIA was unable to protect the rats from metamidophos toxicity. Further studies are needed involving a combination of PAM and/or atropine with repeated doses of PIA to clarify the efficacy of adenosine agonists in OP poisoning.

Norepinephrine induced calcitonin secretion in rat medullary thyroid carcinoma 6-23 cells: interaction between intracellular calcium and cAMP

Catecholamines are known to stimulate calcitonin secretion in C-cells by a receptor mediated pathway, but details regarding the postreceptor events are unknown. Since norepinephrine (NE) influences intracellular calcium concentration [Ca2+]i and cAMP levels in C-cells, we used different adrenergic agonists and antagonists to investigate the effect of NE on [Ca2+]i and cAMP accumulation, and on calcitonin secretion in rat MTC-6-23 cells. NE stimulated intracellular cAMP accumulation and calcitonin secretion dose-dependent, with 10(-7) mol/l causing maximal stimulation. The NE induced increase in cAMP accumulation/calcitonin secretion could be decreased to baseline by equimolar amounts of the beta-adrenergic blocker propanolol. The alpha-blocker phentolamine did not significantly influence NE stimulated calcitonin secretion even at high concentrations. The beta-adrenergic agonist fenoterol proved to be as effective as NE in stimulating cAMP accumulation/calcitonin secretion. Activation of inhibitory G-proteins by the adenosine A1 receptor analogue N6-phenylisopropyladenosine at 10(-6) mol/l completely blocked NE stimulated calcitonin secretion. NE stimulated calcitonin secretion was also completely blocked by the cAMP antagonist RpcAMPs. The calcium channel blocker verapamil significantly inhibited NE stimulated calcitonin secretion, but interestingly increased NE stimulated cAMP accumulation. We conclude that NE induced calcitonin secretion is mediated through beta-receptors coupled to adenylate cyclase via G-proteins. cAMP and changes in [Ca2+]i are necessary for NE induced calcitonin secretion. There seems to be a complex interaction between the two pathways even regarding events occurring distal to cell membrane.

mRNA expression and antilipolytic role of phosphodiesterase 4 in rat adipocytes in vitro

Adipocyte lipolysis is dependent on an increase in the intracellular concentration of cAMP. Intracellular phosphodiesterases (PDEs) hydrolyze cAMP and limit stimulation of lipolysis. In the present study, the mRNA expression of PDE4 subtypes and the antilipolytic role of PDE4 in rat adipocytes were investigated. Fragments encoding PDE4A (233 bp), PDE4B (786 bp), PDE4C (539 bp), and PDE4D (262 bp) sequences were amplified by RT-PCR. The mRNA expression of PDE4 subtypes (A, B, C, D) determined by real-time quantitative PCR was 7, 18.7, 18.9, and 7.2% relative to PDE3B. Inhibition of PDE4 by rolipram increased basal lipolysis and reversed in part prostaglandin E2 antilipolysis. The combination of PDE3 and PDE4 inhibitors synergistically reversed both prostaglandin E2 and phenylisopropyl adenosine antilipolysis. Stimulation of adipocytes with prostaglandin E2 increased total PDE activity and PDE3 activity measured by hydrolysis of 3[H]cAMP by the particulate fraction of adipocytes. The present study confirmed that mRNAs for all four PDE4 subtypes were expressed in rat adipocytes, with PDE4B and PDE4C predominant. Moreover, PDE4 not only limits the rate of basal lipolysis but also contributes to prostaglandin E2 antilipolysis in rat adipocytes.

Interaction of exercise and adenosine receptor agonist and antagonist on rat heart antioxidant defense system

This study investigated the interactive effects of acute exercise and adenosine receptor agonist and antagonist on antioxidant enzyme activities, glutathione and lipid peroxidation in the heart of the rat. Male Fisher-344 rats were divided into six groups and treated as follows: (1) saline control; (2) acute exercise (100% VO2max); (3) R-Phenyl isopropyl adenosine (R-PIA) (3.46 micromol/kg, i.p.); (4) theophylline (1.70 micromol/kg, i.p.) plus acute exercise; (5) theophylline plus R-PIA; and (6) theophylline. Animals were sacrificed 1 h after treatments; hearts were isolated and analyzed. The results show that acute exercise as well as adenosine receptor agonist R-PIA significantly enhanced cardiac superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and glutathione reductase (GR) activity by 36-135% and 16-51%, respectively. Adenosine receptor agonist R-PIA significantly decreased cardiac GSSG concentration and enhanced GSH/GSSG ratio by 22 and 30%, respectively. Whereas theophylline treatment blocked the activation of antioxidant enzyme activities enhanced by acute exercise and R-PIA. Theophylline treatment significantly increased lipid peroxidation by 43% in the heart of exercised rats. The study concluded that the adenosine receptors are involved in the upregulation of cardiac antioxidant defense system and attenuation of lipid peroxidation due to acute exercise in rats.

Effects of adenosine agonist R-phenylisopropyl-adenosine on halothane anesthesia and antinociception in rats

AIM

To investigate the antinociceptive effect of adenosine agonist R-phenylisopropyl-adenosine (R-PIA) given to conscious rats by intracerebroventricular (ICV) and intrathecal (IT), and identify the effect of R-PIA on minimum alveolar concentration (MAC) of halothane with pretreatment of A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) or K+ channel blocker 4-aminopyridine (4-AP).

METHODS

Sprague-Dawley rats were implanted with 24-gauge stainless steel guide cannula using stereotaxic apparatus and ICV method, and an IT catheter (PE-10, 8.5 cm) was inserted into the lumbar subarachnoid space, while the rats were under pentobarbital anesthesia. After one week of recovery from surgery, rats were randomly assigned to one of the following protocols: MAC of halothane, or tail-flick latency. All measurements were performed after R-PIA (0.8-2.0 microg) microinjection into ICV and IT with or without pretreatment of DPCPX or 4-AP.

RESULTS

Microinjection of adenosine agonist R-PIA in doses of 0.8-2.0 microg into ICV and IT produced a significant dose- and time-dependent antinociceptive action as reflected by increasing latency times and ICV administration of adenosine agonist R-PIA (0.8 microg) reducing halothane anesthetic requirements (by 29%). The antinociception and reducing halothane requirements effected by adenosine agonist R-PIA was abolished by DPCPX and 4-AP.

CONCLUSION

ICV and IT administration of adenosine agonist R-PIA produced an antinociceptive effect in a dose-dependent manner and decreased halothane MAC with painful stimulation through activation of A1 receptor subtype, and the underlying mechanism involves K+ channel activation.

Prolonged treatment with prostaglandin E1 increases the rate of lipolysis in rat adipocytes

Prolonged treatment of adipocytes with certain inhibitors of lipolysis, including N(6)-phenylisopropyl adenosine (PIA) and prostaglandin E(1) (PGE(1)) leads to down-regulation of G(i). Prolonged treatment with PIA increases the rate of lipolysis, and we have reported that tumor necrosis factor-alpha (TNF alpha) stimulates lipolysis by down-regulating G(i). To determine the relationship between G(i) concentration and lipolysis, we have investigated the effect of two other acute inhibitors of lipolysis; PGE(1), which down-regulates G(i), and nicotinic acid (NA), which does not down-regulate G(i). Rat adipocytes were incubated with PIA (300 nM), PGE(1) (3 microM) or nicotinic acid (1 mM) for 24 h. The rate of lipolysis (glycerol release) was increased approximately 2 to 3-fold in PIA-treated cells, and in PGE(1)-treated cells. Conversely, the rate of lipolysis was not altered by the prolonged nicotinic acid treatment. These findings support the hypothesis that the rate of lipolysis in adipocytes is determined, at least partly, by the cellular concentration of G(i) proteins.

The protective effect of Allium sativum L. clove aqueous and methanolic extracts against hypoxia-induced lethality in mice

The antihypoxic activity of Allium sativum clove (garlic) aqueous and methanolic extracts was studied in mice. The extracts of garlic showed that the antihypoxic effect was dose-dependent. The minimum effective doses of aqueous and methanolic extracts were 0.2 g/kg and 5.12 g/kg, respectively. Phenytoin, 50 mg/kg, and R-phenylisopropyladenosine (R-PIA), 1.6 mg/kg (R-PIA) as positive controls increased survival time up to 52.5 +/- 2.9 min and 120.5 +/- 6 min, respectively, compared to normal saline (34.73 +/- 0.71 min). The high doses of aqueous (16.9 g/kg) and methanolic (12.8 g/kg) extracts increased survival time up to 73.17 +/- 4.9 and 68.41 +/- 3.7, respectively. These results indicated that the extracts of A. sativum cloves have a protective effect against hypoxia-induced lethality in mice.

