Direct thermography-a new in vitro method to characterize temperature kinetics of ablation catheters

PURPOSE

For the treatment of increasingly complex cardiac arrhythmias, new catheter designs as well as alternative energy sources are constantly being developed. However, there is presently no in vitro method available for assessment of the temperature changes induced at various myocardial levels during energy delivery. Therefore, our study was aimed at developing an in vitro model to record and display the temperature kinetics during ablation in the entire muscle cross section.

METHODS AND RESULTS

A sapphire glass pane was inserted into one wall of the in vitro experimental set-up. Due to its thermodynamic properties, the temperature distribution in an adjacent cross section of the cardiac muscle can be measured exactly ( 1 °C) through this pane by means of a thermography camera. Computer-supported image processing enables the colour-coded and two-dimensional display of the temperature kinetics during the energy application at any location of the myocardial cross section (± 0.5 mm). This new measuring methodology was validated by direct temperature measurements utilizing several intramyocardial thermo elements.

CONCLUSION

This new method allows a temporal and spatial analysis of the temperature phenomena during ablation without the interference and spatial limitation of intramyocardial temperature probes. New ablation technologies can thus be evaluated, independent of the catheter configuration or source of energy used.

[Invasive electrophysiology: complications, nightmares and their management]

Most minor side effects of ablation in the right atrium and right ventricle relate to femoral venous catheterization but there is a small risk of severe complications including atrioventricular (AV) block, damage of surrounding structures and thromboembolic events. Impairment of AV conduction can occur during ablation of atrioventricular re-entrant tachycardia, ablation of anteroseptal, mid-septal and parahisian accessory pathways, ablation of ectopic atrial tachycardia originating from the vicinity of the atrioventricular node and when ablating the septal isthmus for typical atrial flutter. Damage of the right coronary artery is a very rare complication after inferior isthmus ablation with high energy. The thromboembolic risk during and after cardioversion and ablation of atrial flutter is higher than previously recognized and anticoagulation therapy decreases this risk. The risk of perforation and tamponade during ablation in the right atrium and right ventricle is very low but particular caution is necessary in thin-walled structures such as the coronary sinus and the upper right ventricular outflow tract. Phrenic nerve injury can be avoided by pacing from the mapping electrode before application of radiofrequency energy at the right atrial free wall. Limitation of power output depending on the site of ablation and titration of energy application with continuous control of temperature and impedance should be considered to minimize the risk of complications.

[Catheter ablation of atrial fibrillation]

In patients with drug-refractory atrial fibrillation, left-atrial catheter ablation represents a new curative therapeutic option. Segmental ostial or circumferential pulmonary vein isolation can achieve stable sinus rhythm in some 70% of patients with paroxysmal atrial fibrillation but no severe structural heart disease. In patients with chronic atrial fibrillation, complex left-atrial linear, or substrate-oriented ablation strategies may additionally be applied. In patients with cardiac insufficiency or more severe systolic left-ventricular dysfunction, restoration of a stable sinus rhythm through the use of left-atrial catheter ablation can improve the left-ventricular ejection fraction and reduce the severity of cardiac failure. Potential complications of ablation include, in particular, pulmonary veins stenosis, iatrogenic left-atrial tachycardia, thromboembolic events and fatal atrio-esophageal fistulas.

[Analysis of the cause of death of ICD patients during long-term follow-up]

BACKGROUND

Implantable cardioverter defibrillators represent an effective therapy to prevent sudden cardiac death in patients with ventricular arrhythmias. This implies a change in the spectrum of causes of death. There exist no large studies providing an exact differentiation of these causes of death.

METHODS

During a mean observation period of 41+/-29 months (3 d-12 yrs), we analyzed the outcome of 429 ICD patients (CAD n=274, dilative CMP n=97, others n=23, no structural heart disease n=35), mean age 62+/-12 years (23-87 yrs).

RESULTS

The overall survival rate was 79.7% whereby 3.5% of these patients underwent successful heart transplantation and 2.1% died suddenly. 60.9% of all deaths showed a non-sudden cardiac cause (heart failure 56.3%, myocardial infarction 4.6%). In 28.7% a non-cardiac fatal event was present (cancer 11.5%, septicemia 6.9%, stroke 4.6%). The deceased showed significantly more often structural heart disease (100 vs 92%, p<0.02) and a lower left ventricular ejection fraction (37+/-14 vs 44+/-18%, p<0.02). Significant higher survival rates were associated with medication with beta blockers (88 vs 75%, p<0.02) and the class III antiarrhythmic agents sotalol und amiodarone (85 vs 77%, p<0.03).

CONCLUSIONS

Mortality due to sudden death was reduced to 2.1% in ICD patients associated with a shift of causes of death towards severe heart failure. Therefore, special attention should be paid to the signs of heart failure in these patients to enable early initiation of appropriate therapeutic strategies.

Bundle branch reentrant tachycardia in a patient with a calcified bicuspid aortic valve and normal ventricular function

We report the case of a bundle branch reentrant tachycardia (BBRT) in a 40-yearold patient with a calcified bicuspid aortic valve and normal left ventricular function. The ventricular tachycardia was eliminated by successful radiofrequency ablation of the right bundle branch. As the aortic valve annulus is in close proximity to the specialized conduction system, premature degeneration of a bicuspid aortic valve may involve the bundle of His and the proximal bundle branches by invading calcifications. We speculate that calcifications invading the proximal bundle branches from the bicuspid aortic valve may have created the substrate for the BBRT in this patient.

[Unusual infection with pulmonary round foci obtained through "hay packs"]

A 71-year-old female, who has been treated with steroids for 3 weeks, developed a severe pneumonia with septic shock and acute respiratory distress syndrome (ARDS). Despite broad antibiosis due to the detection of pseudomonas aeruginosa (blood culture) the inflammatory markers remained high and the respiratory situation went critical. No proof of a malignoma or autoimmune process was found, despite multiple round foci in x-ray and computer tomography. Finally the delayed diagnosis of pulmonary nocardiosis was adjusted, due to the long incubation period of the pathogen Nocardia farcinica. The infectious origin may be assumed in so called "hay packs" used during the patient's residence at a health resort. After adequate change of the antibiotic regimen stabilization was achieved and the patient meanwhile recovered from the disease.

[Syncope and epileptic seizures]

Syncope is one of the most common symptoms leading to hospital admission. Thereby syncope can be induced by several diseases. It is crucial to detect underlying structural heart disease or high grade arrhythmias, as these are associated with an increased mortality. The careful history and physical examination can often give sufficient evidence to evaluate the origin of syncope. Additional examinations should only be applied selectively. In patients with structural heart disease the specific treatment should be initiated, in patients with cardiac arrhythmias the implantation of a pacemaker or ICD might be indicated. The most common neurally-mediated and orthostatic syncopes can often be treated successfully by physical training. Beside syncope epilepsy might be responsible for a transient loss of consciousness. Again careful history taking helps to differentiate between these two entities.

