Carvedilol blocks β2- more than β1-adrenoceptors in human heart

OBJECTIVE

To understand the basis of the effectiveness of carvedilol in heart failure by determining its specific properties at human heart beta1- and beta2-adrenoceptors.

METHODS

The positive inotropic effects of noradrenaline (in the presence of the beta2-selective antagonist ICI118551) and adrenaline (in the presence of the beta1-selective antagonist CGP20712), mediated through beta1- and beta2-adrenoceptors, respectively, were investigated in atrial and ventricular trabeculae. The patch-clamp technique was used to investigate effects of noradrenaline and adrenaline on L-type Ca2+ current in human atrial myocytes.

RESULTS

Carvedilol was a 13-fold more potent competitive antagonist of the effects of adrenaline at beta2-adrenoceptors (-logKB=10.13+/-0.08) than of noradrenaline at beta1-adrenoceptors (-logKB=9.02+/-0.07) in human right atrium. Chronic carvedilol treatment of patients with non-terminal heart failure reduced the inotropic sensitivity of atrial trabeculae to noradrenaline and adrenaline 5.6-fold and 91.2-fold, respectively, compared to beta1-blocker-treated patients, consistent with persistent preferential blockade of beta2-adrenoceptors. In terminal heart failure carvedilol treatment reduced 1.8-fold and 25.1-fold the sensitivity of right ventricular trabeculae to noradrenaline and adrenaline, respectively, but metoprolol treatment did not reduce the sensitivity to the catecholamines. Increases of current (ICa,L) produced by noradrenaline and adrenaline were not different in atrial myocytes obtained from non-terminal heart failure patients treated with metoprolol or carvedilol, consistent with dissociation of both beta-blockers from the receptors.

CONCLUSIONS

Carvedilol blocks human cardiac beta2-adrenoceptors more than beta1-adrenoceptors, thereby conceivably contributing to the beneficial effects in heart failure. The persistent blockade of beta-adrenoceptors is attributed to accumulation of carvedilol in cardiac tissue.

The G protein-gated potassium current I(K,ACh) is constitutively active in patients with chronic atrial fibrillation

BACKGROUND

The molecular mechanism of increased background inward rectifier current (IK1) in atrial fibrillation (AF) is not fully understood. We tested whether constitutively active acetylcholine (ACh)-activated I(K,ACh) contributes to enhanced basal conductance in chronic AF (cAF).

METHODS AND RESULTS

Whole-cell and single-channel currents were measured with standard voltage-clamp techniques in atrial myocytes from patients with sinus rhythm (SR) and cAF. The selective I(K,ACh) blocker tertiapin was used for inhibition of I(K,ACh). Whole-cell basal current was larger in cAF than in SR, whereas carbachol (CCh)-activated I(K,ACh) was lower in cAF than in SR. Tertiapin (0.1 to 100 nmol/L) reduced I(K,ACh) in a concentration-dependent manner with greater potency in cAF than in SR (-logIC50: 9.1 versus 8.2; P<0.05). Basal current contained a tertiapin-sensitive component that was larger in cAF than in SR (tertiapin [10 nmol/L]-sensitive current at -100 mV: cAF, -6.7+/-1.2 pA/pF, n=16/5 [myocytes/patients] versus SR, -1.7+/-0.5 pA/pF, n=24/8), suggesting contribution of constitutively active I(K,ACh) to basal current. In single-channel recordings, constitutively active I(K,ACh) was prominent in cAF but not in SR (channel open probability: cAF, 5.4+/-0.7%, n=19/9 versus SR, 0.1+/-0.05%, n=16/9; P<0.05). Moreover, IK1 channel open probability was higher in cAF than in SR (13.4+/-0.4%, n=19/9 versus 11.4+/-0.7%, n=16/9; P<0.05) without changes in other channel characteristics.

CONCLUSIONS

Our results demonstrate that larger basal inward rectifier K+ current in cAF consists of increased IK1 activity and constitutively active I(K,ACh). Blockade of I(K,ACh) may represent a new therapeutic target in AF.