Regulation of renal A(1) adenosine receptors in vivo

We have compared renal A(1) adenosine receptor (AR) regulations in rats after chronic agonist and antagonist treatments. In one group, R-phenylisopropyladenosine (R-PIA), a selective A(1) AR agonist, was infused subcutaneously for 7 days. Another group was fed theophylline, a non-selective AR antagonist, for 2 weeks. Renal cortex membrane A(1) AR binding with 1,3-[(3)H]-dipropyl-8-cyclopentylxanthine demonstrated approximately 40% reduction in the B(max ) for the R-PIA group without any changes in the K(d) values. Neither the B(max) nor the K(d) changed following chronic theophylline treatment. Renal cortex G(i)alpha-proteins from the R-PIA treated rats decreased by approximately 30%. Renal G(i)alpha levels did not change in theophylline-treated rats. Consistent with the A(1) AR desensitization, R-PIA-treated rats had significantly higher basal renin release and showed attenuated A(1) AR-mediated inhibition of renin release. These data suggest that prolonged A(1) AR stimulation results in downregulation of renal A(1) ARs and G(i)alpha, accompanied by desensitization of A(1) AR-mediated inhibitory effects on renin release. Unlike cardiac and brain A(1) ARs, renal A(1) receptors are not subject to up-regulation following chronic antagonist treatment.

Inhibition of proteasome activity blocks the ability of TNF alpha to down-regulate G(i) proteins and stimulate lipolysis

Prolonged treatment of rat adipocytes with TNF alpha increases lipolysis through a mechanism mediated, in part, by down-regulation of inhibitory G proteins (G(i)). Separately, down-regulation of G(i) by prolonged treatment with an A(1)-adenosine receptor agonist, N(6)-phenylisopropyl adenosine (PIA) increases lipolysis. To investigate the role of proteolysis in TNF alpha and PIA-mediated G(i) down-regulation and stimulation of lipolysis, we used the protease inhibitors lactacystin (proteasome inhibitor) and calpeptin (calpain inhibitor). Rat adipocytes were preincubated for 1 h with lactacystin (10 microM) or calpeptin (50 microM), before 30-h treatment with either TNF alpha (50 ng/ml) or PIA (300 nM). We then measured lipolysis (glycerol release), abundance of alpha-subunits of G(i)1 and G(i)2 in plasma membranes (Western blotting) and protease activities (in specific fluorogenic assays). TNF alpha and PIA stimulated lipolysis approximately 2-fold and caused G(i) down-regulation. Although neither lactacystin nor calpeptin affected basal lipolysis, lactacystin completely inhibited both TNF alpha and PIA-stimulated lipolysis (the 50% inhibitory concentration was approximately 2 microM), whereas calpeptin had no effect. Similarly, lactacystin but not calpeptin blocked both PIA and TNF alpha-induced G(i) down-regulation. These findings provide further evidence that the chronic lipolytic effect of TNF alpha and PIA is secondary to G(i) down-regulation and suggest that the mechanism involves proteolytic degradation mediated through the proteasome pathway.

Adenosine A(1) receptors mediate plasma exudation from the oral mucosa

The purpose of this study was to pharmacologically characterize the adenosine receptor subtype(s) that mediates adenosine-induced increases in macromolecular efflux from the intact hamster cheek pouch. Using intravital microscopy, we found that 1,3-dipropyl-8-(2-amino-4-chlorophenyl)-xanthine (PACPX), a selective adenosine receptor-1 antagonist, but not 3,7-dimethyl-1-propargylxanthine (DMPX), a selective adenosine receptor-2 antagonist, significantly attenuated adenosine-induced leaky site formation and increased clearance of fluorescein isothiocyanate-labeled dextran (molecular mass, 70 kDa) from the intact hamster cheek pouch (P < 0.05). Both compounds had no significant effects on bradykinin-induced responses. Nanomolar concentrations of R(-)-N(6)-(2-phenylisopropyl)-adenosine [R(-)-PIA], a selective adenosine A(1) agonist, evoked significant, concentration-dependent increases in macromolecular efflux. This response was significantly attenuated by PACPX but not by DMPX. In contrast, CGS-21680, a selective adenosine A(2) agonist, increased macromolecular efflux but only at micromolar concentrations. This response was significantly attenuated by DMPX but not by PACPX. Suffusion of nitroglycerin had no significant effects on R(-)-PIA- and CGS-21680-induced responses. In addition, suffusion of N(G)-nitro-L-arginine methyl ester, a nitric oxide synthase inhibitor, had no significant effects on adenosine-induced responses. Indomethacin had no significant effects on adenosine-, R(-)-PIA-, and CGS-21680-induced increases in macromolecular efflux. Collectively, these data indicate that adenosine increases macromolecular efflux from the intact hamster cheek pouch by stimulating high-affinity adenosine A(1) receptors in a specific, nitric oxide- and prostaglandin-independent fashion.

Increase of waking and reduction of NREM and REM sleep after nitric oxide synthase inhibition: prevention with GABAA or adenosine A1 receptor agonists

The effect of N(G)-nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of enzyme nitric oxide synthase (NOS), on spontaneous sleep during the light period, was studied in adult rats implanted for chronic sleep recordings. L-NAME was injected by subcutaneous (s.c.) route or was infused directly into the dorsal raphe nucleus (DRN). Subcutaneous (46.0--185.0 micromol/kg) administration of L-NAME increased waking (W), slow wave sleep (SWS) and rapid-eye-movement sleep (REMS) latency, whereas SWS, REMS and the number of REM periods were reduced. Direct application of L-NAME into the DRN (0.37--1.1 micromol) induced an increment of W and a reduction of SWS and REMS. Values corresponding to SWS and REMS latency, and the number of REM periods remained within control levels. Subcutaneous administration of the GABA(A) receptor agonist muscimol (1.7--3.5 micromol/kg) or the adenosine A(1) receptor agonist L-PIA [L(-)N(6)-(2-phenylisopropyl)adenosine] (0.1--0.3 micromol/kg) induced slight but inconsistent changes of W, light sleep (LS), SWS and REMS that did not attain significance. Pretreatment with muscimol (1.7--3.5 micromol/kg, s.c.) or L-PIA (0.1--0.3 micromol/kg, s.c.) antagonized the increase of W and reduction of SWS and REMS induced by s.c. (92.0 micromol/kg) or intra-DRN (0.74 micromol) administration of L-NAME. However, neither muscimol nor L-PIA prevented the increase of REMS latency induced by L-NAME 92.0 micromol/kg, s.c. Our findings tend to indicate that the change of behavioral state observed after systemic or intra-DRN administration of L-NAME is partly related to the reduction of GABA and adenosine at critical sites in the CNS.

Protective effect of adenosine receptor agonists in a new model of epilepsy – seizures evoked by mitochondrial toxin, 3-nitropropionic acid, in mice

The role of adenosine receptor agonists in the convulsant activity of mitochondrial toxin, 3-nitropropionic acid (3-NPA), was studied in mice. The occurrence of seizures evoked by peripheral application of 3-NPA was inhibited with the use of A1 adenosine receptor agonist, R-N6-phenylisopropyladenosine and A1/A2 agonist, 2-chloroadenosine. Moreover, both drugs prevented 3-NPA-induced mortality. Similarly, A1/A2 agonist, 5'-N-ethylcarboxamidoadenosine, protected against seizures evoked by the intracerebral administration of 3-NPA, and this effect was reversed by the co-application of adenosine receptor antagonist, 8-(p-sulfophenyl)theophylline. Obtained results suggest that A1 adenosine receptor activation may modulate the chain of events leading to the development of 3-NPA-induced seizures.

Adenosine inhibits norepinephrine release in the postischemic rat heart: the mechanism of neuronal stunning

OBJECTIVE

Numerous studies support the concept of impaired postischemic sympathetic neurotransmission in the heart. We hypothesized that postischemic neuronal dysfunction (neuronal stunning) is caused by a transient suppression of exocytotic norepinephrine (NE) release from sympathetic nerve terminals. Furthermore, we assessed the role of presynaptic adenosine-receptors and alpha2-adrenoceptors in neuronal stunning.

METHODS AND RESULTS

Exocytotic NE release was induced by two electrical field stimulations (S(1) and S(2)) in isolated perfused rat hearts. S(1) was performed under baseline conditions and S(2) either during or following intervention. Results are expressed as mean S(2)/S(1) ratios+/-S.E.M. Stepwise increase of global ischemic periods (10, 20, and 30 min) induced a progressive suppression of NE release in the postischemic hearts, which was reversible during reperfusion. Both the degree and duration of NE suppression was dependent on the extent of the preceding ischemic period. Following 10-min ischemia complete recovery of NE release was achieved after 5-min reperfusion (1.07+/-0.12), whereas 5-min reperfusion did not restore NE release after 30 min (0.36+/-0.07) of ischemia. The adenosine-receptor antagonists 8-phenyltheophylline (8-PT; non-selective) and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; adenosine A1-receptor subtype selective) significantly increased NE release after 30-min ischemia and 5-min reperfusion (0.78+/-0.06 and 0.64+/-0.07), while in the same experimental protocol blockade of alpha2-adrenoceptors by yohimbine failed to restore the postischemic release (0.24+/-0.06). In non-ischemic hearts the adenosine analogue R(-)N(6)-(2-phenylisopropyl)adenosine (R-PIA) resulted in a marked suppression of NE release (0.61+/-0.07). The inhibitory effect of R-PIA and 2-chloro-N(6)-cyclopentyladenosine (CCPA; adenosine A1-receptor subtype selective agonist) persisted 5 min after cessation of R-PIA (0.62+/-0.05) and CCPA (0.58+/-0.04). Activation of alpha2-adrenoceptors by 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK 14,304) also caused a reduction of NE release (0.50+/-0.02), but the release increased to control levels 5 min after cessation of UK 14,304 (0.90+/-0.06).