[15-year-old patient with decompensated heart failure and tachycardia with negative P-waves in inferior ECG recordings. An indication for heart transplantation?]

A 15-year-old girl was admitted with signs of severe cardiac failure. There were no symptoms of cardiac insufficiency 4 weeks before hospital admission. She presented with permanent supraventricular tachycardia with negative P-waves in leads II, III and aVF, the heart rate was 150 beats per minute. The electrophysiological examination showed a permanent junctional reentry tachycardia. A postero-septal accessory pathway could be eliminated successfully by radiofrequency catheter ablation. Immediately after the procedure cardiac function deteriorated with slight decrease of the strongly reduced cardiac output. Intensive care and application of dobutamine led to clinical stability. During a follow-up of two years the young patient showed permanent sinus rhythm and an age related physical strain. This case report documents the rapid and severe manifestation of cardiac failure owing to permanent junctional reentry tachycardia in a 15-year-old girl. She was referred for consideration of heart transplantation. Invasive electrophysiological treatment led to permanent sinus rhythm with improvement of left ventricular function.

Risk factors for recurrence of atrial fibrillation in patients undergoing hybrid therapy for antiarrhythmic drug-induced atrial flutter

AIMS

Catheter ablation of the inferior vena cava-tricuspid annulus isthmus and continuation of antiarrhythmic drug therapy have been shown to be an effective hybrid therapy for atrial flutter which results from antiarrhythmic drug treatment of atrial fibrillation. The aim of this study was to determine the risk factors for recurrence of atrial fibrillation in patients undergoing hybrid therapy for antiarrhythmic drug-induced atrial flutter.

METHODS AND RESULTS

90 patients with paroxysmal (n=46) or persistent atrial fibrillation (n=44) developed atrial flutter due to the administration of amiodarone (n=48), flecainide (n=22), propafenone (n=14) or sotalol (n=6). Recurrence of atrial fibrillation after ablation was assessed during follow-up on continued antiarrhythmic drug therapy and during long-term follow-up, irrespective of the initial antiarrhythmic medication. During the follow-up on continued antiarrhythmic drug therapy (16+/-13 months), recurrence of atrial fibrillation was documented in 24 of 90 patients (27%). The presence of accompanying pre-ablation episodes of atrial fibrillation on antiarrhythmic treatment (Odds ratio 7.1, 95% confidence interval 2.3 to 25, p=0.001) and decreased left ventricular ejection fraction (Odds ratio 3.7, 95% confidence interval 1.01 to 12.5, p=0.048) were significant and independent predictors of post-ablation atrial fibrillation. Antiarrhythmic medication was discontinued during long-term follow-up due to adverse drug effects (amiodarone, n=12; flecainide, n=1) in 13 patients (14%). During the long-term follow-up, irrespective of the initial antiarrhythmic medication (21+/-15 months), stable sinus rhythm was maintained in 60 of 90 patients (67%). CONCLUSION Hybrid therapy can be considered as the first line therapy for patients with antiarrhythmic drug-induced atrial flutter but patients should be carefully evaluated for accompanying pre-ablation episodes of atrial fibrillation and possible adverse drug effects before initiation of hybrid therapy.

[Mechanisms of initiation in atrial fibrillation]

Atrial fibrillation is the most relevant arrhythmia in daily clinical practice. The pathophysiology is determined by multiple independent reentrant wavelets in both atria. Repetitive triggers and an underlying substrate may favor the initiation and maintenance of atrial fibrillation. Mapping studies in patients with drug-refractory paroxysmal atrial fibrillation identified potentials from the ostia of pulmonary veins as a main source of triggers that initiate atrial fibrillation. In ongoing clinical trials, catheter ablation of pulmonary vein foci is used to eliminate atrial premature beats and thereby prevent the initiation of atrial fibrillation. The autonomic modulation of the heart rate and the occurrence of other supraventricular tachycardias that degenerate into atrial fibrillation are also considered as triggering mechanisms of atrial fibrillation. Since symptomatic bradycardia is associated with an increased incidence of atrial fibrillation, atrial pacing therapies for prevention of atrial fibrillation are another concept. Ongoing clinical trials evaluating the efficacy of pacing in patients with and without a primary pacemaker indication are currently under investigation. To date, data to which extent anatomical and electrophysiological characteristics of the atria influence the initiation and maintenance of atrial fibrillation are still missing. The myocardial adaptation to atrial fibrillation, the so-called "atrial remodeling", includes shortening of the atrial refractory period, slowing of atrial conduction, shortening of the atrial action potential, a progressive reduction of L-type calcium channel expression and microfibrosis of the myocardial tissue. New drug developments target atrial remodeling by modulating ion channel function and receptors of the angiotensin metabolism.

Early reinitiation of atrial fibrillation following external electrical cardioversion in amiodarone-treated patients

Early reinitiation of atrial fibrillation (ERAF) following external or internal electrical cardioversion is one of the factors determining unsuccessful electrical cardioversion. Prevention of ERAF has not been studied systematically in patients on amiodarone therapy.

METHODS AND RESULTS

22 patients had ERAF within 1 min after external electrical cardioversion of atrial fibrillation. 11 patients were on amiodarone therapy and 11 patients had no antiarrhythmic medication. The effect of atropine, post-shock atrial pacing and intravenous ajmaline on ERAF was consecutively tested in these patients. Administration of atropine before repeated defibrillation or post-shock atrial pacing prevented ERAF in 9 of the 11 patients (82%) on amiodarone therapy but in only 3 of 11 patients (27%) without amiodarone (p<0.05). In the remaining patients, intravenous ajmaline was effective in the suppression of ERAF in 5 patients without amiodarone and in 1 patient with amiodarone. The PP interval preceding the atrial premature beat reinitiating atrial fibrillation was nonsignificantly longer in amiodarone-treated patients (1127+/-419 ms) in comparison to patients without amiodarone (896+/-271ms). 27% of patients without amiodarone at the time of electrical cardioversion and 55% of patients with amiodarone remained in sinus rhythm during the follow-up of 29+/-14 and 30+/-14 months, respectively.

CONCLUSIONS

ERAF in patients on amiodarone can be treated by atropine or atrial pacing to prevent bradycardia-dependent ERAF. ERAF in amiodarone-treated patients does not apparently predict late recurrence of atrial fibrillation on continued amiodarone therapy.

