Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:4.6.1.1 (adenylate cyclase)
 19,190 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

At least three mechanical changes characterize the response of cardiac muscle to agents that enhance cyclic AMP production. In common with other inotropic interventions, tension is augmented and the rate of tension rise is increased. The third response, acceleration of the rate of relaxation, is characteristic of the actions of beta-adrenergic agonists. These mechanical effects can be attributed to changes in (1) the amount of Ca2+ released during systole, (2) the rate of Ca2+ release at the onset of systole, and (3) the rate at which Ca2+ is reaccumulated by the sarcoplasmic reticulum at the end of systole. The ability of cyclic AMP-dependent protein kinases to phosphorylate the cardiac sarcoplasmic reticulum in vitro parallels stimulation of both Ca2+ transport and Ca2+-activated ATPase. The phosphoprotein formed in the presence of cyclic AMP and protein kinase has the chemical characteristics of a phosphoester, contains mostly phosphoserine, and has an electrophoretic mobility in SDS polyacrylamide gels that corresponds to a protein of 22,000 daltons. This 22,000-dalton protein, tentatively named phospholamban, thus differs from the acyl phosphooprotein formed by the Ca2+-transport ATPase, which as an apparent molecular weight of 90,000 to 100,000 daltons. Phospholamban has not been found in fast skeletal muscle, nor is Ca2+ transport accelerated by cyclic AMP and protein kinase in sarcoplasmic reticulum from these muslces which do not respond to beta-adrenergic agonists with accelerated relaxation. It thus appears likely that phosphorylation of phospholamban correlates both with an increased rate of Ca2+ transport by cardiac sarcoplasmic reticulum in vitro and accelerated relaxation in the intact myocardium. Preliminary findings are consistent with the view that phosphorylation of phospholamban may be related to other actions on Ca2+ fluxes brought about by agents which activate adenylate cyclase in the myocardium, but these interpretations must remain speculative pending more definitive studies.
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PMID:Control of calcium transport in the myocardium by the cyclic AMP-Protein kinase system. 16 80

The relative effects of drugs which elevate cytosolic cyclic AMP on inotropy and diastolic relaxation (lusitropy) of guinea pig atria were quantified in vitro. There was a temporal difference between these responses in that inotropy reached peak response considerably faster than lusitropy. Also, although the relaxation response was sustained to an elevated steady state, the inotropic responses to beta adrenoceptor agonists were transient and returned to base line over 90 min. However, the inotropic responses to forskolin and dibutyryl cyclic AMP (cAMP) were sustained. For all of the drugs tested, the lusitropic response was at least 4 times more sensitive than the inotropic response (i.e., the concentration response curve for relaxation was shifted to the left of the curve for inotropy). In the case of beta adrenoceptor agonists, these differences were greater, presumably because of the fading inotropic response over 90 min. It was found that although high efficacy beta adrenoceptor agonists such as isoproterenol (and the direct activator of adenylate cyclase forskolin) produced both inotropy and lusitropy, lower efficacy agonists produced predominant lusitropy. The low efficacy agonist prenalterol produced insignificant inotropy but 60% maximal lusitropy. These data were modeled mathematically by a "differential coupling model" which assumed that a uniform cytosolic level of elevated cAMP activated two biochemical processes of differing sensitivity. Thus, the lusitropic response (phosphorylation of phospholamban) was coupled more efficiently to the cAMP response than the inotropic response (phosphorylation of calcium channels). A second model ("differential messenger concentration model") which calculated the effects of a compartmentalization of cAMP concentration within the cardiac cell by restricted diffusion and/or selective degradation by phosphodiesterases also was used.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:The relative efficiency of beta adrenoceptor coupling to myocardial inotropy and diastolic relaxation: organ-selective treatment for diastolic dysfunction. 167 90

Cyclic GMP (cGMP) mediates the relaxing action of a variety of vasodilator drugs and endogenous vasodilator substances. Cyclic AMP (cAMP) mediates relaxation by beta-adrenergic agonists as well as other activators of adenylate cyclase. Both second messengers appear to reduce the concentration of intracellular Ca2+ in vascular smooth muscle cells, thus affecting relaxation. The presence of cGMP-dependent protein kinase in vascular smooth muscle cells is required for the reduction of Ca2+ by cAMP and cGMP, suggesting that this enzyme mediates the relaxing effects of both cyclic nucleotides. Although the specific substrate proteins for cGMP-dependent protein kinase are not well characterized in vascular smooth muscle, new evidence indicates that Ca2(+)-ATPase activation by phosphorylation of phospholamban by the kinase may underlie the mechanism of action of cyclic-nucleotide-dependent relaxation.
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PMID:Towards an understanding of the mechanism of action of cyclic AMP and cyclic GMP in smooth muscle relaxation. 184 22