CONCLUSIONS

The results establish the phenomenon of neuronal stunning in terms of a postischemic suppression of exocytotic NE release and provide evidence that neuronal stunning is mediated by endogenous adenosine through activation of presynaptic adenosine A1-receptors.

K(ATP) channel openers, adenosine agonists and epileptic preconditioning are stress signals inducing hippocampal neuroprotection

Many models of induced ischemic and epileptic tolerance have now been described in the brain. Although detailed mechanisms underlying such protections still remain largely unknown, induction of heat shock proteins is amongst the endogenous responses believed to play an important role in cellular defense mechanisms. This study reveals that the development of epileptic tolerance also coincides with the induction of the 70,000 mol. wt heat shock protein expression within the time window of protection. Adenosine agonists or ATP-sensitive potassium channel openers have also been shown to exert strong neuroprotective effects when injected shortly prior to a severe ischemic or epileptic insult. The present work shows that adenosine receptor activation and ATP-sensitive potassium channel opening induce 70,000 mol. wt heat shock protein expression in the rat hippocampus and are able to mimic neuroprotection driven by preconditioning. R-phenylisopropyladenosine, a purine agonist, or (-)cromakalim, an ATP-sensitive potassium channel opener, was administered three days prior to a lethal ischemic or epileptic episode to mimic preconditioning. Neurodegeneration was assessed using Cresyl Violet staining and cellular DNA fragmentation visualized by the terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate-biotin nick end labeling method. 70, 000 mol. wt heat shock protein expression was analysed by western blotting and immunohistochemistry. The results show a long-lasting neuroprotection induced by activation of adenosine receptors or ATP-sensitive K(+) channels as early as three days prior to induction of a severe ischemic or epileptic challenge. This protective effect is associated with enhanced 70,000 mol. wt heat shock protein expression also occurring three days following administration of R-phenylisopropyladenosine or (-)cromakalim. These findings support the idea that preconditioning doses of R-phenylisopropyladenosine and (-)cromakalim act as mild cellular stresses inducing neuroprotection in a manner similar to a mild kainate treatment prior to a lethal ischemic or severe epileptic insult three days later. They also suggest that a delayed 70,000 mol. wt heat shock protein expression induced by excitatory neuronal stresses such as short ischemia, mild kainic acid treatment or activation of adenosine receptors and ATP-sensitive potassium channels is predictive of neuronal survival against a subsequent lethal injury.

Effects of recombinant ovine leptin on in vitro lipolysis and lipogenesis in subcutaneous adipose tissue from lactating and nonlactating sheep

Direct effects of recombinant ovine leptin on adipose metabolism in sheep were investigated. Lipolytic and lipogenic rates were assessed following preincubation of subcutaneous adipose tissue explants with recombinant ovine leptin. Leptin had no consistent effect on the basal (unstimulated) lipolytic rate in adipose tissue from wethers. Lipolytic rate measured in the presence of combinations of adenosine deaminase, isoprenaline, and N6-phenylisopropyl adenosine was unaffected by pretreatment with leptin. In lactating ewes, there was no relationship between increasing concentrations of leptin and basal lipolytic rate. Leptin had no effect on basal (unstimulated) lipogenesis, or on insulin-stimulation or growth hormone inhibition of lipogenesis in adipose tissue from wethers. Lipogenesis in adipose tissue from lactating ewes was also unaffected by preincubation with leptin; however, at supraphysiological concentrations of leptin, there was a small reduction in the rate of insulin-stimulated lipogenesis. Leptin failed to induce phosphorylation of the signal transducers and activators of transcription, STAT 3 and STAT 5, in sheep adipocytes. These results suggest that leptin does not have a direct physiological effect on subcutaneous adipose tissue metabolism in sheep.

Role for PKC in the adenosine-induced decrease in shortening velocity of rat ventricular myocytes

We previously demonstrated that both adenosine receptor activation and direct activation of protein kinase C (PKC) decrease unloaded shortening velocity (V(max)) of rat ventricular myocytes. The goal of this study was to further investigate a possible link among adenosine receptors, phosphoinositide-PKC signaling, and V(max) in rat ventricular myocytes. We determined that the adenosine receptor agonist R-phenylisopropyladenosine (R-PIA, 100 microM) and the alpha-adrenergic receptor agonist phenylephrine (Phe, 10 microM) increased turnover of inositol phosphates. PKC translocation from the cytosol to the sarcolemma was used as an indicator of PKC activation. Western blot analysis demonstrated an increased PKC-epsilon translocation after exposure to R-PIA, Phe, and the PKC activators dioctanoylglycerol (50 microM) and phorbol myristate acetate (1 microM). PKC-alpha, PKC-delta, and PKC-zeta did not translocate to the membrane after R-PIA exposure. Finally, PKC inhibitors blocked R-PIA-induced decreases in V(max) as well as Ca(2+)-dependent actomyosin ATPase in rat ventricular myocytes. These results support the conclusions that adenosine receptors activate phosphoinositide-PKC signaling and that adenosine receptor-induced PKC activation mediates a decrease in V(max) in ventricular myocytes.

Further characterization of an adenosine transport system in the mitochondrial fraction of rat testis

Previous work from our laboratory has demonstrated the presence of high-affinity binding sites for [3H]nitrobenzylthioinosine ([3H]NBTI), a marker of adenosine uptake systems, in the mitochondrial fraction of rat testis. Here, we characterize this system functionally through [3H]adenosine uptake assays. This system (K(m)=2+/-1.3 microM; V(max)=86.2+/-15.5 pmol/mg protein/min) was found to be saturable, non sodium-dependent and sensitive to temperature, pH and osmolarity. [3H]Adenosine incorporation was potently inhibited by hydroxynitrobenzylthioguanosine (HNBTG, IC(50)=3 nM) although NBTI inhibited this uptake weakly (IC(50)=72. 7+/-37.1 microM). Dilazep>dipyridamole>/=hexobendine inhibited [3H]adenosine incorporation at low micromolar concentrations. The nucleosides inosine and uridine were weak inhibitors of this system. The adenosine receptor ligands N(6)-phenylisopropyladenosine (PIA) and 2-chloroadenosine inhibited the uptake only at micromolar concentrations. Neither 5'-(N-ethylcarboxamido)-adenosine (NECA) nor theophylline inhibited adenosine uptake by more than 60% but the mitochodrial benzodiazepine receptor ligands 4'-chloro-diazepam (Ro 5-4864) and 1-(2-chlorophenyl)-N-methyl-N-(1-methyl-propyl) isoquinoline carboxamide (PK 11195) were able to inhibit it. The lack of inhibition by the blockers of the mitochondrial adenine-nucleotide carrier, atractyloside and alpha, beta-methylene-ATP, indicates that [3H]adenosine uptake occurs via a transporter other than this carrier. All these results support the existence of an equilibrative adenosine transport system, which might mediate the passage of adenosine formed in the mitochondria to the cytoplasm.

Subtype-specific kinetics of inhibitory adenosine receptor internalization are determined by sensitivity to phosphorylation by G protein-coupled receptor kinases

Despite coupling to the same class of inhibitory G proteins and binding the same physiological ligand, the human A(1) and rat A(3) adenosine receptors (ARs) desensitize at different rates in response to sustained agonist exposure. This is due to the ability of the A(3)AR, but not the A(1)AR, to serve as a substrate for rapid phosphorylation and desensitization by members of the G protein-coupled receptor kinase (GRK) family. The aim of this study was to investigate whether these differences were also manifested in their abilities to undergo agonist-dependent receptor internalization. For the first time, we report that A(3)ARs internalize profoundly in response to short-term exposure to agonist but not activators of second messenger-regulated kinases. The A(3)AR-selective antagonist MRS1523 blocked both A(3)AR phosphorylation and internalization. Moreover, in contrast to the A(1)AR, which internalized quite slowly (t(1/2) = 90 min), A(3)ARs internalized rapidly (t(1/2) = 10 min) over a time frame that followed the onset of receptor phosphorylation. A nonphosphorylated A(3)AR mutant failed to internalize over a 60-min time course, suggesting that receptor phosphorylation was essential for rapid A(3)AR internalization to occur. In addition, fusion onto the A(1)AR of the A(3)AR C-terminal domain containing the sites for phosphorylation by GRKs conferred rapid agonist-induced internalization kinetics (t(1/2) = 10 min) on the resulting chimeric AR. In conclusion, these data suggest that GRK-stimulated phosphorylation of threonine residues within the C-terminal domain of the A(3)AR is obligatory to observe rapid agonist-mediated internalization of the receptor.

