Noradrenaline-induced desensitization in cultured heart cells as a model for the defects of the adenylate cyclase system in severe heart failure

PKA delivery to the distal lung air spaces increases alveolar liquid clearance after isoproterenol-induced alveolar epithelial PKA desensitization

Isoproterenol (Iso) infusion for 48 h in rats decreases the ability of beta-adrenoceptor (beta-AR) agonists to increase alveolar liquid clearance (ALC). An impairment in protein kinase A (PKA) function appears to be critical in producing the desensitized ALC response. To test this hypothesis, we used a novel protein delivery reagent (Chariot, Active Motif) to deliver either the PKA catalytic subunit or the PKA holoenzyme to the distal lung epithelium of Iso-infused rats (400 microg.kg(-1).h(-1), 48 h). After this infusion, ALC was measured by mass balance over 2 h. ALC in Iso-infused rats was 27.9% (SD 5.8) of instilled volume absorbed. Delivery of the catalytic PKA subunit to Iso-infused rats increased ALC to 47.7% (SD 8.9) (P < 0.05). ALC in Iso-infused rats delivered the inactive PKA holoenzyme [29.6% (SD 2.5)] was not increased above baseline values. Subsequent holoenzyme activation by intravenous infusion of the stable cAMP analog Sp-8-Bromo-cAMPS increased ALC to 41.7% (SD 8.8) (P < 0.05). Immunohistochemical localization of Chariot-delivered PKA revealed staining in the alveolar and distal airway epithelium. These data indicate that protein delivery reagents can be used to rapidly deliver biologically active proteins to the distal lung epithelium and that PKA desensitization may be an important rate-limiting event in the development of Iso-induced desensitization of the alveolar epithelial beta-AR signaling pathway.

Postreceptor defects in alveolar epithelial beta-adrenergic signaling after prolonged isoproterenol infusion

We previously found that prolonged isoproterenol (Iso) infusion in rats impaired the ability of beta-adrenoceptor (beta-AR) agonists to increase alveolar liquid clearance (ALC). Here, we determined if postreceptor defects in beta-AR signaling contribute to this impairment. Iso was infused using subcutaneous miniosmotic pumps (4, 40, or 400 microg. kg-1. h-1) in rats for 48 h. At this time, forskolin-stimulated ALC was measured by mass balance. Forskolin-stimulated ALC [33.4 +/- 2.1%/h (mean +/- SE) in vehicle-infused rats] was reduced by 25 and 38%, respectively, after the 40 and 400 microg. kg-1. h-1 Iso infusions. The ability of forskolin to increase cAMP was reduced by 70% in alveolar type II (ATII) cells isolated from rats infused with 400 microg. kg-1. h-1 Iso. Additionally, the ability of the stable cAMP analog 8-bromoadenosine-3',5'-cyclic monophosphorothioate, Sp-isomer, to increase ALC (48.7 +/- 3.0% in vehicle-infused rats) was reduced by 25 and 51%, respectively, after the 40 and 400 microg. kg-1. h-1 infusions. Finally, the ability of cAMP to increase protein kinase A activity was eliminated in ATII cells isolated from rats infused with Iso at 400 microg. kg-1. h-1. These data demonstrate that prolonged beta-AR agonist exposure can impair alveolar epithelial beta-AR signaling downstream of the beta-AR.

Myocardial dysfunction in sepsis

1. Sepsis is the leading reversible cause of death in patients requiring modern intensive care services. 2. In this group of patients, death usually results from progressive multiple organ failure, rather than overwhelming primary infection. 3. The pathophysiology of sepsis-induced remote organ dysfunction is incompletely understood, although it is believed to result from a systemic inflammatory process that causes tissue injury in the absence of septic shock. 4. As septic shock is the most common early manifestation of severe sepsis, an understanding of mechanisms of myocardial dysfunction is of clinical relevance. In the present review, we will discuss mechanisms of remote organ failure in sepsis, focusing in particular on the pathogenesis of myocardial dysfunction.

Cyclic AMP levels in ventricular myocytes from noradrenaline-treated guinea-pigs

Chronic activation of the sympathetic nervous system in human heart failure is believed to cause cardiac beta-adrenoceptor desensitisation. We have investigated the relationship between beta-adrenoceptor desensitisation and cyclic AMP levels in cardiac myocytes isolated from the ventricle of guinea-pigs chronically infused with noradrenaline hydrochloride for 7 days. Functional beta-adrenoceptor desensitisation was confirmed by a significant decrease in the maximum isoprenaline-stimulated contraction amplitude and an increased EC50 for isoprenaline. In the absence of beta-adrenoceptor stimulation, basal cyclic AMP levels were significantly depressed in populations of myocytes from noradrenaline-treated animals compared to sham-operated controls, and this was not accounted for by myocyte hypertrophy or necrosis. Similarly, there was a significant decrease in cyclic AMP levels at maximally inotropic isoprenaline concentrations. Threshold and maximum inotropic concentrations of the phosphodiesterase inhibitor, 3-isobutyl-l-methylxanthine (IBMX), restored isoprenaline-stimulated cyclic AMP levels in noradrenaline-treated guinea-pig cardiac myocytes, although we have previously reported no increase in maximum inotropic effect of isoprenaline with these compounds.