Acceleration of cardiac relaxation upon beta adrenergic stimulation is due, in part, to enhancement in the rate of Ca2+ sequestration by the sarcoplasmic reticulum (SR) Ca2+ pump resulting from cAMP-mediated phosphorylation of the SR protein phospholamban. Our previous studies have shown that in rat myocardium, beta adrenergic activation of adenylate cyclase and the Ca2+ pump activity of SR decline with aging (Mech. Ageing Dev., 19 (1982) 127-139; 38 (1987) 127-143). In the present study, the effect of aging on phospholamban phosphorylation and consequent changes in SR Ca2+ pump activity were evaluated using cardiac SR from 6 (young adult), 12 (adult) and 28 (aged) months old rats. No age-related differences were observed in the rate or maximum level of phospholamban phosphorylation by exogenous cAMP-dependent protein kinase. The rates of ATP-dependent Ca2+ uptake by SR from young adult and aged rats were stimulated upon phospholamban phosphorylation, the percentage stimulation of Ca2+ uptake at varying Ca2+ concentrations (0.24-11.9 microM) was not diminished with aging. However, the rates of Ca2+ uptake by phosphorylated and unphosphorylated SR were remarkably lower (35-50%) in the aged. Regardless of the age of rats, the stimulatory effect of phosphorylation on Ca2+ uptake by SR was due to increase in Vmax of Ca2+ transport with no appreciable changes in K0.5 for Ca2+. These findings imply that in spite of the age-associated decline in SR Ca2+ pump activity, the ability of phospholamban to undergo cAMP-mediated phosphorylation and the relative responsiveness of the SR Ca2+ pump to phospholamban phosphorylation are not diminished in the aging heart.
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PMID:Effects of aging on phospholamban phosphorylation and calcium transport in rat cardiac sarcoplasmic reticulum. 236 95

The effects of muscarinic cholinergic stimulation on beta-adrenergic induced increases in phospholamban phosphorylation and Ca2+ transport were studied in intact myocardium. Isolated guinea pig ventricles were perfused via the coronary arteries with 32Pi, after which membrane vesicles were isolated from individual hearts. Isoproterenol produced reversible increases in 32P incorporation into phospholamban. Associated with the increases in 32P incorporation were increases in the initial rate of phosphate-facilitated Ca2+ uptake measured in aliquots of the same membrane vesicles isolated from the perfused hearts. The increases in 32P incorporation and calcium transport were significantly attenuated by the simultaneous administration of acetylcholine. Acetylcholine also attenuated increases in phospholamban phosphorylation and Ca2+ uptake produced by the phosphodiesterase inhibitor isobutylmethylxanthine and forskolin. The contractile effects of all agents which increased cAMP levels (increased contractility and a reduction in the t1/2 of relaxation) were also attenuated by acetylcholine. The inhibitory effects of acetylcholine were associated with attenuation of the increases in cAMP levels produced by isoproterenol and isobutylmethylxanthine but not by forskolin. Acetylcholine also increased the rate of reversal of the functional and biochemical effects of isoproterenol by propranolol without affecting cAMP levels. These results suggest that cholinergic agonists inhibit the functional effects of beta-adrenergic stimulation in part by inhibition of phospholamban phosphorylation. This inhibition may be mediated by two potential mechanisms: inhibition of beta-adrenergic activation of adenylate cyclase and stimulation of dephosphorylation.
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PMID:Muscarinic cholinergic inhibition of beta-adrenergic stimulation of phospholamban phosphorylation and Ca2+ transport in guinea pig ventricles. 241 74