Mutually protective actions of kainic acid epileptic preconditioning and sublethal global ischemia on hippocampal neuronal death: involvement of adenosine A1 receptors and K(ATP) channels

Preconditioning with sublethal ischemia attenuates the detrimental effects of subsequent prolonged ischemic insults. This research elucidates potential in vivo cross-tolerance between different neuronal death-generating treatments such as kainate administration, which induces seizures and global ischemia. This study also investigates the effects of a mild epileptic insult on neuronal death in rat hippocampus after a subsequent, lethal epileptic stress using kainic acid (KA) as a model of epilepsy. Three preconditioning groups were as follows: group 1 was injected with 5 mg/kg KA before a 6-minute global ischemia; group 2 received a 3-minute global ischemia before 7.5 mg/kg KA; and group 3 was injected with a 5-mg/kg dose of KA before a 7.5-mg/kg KA injection. The interval between treatments was 3 days. Neuronal degeneration, revealed by the silver impregnation method and analysis of cresyl violet staining, was markedly reduced in rats preconditioned with a sublethal ischemia or a 5-mg/kg KA treatment. Labeling with terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'triphosphate-biotin nick-end labeling and DNA laddering confirmed the component of DNA fragmentation in the death of ischemic and epileptic neurons and its reduction in all preconditioned animals. The current study supports the existence of bidirectional cross-tolerance between KA excitotoxicity and global ischemia and suggests the involvement of adenosine A1 receptors and sulfonylurea- and ATP-sensitive K+ channels in this protective phenomenon.

Regulation of porcine adipocyte metabolism by insulin and adenosine

The acute effects of insulin and adenosine on rates of lipolysis and lipogenesis in pig adipocytes were investigated to determine what limits the expression of the insulin response in vitro. Adenosine and insulin independently inhibited isoproterenol-stimulated lipolysis. Adenosine, acting through the pertussis toxin-sensitive G-protein Gi, was more effective than insulin and could completely inhibit lipolysis. Fatty acid synthesis from glucose was increased by both adenosine and insulin. Neutralization of endogenous adenosine with adenosine deaminase decreased basal rates of lipogenesis and increased the insulin response from 30 to 60% above basal. Neutralization of Gi with pertussis toxin further decreased the basal rate and increased the insulin response to 160% above basal. These data indicate that Gi, and the ligands that signal through Gi, stimulate glucose incorporation into fatty acids and can attenuate the insulin response. It seems likely that an exaggerated rate of glucose metabolism in the absence of insulin contributes to the inconsistent insulin responses exhibited in pig adipose tissue in vitro. These data also demonstrate that insulin and adenosine have major roles in regulating pig adipose tissue metabolism.

Transregulation of adenylyl-cyclase-coupled inhibitory receptors in heart failure enhances anti-adrenergic effects on adult rat cardiomyocytes

OBJECTIVE

In congestive heart failure (CHF), a desensitisation of stimulatory beta-receptors and of adenylyl cyclase in the heart is associated with an increase in inhibitory Gi proteins. To investigate whether the regulation of the Gi-mediated inhibitory side of the adenylyl cyclase system may be of functional importance in the failing myocardium, the contractile response of isolated adult cardiomyocytes to stimulation of inhibitory muscarinic M2 and A1 adenosine receptors was analysed.

METHODS

CHF was induced in rats by banding of the ascending aorta and was verified by doubling of lung wet weight. After four weeks, contraction amplitude (delta L) and the velocity (dL/dtmax) of isolated ventricular cardiomyocytes during electrical field stimulation in the presence of 1 mM Ca2+ were measured using video micrometry.

RESULTS

Contractile responses of failing cardiomyocytes to 5 mM Ca2+ were unchanged. The response to increasing concentrations of the beta-adrenergic agonist, isoproterenol (0.1-30 nM), and to forskolin (0.1 nM-1 microM) were significantly blunted. When A1 receptors were activated with N6-(R-phenyl-isopropyl)-adenosine (PIA; 0.01-1 microM) in the presence of 3 nM isoproterenol, contractility was unchanged in cells compared with those from sham-operated rats, but delta L was reduced by up to 23% and dL/dtmax by 35% in failing cardiomyocytes (P < 0.01), demonstrating an enhanced inhibitory effect of A1 receptors. The response to the M2 receptor agonist, carbachol (0.01-3 microM), was augmented to a comparable extent (delta L, -22%, dL/dtmax, -39%; P < 0.01).

CONCLUSIONS

In CHF, the inotropic responses to beta-receptor-stimulation and to direct stimulation of adenylyl cyclase, but not to Ca2+, are diminished due to desensitisation of the stimulatory side of the adenylyl cyclase signal transduction system. In parallel, the responses to inhibitory receptors are augmented, leading to a pronounced Gi-mediated negative inotropic effect on failing heart muscle cells. Those anti-adrenergic effects could contribute to the contractile dysfunction of the failing heart. Reversal of the sensitisation to inhibitory stimuli might be one of the desirable mechanisms of medical therapy in CHF.

Enhancement by adenosine of insulin-induced activation of phosphoinositide 3-kinase and protein kinase B in rat adipocytes

The role of adenosine receptor in regulation of insulin-induced activation of phosphoinositide 3-kinase (PI 3-kinase) and protein kinase B was studied in isolated rat adipocytes. Rat adipocytes are known to spontaneously release adenosine, which in turn binds and stimulates the adenosine A1 receptors on the cells. In the present study, we observed that degradation of this adenosine by adenosine deaminase attenuated markedly the insulin-induced accumulation of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3), a product of PI 3-kinase. p-Aminophenylacetyl xanthine amine congener (PAPA-XAC), an inhibitor of the adenosine A1 receptor, also inhibited the insulin-induced PtdIns(3,4,5)P3 accumulation. When extracellular adenosine was inactivated by adenosine deaminase, phenylisopropyladenosine, an adenosine A1 receptor agonist, potentiated the insulin-induced accumulation of PtdIns(3,4,5)P3. Insulin-induced activation of protein kinase B, the activity of which is controlled by the lipid products of PI 3-kinase, was also potentiated by adenosine. Prostaglandin E2, another activator of a pertussis toxin-sensitive GTP-binding protein in these cells, potentiated the insulin actions. Thus, the receptors coupling to the GTP-binding protein were found to positively regulate the production of PtdIns(3,4,5)P3, a putative second messenger for insulin actions, in physiological target cells of insulin.

Effects of ethanol on basal and adenosine-induced increases in beta-endorphin release and intracellular cAMP levels in hypothalamic cells

Recently we have shown that the cAMP system is involved in ethanol-regulated beta-endorphin (beta-EP) release from rat hypothalamic neurons in primary cultures. The cascade of events that leads to activation of cAMP following ethanol treatment in hypothalamic beta-EP neurons is not apparent. In this study the role of adenosine, a cAMP regulator, in ethanol-regulated beta-EP release was determined by measuring the cellular incorporation of [3H]adenosine, intracellular cAMP levels and media immunoreactive (IR) beta-EP levels in cultures of rat hypothalamic cells following ethanol treatments in the presence and absence of an adenosine agonist and antagonist. Acute exposure to a 50 mM dose of ethanol for a period of 1 h increased media levels of IR-beta-EP and cellular contents of cAMP, but the ethanol treatment decreased [3H]adenosine uptake. Constant exposure to a 50 mM dose of ethanol for a period of 48 h, failed to alter media levels of IR-beta-EP, cell content of cAMP and [3H]adenosine uptake. The media level of IR-beta-EP was elevated following treatment with adenosine receptor agonist phenyl-isopropyl adenosine (PIA) and was reduced following treatment with adenosine receptor antagonist isobutylmethylxanthine (IBMX) or with adenosine uptake inhibitor adenosine deaminase. The level of cellular cAMP was also increased by PIA but was decreased by IBMX and adenosine deaminase. The stimulatory actions of the adenosine agonist PIA on IR-beta-EP release and on cAMP production were potentiated by simultaneous incubation with ethanol for 1 h. However, chronic ethanol exposure reduced PIA-induced IR-beta-EP release and cAMP production. Additionally, both IBMX and adenosine deaminase reduced ethanol-induced IR-beta-EP release and cAMP levels. These results suggest that ethanol inhibits adenosine uptake in IR-beta-EP neurons in the hypothalamus, thereby increasing extracellular levels of adenosine and leading to activation of membrane adenosine receptors, cAMP production and IR-beta-EP secretion from these neurons. Chronic ethanol desensitizes the adenosine-regulated cAMP production and IR-beta-EP release from hypothalamic neurons.

