Cyclic AMP phosphodiesterase activity at the external surface of intact skeletal muscles and stimulation of the enzyme by insulin

New perspectives in signaling mediated by receptors coupled to stimulatory G protein: the emerging significance of cAMP efflux and extracellular cAMP-adenosine pathway

G protein-coupled receptors (GPCRs) linked to stimulatory G (Gs) proteins (GsPCRs) mediate increases in intracellular cyclic AMP as consequence of activation of nine adenylyl cyclases , which differ considerably in their cellular distribution and activation mechanisms. Once produced, cyclic AMP may act via distinct intracellular signaling effectors such as protein kinase A and the exchange proteins activated by cAMP (Epacs). More recently, attention has been focused on the efflux of cAMP through a specific transport system named multidrug resistance proteins that belongs to the ATP-binding cassette transporter superfamily. Outside the cell, cAMP is metabolized into adenosine, which is able to activate four distinct subtypes of adenosine receptors, members of the GPCR family: A₁, A_2A, A_2B, and A₃. Taking into account that this phenomenon occurs in numerous cell types, as consequence of GsPCR activation and increment in intracellular cAMP levels, in this review, we will discuss the impact of cAMP efflux and the extracellular cAMP-adenosine pathway on the regulation of GsPCR-induced cell response.

Cyclic AMP and cyclic GMP concentrations in, and efflux from, preimplantation cattle embryos

Basal embryonic cAMP and cGMP concentrations and cAMP and cGMP accumulation in embryos and in the incubation medium were measured in cattle blastocysts recovered at days 14 (n = 23), 15 (n = 29) and 16 (n = 23) of pregnancy. Cyclic AMP and cGMP concentrations were measured by radioimmunoassay and the results expressed per microgram of protein, which was determined by the Pierce Micro BCA protein assay. Cyclic AMP and cGMP were present on each day. Basal embryonic cAMP was similar on days 14, 15, and 16, at 2.3., 2.5 and 2.6 fmol per microgram protein, respectively, while the concentration of cGMP was higher at day 14 (0.14 fmol per microgram protein than at either day 15 or 16 (0.06 and 0.05 fmol per microgram protein, respectively; P < 0.05), which were similar (P > 0.10). Basal embryonic cAMP concentrations were 15 to 60-fold higher than cGMP concentrations. Following a 2 h culture period in the presence or absence of the phosphodiesterase (PDE) inhibitor, isobutylmethylxanthine (IBMX), the accumulated concentrations of cAMP and cGMP were measured in the embryos and in the incubation medium. IBMX did not affect the concentrations of either cAMP or cGMP in the embryos but increased the concentrations of cAMP (P < 0.005) and cGMP (P < 0.01) in the incubation medium. There was no effect of day on either embryonic or medium cAMP, but both embryonic and medium cGMP were higher at day 14 than at day 15 (P < 0.05) or 16 (P < 0.005). Differences between embryonic and medium cAMP and cGMP accumulation were examined. There was no effect of day or treatment. cGMP accumulation in the medium was higher than in the embryos (P < 0.005) whereas cAMP accumulation in the medium was lower than in the embryos (P < 0.05). Whether the efflux of cAMP and cGMP is active or passive is not clear but it may indicate a possible role in embryo-maternal signalling.

Cyclic AMP-reactive proteins in human saliva

This study was conducted to compare cyclic AMP-reactive proteins (cARP), the secretory form of regulatory (R) subunits of cyclic AMP-dependent protein kinase (PKA), and cyclic nucleotide phosphodiesterase (PDE) activity in human whole saliva with that of parotid fluid. Additionally, experiments were done to determine whether secretory cARP is altered by environmental stimuli. Earlier work showed that R subunits are present in parotid fluid and in salivary glands of rats. No previous information is available about secretory PDE in saliva. Whole and parotid ductal saliva samples were collected by a non-invasive procedure from healthy volunteers. After photoaffinity labelling with [32P]-8-N3-cAMP, the R subunits were identified by autoradiography. Cyclic nucleotide PDE activity was measured as a function of the conversion of the cyclic nucleotide to the tritiated 5'-nucleotide. The results showed that R of the type II cAPK, RII (M(r) 50-54 kDa) and/or a slower-moving isoform (M(r) 54-56 kDa, RIIa) were present in all parotid saliva samples tested. Whole saliva was positive for RII in more than 95% of the samples tested (n = 62), but with 50-90% reduction in concentration compared to parotid fluid. Both female and male subjects exposed to controlled auditory (60-80 dB) stimuli responded by a two- to five-fold increase in photoaffinity labelling of cARP (salivary RII, RIIa and RIIfr). There was considerable individual variability, but in all cases the differences in the results were significant (p < 0.05, n = 20). Whole saliva showed measurable PDE activity in fresh or frozen samples, whereas no PDE activity was detected in parotid fluid.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin regulation of sugar transport in giant muscle fibres of the barnacle