Muscarinic cholinergic agonists such as acetylcholine attenuate phosphorylation of phospholamban induced by agents that activate cAMP-dependent protein kinase. However, cAMP accumulation is variably affected or only slightly reduced; thus, the choline ester might produce effects in addition to inhibition of adenylate cyclase. We hypothesized that acetylcholine might regulate a phosphatase in mammalina myocardium. Exposure of Langendoff-perfused guinea pig ventricles to isoproterenol (10 nM) for 45 s increased phosphatase inhibitor-1 activity 2-fold. Co-administration of acetylcholine (100 nM) antagonized the effect of isoproterenol, and atropine (1 microM) blocked the effect of acetylcholine. Forskolin (1 microM) caused a 3-fold increase in inhibitor-1 activity, and acetylcholine markedly attenuated the effect of forskolin. However, acetylcholine did not lower cAMP levels in the same tissues. Both isoproterenol and forskolin reduced the type 1 phosphatase activity intrinsic to sarcoplasmic reticulum by 25-50%, using [32P]phosphorylase a or 32P-labeled membrane vesicles as a substrate for the phosphatase. Co-administration of acetylcholine markedly attenuated these effects of isoproterenol and forskolin. Acetylcholine alone caused a 50% increase in type 1 phosphatase activity. We concluded that inhibitor-1 and type 1 phosphatase can be regulated in intact cardiac muscle by agents that increase intracellular cAMP and by acetylcholine.
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PMID:Autonomic regulation of type 1 protein phosphatase in cardiac muscle. 253 94

Spontaneously beating heart myocytes were prepared from adult rat ventricular tissues to study the correlation between beta-adrenergic receptor-stimulated changes in contractile performance and protein phosphorylation in vitro. The plasma membrane of isolated myocardial cells was permeabilized by saponin in the presence of EGTA and Mg-ATP. The permeabilized myocytes, which formed a homogeneous cell population, retained the rod-cell morphology of heart cells in situ and showed spontaneous cyclic contractions. Their contractile activity in response to extracellularly added cAMP mimicked the effects caused by beta-adrenergic stimulation of the whole heart: both the frequency and longitudinal velocity of free contraction and relaxation of the cells increased. Similar increases were observed when beta-agonist, isoproterenol, and GTP were added to suspending medium. In addition, isoproterenol maximally enhanced the adenylate cyclase activity of the cells in the presence of GTP. Both of these effects of isoproterenol were completely blocked by the beta-antagonist propranolol. cAMP-mediated phosphorylation of proteins in the permeabilized myocytes was investigated under conditions in which the beating frequency increased. cAMP elevated the phosphorylation level of five proteins; three of them with apparent molecular masses of 24, 15, and 12 kDa were membrane proteins and the other two with apparent molecular masses of 150 and 28 kDa were myofibrillar proteins. The 24-kDa phosphoprotein dissociated into 12-kDa molecules when boiled in sodium dodecyl sulfate, suggesting that these proteins are oligomeric and monomeric forms of phospholamban. The phosphorylation of these five proteins was stimulated by isoproterenol. The effect of isoproterenol was enhanced by GTP but completely blocked by propranolol. The time course of their phosphorylation correlated well with that of the increase in the beating frequency of the cells; both were measured after the administration of isoproterenol and GTP. When propranolol was added after the start of the stimulation by isoproterenol, only phospholamban and the 15-kDa protein were rapidly dephosphorylated in close correlation with the decrease of the beating frequency. These results demonstrate for the first time that the permeabilized myocytes retain the functional beta-adrenergic receptor and cellular responses to beta-adrenergic stimulation. They also suggest that cAMP-mediated phosphorylation of proteins, possibly phospholamban and/or the 15-kDa protein, is involved in the increased contractile activity of permeabilized heart cells.
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PMID:Beta-adrenergic regulation of contractility and protein phosphorylation in spontaneously beating isolated rat myocardial cells. 282 81

Muscarinic agonists inhibit cyclic AMP (cAMP)-induced phosphorylation of the cardiac protein phospholamban. The mechanism of this muscarinic inhibition of phosphorylation of phospholamban appears to occur at more than one level in the series of reactions comprising the adenylate cyclase, cAMP-dependent protein kinase system. Muscarinic agonists attenuate hormone and drug stimulation of cardiac adenylate cyclase. This results in reduced tissue levels of cAMP and diminished phosphorylation of cardiac proteins and consequent inhibition of biochemical and inotropic effects of drugs that act via cAMP. The mechanism of muscarinic inhibition of adenylate cyclase is only partially understood, but probably involves the inhibitory guanine nucleotide-binding regulatory protein. In addition to the inhibition of adenylate cyclase, muscarinic agonists appear to be able to inhibit the effects of cAMP. The mechanism for this second effect of muscarinic agonists is unknown.
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PMID:Mechanisms of muscarinic modulation of protein phosphorylation in intact ventricles. 608 11