A comparison of the adenosine-mediated synaptic inhibition in the CA3 area of immature and adult rat hippocampus

We compared the effects of the adenosine A1 receptor activation on the postsynaptic potentials (psps) recorded from the CA3 area of immature (postnatal days 10-20) and adult rat hippocampal neurons in vitro. The adenosine A1 receptor agonist 2-phenyl-isopropyl-adenosine (PIA, 1 microM) depressed the stimulus-induced psps less in immature and more in adult neurons. In the presence of the GABAA receptor antagonist bicuculline methiodide (BMI, 10 microM), PIA reduced the duration and number of action potentials of the stimulus-induced paroxysmal depolarizations (PDs) in immature neurons, while it blocked PDs in adult neurons. Spontaneous BMI-induced PDs, were blocked by PIA in less than half (5/12) immature and all (6/6) adult neurons. The adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1 microM) enhanced the stimulus-induced psps in immature and adult neurons alike; this effect did not lead to stimulus-induced bursting in immature neurons. DPCPX induced spontaneous bursts (proconvulsant effect) in only 2/16 immature but in all adult (12/12) neurons. In BMI, DPCPX increased the duration and number of action potentials of the stimulus-induced PDs in immature and adult neurons alike (by about 30%), but it increased the rates of occurrence of spontaneous PDs in significantly more adult neurons. In conclusion, our results suggest that adenosine, acting via A1 receptors, is a more effective endogenous anti-epileptic in adult than in immature hippocampus, a fact which may contribute to the susceptibility of the latter to epileptogenesis.

Regulation of the GTP-binding protein-based antilipolytic system of sheep adipocytes by growth hormone

Chronic exposure of sheep adipose tissue to growth hormone (GH) in vitro decreases the ability of the adenosine analogue, N6-phenylisopropyladenosine (PIA), to inhibit isoprenaline-stimulated lipolysis by a mechanism which is dependent on both gene transcription and protein serine/threonine phosphorylation. The inhibition is not due to a change in ligand binding to the adenosine receptor, the amounts of the three isoforms of the inhibitory GTP-binding protein, Gi, or the maximum (forskolin-stimulated) adenylate cyclase activity. The ability of GH to modulate the PIA-activated adenosine receptor to stimulate dissociation of heterotrimeric Gi was assessed by measurement of pertussis toxin-catalysed ADP-ribosylation of Gi; GH does not appear to alter the interaction between the activated receptor and Gi. The ability of GH to alter the ability of activated Gi to inhibit adenylate cyclase activity was assessed by measuring the ability of a GTP analogue, guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG), to inhibit forskolin-stimulated adenylate cyclase activity; chronic exposure to GH prevented this effect of p[NH]ppG. Thus the attenuation of the inhibition of lipolysis by PIA by chronic exposure of adipocytes to GH appears to be due to an impairment in the interaction between adenylate cyclase and the alpha subunit of one or more isoforms of Gi.

Mechanisms of thyroid hormone control over sensitivity and maximal contractile responsiveness to beta-adrenergic agonists in atria

This paper discusses the mechanisms of two basic effects of thyroid hormones on atrial responses to beta-adrenergic agonists, i.e. increased inotropic sensitivity and decreased maximal contractile responsiveness. The increased sensitivity of atria to beta-adrenergic agonists under thyroid hormones appears to be related to increases in beta-adrenoceptor density and Gs/Gi protein ratio, leading to activation of Gs-mediated pathway, but suppression of Gi-mediated pathway of adenylate cyclase regulation. Therefore, the i/c concentrations of cAMP and corresponding inotropic responses achieve their maximums at lower doses of beta-adrenergic agonist. Thyroid hormones also decrease the expression of phospholamban, but increase the expression of sarcoplasmic reticulum Ca2+-pump. As a result, the basal activity of sarcoplasmic reticulum Ca2+-pump increases, but its beta-adrenergic activation through phosphorylation of phospholamban decreases. It is suggested that these changes are causal for decreased maximal inotropic and lusitropic responses of atria to beta-adrenergic agonists.

Differential effect of thyroid-stimulating hormone (TSH) on intracellular free calcium and cAMP in cells transfected with the human TSH receptor

The thyroid-stimulating hormone (TSH) binds to a receptor which activates adenylate cyclase and elevates cAMP concentration. In addition, effects of TSH on intracellular calcium and inositol phosphate accumulation have been reported. However, the mechanism of TSH-stimulated accumulation of inositol phosphates and elevation of calcium levels is unresolved. Previous work from this laboratory has shown TSH to cause acute transient increases in intracellular calcium in pig, human and FR TL-5 rat thyroid cells as well as in cell transfected with the human TSH receptor (JPO9 cells) in some (but not all) experiments. The aim of this study was to investigate the variability of the calcium response to TSH in JPO9 cells to learn more about the nature of this calcium signal induction. Calcium responses to TSH were determined using the fluorochrome fura-2 in both monolayers of adherent cells and adherent single cells. The responses to a single addition and to repetitive additions of TSH were compared. We also determined the cAMP response to TSH using these two protocols of TSH addition. Our data show that, whereas the cAMP response to TSH is highly predictable and consistent and does not require multiple exposures to TSH, cells were unlikely to respond to TSH with an increase in calcium unless they received multiple challenges with the hormone. A single addition of 10 mU/ml TSH failed to increase calcium in any of 40 single cells examined and in only 4 of 15 monolayers of cells (27%) examined; in contrast, 10 of 12 monolayers eventually responded with an increase in calcium after multiple exposure to TSH and 18 of 67 single cells. Similar data were obtained whether calcium was measured in single cells or in populations of cells. We also demonstrated cooperativity between an adenosine derivative, N6-(L-2-phenylisopropyl)adenosine, and TSH such that their co-administration resulted in a consistent and marked elevation in calcium levels not achieved with either agonist alone. In summary, we suggest that the coupling between the TSH receptor and the intracellular signalling system that leads to activation of intracellular calcium in JPO9 cells requires repetitive stimulation or the influence of other agonists, in contrast with the coupling between the TSH receptor and activation of the adenylate cyclase enzyme.

Effects of purinergic agonists on aromatase and gamma-glutamyl transpeptidase activities and on transferrin secretion in cultured Sertoli cells

To study the role of extracellular nucleosides and nucleotides in the regulation of Sertoli cells, the effects of agonists which occupy A1 and P2 purinergic receptors on aromatase and gamma-glutamyl transpeptidase (gamma-GTP) activities and on transferrin secretion were tested. Sertoli cell treatment with purinergic agonists for a prolonged period of time (72 h) resulted in an increase in aromatase activity under basal conditions. In cultures stimulated with FSH, purinergic agonists counteracted the inhibitory effect on aromatase activity that long-term treatment with FSH promoted. The effects of prolonged treatments with purinergic agonists on the other two parameters of Sertoli cell function were less pronounced. Neither gamma-GTP activity nor transferrin secretion was modified under basal conditions. On the other hand, under conditions where cell differentiation was favored by FSH treatment, reductions in gamma-GTP activity and transferrin secretion were usually observed. The results obtained in dbcAMP-stimulated cultures suggested that A1 agonists exert their regulatory function at the level of cAMP formation while P2 agonists act at a more distal point. The fact that morphological changes induced by FSH were reversed by both types of agonists, while those induced by dbcAMP were only abrogated by P2 agonists, supports this hypothesis. In summary, these results demonstrate that purinergic agonists may be important in the regulation of Sertoli cell function.

Reconstitution of bovine A1 adenosine receptors and G proteins in phospholipid vesicles: betagamma-subunit composition influences guanine nucleotide exchange and agonist binding

We have studied the interactions of purified A1 adenosine receptors and G proteins reconstituted into phospholipid vesicles to investigate how the betagamma composition of G protein heterotrimers influences coupling. Recombinant hexahistidine-tagged bovine A1 adenosine receptors were expressed in Sf9 cells and purified to homogeneity by sequential chromatography over heparin-sepharose, xanthine amino congener-agarose, and nickel-nitrilotriacetic acid columns. These receptors were reconstituted with pure recombinant G proteins of defined subunit composition. Receptor-G protein complexes containing alphai2 and beta1gamma2 or beta1gamma3 and stimulated with the agonist, (R)-phenylisopropyladenosine, exchange guanine nucleotide 2-3 times more rapidly than do complexes containing beta1gamma1. This difference is not overcome by increasing the concentration of betagamma subunits. Receptor-G protein complexes containing beta1gamma1 also bind less of the agonist, [125I]-iodoaminobenzyladenosine (125I-ABA), than do complexes containing beta1gamma3. Kinetic experiments show that 125I-ABA dissociates 2-fold more rapidly from receptor-G protein complexes containing beta1gamma1 than from complexes containing the other betagamma subunits. The affinity of the interaction between immobilized Galphai2 subunits and beta1gamma1 or beta1gamma2 measured with an optical biosensor in the absence of receptor is similar. Taken together, these data implicate the gamma-subunit in influencing the interaction between the A1 adenosine receptor and G proteins.

Reduced inhibitory actions of adenosine A1 and kappa 1-opioid receptor agonists on beta-adrenoceptors in spontaneously hypertensive rat heart

1. The modulatory actions of both adenosine A1 and kappa 1-opioid receptor agonists on beta-adrenoceptor stimulation in the heart of both spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were compared. 2. In both types of rats, both R(-)-N6-(2-phenylisopropyl)adenosine (R-PIA), an adenosine A1 receptor agonist, and U50 488H, a kappa 1-opioid receptor agonist, inhibited the stimulatory effects of beta-adrenoceptor activation on electrically induced [Ca2+]i transients measured by a spectrofluorometric method with fura-2/AM as the calcium indicator. The effects of these two agonists were blocked by their respective antagonists, namely 8-cyclopentyl-1,3-diprolxanthine and norbinaltorphimine. 3. The inhibitory actions of both R-PIA and U50 488H on beta-adrenoceptor augmentation of electrically induced [Ca2+]i transients in the heart were more significantly reduced in SHR than in WKY rats, suggesting the negative modulatory actions of endogenous substances on beta-adrenoceptors were impaired in SHR, which may contribute to hypertension.