Electroanatomical mapping for visualization of atrial activation in patients with incisional atrial tachycardias

AIMS

Incisional atrial tachycardias in patients following surgery for congenital heart disease are based on complex structural abnormalities in these hearts. The aim of this study was to evaluate the use of the electroanatomical mapping system, CARTO, in consecutive patients with different forms of incisional atrial tachycardia.

METHODS AND RESULTS

The electroanatomical mapping system combines electrophysiological and spatial information and allows visualization of atrial activation in a three-dimensional anatomical reconstruction of the atria. Electroanatomical mapping of right atrial activation was performed in 10 patients after surgery for congenital heart disease, surgery for Wolff-Parkinson-White syndrome, or heart transplantation presenting with 13 incisional atrial tachycardias. The three-dimensional mapping allowed a rapid distinction between focal (n=3) and reentrant mechanisms (n=10) and visualization of the activation wavefronts along anatomical and surgically created barriers. Electroanatomical activation maps (mean right atrial activation time 213+/-107 ms) were constructed with 89+/-60 catheter positions during an average mapping time of 48+/-33 min. Reentrant tachycardias propagating through the tricuspid annulus-vena cava inferior isthmus (n=6) or along periatriotomy loops (n=4) were identified in eight patients. Ectopic atrial foci near surgical scars could be localized in three patients. Catheter ablation by creation of a lesion in a critical isthmus of conduction or by targeting the arrhythmogenic focus eliminated 11 of 13 incisional atrial tachycardias.

CONCLUSION

Visualization of atrial activation in a three-dimensional reconstruction of the right atrium using the electroanatomical mapping system CARTO facilitates understanding of the mechanism and defines the reentrant circuits of incisional atrial tachycardias. This new method may improve the success rate of electrophysiologically guided and anatomically guided catheter ablation of incisional atrial tachycardias.

Catheter ablation of atrial flutter due to amiodarone therapy for paroxysmal atrial fibrillation

AIMS

Antiarrhythmic drug treatment for atrial fibrillation can cause atrial flutter-like arrhythmias. The aim of this study was to clarify the effect of catheter ablation of the tricuspid annulus-vena cava inferior isthmus on amiodarone-induced atrial flutter and to determine the incidence of atrial fibrillation after catheter ablation of amiodarone-induced atrial flutter in comparison to regular typical flutter.

METHODS AND RESULTS

Among 92 consecutive patients with typical atrial flutter who underwent isthmus ablation 28 patients had atrial flutter without a history of previous atrial fibrillation (group I), 10 patients had atrial flutter following the initiation of amiodarone therapy for paroxysmal atrial fibrillation (group II) and 54 patients had atrial flutter and atrial fibrillation (group III). Atrial cycle length during atrial flutter in amiodarone-treated patients (group II) (277+/-24 ms) was significantly longer as compared to the cycle length of atrial flutter in group I (247+/-33 ms) and group III patients (235+/-28 ms). The rate of successful transient entrainment and overdrive stimulation to sinus rhythm was not different between patients with (60%) or without amiodarone therapy (group I: 71%, group III: 53%). Successful isthmus ablation with bidirectional conduction block eliminating right atrial flutter was achieved in 90% of amiodarone-treated patients and 93% of patients without amiodarone therapy. In the amiodarone-treated patient group atrial conduction times during pacing in sinus rhythm were significantly prolonged by 20-30% before and after ablation in all regions of the reentrant circuit. During a mean follow-up of 8+/-3 months post-ablation, atrial fibrillation recurred in two of 10 patients on continued amiodarone therapy after successful isthmus ablation. Thus, successful catheter ablation of atrial flutter due to amiodarone therapy was associated with a markedly lower recurrence rate of paroxysmal atrial fibrillation (20%) as compared to patients with atrial flutter plus preexisting paroxysmal atrial fibrillation (76%) and was similar to the outcome of patients with successful atrial flutter ablation without preexisting atrial fibrillation (25%).

CONCLUSION

These data suggest that isthmus ablation with bidirectional block and continuation of amiodarone therapy is an effective therapy for the treatment of atrial flutter due to amiodarone therapy for paroxysmal atrial fibrillation.

New mapping technology for atrial tachycardias

Prerequisite for succesful radiofrequency catheter ablation of tachycardias is the exact mapping during the electrophysiological study. The new mapping system CARTO allows a three-dimensional color-coded electroanatomic map of impulse propagation using electromagnetic technology. The aim of this study was to determine the feasibility and safety of the new electromagnetic mapping technology CARTO for atrial tachycardias.

RESULTS

Electrophysiologic study and CARTO mapping was performed in 38 atrial tachycardias. The mapping procedure took 26 +/- 23 min. We created 33 maps within the right atrium and 5 maps within the left atrium with a mean of 74 +/- 38 different catheter positions. The mechanism was determined as reentrant in 9, junctional in 1 and focal in 28 tachycardias. In focal tachycardias the tachycardia cycle length (CL) and the total atrial activation time (AT) were clearly different (352 +/- 98 ms vs 99 +/- 25 ms). Reentrant tachycardias had a comparable CL and AT (236 +/- 44 ms vs 240 +/- 56 ms). In 83% of the focal tachycardias and in 67% of the reentrant tachycardias, ablation was performed successfully. No complications occured.

CONCLUSION

The electroanatomic mapping system allows high resolution visualization of electrical activity and may therefore improve precision and simplify the determination of the arrhythmogenic substrate during tachycardias for successful catheter ablation.

Slow pathway ablation in a patient with common AV nodal reentrant tachycardia and complete situs inversus

A 72-year-old woman with complete situs inversus underwent successful slow pathway ablation of typical AV nodal reentrant tachycardia. Catheter ablation of AV nodal reentrant tachycardia in dextrocardia required a lengthy procedure but was safe and without complications.

Fast pathway ablation in patients with common atrioventricular nodal reentrant tachycardia and prolonged PR interval during sinus rhythm

AIMS

This study aimed to clarify the safety and efficacy of selective fast pathway ablation in patients with atrioventricular nodal reentrant tachycardia and a prolonged PR interval during sinus rhythm. Such patients have been reported to have an increased incidence of complete atrioventricular block.