Effect of rilmakalim on detrusor contraction in the presence and absence of urothelium

Openers of K(ATP) channels are known to inhibit KCl-, carbachol- and also electrically induced contractions in detrusor muscle strips from various species. Contractions of isolated strips of urinary bladder are usually of higher amplitude when the urothelium has been removed. This has been explained by the release of an urothelium-derived relaxing factor. In this study we examined whether intact urothelium may modulate the effect of the selective KATP channel opener rilmakalim. Contractile responses to 85 mM KCl and 10 microM carbachol were measured in detrusor strips from mouse, pig and man. In the presence of an intact urothelium, contractions were significantly reduced in strips from all three species investigated. In preparations with urothelium rilmakalim reduced KCl contractions with similar potency and efficacy [-logIC50 (M) 4.6 to 5.1; Effmax reduction to 14-30% of control]. However, in urothelium-denuded strips rilmakalim was more potent in pig (-logIC50 5.5) than in mouse and man (-logIC50 4.7 and 4.4, respectively). The order of potency for rilmakalim to suppress carbachol-induced contractions was pig (-logIC50 6.7)>man (5.8)>mouse (4.7); contractions were significantly more reduced in pig (Effmax reduction to 11+/-2%, n=10) and in mouse (21+/-2%, n=8) than in human detrusor (55+/-5%, n=5). The presence of urothelium did not affect the concentration-response curves for rilmakalim, with the exception of KCl-induced contractions in pig. Only the rilmakalim-induced relaxation of carbachol-mediated contractions in pig were prevented by the KATP channel blocker glibenclamide. We conclude that with this one exception, the responses to rilmakalim in detrusor contractions were not mediated by KATP channel opening.

Juvenile pig detrusor: Effects of propiverine and three of its metabolites

In isolated detrusor strips, propiverine is known to be effective to decrease contractions elicited by electric field stimulation (EFS). Here we investigated whether the metabolites M-5, M-6 and M-14 of propiverine retain the pharmacological properties of the parent compound also in juvenile organisms. EFS-induced contractions of detrusor strips from juvenile pigs are more sensitive to atropine than strips from mature pigs. The atropine-resistant component of contraction is also significantly larger in juvenile pigs. Propiverine, its metabolites M-5, M-14 and also tolterodine completely reduced detrusor contraction in juvenile pigs. M-6 almost did not affect atropine-resistant contractions. We conclude that juvenile pig detrusors possess a higher atropine-resistant component of EFS-elicited contraction. Nevertheless order of potency and efficacy of propiverine and its metabolites M-5 and M-14 are similar in juvenile and mature pigs, while M-6 only reduces atropine-sensitive contractions in the juvenile organism.

Activation of Human ether-a-go-go-Related Gene Potassium Channels by the Diphenylurea 1,3-Bis-(2-hydroxy-5-trifluoromethyl-phenyl)-urea (NS1643)

The cardiac action potential is generated by a concerted action of different ion channels and transporters. Dysfunction of any of these membrane proteins can give rise to cardiac arrhythmias, which is particularly true for the repolarizing potassium channels. We suggest that an increased repolarization current could be a new antiarrhythmic principle, because it possibly would attenuate afterdepolarizations, ischemic leak currents, and reentry phenomena. Repolarization of the cardiac myocytes is crucially dependent on the late rapid delayed rectifier current (I(Kr)) conducted by ether-a-go-go-related gene (ERG) potassium channels. We have developed the diphenylurea compound 1,3-bis-(2-hydroxy-5-trifluoromethyl-phenyl)-urea (NS1643) and tested whether this small organic molecule could increase the activity of human ERG (HERG) channels expressed heterologously. In Xenopus laevis oocytes, NS1643 increased both steady-state and tail current at all voltages tested. The EC(50) value for HERG channel activation was 10.5 microM. These results were reproduced on HERG channels expressed in mammalian human embryonic kidney 293 cells. In guinea pig cardiomyocytes, studied by patch clamp, application of 10 microM NS1643 activated I(Kr) and significantly decreased the action potential duration to 65% of the control values. The effect could be reverted by application of the specific HERG channel inhibitor 4'-[[1-[2-(6-methyl-2-pyridyl)ethyl]-4-piperidinyl]carbonyl]-methanesulfonanilide (E-4031) at 100 nM. Application of NS1643 also resulted in a prolonged postrepolarization refractory time. Finally, cardiomyocytes exposed to NS1643 resisted reactivation by small depolarizing currents mimicking early afterdepolarizations. In conclusion, HERG channel activation by small molecules such as NS1643 increases the repolarization reserve and presents an interesting new antiarrhythmic approach.