1. Sugar transport in the giant muscle cells of Balanus nubilus is accelerated during contractile activity and exposure to porcine insulin. The characteristics of hexose-transfer regulation in the giant muscle cells have been examined by studying the transport of 3-O-methylglucose (a non-metabolized sugar) in both intact giant fibres and fibres subjected to internal solute control by internal dialysis.2. Sugar transport in barnacle muscle is mediated by a saturable process which is inhibited by both phloretin and cytochalasin B. Insulin increases the capacity of the transport system with little effect on its apparent affinity for sugar. Under the same conditions insulin increases 3-O-methylglucose-displaceable cytochalasin B binding. The effects of insulin on transport are half-maximal at 5 muM-insulin and are abolished by both insulin antibody and phloretin. The intact barnacle releases an insulin-like material in response to a rise in blood glucose levels.3. Insulin increases the cyclic GMP (cGMP) content and reduces the cyclic AMP (cAMP) content of barnacle muscle. Experiments with fibres injected with aequorin show that insulin also lowers cytosolic ionized Ca levels. The changes in cyclic nucleotide levels induced by insulin precede the effects on sugar transport and cytosolic ionized Ca. During repetitive contractile activity, cAMP, cGMP and ionized Ca levels are raised.4. Agents which raise the cAMP content of barnacle muscle normally inhibit sugar transport. Dibutyryl cAMP also inhibits transport. Alterations in cytosolic ionized Ca levels in intact fibres are without effect on sugar transport. Nevertheless, stimulation of transport by insulin is blunted when cytosolic ionized Ca is lowered by intracellular injection of the Ca-chelating agent, EGTA.5. Sugar uptake in the internally dialysed fibre is inhibited by intracellular application of cAMP. Internal application of Ca and cGMP stimulate sugar uptake in the dialysed fibre. Cyclic AMP reduces the capacity of the transport system whereas Ca and cGMP increase the capacity of the saturable transfer system. Cyclic AMP and cGMP act at kinetically independent sites. Internal ATP (2 mM) inhibits sugar uptake in the dialysed fibre by some 40%, possibly through the production of cAMP.6. External insulin stimulates sugar uptake in the dialysed fibre even when ionized Ca levels are buffered using EGTA. Stimulation by insulin requires the presence of cytosolic ATP and is potentiated by internal application of 1 mM-GTP. In the dialysed fibre stimulation of transport by insulin is greater than that brought about by Ca and cGMP.7. The stimulation of transport by insulin in the intact fibre and its inhibition by dibutyryl cAMP are abolished by intracellular injection of Gpp(NH)p. Injection of intact fibres with GTPgammaS potentiates the stimulation of transport by insulin and renders insulin-activation of transport irreversible. Injection of intact fibres with ATPgammaS leads to the irreversible inhibition of transport.8. Injection of intact fibres with cAMP phosphodiesterase lowers cAMP levels close to zero and stimulates sugar transport. Application of insulin to diesterase-injected fibres still stimulates transport in the absence of altered cytosolic cAMP.

Localization of 3',5'-cyclic adenosine monophosphate phosphodiesterase (cAMP-PDEase) activity in isolated bovine thyroid plasma membranes

Isolated bovine thyroid plasma membrane preparations were obtained by isopycnic density gradient centrifugation. Cyclic AMP-PDEase (EC 3.1.4.c) activity has been demonstrated by electron microscopic histochemistry on the unit membrane of isolated bovine thyroid cells. 3-isobutyl-1-methyl-xanthine (IBMX) produced partial inhibition, while omission of the substrate revealed no reaction product deposition. These observations correlated well with biochemical studies that showed 0.4% of the total cAMP-PDEase activity to be present in the plasma membrane preparations. Kinetic analysis of cAMP hydrolysis yielded two apparent Michaelis constants for the homogenate and the plasma membrane-rich fraction. Dose-response curves for IBMX inhbition showed cAMP-PDEase of the homogenate to be more sensitive to inhibition than that of the plasma membrane-bound enzyme. Furthermore, wash experiments indicate that the plasma membrane-associated enzyme is tightly bound. This investigation strengthens our previous study and suggests that bovine thyroid cell plasma membranes contain a cAMP-PDEase that may be involved in interactions between the cell and the external environment in a manner yet to be determined.