METHODS AND RESULTS

In this study, the earliest retrograde atrial activation during atrioventricular nodal reentrant tachycardia and right ventricular stimulation was localized. Fast pathway ablation was then performed in five patients with the common form of atrioventricular nodal reentrant tachycardia and a prolonged PR interval. Three of the five patients had almost incessant atrioventricular nodal reentrant tachycardia. Radiofrequency catheter ablation induced a complete ventriculo-atrial block during right ventricular stimulation in four patients and a marked prolongation of ventriculo-atrial conduction during right ventricular stimulation in one. Non-inducibility of common atrioventricular nodal reentrant tachycardia with and without isoproterenol was achieved in all five patients. The PR interval increased from 254 +/- 53 ms to 276 +/- 48 ms and the atrio-His interval from 172 +/- 46 ms to 192 +/- 45 ms. Second- or third-degree atrioventricular block did not occur during the ablation procedure. During the followup of 19 +/- 20 months none of the patients developed symptoms suggestive of atrioventricular nodal reentrant tachycardia or evidence of second- or third-degree atrioventricular block.

CONCLUSION

These data suggest that atrioventricular node (retrograde) fast pathway ablation can apparently be safely performed in patients with common atrioventricular nodal reentrant tachycardia and a prolonged PR interval during sinus rhythm.

[Radiofrequency catheter ablation of atrial flutter and atrial fibrillation]

Radiofrequency catheter ablation is now considered as a curative approach in patients with typical atrial flutter. Typical atrial flutter is due to a macrore-entrant circuit within the right atrium and it can be eliminated by a linear lesion in the isthmus between the tricuspid annulus and the vena cava inferior. The electrophysiological criterion of a bidirectional isthmus block has been shown to reduce the recurrence rate of atrial flutter after catheter ablation, thus achieving long-term cure of typical atrial flutter. Acute success rates of 85 to 90% and recurrence rates of 10 to 15% have been reported. The risk of paroxysmal atrial fibrillation continues to be clinically relevant in patients who underwent successful ablation of atrial flutter, in particular in patients with previously documented atrial fibrillation. The incidence of a new onset of atrial fibrillation after ablation of atrial flutter seems to be approximately 20%. Isthmus ablation has also been shown to be beneficial for the majority of patients with typical atrial flutter and atrial fibrillation: In addition to an elimination of typical atrial flutter the isthmus ablation apparently reduces the incidence of paroxysmal atrial fibrillation. At present, atrial fibrillation can only be treated by catheter ablation as a curative approach in the rare cases where an accessory pathway, an AV nodal re-entrant tachycardia, typical atrial flutter or an ectopic atrial tachycardia is the induction mechanism of the atrial fibrillation. The majority of patients with atrial fibrillation is apparently not amenable to a curative local ablation. While AV junction ablation and AV node modification can palliate some of the symptoms of atrial fibrillation by a control of ventricular rate, the arrhythmia persists with the loss of AV synchrony and continued risk of thromboembolism. The surgical MAZE procedure implies a compartimentation of the atria by surgical incisions resulting in areas to small to sustain the arrhythmia. Based on this procedure experimental and clinical studies are currently performed in order to develop catheter ablation cure of atrial fibrillation.

[Repetitive monomorphic ventricular tachycardia (Gallavardin type): clinical and electrophysiological characteristics in 20 patients]

Repetitive monomorphic ventricular tachycardia (RMVT) is defined by the presence of numerous monomorphic isolated, premature ventricular complexes, couplets, and runs of unsustained ventricular tachycardia having the same morphology in patients without structural heart disease. Patients with RMVT mostly demonstrate the typical left bundle branch block morphology with normal or rightward axis during tachycardia. At our institution, 20 patients with RMVT have been systematically studied: a syncope had occurred in 35% of our patients, in three cases a syncope was the first manifestation of the RMVT. Of our RMVT patients, 25% developed sustained episodes (> 3 min) of ventricular tachycardia as documented by Holter ECG. The salvos of ventricular tachycardia are generally short in RMVT. This behavior and the typical exercise dependence differentiates RMVT from paroxysmal sustained idiopathic ventricular tachycardia. Exercise testing is mandatory for correct diagnosis of RMVT. In our institution, 85-90% of RMVT patients demonstrated runs of ventricular tachycardia or sustained ventricular tachycardia while on a treadmill (exercise test) or during isoproterenol infusion. RMVT was inducible by programmed electrical right ventricular stimulation in only 13% of our patients. Therefore, in patients with suspected RMVT programmed electrophysiological stimulation is only useful to differentiate a ventricular tachycardia from a supraventricular tachycardia with bundle brunch block or in patients with unexplained syncope. The prognosis is considered generally good; in our patients no life threatening ventricular tachyarrhythmias were observed during a follow-up of up to 4 years. Verapamil and beta-adrenoceptor antagonists generally offer symptomatic improvement. In some cases treatment with a class III antiarrhythmic agent is necessary. While drug-refractory paroxysmal sustained idiopathic ventricular tachycardia can be abladed with both immediate and long-term success, catheter ablation of RMVT is only rarely indicated.

[Electro-anatomic mapping of the sinoatrial activation: initial experiences with the new CARTO mapping system]

Prerequisite for successful radiofrequency catheter ablation on tachycardias is the exact mapping during the electrophysiologic study. The new mapping system CARTO allows a three-dimensional color-coded electroanatomic map of impulse propagation using electromagnetic technology. Mapping of sinuatrial activation in the right atrium of 11 patients represents the first clinical experience with this new system. The physiological activation sequence could be determined in all patients three-dimensionally, and the sinus node could be localized as a physiological activation focus with interindividual variability only in the sagital plane without complications. The nonfluoroscopic mapping system allows high resolution visualization of electrical activity and may therefore improve precision and simplify the determination of the arrhythmogenic substrate during tachycardias for successful catheter ablation.

Tumor necrosis factor alpha (TNF alpha) is cardiodepressant in pathophysiologically relevant concentrations without inducing inducible nitric oxide-(NO)-synthase (iNOS) or triggering serious cytotoxicity

Cardiac hypertrophy and heart failure are frequently accompanied by elevated plasma levels of tumor necrosis factor alpha (TNF alpha), the pathogenetic relevance of this finding being a matter of debate. In human acute septic cardiomyopathy, on the other hand, the negative inotropic impact of TNF alpha on the heart is well documented and frequently ascribed to the induction of inducible nitric oxide (NO) synthase (iNOS) and an enhanced production of NO in the heart. Yet the present study presents evidence that in cardiomyocytes TNF alpha in non-toxic concentrations specifically depresses contractile performance independent of NO. In spontaneously beating neonatal rat cardiomyocytes, TNF alpha in a low, pathophysiologically relevant concentration (10 U/ml, 1-3 days) does not alter basal pulsation amplitude, but blocks alpha- and beta-adrenoceptor-stimulated increase in contractility and beating irregularity and impairs the impact of high extracellular calcium on contractile performance. However, this low TNF alpha-concentration does not suffice to induce iNOS - documented by reverse transcriptase polymerase chain reaction - or enhance nitrite concentrations in the cell culture supernatants as a measure of cellular NO production, neither in the presence nor absence of dexamethasone (0.1 micro M). Only in high concentration - the specific proinflammatory action being documented by an enhanced release of interleukin-6 from cardiomyocytes - TNF alpha (1000 U/mol; 6, 24 h) weakly induces the mRNA for iNOS, with a consecutive moderate rise in cellular nitrite production. TNF alpha-incubation (10-1000 U/ml) does not alter the morphological appearance of the cells displayed by phase contrast microscopy or evoke gross cytotoxicity.