Modulation of GH4 cell cycle via A1 adenosine receptors

Identification and characterization of A1 adenosine receptors (A1Rs) in a tumor cell line derived from rat pituitary (GH4 cells) was performed by ligand binding and immunocytochemistry. Subsequently, the involvement of A1Rs in the regulation of cell proliferation was studied in these cells. The agonist N6-(R)-phenylisopropyladenosine (R-PIA) did not modify the number of cultured cells, but it regulated the kinetics of the cell cycle. By means of experiments of pulse and of pulse and chase with bromodeoxyuridine and further labeling with Hoechst 33258, propidium iodide, and/or fluorescein-conjugated antibodies against bromodeoxyuridine, it was demonstrated that R-PIA, via A1Rs, accelerated progression from G0/G1 to S phase and from S to G2/M phase of the cell cycle, whereas the initiation of a new cycle occurred at the same time in treated and untreated cells. As a consequence, R-PIA did not change the total length of the cycle. This is the first description of cell cycle regulation without modification of cell proliferation. Although pertussis toxin blocked the R-PIA-induced inhibition of cyclic AMP production in these cells, it did not affect the R-PIA action on the cell cycle. In contrast, cholera toxin mimicked the action of R-PIA. Thus, it is likely that regulation of the cell cycle via A1Rs is mediated by heterotrimeric G proteins different from those that mediate inhibition of adenylate cyclase. Due to the fact that cells in G0/G1 phase were less susceptible to secretory signals, adenosine, in an autocrine manner and by regulating the cell cycle kinetics, may contribute to the modulation of the secretory capacity of pituitary cells.

Betagamma subunits of pertussis toxin-sensitive G proteins mediate A1 adenosine receptor agonist-induced activation of phospholipase C in collaboration with thyrotropin. A novel stimulatory mechanism through the cross-talk of two types of receptors

COS-7 cells were transiently transfected with human thyrotropin receptor and dog A1 adenosine receptor cDNAs. An A1 agonist, N6-(L-2-phenylisopropyl) adenosine (PIA), which is ineffective alone, enhanced the thyrotropin (TSH)-induced inositol phosphate production, reflecting phospholipase C (PLC) activation, but inhibited the TSH-induced cAMP accumulation, reflecting adenylyl cyclase inhibition. These PIA-induced actions were completely inhibited by pertussis toxin (PTX) treatment. Moreover, in the cells expressing a PTX-insensitive mutant of Gi2alpha or Gi3alpha, in which a glycine residue was substituted for a cysteine residue to be ADP-ribosylated by PTX, at the fourth position of the C terminus, PIA effectively exerted both stimulatory and inhibitory effects on the TSH-induced actions although the cells were treated with the toxin. Overexpression of the betagamma subunits of the G proteins enhanced the TSH-induced inositol phosphate production without any significant effect on the cAMP response; under these conditions, PIA did not further increase the elevated inositol phosphate response to TSH. On the contrary, overexpression of a constitutively active mutant of Gi2alpha, in which the guanosine triphosphatase activity is lost, inhibited the TSH-induced cAMP accumulation but hardly affected the inositol phosphate response; under these conditions, PIA never exerted further inhibitory effects on the cAMP response to TSH. In contrast to the case of the TSH-induced inositol phosphate response, the response to a constitutively active G11alpha mutant was not appreciably affected, and that to NaF was rather inhibited by PIA and overexpression of the betagamma subunits. Taken together, these results suggest that a single type of PTX-sensitive G protein mediates the A1 adenosine receptor-linked modulation of two signaling pathways in collaboration with an activated thyrotropin receptor; alpha subunits of the PTX-sensitive G proteins mediate the inhibitory action on adenylyl cyclase, and the betagamma subunits mediate the stimulatory action on PLC. In the case of the latter stimulatory action on PLC, the betagamma subunits may not directly activate PLC. The possible mechanism by which betagamma subunits enhance the TSH-induced PLC activation is discussed.

Effector mechanism of adenosine in acute ischemic preconditioning of skeletal muscle against infarction

We used adenosine A1 receptor agonist N6-1(phenyl-2R-isopropyl)-adenosine (PIA), A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and ATP-sensitive K+ (KATP) channel blockers sodium 5-hydroxydecanoate (5-HD) and glibenclamide (Glib), as probes to investigate the role and mechanism of adenosine in ischemic preconditioning (IPC) of noncontractile skeletal muscle against infarction, using the pig latissimus dorsi muscle flap model. Except for Glib, all drugs were delivered to each muscle flap by 10-min local intra-arterial infusion to avoid systemic effects. Muscle flaps that were subjected to 4 h of global ischemia and 48 h of reperfusion sustained 40 +/- 2% infarction. IPC with three cycles of 10 min ischemia and reperfusion, preischemic adenosine, or PIA treatment reduced (P < 0.05) muscle infarction to 24 +/- 2, 18 +/- 2, and 24 +/- 2%, respectively. The anti-infarction effect of IPC and adenosine was blocked by DPCPX, 5-HD, and Glib (P < 0.05). Preischemic adenosine treatment also maintained higher muscle contents of phosphocreatine, ATP, and energy charge potential and lower muscle contents of dephosphorylated metabolites and lactate during ischemia and a lower muscle myeloperoxidase (MPO) activity during reperfusion compared with the control (P < 0.05). Preischemic adenosine treatment did not increase muscle content of adenosine during ischemia or reperfusion. Furthermore, adenosine given at the onset of reperfusion was not effective in attenuating muscle MPO activity or infarction. Taken together, these observations indicate that adenosine, through A1 receptors, initiates the mechanism of IPC with postreceptor involvement of KATP channels in skeletal muscle. However, adenosine is unlikely to play a key role in the effector mechanism. Presently, the cause and role of energy sparing and neutrophil inhibitory effects associated with the anti-infarction effect of preischemic adenosine treatment are unknown.

Sodium saccharin inhibits adenylyl cyclase activity in non-taste cells

We have studied the in vitro effect of sodium saccharin (NaSacch) on the rat adipocyte adenylyl cyclase complex. NaSacch (2.5-50 mM) inhibited significantly in a dose-dependent manner basal and isoproterenol-stimulated cAMP accumulation on isolated rat adipocytes. Similarly, NaSacch (2.5-50 mM) inhibited forskolin-stimulated adenylyl cyclase activity measured in the presence of Mg(2+)-ATP on adipocyte, astrocyte and thyrocyte membrane fractions. In contrast, NaSacch did not inhibit but slightly increased the forskolin-stimulated adenylyl cyclase activity measured in the presence of Mn(2+)-ATP and GDP beta S, a stable GDP analogue. The effect of NaSacch was not mediated through either the A1-adenosine receptor (A1R) or the alpha 2-adrenergic receptor (alpha 2AR). The inhibitory effect of NaSacch was additive to that of A1R agonist and was not blocked by the addition of the alpha 2AR antagonist RX 821002. Pretreatment of adipocytes with pertussis toxin slightly attenuated but did not abolish the inhibitory effect of NaSacch on forskolin-stimulated adenylyl cyclase activity on membrane fractions. These data suggest that the inhibitory effect of NaSacch on forskolin stimulated-adenylyl cyclase in adipocytes does not imply only Gi protein but also other direct or indirect inhibitory pathway(s) which remain to be determined.

Short term desensitization of the A1 adenosine receptors in DDT1MF-2 cells

Previous studies have indicated that desensitization of the A1 adenosine receptor (A1AR), unlike other adenosine receptor subtypes and G protein-coupled receptors, required prolonged exposure to agonists. We more closely studied this observation by focusing on changes in the A1AR signal transduction pathway after short term agonist exposure (0.5-4 hr) in the hamster vas deferens smooth muscle cell line (DDT1MF-2 cells). Incubation of these cells with 1 microM (R)-phenylisopropyladenosine [(R)-PIA] produced a time-dependent loss in binding of the agonist radioligand [125I]N6-2-(4-amino-3-iodophenyl)ethyladenosine but not of the antagonist radioligand [3H]8-cyclopentyl-1,3-dipropylxanthine. This was accompanied by a reduction in the high affinity (G protein-coupled) state of this receptor from 63 +/- 8% to 37 +/- 12% after treatment for 4 hr. Moreover, cells treated with (R)-PIA demonstrated reduced agonist-stimulated GTPase activity and diminished inhibition of adenylyl cyclase activity but no change in expression of alphai and beta subunits. The decreases in agonist binding in the desensitized cells were reversible after treatment of DDT1MF-2 cell membranes with alkaline phosphatase or protein phosphatases 1 and 2A, suggesting a role of phosphorylation in the uncoupling and desensitization of the A1AR. Incubation of cells with (R)-PIA led to rapid translocation of G protein-coupled receptor kinase (GRK) from the cytosol to the plasma membrane within 1 hr of exposure. In addition, purified preparations of the A1AR that were phosphorylated with purified recombinant GRK-2 demonstrated enhanced affinity for arrestin over Gi/Go. These results indicate rapid and functional desensitization of the A1AR by brief exposure to agonist. The mechanism underlying this event seems to involve phosphorylation of the A1AR, presumably by the GRK or GRKs.