Effects of adenosine 5'-triphosphate (ATP) and beta-gamma-methylene ATP on the rat urinary bladder

1 High concentrations of adenosine 5'-triphosphate (ATP, 100 to 1000 micrometer) were required to cause contraction of the rat urinary bladder, while adenosine and adenosine 5'monophosphate (AMP, 1 to 50 micrometer) produced relaxation. 2 One hundred fold lower concentrations of beta-gamma-methylene ATP, which is resistant to degradation to AMP and adenosine, caused dose-dependent, phasic contractions which mimicked atropine-resistant responses to nerve stimulation. 3 Adenosine and AMP caused dose-dependent inhibition of carbachol-induced contractions; theophylline competitively antagonized this inhibition but not the contractile responses to beta-gamma-methylene ATP, ATP or atropine-resistant nerve stimulation. 4 These results suggest that the insensitivity of the rat bladder to ATP is due to its rapid degradation to AMP and adenosine and support the hypothesis that the bladder receives a purinergic excitatory innervation.

Effect of human leucocyte migration inhibitory factor (LIF) on 3', 5'-cyclic AMP levels of peripheral blood leucocytes

The possible involvement of 3', 5'-cyclic AMP (cyclic AMP) in the mechanism of action of leucocyte migration inhbitory factory (LIF) was tested. LIF-treated human peripheral blood leucocytes were incubated at 37 degrees C for various times between 0 and 22 h. The concentrations of cyclic AMP in these cultures did not differ from those in controls. Furthermore, LIF did not affect the cellular release of cyclic AMP. Neither when testing purified neutrophils, which are the prime targets of LIF action, an effect of LIF was found. Cyclic AMP levels decreased with time of incubation, whether testing mononuclear cells (92 per cent lymphocytes), purified neutrophils (98 per cent), or buffy coat cells (72 per cent neutrophils; 22 per cent lymphocytes). However, a transient and as yet unexplained increase in the cyclic AMP levels of the latter mixed population of cells was manifested within 10 to 30 min of incubation.

Efflux of cyclic nucleotides from rat pineal: release of guanosine 3',5'-monophosphate from sympathetic nerve endings

Potassium and norepinephrine stimulate the efflux of adenosine 3',5'-monophosphate (cyclic AMP) and guanosine 3',5'-monophosphate (cyclic GMP) from intact pineal glands. The postsynaptic beta-adrenergic receptor mediates the efflux of cyclic AMP. In contrast, the efflux of cyclic GMP requires calcium and intact nerve endings. It appears that sympathetic nerve endings may release cyclic GMP into the synaptic space.

Human leukocyte migration inhibitory factor (LIF). II. Partial biochemical characterization of the substrate specificities for this lymphokine

Previously reported experiments suggested that an esterase or a protease, or both, might participate in the expression of human leukocyte migration inhibitory factor (LIF). To clarify this further, a wide variety of simple ester were tested for the ability to protect LIF against inactivation by the serine esterase inhibitor phenylmethylsulfonyl fluoride (PMSF). alpha-N-benzoyl-L-arginine ethylester (BAEE), a typical trypsin substrate, and bis-p-nitrophenyl phosphate (BNPP), a phosphodiester, were the only esters capable of retaining LIF activity in the presence of PMSF. Agents chemically closely related to these esters were inactive. Moreover, the protection afforded by BAEE and BNPP was the king that would be anticipated if the esters and irreversible inhibitor competed for the same site on LIF. Baee and BNPP also protected against inactivation by di-isopropylfluorophosphate (DFP), another irreversible serine esterase inhibitor. In addition, LIF-treated leukocytes partly escaped migration inhibition in the presence of BAEE and BNPP, respectively. These results indicate that human LIF contains a serine residue necessary for lymphokine activity. It is still not proved, however, that LIF as an enzyme is capable of hydrolyzing BAEE and BNPP, although it seems highly possible. The substrate specificities of a putative LIF enzyme are discussed on the basis of the chemical structure of BAEE and BNPP.

Study of the characteristics of the inotropic effect of insulin in rabbit papillary muscle

The effect of insulin was examined with emphasis on the alteration in the force-frequency relation. The results show that insulin does not change the time to peak tension nor the time of contraction. The inotropic effect was significant and did not depend upon the frequency of stimulation. However, there was a definite dependence of the magnitude of the inotropic effect on temperature. Previous studies have indicated that the inotropic effect is not a result of increased substrate availability or changes in cAMP phosphodiesterase activity. These results and those reported here are consistant with the hypothesis that insulin's inotropic effect is due to increases in intracellular Ca++.