A post-receptor defect of adenylyl cyclase in severely failing myocardium from children with congenital heart disease

The aim of this study was to determine whether a defect at the post-receptor level of adenylyl cyclase may also contribute to the decreased effectiveness of cAMP-increasing agents in severely failing patients with congenital heart disease. The severity of congestive heart failure in 31 patients with congenital heart disease was graded by a scoring system which included a description of historical and clinical variables. Patients were divided into a group with no or mild heart failure (score < or = 6) and a group with severe heart failure (score > 6). beta-Adrenoceptor-stimulated adenylyl cyclase activity was significantly decreased by 65% in patients with severe heart failure in comparison to the group of patients with no or mild heart failure. In addition, receptor-independent adenylyl cyclase stimulation by forskolin was reduced by 52% in patients with score > 6 compared to patients with score < or = 6. This post-receptor defect of adenylyl cyclase was apparently due to a decrease in the activity of catalytic subunit of adenylyl cyclase as adenylyl cyclase stimulation by forskolin in the presence of Mn2+ which uncouples catalytic subunit from the G proteins, G(s) and G(i), was also significantly diminished in the patients with severe heart failure. In contrast, the level of inhibitory G protein alpha-subunits was apparently not different in the two groups. In summary, the data indicate that a defect at the catalytic subunit of adenylyl cyclase apparently contributes to the decreased effectiveness of cAMP-increasing agents in severely failing patients with congenital heart disease.

Partial inhibition of protein synthesis by Pseudomonas exotoxin A deranges catecholamine sensitivity of cultured rat heart myocytes

To elucidate cellular mechanisms of myocardial depression in Pseudomonas sepsis the effects of sublethal concentrations of P. aeruginosa exotoxin A--a main virulence factor--were studied in cultured neonatal rat cardiomyocytes. It is known that this toxin exerts its pathogenic effect by inhibition of protein synthesis via ADP-ribosylation and thereby inactivation of elongation factor 2 (EF-2). Within 48 72 h, half maximal inhibition of protein synthesis occurs at 4-10 ng/ml. The toxin prevents the beta-adrenoceptor(AR)-mediated myosin heavy chain isozyme shift (V3/V1), while the T3-induced myosin shift is not suppressed. While beta 1-AR-downregulation by excess of norepinephrine (NE) is not affected, protein synthesis-dependent receptor upregulation in the recover period after removal of NE is completely suppressed by P. aeruginosa exotoxin A. Thus, a non-lethal, partial inhibition of global cellular protein synthesis by P. aeruginosa exotoxin A: (1) completely prevents beta 1-AR-mediated myosin isozyme shift and beta-AR upregulation: (2) sustains the cardiomyocytes in a catecholamine-refractory contractile state in the recovery period after catecholamine desensitization: (3) suggests cellular mechanisms by which P. aeruginosa exotoxin A might impair heart function in Pseudomonas sepsis: and (4) may help reveal the possible influence of endogenous inhibitors of EF-2.

Beta-adrenoceptor desensitization despite cardiac denervation in human failing transplanted myocardium

The aim of this study, using right ventricular endomyocardial biopsies, was to determine whether adenylyl cyclase activity is decreased-despite cardiac denervation-in human failing transplanted myocardium. beta-Adrenoceptor-stimulated adenylyl cyclase activity in membranes of moderately failing transplanted hearts was significantly decreased by 49% in comparison to non-failing transplanted hearts. Receptor-independent adenylyl cyclase stimulation (forskolin + GTP, forskolin + Mn2+) and the level of inhibitory G protein alpha-subunits were not significantly different in membranes of failing vs. non-failing transplanted myocardium. The results indicate that similar to failing non-transplanted myocardium the responsiveness to beta-adrenoceptor agonists is also decreased in failing transplanted myocardium.

Exposure to the n-3 polyunsaturated fatty acid docosahexaenoic acid impairs alpha 1-adrenoceptor-mediated contractile responses and inositol phosphate formation in rat cardiomyocytes

The beneficial effects of n-3 polyunsaturated fatty acids of fish oil in the prevention of fatal arrhythmias in myocardial ischemia were suggested to be at least in part mediated by a modulation of dihydropyridine-sensitive L-type calcium channels. As cardiac alpha 1-adrenoceptor stimulation has been suggested to have no significant effect on L-type calcium channels, the aim of this study using cultured neonatal rat cardiomyocytes was to investigate whether chronic n-3 polyunsaturated fatty acid exposure may have an influence on alpha 1-adrenoceptor-induced positive inotropic effects and induction of arrhythmias. Pretreatment of the rat cardiomyocytes for 3 days in the presence of the n-3 polyunsaturated fish oil-derived fatty acid docosahexaenoic acid (60 mumol/l) markedly decreased alpha 1-adrenoceptor-stimulated increase in contraction velocity and induction of arrhythmias. The increase in contraction velocity of the cardiomyocytes induced by the beta-adrenoceptor agonist isoprenaline was also markedly reduced by the n-3 fatty acid pretreatment. Basal contractile amplitude and spontaneous beating frequency of the cardiomyocytes were not significantly altered by the docosahexaenoic acid exposure. The pretreatment of the rat cardiomyocytes for 3 days in the presence of docosahexaenoic acid (60 mumol/l) decreased alpha 1-adrenoceptor-stimulated formation of the calcium-mobilizing second messenger IP3 and its metabolites IP2 and IP1 by 55%. The depression of IP3 formation by docosahexaenoic acid treatment was not mediated by a decreased uptake of myo-inositol into the cardiomyocytes nor by a decreased synthesis of phosphatidylinositol bisphosphate (PIP2), the substrate of phospholipase C. The level of glycerol-3-phosphate, an important substrate of the phosphoinositide cycle, was unaltered by the docosahexaenoic acid pretreatment. Receptor binding studies revealed that the dissociation constant and maximal binding capacity of the alpha 1-adrenoceptor antagonist (3H)prazosin was unchanged by the n-3 polyunsaturated fatty acid exposure. Beta-Adrenoceptor-and forskolin-stimulated adenylyl cyclase activities were not diminished by the docosahexaenoic acid pretreatment. Chronic exposure of the cardiomyocytes to the n-6 polyunsaturated fatty acid arachidonic acid (60 mumol/l) did neither significantly alter alpha 1-adrenoceptor-induced inositol phosphate formation nor alpha 1-adrenoceptor-stimulated increase in contraction velocity. The results presented show that chronic n-3 polyunsaturated fatty acid pretreatment of rat cardiomyocytes leads to a marked impairment of alpha 1-adrenoceptor-induced positive inotropic effects and induction of arrhythmias concomitant with a n-3 fatty acid-induced decrease in IP3 formation. This derangement of the phosphoinositide pathway by chronic n-3 fatty acid exposure may, thus, contribute to the beneficial effects of fish oil-derived fatty acids in the prevention of fatal arrhythmias in myocardial ischemia.