Insulin resistance in adipocytes after downregulation of Gi subtypes

To determine whether downregulation of Gi proteins is associated with insulin resistance, we incubated isolated adipocytes with N6-(2-phenylisopropyl)adenosine (PIA; an A1-adenosine receptor agonist; 300 nM), prostaglandin E1 (PGE1; 3 microM), or nicotinic acid (1 mM) for 4 days in primary culture. The cells were washed, and the rate of glucose transport (2-deoxy-[3H]glucose uptake) was measured after incubation with various concentrations of insulin for 45 min. Both PIA and PGE1 (which downregulate Gi) decreased the maximal responsiveness of the cells to insulin by approximately 30% and caused a rightward shift in the dose-response curve. By contrast, nicotinic acid (which does not downregulate Gi) did not alter the insulin sensitivity of the cells. Prolonged treatment of adipocytes with either PIA or PGE1 (but not nicotinic acid) rendered the cells completely resistant to the antilipolytic effect of insulin. The ability of insulin to stimulate autophosphorylation of the beta-subunit of the insulin receptor was decreased by approximately 30% in PIA-treated cells, and the dose-response curve was shifted to the right. Similarly, the ability of the receptor to phosphorylate poly(Glu4-Tyr1) was decreased by approximately 35%. This decrease in tyrosine kinase activity of the receptor may account for the decrease in insulin sensitivity of glucose transport but cannot account for the complete loss of antilipolysis. The findings suggest both a direct and indirect involvement of Gi proteins in insulin action.

Adenosine receptor agents and conditioned place preference

1. The effects of different doses of the adenosine agonists N5-ethylcarboxamido-adenosine (NECA), R-isomer of N6-phenylisopropyladenosine (R-PIA), and N6-cyclohexyladenosine (CHA) or of the antagonists theophylline and 8-phenyltheophylline (8-PT) on conditioned place preference (CPP) have been studied. 2. The results show that R-PIA and CHA induced conditioned place aversion (CPA) whereas NECA induced conditioned place preference (CPP). 3. Low doses of theophylline elicit CPP, but high doses of the drug induced CPA. 8-PT also produced the CPP. 4. The responses of R-PIA and CHA but not NECA was decreased by theophylline and 8-PT administration. 5. It is concluded that the induction of CPP an CPA by adenosine antagonsists may be mediated by different adenosine receptors.

Adenosine modulates inspiratory neurons and the respiratory pattern in the brainstem of neonatal rats

The role of adenosine in the modulation of respiration-related neurons was examined using an in vitro brainstem-spinal cord preparation from neonatal rats (0-4 d old). Respiratory activity was recorded from the C4 or C5 ventral roots by suction electrodes and from inspiratory related neurons (I neurons) in the rostral ventrolateral medulla by microelectrodes. The following substances were added to the preparation superfusate, and their effect was evaluated: the adenosine A1 receptor agonist N6-(2-phenylisopropyl)adenosine, R(-)isomer (R-PIA), the adenosine uptake blocker dipyridamole, the adenosine receptor antagonist theophylline, and the specific A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). R-PIA and dipyridamole decreased the activity of I neurons and the C4 respiratory burst rate. Furthermore, these compounds induced a significantly more irregular respiratory rate in three-quarters of preparations from the youngest animals (<48 h old) compared with that of controls. Theophylline or DPCPX reversed the effects of both R-PIA and dipyridamole on respiratory rate, regularity of respiratory rate, inspiratory time, amplitude, and intra-burst frequency of I neurons. Thus, adenosine depresses both the I neurons in the rostral ventrolateral medulla and the respiratory motor output. This depression of I neurons and respiratory rate can be abolished by theophylline primarily through a blockade of medullary adenosine A1 receptors. An age-dependent correlation of the effects of R-PIA and dipyridamole, with a more pronounced decrease in respiratory activity in preparations from younger animals, indicates that adenosinergic modulation of medullary respiration-related neurons changes during the first days of postnatal life.

Effects of adenosine agonists on intraocular pressure and aqueous humor dynamics in cynomolgus monkeys

The effects of single or multiple topical doses of the relatively selective A1 adenosine receptor agonists (R)-phenylisopropyladenosine (R-PIA) and N6-cyclohexyladenosine (CHA) on intraocular pressure (IOP), aqueous humor flow (AHF) and outflow facility were investigated in ocular normotensive cynomolgus monkeys. IOP and AHF were determined, under ketamine anesthesia, by Goldmann applanation tonometry and fluorophotometry, respectively. Total outflow facility was determined by anterior chamber perfusion under pentobarbital anesthesia. A single unilateral topical application of R-PIA (20-250 micrograms) or CHA (20-500 micrograms) produced ocular hypertension (maximum rise = 4.9 or 3.5 mmHg) within 30 min, followed by ocular hypotension (maximum fall = 2.1 or 3.6 mmHg) from 2-6 hr. The relatively selective adenosine A2 antagonist 3,7-dimethyl-1-propargylxanthine (DMPX, 320 micrograms) inhibited the early hypertension, without influencing the hypotension. Neither 100 micrograms R-PIA nor 500 micrograms CHA clearly altered AHF. Total outflow facility was increased by 71% 3 hr after 100 micrograms R-PIA. In conclusion, the early ocular hypertension produced by topical adenosine agonists in cynomolgus monkeys is associated with the activation of adenosine A2 receptors, while the subsequent hypotension appears to be mediated by adenosine A1 receptors and results primarily from increased outflow facility.

Adenylyl cyclase inhibitory pathway is differentially modified in rat white and brown fat by high-energy diets

Incubation of white adipose tissue (WAT) adipocytes from rats fed a high-energy diet (Exp group) with antilipolytic Gi-coupled adenylyl cyclase inhibitory agonists, nicotinic acid (Nic) and N8-(L-2-phenylisopropyl)adenosine (PIA), resulted in lower cellular adenosine 3',5'-cyclic monophosphate (cAMP) levels than in stimulated adipocytes from rats fed a nutritionally balanced diet (Con group). In contrast to WAT, incubation of brown adipose tissue (BAT) adipocytes with Nic yielded higher cAMP levels in the Exp vs. Con rats. In both WAT and BAT adipocytes, pertussis toxin treatment abolished the differences in Nic- and PIA-inhibited cAMP formation between Exp and Con animals. Immunoblotting of adipocyte membranes indicated a lower content of Gi alpha but not Gs alpha in BAT membranes of Exp vs. Con animals after 6 and 10 wk of feeding. No such differences were found in the Gs alpha or Gi alpha contents of WAT membranes. Thus the inhibitory pathway of adenylyl cyclase is proposed to be sensitized in WAT and desensitized in BAT of rats fed high-energy diets. These modifications in sensitivity are in line with reduced cAMP and lipolysis in WAT and increased cAMP and thermogenesis in BAT during obesity.

New inotropic agents: milrinone analogs

1. Two new milrinone analogs, 3-acetyl-6-phenyl-2(1H)-pyridone (SF 348) and 3-acetyl-7-methyl-7,8-dihydro-2,5(1H, 6H) quinolinone (SF 349), increase the contractile activity of spontaneously beating and electrically driven atria isolated from reserpine-treated guinea-pigs. 2. Propranolol 0.1 microM drastically inhibits the contractile effect of SF 348, whereas that of SF 349 is unaffected. Preincubation of the atria with adenosine-deaminase suppresses the cardiac activity of SF 349, but does not affect that of SF 348. 3. SF 349 competitively antagonizes the negative inotropic effect induced by N6-(R-phenylisopropyl)-adenosine (R-PIA) and displaces N6-cyclohexyl[3H]-adenosine (3H-CHA) from its binding sites to A1 receptors in the guinea-pig heart. 4. The positive inotropic effect of SF 348 is largely sustained by activation of beta-adrenoceptors, whereas SF 349 acts by displacing endogenous adenosine from its inhibitory (A1) receptors in the atria.

Adenosine A2 receptor function in rat ventricular myocytes

OBJECTIVE

This study was undertaken to investigate the functional significance of adenosine A2 receptor stimulation in a mammalian ventricular myocyte preparation.

METHODS

Isolated contracting rat ventricular myocytes were employed to assess the contractile, adenylyl cyclase and cyclic AMP responses to adenosine receptor stimulation.