Tissue distribution of a human Ca v 1.2 alpha1 subunit splice variant with a 75 bp insertion

Mesenchymal stem cells from human bone marrow (MSC) express mRNA encoding the L-type Ca2+ channel Ca v 1.2 alpha1 subunit (alpha(1)1.2). We now describe a splice variant including an alternative exon of 75 bp in the region between exons 9 and 10, which we identified in MSC by semi-quantitative RT-PCR. With primers specific for variants including (+9*) or excluding the 75 bp insertion (-9*), we found comparable mRNA expression patterns in MSC and in primary cultures of related connective tissue cells (chondrocytes, osteoblasts and fibroblasts). Since culture conditions might have altered variant expression, we investigated mRNA levels in various native human tissue samples (cartilage, bone, fat, liver, kidney, aorta, bladder, cardiac ventricle and atrium, CNS). We found highest levels of the +9* variant in aorta, containing smooth muscle and connective tissue cells, but the variant was expressed in all tissues. We therefore hypothesized that broad expression of +9* might be linked to the presence of vasculature and/or connective tissue structures, rather than to tissue-specific parenchymal cells (e.g. cardiomyocytes). To test this hypothesis we separated human atrium into a cardiomyocyte-enriched fraction and a cardiomyocyte-depleted fraction. RT-PCR demonstrated significantly larger levels of the +9* variant in the non-cardiomyocyte fraction. The result was even more clear in single cell RT-PCR experiments, where the +9* variant was undetectable in cardiomyocytes but present in non-cardiomyocytes. We conclude that the +9* variant is present in all human tissues investigated so far, and suggest that expression in human atrium is associated with vascular smooth muscle and/or connective tissue cells.

Teaching antiarrhythmic therapy and ECG in simulator-based interdisciplinary undergraduate medical education

BACKGROUND

Third-year students in the Dresden Medical School Programme undergo a 6 week course 'Basics of Drug Therapy' in a problem-based learning curriculum. As part of this course a practical seminar about antiarrhythmic drugs and ECG was set up. This study was conducted to evaluate the use of a simulator in this course.

METHODS

A total of 234 students were randomly allocated to receive instructions with (Group S) or without (Group C [control]) the use of a simulator. After a lecture on antiarrhythmic drugs, arrhythmias were presented to Group S using an advanced life support (ALS) manikin. The students were asked to administer a drug or to defibrillate, and the outcome was shown on the monitor. The students in Group C were presented with ECG charts without a simulator. The course was evaluated by a questionnaire and multiple-choice questions (MCQ) about arrhythmias.

RESULTS

We received 222 questionnaires. The content-time ratio was rated almost perfect in both groups, but the students in Group S rated the course better suited to link theory and practice. Students in Group S considered the simulator helpful and a good tool for teaching, and the extra effort to be worthwhile. A significantly higher number of students in Group S preferred electric cardioversion as therapy for ventricular tachycardia.

CONCLUSIONS

An ALS manikin can be an effective tool in teaching clinical pharmacology.

Molecular and functional expression of voltage-operated calcium channels during osteogenic differentiation of human mesenchymal stem cells

We used the patch-clamp technique and RT-PCR to study the molecular and functional expression of VOCCs in undifferentiated hMSCs and in cells undergoing osteogenic differentiation. L-type Ca2+ channel blocker nifedipine did not influence alkaline phosphatase activity, calcium, and phosphate accumulation of hMSCs during osteogenic differentiation. This study suggests that osteogenic differentiation of hMSCs does not require L-type Ca2+ channel function.

INTRODUCTION

During osteogenic differentiation, mesenchymal stem cells from human bone marrow (hMSCs) must adopt the calcium handling of terminally differentiated osteoblasts. There is evidence that voltage-operated calcium channels (VOCCs), including L-type calcium channels, are involved in regulation of osteoblast function. We therefore studied whether VOCCs play a critical role during osteogenic differentiation of hMSCs.