Chronic muscarinic cholinoceptor stimulation increases adenylyl cyclase responsiveness in rat cardiomyocytes by a decrease in the level of inhibitory G-protein alpha-subunits

Exposure of neonatal rat cardiomyocytes for 3 days to the muscarinic cholinoceptor agonist carbachol led to a concentration-dependent increase in adenylyl cyclase stimulation by the beta-adrenoceptor agonist isoproterenol by up to 115% (at 1 mmol/l carbachol). In addition, direct adenylyl cyclase stimulation by forskolin was increased in carbachol (1 mmol/l)-treated cells by 32%. Pretreatment of the rat cardiomyocytes with pertussis toxin, which enhances adenylyl cyclase activity by a functional inactivation of the inhibitory G-protein (Gi), was performed to investigate the possible role of Gi-proteins in carbachol-induced sensitization of adenylyl cyclase stimulation. After pretreatment of the cells with pertussis toxin, the carbachol-mediated increase in forskolin-stimulated adenylyl cyclase activity was lost and the carbachol-mediated increase in beta-adrenoceptor-stimulated adenylyl cyclase activity was attenuated. Labelling of the 40 kDa pertussis toxin substrates in cardiomyocyte membranes was decreased by carbachol in a concentration-dependent manner by up to 34% (at 1 mmol/l carbachol). The number and affinity of beta 1-adrenoceptors was unaltered following the chronic carbachol treatment. The specific protein synthesis inhibitor Pseudomonas exotoxin A was used to study whether the carbachol-induced decrease in the level of pertussis toxin-sensitive G-proteins and increase in adenylyl cyclase activity depend on de-novo protein synthesis. Pseudomonas exotoxin A inhibits peptide chain elongation by ADP-ribosylating elongation factor 2. Treatment of the cells with 1 ng/ml Pseudomonas exotoxin A for 3 days led to a reduction in the subsequent ADP-ribosylation of elongation factor 2 in the cytosol of the heart muscle cells by 57%.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Desensitization of rat cardiomyocyte adenylyl cyclase stimulation by plasma of noradrenaline-treated patients with septic shock

The purpose of this study was to test the possibility that the mechanisms of catecholamine-induced desensitization of cardiac beta-adrenoceptor stimulation are modified in septic shock. Exposure of neonatal rat cardiomyocytes for 48 hr to plasma of noradrenaline-treated patients with septic shock led to a down-regulation of beta-adrenoceptors by 35%, an increase in the level of inhibitory G protein alpha-subunits by 60%, and a decrease in isoproterenol-stimulated adenylyl cyclase activity by 50% in membranes prepared from the rat cardiomyocytes. Similar alterations were observed following pretreatment of the cells with plasma of adrenaline-treated patients with cardiogenic shock. In contrast, exposure of the cardiomyocytes to plasma of intensive care patients without shock, and to plasma of dopamine-treated patients with septic shock did not induce alterations of the cardiomyocyte adenylyl cyclase system. The dosage of the catecholamines had to be increased within the first two days of treatment in the noradrenaline-treated patients, but not in the dopamine-treated patients with septic shock. Thus, this observed tolerance to noradrenaline in the treatment of septic shock may, in part, be due to a desensitization of cardiac beta-adrenoceptor stimulation induced by the beta-adrenoceptor stimulatory effect of noradrenaline.

Distinct pathways for beta-adrenoceptor-induced up-regulation of muscarinic acetylcholine receptors and inhibitory G-protein alpha-subunits in chicken cardiomyocytes

Exposure of cardiomyocytes from chicken embryos for 3 days to the beta-adrenoceptor agonist, isoproterenol, lead to a down-regulation of beta-adrenoceptors by about 70% and to a decrease in isoproterenol-stimulated adenylyl cyclase activity by about 40% (homologous desensitization). In addition, the isoproterenol treatment induced an increase in the level of muscarinic acetylcholine receptors by about 30% and an increase in pertussis toxin-catalyzed ADP-ribosylation of two about 40 kDa proteins, most probably alpha-subunits of the inhibitory G-protein (Gi), by about a factor of two (heterologous desensitization). The purpose of the present study was to characterize the role of beta-adrenoceptor-dependent and -independent mechanisms in heterologous desensitization of adenylyl cyclase. Therefore, the effect of pretreatment with the beta-adrenoceptor antagonist, propranolol, with the partial agonists, celiprolol and xamoterol, and with the beta-adrenoceptor-independent adenylyl cyclase activators, prostaglandin E1 and forskolin, on beta-adrenoceptors, muscarinic acetylcholine receptors and pertussis-toxin-catalyzed ADP-ribosylation of G-protein alpha-subunits was studied. Pretreatment of the cardiomyocytes for 3 days with xamoterol or celiprolol, but not with propranolol, induced a small decrease in beta-adrenoceptor number and in isoproterenol-stimulated adenylyl cyclase activity by about 15-20%. Exposure to prostaglandin E1 and forskolin lead to a more pronounced decrease in beta-adrenoceptor binding and in isoproterenol-mediated adenylyl cyclase stimulation by about 40-60% (heterologous desensitization). An increase in the level of muscarinic acetylcholine receptors, similar to that induced by isoproterenol exposure, was only observed after pretreatment with the partial agonists, celiprolol and xamoterol, but not after pretreatment with the beta-adrenoceptor-independent agonists, prostaglandin E1 and forskolin, nor after pretreatment with propranolol. In contrast, prostaglandin E1 and forskolin exposure lead to a similar increase in pertussis toxin-catalyzed ADP-ribosylation of about 40 kDa G-proteins as isoproterenol exposure whereas treatment with propranolol, celiprolol and xamoterol had no or only a very small effect on pertussis toxin substrates. In summary, the data suggest that, similar as shown for homologous desensitization, cyclic AMP-dependent and -independent mechanisms are also involved in heterologous desensitization of adenylyl cyclase stimulation. The beta-adrenoceptor-induced upregulation of muscarinic acetylcholine receptors and of the alpha-subunits of pertussis toxin-sensitive G-proteins, most probably of Gi, seem to be mediated via distinct pathways.