RESULTS

In single myocytes the presence of A1 receptors was confirmed, as indicated by the A1 receptor agonist, phenylisopropyladenosine (PIA), reducing by 60 and 74% the inotropic response and activation of adenylyl cyclase, respectively, elicited by the beta-adrenergic agonist, isoproterenol. An A1 receptor antagonist, dipropylcyclopentylxanthine (DPCPX), prevented the antiadrenergic action of PIA. The A2 receptor agonist, carboxyethylphenethyl-aminoethyl-carboxamido-adenosine (CGS-21680; 0.01-10 microM) increased myocyte inotropy in a concentration-dependent manner, reaching a maximum of 41-45%. Ethylcarboxamidoadenosine (NECA), naphthyl-substituted aralkoxy-adenosine (SHA-082) and adenosine in the presence of DPCPX also increased myocyte inotropy, as evidenced by increases in myocyte shortening, duration of shortening, time-to-peak shortening, time-to-75% relaxation and rate of maximal shortening. The agonists, however, did not effect the maximal rate of relaxation. The A2 receptor antagonists, chlorofuranyldihydrotri-azoloquinazolinimine (CGS-15943) and chlorostyrylcaffeine (CSC), the latter selective for the A2a receptor, prevented the contractile responses elicited by the A2 agonists. Compared to the concentrations of A2 receptor agonists necessary to increase myocyte contractile variables, 3-12 times greater concentrations of the agonist were required to increase myocyte adenylyl cyclase activity and cAMP levels.

CONCLUSIONS

The results suggest the presence of adenosine A2a receptors in the rat ventricular myocyte that appear to be responsible for an increase in inotropy via cAMP-dependent and -independent mechanisms.

Contribution of PKC to beta-hexosaminidase-induced airway smooth muscle proliferation

beta-Hexosaminidases (Hex) A and B promote mitogenesis via airway smooth muscle (ASM) mannose receptor. The objective of this study was to elucidate the contribution of protein kinase C (PKC) in Hex-induced mitogenesis in ASM cells (ASMC). Exposure of ASMC to Hex caused increases in both the calcium-dependent and the calcium-independent PKC activities. Both downregulation of PKC and PKC inhibitors staurosporine and calphostin C diminished Hex-induced DNA synthesis and cell number. Hex-induced DNA synthesis was enhanced by a diacylglycerol kinase inhibitor, R-59022, which was blocked by calphostin C. These data suggest that activation of PKC in part mediates Hex-induced mitogenesis in ASMC.

Epidermal growth factor protects GH3 cells from adenosine induced growth arrest

We have demonstrated that high doses of adenosine (0.1 mM) inhibit the growth of the rat pituitary GH(3) cell line. This effect was not mediated via cell surface adenosine receptors as the adenosine agonists N(6)-(2-phenylisopropyl)adenosine (PIA, 0.1 mM) and 5'-N- ethylcarboxamidoadenosine (NECA, 0.1 mM) were unable to reproduce the growth inhibitory effect of adenosine. The adenosine uptake inhibitor dipyridamole completely blocked the growth inhibitory effect of adenosine suggesting that its site of action is intracellular. Epidermal growth factor (EGF) was able to completely reverse the growth inhibitory effect, restoring the growth rate of cultures treated with adenosine and EGF to that of control cultures. The ratio of cells in G(0)/G(1):S:G(2)/M phases of the cell cycle was unaltered in adenosine treated compared with control cultures suggesting that there was no change in the rate of cell division, however the degree of apoptosis was markedly increased in adenosine treated cultures. EGF was able to reduce the adenosine induced apoptosis almost to levels seen in the control cultures. Thus the mechanism of the growth inhibitory effect of adenosine does not appear to be by reducing the rate of cell division but rather by increasing the rate of cell death and EGF restores the growth rate of adenosine treated cells to that of untreated cells by preventing adenosine induced apoptosis.

Regulation of lipolysis by somatotropin: functional alteration of adrenergic and adenosine signaling in bovine adipose tissue

To investigate the cellular mechanisms of somatotropin (ST) action on adipose tissue lipolysis, experiments were conducted using adipose tissue taken from lactating cows treated with excipient or ST (40 mg/day). Stimulation of lipolysis in vitro by the effectors isoproterenol with or without adenosine deaminase, dibutyryl cAMP with or without isobutylmethylxanthine, and forskolin was not altered by ST treatment. Conversely, the response to the antilipolytic effector, phenylisopropyladenosine (PIA), was significantly reduced in adipose tissue explants from ST or fasted cows. The different responses to adrenergic-stimulating agents (in vivo) and PIA (in vitro) were not due to differences in the abundance of alpha, beta or gamma subunits of the stimulatory (Gs) and inhibitory (Gi) subunits of the heterotrimeric G-proteins which bind to the beta-adrenergic and adenosine receptors respectively. However, the functionality of Gi proteins, as assessed by their ability to be ADP-ribosylated by pertussis toxin, was significantly reduced in ST-treated but not fasted cows. These data highlight differential regulation of signaling proteins by ST and fasting, both of which result in enhanced in vivo response to adrenergic stimulation of lipolysis.

Expression and function of adenosine receptors in the chinchilla cochlea

Previous studies indicate the presence of adenosine receptors in the cochlear tissues obtained from different animals. This study was initiated to determine the subtypes of adenosine receptor (AR) present in the chinchilla cochlea and to assess their function. Radioligand binding studies demonstrate the presence of both the A1AR and A3AR in membranes prepared from the cochlea, using the radioligands [3H]DPCPX and [125I]APNEA. Estimates of the number (Bmax) of A1AR and A1AR plus A3AR by saturation curves were 118 +/- 13 and 417 +/- 120 fmol/mg, respectively, with the respective equilibrium dissociation constants (Kd) averaging 2.7 +/- 0.2 and 26.3 +/- 13.8 nM. No significant number of A2aAR were detected using [3H]CGS21680. The nonhydrolyzable adenosine analog R-phenylisopropyladenosine (R-PIA, 1 microM) elicited a small but significant degree of inhibition of forskolin-stimulated adenylyl cyclase activity (10.4 +/- 2.5%) in cochlear membrane preparations, which was insensitive to blockade by theophylline (100 microM). Furthermore, R-PIA elicited an increase in inositol 1,4,5-trisphosphate production in dissociated cell preparations obtained from the cochlea. No significant effect of R-PIA was observed on auditory measures such as auditory brainstem evoked response, cochlear action potential and endocochlear potential following round window application. However, round window application of R-PIA elicited significant increases in the activities of antioxidant enzymes such as superoxide dismutase and glutathione peroxidase and significantly reduced the levels of malondialdehyde, a marker of lipid peroxidation. These results suggest a potential cytoprotective role of adenosine in the cochlea against oxidative damage.

Preconditioning cultured human pediatric myocytes requires adenosine and protein kinase C

We showed previously that 20 min of low-volume anoxia ("ischemia") and 20 min of "reperfusion" preconditions quiescent pediatric myocyte cultures against damage resulting from 90 min of subsequent prolonged ischemia and 30 min of reperfusion. The purpose of this study was to assess the roles of adenosine and protein kinase C (PKC) in this preconditioning model. Our results suggest that 1) preconditioned myocytes secrete a protective mediator(s) into the "ischemic" supernatant that is transferable to other cells, and adenosine is released into the supernatant in quantities sufficient for adenosine-receptor activation (2) preconditioning is inhibited by adenosine-receptor antagonism, and myocyte protection similar to preconditioning can be achieved with exogenously administered adenosine or adenosine-receptor stimulation; (3) brief ischemic and adenosine-induced myocyte preconditioning is mimicked by the phorbol ester 4beta-phorbol 12-myristate 13-acetate (PKC agonist) and inhibited by PKC antagonists; and (4) brief ischemic and adenosine-induced myocyte preconditioning both induce PKC translocation to myocyte membranes and increase the PKC phosphorylation rate. These data suggest that adenosine released from ischemic human pediatric myocytes mediates preconditioning through activation of PKC.

Adenosine receptor activation suppresses tactile hypersensitivity and potentiates spinal cord stimulation in mononeuropathic rats

The aim of the present study was to investigate the intrathecal (i.t.) action of a selective A1-adenosine receptor agonist, R-phenylisopropyl adenosine (R-PIA), on tactile withdrawal thresholds in a rat model of mononeuropathy produced by sciatic chronic constriction injury (CCI). An additional aim was to examine whether adenosine receptor activation is involved in the effects of spinal cord stimulation (SCS), which activates low-threshold fibers and suppresses touch-evoked pain both in patients and in experimental animals with neuropathy. Animals presenting hindlimb withdrawal to von Frey filaments with a bending force of < 7.5 g on the lesioned side (compared to > or = 35 g in the normal limb), were considered as having tactile hypersensitivity ("allodynia'). R-PIA (1-10 nmol i.t.) induced a dose-dependent suppression of the tactile allodynia without producing impairment of motor function. The effect of R-PIA (3 nmol i.t.), a clearly submaximal dose, was abolished by concomitant treatment with the selective A1-adenosine receptor antagonist cyclopentylxanthine (10 nmol i.t.). In animals where SCS failed to influence tactile allodynia, concomitant i.t. administration of R-PIA (3 nmol) and SCS induced a clear-cut and long-lasting suppression of the hypersensitivity to tactile stimulation. In conclusion, adenosine receptor stimulation antagonizes tactile hypersensitivity in a CCI model of mononeuropathy and potentiates the action of spinal cord stimulation.