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

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

Tumor necrosis factor alpha decreases inositol phosphate formation and phosphatidylinositol-bisphosphate (PIP2) synthesis in rat cardiomyocytes

Treatment of neonatal rat cardiomyocytes for 72 h in the presence of tumor necrosis factor alpha (TNF alpha) (10 U/ml) lead to a decrease in basal and alpha 1-adrenoceptor-induced formation of the calcium-mobilizing second messenger inositol trisphosphate (IP3) and its metabolites, IP2 and IP1, by 35 and 26%, respectively. The synthesis of phosphatidylinositol bisphosphate (PIP2), the substrate of PI-specific phospholipase C, was decreased by 45% following the TNF alpha (10 U/ml) exposure. Time courses of TNF alpha (10 U/ml)-induced alterations in rat cardiomyocytes showed a parallel decline of basal inositol phosphate formation and PIP2 synthesis suggesting that the decrease in inositol phosphate formation was due to the reduction in PIP2 synthesis. As the TNF alpha-induced decrease of PIP2 synthesis was associated with a decreased synthesis of the phospholipid phosphatidylinositol (PI), the precursor of PIP2, by 33%, the decreased availability of PIP2 is apparently, at least in part, the result of the decreased synthesis of PI. As an apparent functional consequence of the decrease in IP3 formation following the TNF alpha exposure, the alpha 1-adrenoceptor-mediated induction of arrhythmias by 100 mumol/l noradrenaline + 10 mumol/l timolol was abolished in TNF alpha-pretreated rat cardiomyocytes. To investigate one of the possible mechanisms of the TNF alpha-induced decrease of PIP2 formation, the effect of TNF alpha pretreatment on glycerol-3-phosphate dehydrogenase (GDH), a key enzyme of lipogenesis, was studied: Exposure of the rat cardiomyocytes for 72 h to TNF alpha induced a concentration-dependent decrease in GDH activity by maximally 55%.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation of alpha-adrenoceptor-mediated responses following chronic beta-adrenoceptor stimulation in the rat heart

1. Noradrenaline and isoprenaline were infused subcutaneously in rats by use of Alzet osmotic minipumps. The effects of catecholamine infusion on ventricular alpha- and beta-adrenoceptor density and also the responses of isolated cardiac tissues were compared with saline infusion. 2. Noradrenaline (1 mg kg-1) or isoprenaline (40 micrograms kg-1) infused for 3 days resulted in a desensitization of beta-adrenoceptor-mediated responses of isolated left atria and papillary muscles. Concentration-response curves to isoprenaline were shifted to the right in left atria whilst maximum responses were reduced in papillary muscles. Right atrial rate responses were not affected by infusions of catecholamines. 3. Infusions of either noradrenaline or isoprenaline resulted in a supersensitivity of alpha-adrenoceptor-mediated responses in isolated papillary muscles with leftward displacements of concentration-response curves to phenylephrine. 4. The density of both ventricular [3H]-dihydroalprenolol and [3H]-prazosin binding sites was reduced following noradrenaline infusion. Isoprenaline infusion reduced only the density of [3H]-dihydroalprenolol binding sites. 5. Noradrenaline infusion therefore 'down-regulates' both alpha- and beta-adrenoceptors in the rat heart but at the same time ventricular alpha-adrenoceptor-mediated responses are enhanced. Isoprenaline similarly enhances responses to phenylephrine and possible mechanisms for this phenomenon are discussed.

Alterations of beta-receptor-adenylyl cyclase coupling by long-term ATP depletion in cultured rat cardiomyocytes

In cultured rat heart muscle cells, reversible long-term ATP depletion induces a decrease in beta-adrenoceptor density and a fall in isoproterenol- as well as forskolin-stimulated cAMP formation. However, isoproterenol-stimulated adenylyl cyclase activity in membrane preparations is not reduced after ATP depletion. These results suggest that the decreased responsiveness to catecholamines during myocardial ischemia cannot be explained by alterations of the beta-adrenoceptor-adenylyl cyclase system alone.