Regulation of adenylyl cyclase by noradrenaline and tumour necrosis factor alpha in rat cardiomyocytes

The regulation of adenylyl cyclase components and of adenylyl cyclase activity by noradrenaline and tumour necrosis factor alpha (TNF alpha) was studied in rat cardiomyocytes. Long-term treatment of rat cardiomyocytes in the presence of noradrenaline leads, in addition to a down-regulation of beta 1-adrenoceptors, to an increase in the level of inhibitory G protein alpha-subunits and to a heterologous desensitization of adenylyl cyclase stimulation. Similar to the noradrenaline exposure, incubation of the cardiomyocytes in the presence of the cytokine TNF alpha (10 U.ml-1) also increases the level of Gi alpha proteins. However, in contrast to the noradrenaline treatment, which apparently induces a selective up-regulation of Gi alpha, the TNF alpha exposure also increases the level or activity of other components of adenylyl cyclase, such as the level of membrane beta 36-subunits of G proteins and, most likely, the level of the alpha-subunits of the stimulatory G protein (Gs alpha) and the activity of adenylyl cyclase catalytic subunit. While noradrenaline treatment desensitizes receptor-dependent and independent adenylyl cyclase activity, treatment of the cells with TNF alpha induces a sensitization of adenylyl cyclase stimulation. The data indicate that only a selective increase in the level of inhibitory G protein alpha subunits decreases adenylyl cyclase activity. The hypersensitivity of adenylyl cyclase induced by TNF alpha exposure may be due to concomitant alterations of other components of the adenylyl cyclase signal transduction system.

Mechanisms in acute septic cardiomyopathy: evidence from isolated myocytes

Although often not considered, the heart is one of the targets of multiple organ failure in sepsis and septic shock, with myocardial depression being a prominent component of this "acute septic cardiomyopathy". Hypotheses concerning the etiology of this depression are increasingly elucidated on a cellular level, including dysfunction of the beta-adrenoceptor/G protein/adenylate cyclase system, calcium channel blockade by cardiodepressant factor, contractile impairment by activated leucocytes, as well as inhibition of protein synthesis by Pseudomonas exotoxin A. In the search for "mechanisms of myocardial depression in sepsis", isolated cardiomyocytes may play a role as research tools with respect to: a) discrimination between direct and indirect cardiodepressant effects; b) identifying not only the acute, but also chronic toxin- and mediator-induced cardiodepression; c) clarification of the mechanism of action of cardiodepressant bacterial toxins and sepsis mediators; d) establishment of in vitro models of leucocyte-mediated cardiodepression in sepsis.

The oxime HGG-12 as a muscarinic acetylcholine receptor antagonist without intrinsic activity in cardiac membranes

Direct interactions of the bispyridinium oxime HGG-12 with muscarinic acetylcholine receptors were investigated in porcine cardiac atrial membranes. Competition binding experiments using the radiolabeled muscarinic receptor antagonist (3H)QNB revealed specific binding of HGG-12 to muscarinic acetylcholine receptors of porcine atrial membranes with a dissociation constant of 3.8 x 10(-7) mol/l. Muscarinic acetylcholine receptor-stimulated binding of the radiolabeled GTP analog (35S)GTP[S] to guanine nucleotide binding proteins (G-proteins) was used to study antagonistic and possible agonistic effects of HGG-12 at muscarinic acetylcholine receptors. HGG-12 completely inhibited carbachol- and oxotremorine-stimulated (35S)GTP[S] binding to pertussis toxin sensitive and insensitive G-proteins in a competitive manner. Inhibition constants (K1) of HGG-12 for blockade of carbachol- and oxotremorine-stimulated GTP[S]-binding (9.7 x 10(-7) mol/l and 1.7 x 10(-6) mol/l, respectively) were higher by about a factor of 100 than those of the muscarinic acetylcholine receptor antagonist atropine. In the absence of muscarinic acetylcholine receptor agonists. HGG-12 by itself had no stimulatory effect on (35S)GTP[S] binding in porcine atrial membranes. The results of this study show that the oxime HGG-12 is a competitive antagonist without intrinsic activity at porcine atrial muscarinic acetylcholine receptors. The stimulatory action of HGG-12 on muscarinic acetylcholine receptors which has been described by several authors is, therefore, suggested to be due to partial inhibition of acetylcholinesterase by the oxime rather than to direct agonism at muscarinic acetylcholine receptors.

Tumor necrosis factor alpha up-regulates Gi alpha and G beta proteins and adenylyl cyclase responsiveness in rat cardiomyocytes

Treatment of cultured rat cardiomyocytes in serum-free medium for 48 h with recombinant human tumor necrosis factor alpha (TNF alpha) led to a concentration-dependent increase in the level of membrane-inhibitory guanine nucleotide-binding protein (Gi) alpha-subunits and in pertussis toxin-catalyzed [32P]ADP ribosylation of 40 kDa proteins. Both Gi alpha protein subtypes present in rat cardiac myocyte membranes, Gi alpha 40 and Gi alpha 41, were up-regulated by the cytokine, with the maximal increase occurring at 10 U/ml TNF alpha. In contrast to noradrenaline exposure, which causes a similar, but apparently exclusive, increase in alpha i-subunits, treatment with TNF alpha in addition increased the level of membrane G protein beta 36-subunits. Furthermore, while noradrenaline exposure led to a decrease in receptor-dependent and -independent adenylyl cyclase activity, treatment of cardiomyocytes with TNF alpha caused a concentration-dependent increase in cyclase responsiveness to either forskolin, guanosine 5'-O-(3-thiotriphosphate) or isoproterenol, even though beta-adrenoceptor density was decreased by TNF alpha. The increase in adenylyl cyclase activity induced by TNF alpha was completely suppressed when the cells were cocultured with noradrenaline, a condition leading to an additive increase in Gi alpha level. The data indicate that the cytokine TNF alpha can potently modulate G protein-mediated signal transduction in rat cardiac myocytes. Although TNF alpha, like noradrenaline, exposure of the cells increased the level of membrane Gi alpha proteins, it did not decrease but rather caused an increase in adenylyl cyclase responsiveness.(ABSTRACT TRUNCATED AT 250 WORDS)

Pseudomonas exotoxin A prevents beta-adrenoceptor-induced upregulation of Gi protein alpha-subunits and adenylyl cyclase desensitization in rat heart muscle cells