Extracellular metabolism of cyclic AMP

Intravenously administered cyclic [8-3H]AMP to rats was quickly eliminated from the circulation. After 2 min 93% of the administered radioactivity disappeared from the plasues was recovered mainly in the form of nucleotides, ATP, ADP, AMP and IMP. In vitro contact of cyclic AMP with perfused liver, isolated liver cells and adipose tissue resulted in a rapid breakdown of the nucleotide, presumably on the outer surface of the cells. The degradation products have been identified mainly as adenosine and inosine. Incubation of adipose tissue and isolated liver cells with [3H] AMP also resulted in the breakdown of the nucleotide in themedium. The rate of AMP degradation by these tissues was faster than that for cyclic AMP degradation. The data suggest that cyclic AMP is readily metabolized on the outer surface of cells to products which may be converted within the cells to nucleotides. These findings seem of importance for the quantitative assessments of cellular cyclic AMP outflow during hormonal stimulation.

Effect of adrenaline on the release of cyclic AMP by the rat diaphragm

An investigation was made of the effect of adrenaline on the tissue level and release of cyclic AMP (cAMP) into the incubation medium by the isolated rat diaphragm preparation. Adrenaline, added in vitro, caused a dose-dependent biphasic increase in the tissue level of the nucleotide. A significant elevation of the level of cAMP in the tissue was observed throughout a 60 min incubation period in the presence of 0.01, 0.1 and 10 mug/ml of adrenaline. The hormone also produced a small but significant increase in the release of cAMP into the incubation medium. The accumulation of cAMP in the incubation medium ceased after 10 min of incubation in the presence of adrenaline, although the tissu e level of the nucleotide remained considerably elevated throughout the entire incubation period. Theophylline potentiated the effect of adrenaline on both the accumulation of cAMP in the tissue and in the incubation medium. Theophylline by itself also increased the tissue level of cAMP slightly. It is concluded from the present study that the elimination of cAMP from the rat diaphragm muscle by release of the nucleotide from the cells is small.

Characterization of the metabolism of exogenous cyclic AMP by perfused rat heart and incubated prepubertal rat ovary

In order to study the metabolism of extracellular 3',5'-adenosine monophosphate (cAMP), rat hearts were perfused and prepubertal rat ovaries incubated with 3H- and 32P-labelled cAMP (0.025-1 muM). The rate of disappearance of cAMP from the medium was determined by "Ba-Zn-precipitation" and degradation products of 3H- and 32P-CAMP by paper chromatography. Both tissues degraded cAMP to 5'-adenosine monophosphate (AMP), but the enzyme kinetic for this phosphodiesterase activity was different (apparent Km value for the heart 3.95 muM and for the ovary 0.2 muM). AMP was further degraded, since also other labelled substances were found in the medium. An uptake of both 3H- and 32P-labelled substance(s) into the heart and the ovary was noticed. Tissue extracts contained several labelled purines, but the amounts of labelled cAMP did not exceed expected amounts in the extracellular space. In the ovary the uptake of cAMP and AMP seemed to be low, since the uptake of labelled substances was inhibited by high concentrations of unlabelled AMP or adenosine. The degradation of 32P-cAMP was unchanged when AMP was present, strongly suggesting that the phosphodiesterase enzyme was acting extracellularly. In the heart added AMP was very rapidly degraded making it impossible to elucidate whether cAMP was degraded extracellularly or not. It is concluded that elimination of extracellular cAMP under physiological conditions can be due to degradation of cAMP by various tissues. At least for the ovary this phosphodiesterase enzyme is extracellularly active.

5'-nucleotidase: an ecto-enzyme of frog skeletal muscle

Using 1-4C-labeled AMP and IMP as substrates, 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) activity was detected at the external surface of frog skeletal muscle with the active site facing toward the extracellular space. The enzyme was firmly bound to the muscle membrane. Its activity was dependent on Ca2+ or Mg2+ and was inhibited by non-radioactive ribonucleoside 5'-monophosphates, or theophylline, while adenosine 3'-monophosphate and p-nitrophenylphosphate had little or no effect. 5'-Nucleotidase with similar properties was also found in the isolated plasma membrane fraction of the muscle.

Cyclic AMP-mediated induction of the cyclic AMP phosphodiesterase of C-6 glioma cells

Long-term regulation of the cyclic nucleotide phosphodiesterase of the C-6 rat glioma cell line has been studied. Both the low K(m) and high K(m) activities can be induced by elevation of intracellular cyclic AMP levels following either dibutyryl cyclic AMP or norepinephrine treatment of the cells. The enzymes are maximally induced by 3-4 hr. The presence of either cycloheximide or actinomycin D prevents induction by either dibutyryl cyclic AMP or norepinephrine. Evidence is presented that the norepinephrine effect is mediated by the beta-catecholamine receptor. The increased phosphodiesterase activity causes a partial refractoriness to a second challenge with norepinephrine, which can be overcome by blockade of the induction with cycloheximide. The results suggest that just as short-term regulation of cyclic AMP levels occurs via changes in the rates of synthesis or degradation, long-term alterations of the system may also involve either the adenylate cyclase or the phosphodiesterase.