Hypoxia and glucose independently regulate the beta-adrenergic receptor-adenylate cyclase system in cardiac myocytes

We explored the effects of two components of ischemia, hypoxia and glucose deprivation, on the beta-adrenergic receptor (beta AR)-adenylate cyclase system in a model of hypoxic injury in cultured neonatal rat ventricular myocytes. After 2 h of hypoxia in the presence of 5 mM glucose, cell surface beta AR density (3H-CGP-12177) decreased from 54.8 +/- 8.4 to 39 +/- 6.3 (SE) fmol/mg protein (n = 10, P less than 0.025), while cytosolic beta AR density (125I-iodocyanopindolol [ICYP]) increased by 74% (n = 5, P less than 0.05). Upon reexposure to oxygen cell surface beta AR density returned toward control levels. Cells exposed to hypoxia and reoxygenation without glucose exhibited similar alterations in beta AR density. In hypoxic cells incubated with 5 mM glucose, the addition of 1 microM (-)-norepinephrine (NE) increased cAMP generation from 29.3 +/- 10.6 to 54.2 +/- 16.1 pmol/35 mm plate (n = 5, P less than 0.025); upon reoxygenation cAMP levels remained elevated above control (n = 5, P less than 0.05). In contrast, NE-stimulated cAMP content in glucose-deprived hypoxic myocytes fell by 31% (n = 5, P less than 0.05) and did not return to control levels with reoxygenation. beta AR-agonist affinity assessed by (-)-isoproterenol displacement curves was unaltered after 2 h of hypoxia irrespective of glucose content. Addition of forskolin (100 microM) to glucose-supplemented hypoxic cells increased cAMP generation by 60% (n = 5; P less than 0.05), but in the absence of glucose this effect was not seen. In cells incubated in glucose-containing medium, the decline in intracellular ATP levels was attenuated after 2 h of hypoxia (21 vs. 40%, P less than 0.05). Similarly, glucose supplementation prevented LDH release in hypoxic myocytes. We conclude that (a) oxygen and glucose independently regulate beta AR density and agonist-stimulated cAMP accumulation; (b) hypoxia has no effect on beta AR-agonist or antagonist affinity; (c) 5 mM glucose attenuates the rate of decline in cellular ATP levels during both hypoxia and reoxygenation; and (d) glucose prevents hypoxia-induced LDH release, a marker of cell injury.

Does treatment with beta-adrenoceptor antagonists in vivo alter human adenylate cyclase responsiveness in vitro?

1. Treatment with beta-adrenoceptor antagonists in vivo can alter adenylate cyclase responsiveness in the human heart. We have determined the effects of treatment with four different beta-adrenoceptor antagonists in vivo on the responsiveness of lymphocyte and platelet adenylate cyclase in vitro in healthy volunteers. 2. Propranolol (non-selective, 4 x 40 mg day), bisoprolol (beta 1-selective, 1 x 10 mg day), and ICI 118.551 (beta 2-selective, 3 x 25 mg day) were tested as drugs without and pindolol (non-selective, 2 x 5 mg day) as a drug with intrinsic sympathomimetic activity. Adenylate cyclase stimulation by GTP, prostaglandin E1 and forskolin was determined before, after a 7 day treatment period and 7 days after drug withdrawal. 3. Neither treatment with or withdrawal of any of the beta-adrenoceptor antagonists altered adenylate cyclase responsiveness. 4. We conclude that adenylate cyclase responsiveness in circulating blood cells underlies different regulatory mechanisms than that in solid tissues such as the human heart. Our data suggest that circulating blood cells do not always reflect alterations in solid tissues.

Contraction of cardiac myocytes from noradrenaline-treated rats in response to isoprenaline, forskolin and dibutyryl cAMP

Rats were treated with noradrenaline at either 300 micrograms/kg per h for 1 week (Group 1) or 600 micrograms/kg per h for 2 weeks (Group 2) using subcutaneously implanted osmotic minipumps. Control rats were sham-operated. Ventricular myocytes were isolated and contraction amplitude and rats of shortening and relaxation measured using a video and edge-detection device. Maximum contraction amplitude achieved in high calcium was similar for all groups, indicating that there was little change in the basic contractility of cells from noradrenaline-treated animals. Cells from Group 1 treated rats showed a depressed concentration-response curve to isoprenaline but maximum contraction amplitudes in forskolin and dbcAMP were unchanged. In cells from Group 2 treated rats the concentration for half-maximal effect (EC50) of isoprenaline was increased and the maximum contraction amplitude was depressed (P less than 0.001). The ratio between maximum response to calcium and that to isoprenaline in the same cell was also significantly reduced (P less than 0.001). There were significant decreases in response to forskolin, with both the maximum contraction amplitude (P less than 0.01) and forskolin/calcium ratio (P less than 0.001) being attenuated in myocytes from noradrenaline-treated rats. The dbcAMP/calcium ratio was also depressed after noradrenaline treatment (P less than 0.02). The extent of the reduction in response was greatest for isoprenaline and least for dbcAMP. These results suggest that desensitisation progresses from a homologous to a heterologous form with increased dose and time of exposure to circulating catecholamines and that, in the latter, lesions occur at several stages of the adenylate cyclase cascade.