Exposure of rat heart muscle cells to noradrenaline (1 microM) for 48 hr led to a decrease in the number of beta 1-adrenoceptors of 50% and a concomitant decrease in adenylyl cyclase stimulation by isoprenaline and forskolin of about 60 and 30%, respectively. In addition, the levels of two inhibitory guanine nucleotide-binding protein (Gi protein) alpha-subunits (Gi alpha 40 and Gi alpha 41) were increased in membranes of noradrenaline-treated cells. Evidence is presented that noradrenaline induces this increase by activation of beta-adrenoceptors. First, the noradrenaline action was mimicked by the beta-adrenoceptor agonist isoprenaline. Second, beta-adrenoceptor blockade by timolol but not alpha-adrenoceptor blockade by prazosin prevented the noradrenaline-induced up-regulation of Gi alpha proteins. Furthermore, timolol but not prazosin abolished the noradrenaline-induced down-regulation of beta 1-adrenoceptors and the decreases in receptor-dependent (isoprenaline) and -independent (forskolin) adenylyl cyclase stimulation. The specific protein synthesis inhibitor Pseudomonas exotoxin A was used to study whether the noradrenaline-induced up-regulation of Gi alpha subunits depends on increased synthesis of these proteins. This toxin inhibits peptide chain elongation by ADP-ribosylating elongation factor 2. Treatment of rat heart muscle cells with Pseudomonas exotoxin A (1 ng/ml) completely prevented the noradrenaline-induced increase in Gi alpha proteins, measured by both pertussis toxin-catalyzed ADP-ribosylation and immunoblotting with anti-Gi alpha antibodies. Most importantly, Pseudomonas exotoxin A also completely prevented the noradrenaline-induced decrease in forskolin-stimulated adenylyl cyclase activity. Furthermore, the noradrenaline-induced decrease in isoprenaline-stimulated adenylyl cyclase activity was significantly attenuated by the toxin, although the down-regulation of beta 1-adrenoceptors caused by noradrenaline treatment was not affected. The data presented suggest that prolonged activation of beta-adrenoceptors in rat heart muscle cells, in addition to causing a receptor down-regulation, induces the synthesis of Gi alpha proteins, which then apparently mediate a decreased adenylyl cyclase responsiveness. The data, additionally, suggest that the synthesis of Gi alpha proteins is under control of the activity of the adenylyl cyclase system and that altered levels of these proteins may play a major role in long term regulation of signal transduction by this enzyme.

Hormonal regulation of Gi alpha level and adenylyl cyclase responsiveness

Prolonged exposure of cells to adenylyl cyclase stimulatory hormonal factors can cause an increase in the level of membrane inhibitory G protein (Gi) alpha-subunits, while inhibitory receptor agonists have been reported to induce the opposite response. As studied in cultured rat cardiomyocytes, the beta-adrenoceptor-induced increase in the level of Gi alpha proteins is protein synthesis-dependent, is apparently not accompanied by an increase in G protein beta-subunits and results in a decreased adenylyl cyclase responsiveness. On the other hand, a decrease in Gi alpha level apparently results in sensitization of adenylyl cyclase stimulation. These data suggest that the up- or down-regulation of the level and activity of Gi protein alpha-subunits is a rather general cellular response, providing an intracellular negative feedback control against prolonged receptor activation.

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.

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

Cultivation of rat heart muscle cells for up to 5 days in the presence of 10(-6) mol/l (-)noradrenaline leads to a desensitization of the cells to inotropic stimulation and cAMP formation at different levels of the cAMP-system: The decrease in the responsiveness to the beta-adrenoceptor agonist isoprenaline quantitatively parallels the down-regulation of beta 1-adrenoceptors. The impairment of the positive inotropic effect of the phosphodiesterase-inhibitor IBMX is associated with a diminished basal cAMP-formation but an unchanged phosphodiesterase-activity in noradrenaline-treated cells. This decrease in basal cAMP-accumulation as well as the attenuation of the forskolin-stimulated cAMP-formation indicate that a defect in the adenylate cyclase system beyond the receptor must be involved in noradrenaline-induced desensitization. In contrast to the diminished effectiveness of cAMP-increasing agents the positive inotropic effect of ouabain and the alpha-adrenoceptor-mediated positive inotropic effect of phenylephrine are unaltered in noradrenaline-treated cells. It is concluded from these results that in addition to the down-regulation of beta 1-adrenoceptors a defect on the post-receptor level of adenylate cyclase occurs in noradrenaline-induced desensitization not affecting mechanisms beyond the catalytic subunit of adenylate cyclase. As these findings are very similar to those observed in isolated preparations from severely failing human hearts, the presumption is confirmed that noradrenaline-induced desensitization might constitute an important etiological factor in the subsensitivity of failing human hearts to inotropic stimulation.

Homologous vs. heterologous desensitization of the adenylate cyclase system in heart cells

Exposure of cultured heart muscle cells to noradrenaline led to a decrease in the effects of isoproterenol and prostaglandin E1 on cAMP formation and contraction velocity. However, heterologous desensitization, as measured by prostaglandin E1 stimulation, only occurred at higher noradrenaline concentrations than homologous desensitization (isoproterenol stimulation). As the defects of the adenylate cyclase system in heart failure are attributed to noradrenaline-induced desensitization, it is concluded from the results that, in comparison to the subsensitivity to beta-adrenoceptor agonists in failing human hearts, a decrease in the responsiveness to other receptor-dependent adenylate cyclase stimulators should also occur but only at higher degrees of heart failure.

Studies on the role of central catecholaminergic mechanisms in the antidotal effect of the oxime HI 6 in soman poisoned mice

The effects of atropine and the oxime HI 6 on running performance, brain and plasma cholinesterase activity and brain catecholamines were investigated in mice intoxicated with sublethal doses of soman (100 micrograms/kg s.c.). The running time on a rotating mash wire drum (total running time 60 min) after injection of soman was reduced to 17.2 min. Treatment with atropine (10 mg/kg i.p.) or HI 6 (55 mg/kg i.p.) improved the running performance to 48.2 and 44.8 min, respectively. Cholinesterase activity was decreased in soman poisoned mice to 47.3% in plasma and 43.5% in brain. Therapy with the oxime HI 6 resulted in a reactivation of soman-inhibited peripheral cholinesterase to 76.6%, but failed to reactivate central cholinesterase. Dopamine levels in mice brain were elevated in soman poisoning by 23.2%, whereas noradrenaline levels remained unchanged. The increase in brain dopamine levels was antagonized by atropine as well as by HI 6. The results of this study lead to the speculation that central dopaminergic mechanisms may be involved in soman toxicity as well as in the antidotal action of atropine and the mainly peripherally acting oxime HI 6.