Adenosine causes negative chronotropic and inotropic effects on cardiac tissue. We have investigated the nature of the cardiac adenosine receptor and its effector mechanisms in preparations of newborn chick heart. The adenosine analog [3H]N6 (L-phenylisopropyl) adenosine (L-PIA), an agonist at R-type adenosine receptors, bound with high affinity to receptors in crude and highly-purified membrane preparations. The KD was 3-5 nM. The receptor density was low in crude membranes (10 fmol/mg protein) but significantly enriched in purified sarcolemma (164 fmol/mg protein). Competition studies showed that N-ethylcarboxamide adenosine and N6(D-phenylisopropyl)adenosine were less potent than N6(L-phenylisopropyl)adenosine at the chick heart adenosine receptor, as expected for an Ri-type adenosine receptor. Gpp(NH)p decreased the binding of [3H]N6(L-phenylisopropyl)adenosine to chick heart membranes, suggesting that the guanine nucleotide converted the receptor to a lower affinity state. N6(L-phenylisopropyl)adenosine inhibited beta-adrenergic receptor stimulated adenylate cyclase activity. The IC50 for cyclase attenuation by N6(L-phenylisopropyl)adenosine was 1 microM. N6(L-phenylisopropyl)adenosine reversed the effect of the beta-receptor agonist isoproterenol on phospholamban phosphorylation in 32P-labelled slices of newborn chick hearts. This effect of N6(L-phenylisopropyl)adenosine was evident by 2 min, had an IC50 of 200 nM, and was prevented by the adenosine receptor antagonist 8-phenyltheophylline. Taken together, the results suggest that the antiadrenergic effects of adenosine on cardiac tissue are mediated by a decrease in membrane protein phosphorylation signalled by activation of Ri-adenosine receptors. The coupling mechanism between receptor activation and protein phosphorylation may be an attenuation of adenylate cyclase.
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PMID:Inhibitory adenosine receptors in the heart: characterization by ligand binding studies and effects on beta-adrenergic receptor stimulated adenylate cyclase and membrane protein phosphorylation. 615 Oct 2

A rat heart sarcolemmal preparation could be obtained in which both 5'-nucleotidase and adenylate cyclase were enriched approx. 9-fold by subjecting a homogenate to a discontinuous sucrose gradient, without the use of a high salt extraction. After incubation of this fraction with Mg[gamma-32P]ATP, the majority of 32P incorporated was present in 24 000- and 9000-dalton protein components. Only when a heart cytosol fraction or a purified cyclic AMP-dependent protein kinase was added, was enhancement of 32P-incorporaton found by addition of cyclic AMP. The 9000- and 24 000-dalton proteins appeared to be interconvertible. The degree of conversion could be affected by changing the temperature during solubilizaion of the membranes in SDS prior to electrophoresis. This suggested that the 24 000-dalton protein does not correspond to phospholamban, first identified by others in canine heart sarcoplasmic reticulum. Moreover, it could be excluded that the 24 000-dalton protein was derived from contaminating myofibrillar troponin I. When the sarcolemmal fraction was preincubated with Ca2+, Mg2+, ATP and oxalate, contaminating sarcoplasmic reticulum vesicles, loaded with calcium oxalate, settled to a greater density in the sucrose gradient. Membrane constituents other than those with enzymatic activity were monitored to confirm the separation between sarcolemmal and sarcoplasmic reticulum membranes: Coomassie blue staining material, sialic acid, cholesterol and phospholipid. The 24 000- and 9000-dalton proteins were equally distributed among the sarolemmal and sarcoplasmic reticulum fractions present in the sucrose gradient. However, the rate of 32P-incorporation in the presence of heart cytosol fraction was much slowr in the sarcoplasmic reticulum than in the sarcolemmal fraction.
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PMID:Phosphorylation of low molecular weight proteins in purified preparations of rat heart sarcolemma and sarcoplasmic reticulum. 625


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