Intrinsic sympathomimetic activity of beta-adrenoceptor antagonists: down-regulation of cardiac beta 1- and beta 2-adrenoceptors

Prolonged treatment of cultured rat heart muscle cells containing beta 1- and non-muscle cells containing beta 2-adrenoceptors with beta-adrenoceptor antagonists devoid of intrinsic sympathomimetic activity had no effect on beta-adrenoceptor density. In contrast, antagonists with intrinsic sympathomimetic activity decreased beta-adrenoceptor density and response (adenylate cyclase stimulation) in both heart muscle (beta 1) and non-muscle cells (beta 2) by a maximum of about 50%. An even larger down-regulation of beta-adrenoceptors and loss of receptor-stimulated adenylate cyclase activity was induced by the full endogenous agonist, noradrenaline, with the beta-adrenoceptors of heart muscle cells (beta 1) being much more sensitive to the beta 1-selective noradrenaline than the heart non-muscle cell beta 2-adrenoceptors. When combined with noradrenaline, the antagonists with intrinsic sympathomimetic activity prevented the action of noradrenaline at both beta 1- and beta 2-adrenoceptors, thereby leading to an apparent up-regulation of receptor density and response. This apparent reversal from an agonist to an antagonist action was observed at much lower concentrations of noradrenaline at beta 1- than at beta 2-adrenoceptors. The data presented indicate that the beta-adrenoceptor antagonists with intrinsic sympathomimetic activity, but not those without, upon prolonged treatment decrease the density and responsiveness of both beta 1- and beta 2-adrenoceptors in cultured rat heart cells. This suggests that the intrinsic sympathomimetic activity of these agents is not a subtype-selective component. Furthermore, the agonist and antagonist activity of these agents apparently depends on the concomitant presence of an endogenous full agonist and an its own affinity and that of the partial agonist for the beta-adrenoceptor subtype.

Mechanism of noradrenaline-induced heterologous desensitization of adenylate cyclase stimulation in rat heart muscle cells: increase in the level of inhibitory G-protein alpha-subunits

The mechanism of heterologous desensitization of adenylate cyclase stimulation was studied in cultured neonatal rat heart muscle cells. After culturing of the cells for 3 days in the presence of 1 microM noradrenaline there was in addition to a 52% decrease in isoproterenol-stimulated adenylate cyclase activity, a lessening of the stimulation of beta-adrenoceptor-independent adenylate cyclase by guanosine-5'-O-(thiotriphosphate) and forskolin by 24 and 34%, respectively. The decrease in receptor-independent adenylate cyclase stimulation by forskolin, but not the attenuation of isoproterenol-stimulated adenylate cyclase activity, was abolished by pertussis toxin (PTX) pretreatment of the cells. Gi, the inhibitory G-protein of adenylate cyclase was therefore quantitated. Labelling of the Mr approximately 40 kDa PTX substrates in membranes of noradrenaline-treated cells was increased by 70% as shown by pertussis toxin-catalyzed ADP ribosylation of heart cell membranes. This increase was also seen in the presence of an excess of purified beta gamma-subunits of transducin and of GTP, suggesting that the increased labelling was not due to elevation of the level of beta gamma-subunits or increase in the concentration of GTP in the membranes of noradrenaline-treated cells. Analysis of the PTX substrates on high resolution urea/SDS-polyacrylamide gels revealed that at least two distinct PTX substrates (40 and 41 kDa) were present in rat heart cell membranes. The labelling of both substrates was increased in membranes of desensitized cells. Immunoblotting of heart cell membranes with anti-Gi alpha-antibodies demonstrated a marked increase in the amount of Gi alpha in membranes of noradrenaline-treated cells. In contrast, immunoblotting with anti-beta-antibodies showed that the level of the beta-subunit of G-proteins (36 kDa) was unchanged after noradrenaline exposure. The data indicate that prolonged treatment of rat heart muscle cells with noradrenaline leads to an increase in the level of alpha-subunits of Gi-proteins. This suggests that this increase is responsible for the observed heterologous desensitization of adenylate cyclase stimulation.