MATERIALS AND METHODS

Osteogenic differentiation was induced in hMSCs cultured in maintenance medium (MM) by addition of ascorbate, beta-glycerophosphate, and dexamethasone (ODM) and was assessed by measuring alkaline phosphatase activity, expression of osteopontin, osteoprotegerin, RANKL, and mineralization. Expression of Ca2+ channel alpha1 subunits was shown by semiquantitative or single cell RT-PCR. Voltage-activated calcium currents of hMSCs were measured with the whole cell voltage-clamp technique.

RESULTS

mRNA for the pore-forming alpha1C and alpha1G subunits of the L-type and T-type Ca2+ channels, respectively, was found in comparable amounts in cells cultured in MM or ODM. The limitation of L-type Ca2+ currents to a subpopulation of hMSCs was confirmed by single cell RT-PCR, where mRNA for the alpha1C subunits was detectable in only 50% of the cells cultured in MM. Dihydropyridine-sensitive L-type Ca2+ currents were found in 13% of cells cultured in MM and in 12% of the cells cultured in ODM. Under MM and ODM culture conditions, the cells positive for L-type Ca2+ currents were significantly larger than cells without Ca2+ currents as deduced from membrane capacitance; thus, current densities were comparable. Addition of the L-type Ca2+ channel blocker nifedipine to the culture media did not influence alkaline phosphatase activity and the extent of mineralization.

CONCLUSION

These results suggest that, in the majority of hMSCs, Ca2+ entry through the plasma membrane is mediated by some channels other than VOCCs, and blockade of the L-type Ca2+ channels does not affect early osteogenic differentiation of hMSCs.

Pharmacodynamics of propiverine and three of its main metabolites on detrusor contraction

1. Besides its antimuscarinic effects, propiverine may possess an additional mode of action. We compared the effects of propiverine, three of its metabolites (M-5, M-6, M-14) and atropine in human, pig and mouse urinary bladder preparations in order to elucidate the nature of a possible additional mode of action. 2. Like the parent compound, M-5, M-6 and M-14 reduced to variable degrees the contractions elicited by electric field stimulation (EFS) of isolated, urothelium-denuded detrusor strips. In mouse the atropine-resistant and therefore the nonadrenergic, noncholinergic component of contractile response to EFS was reduced by M-5, M-14 and propiverine, but was hardly affected by M-6. 3. Atropine, propiverine and M-6 significantly shifted the cumulative concentration-response curves for carbachol (CCh) to higher concentrations. Atropine and M-6 did not affect the maximum tension induced by CCh. Propiverine, M-5 and M-14 reduced the maximum CCh effect, suggesting at least one additional mode of action. This pattern of response was observed in all the three species, albeit with some differences in sensitivity to the various agents. 4. In freshly isolated human detrusor smooth muscle cells, propiverine and M-14 inhibited the nifedipine-sensitive L-type calcium current (I(Ca)) in a concentration-dependent manner. In contrast, the effects of M-5 and M-6 on I(Ca) were insignificant in the concentration range examined. 5. The investigated responses to propiverine and its metabolites suggest that impairment of maximum CCh-induced contractions is due to strong effect on I(Ca) and that this may be associated with the presence of the aliphatic side chain.

Risperidone-induced action potential prolongation is attenuated by increased repolarization reserve due to concomitant block of ICa,L

The neuroleptic risperidone is an effective blocker of the rapidly activating component of the delayed rectifier current (I(Kr)) and hence is expected to prolong cardiac action potential duration (APD). However, unlike with other typical I(Kr) blockers we failed to demonstrate a marked prolongation of late repolarization with risperidone. It is hypothesized that the APD-prolonging effect of risperidone is masked by the high repolarization reserve due to the prominent delayed rectifier currents I(Kr) and I(Ks) in guinea pig papillary muscle. Action potentials and force of contraction were recorded in isolated guinea pig papillary muscles. L-type calcium current I(Ca,L) and I(Kr) were measured using the standard patch clamp technique in single ventricular cardiomyocytes. Reduction of the repolarization reserve by the blocking of I(Ks) with chromanol 239B augmented the effect of the selective I(Kr) blocker E-4031, but not of risperidone, although both drugs completely blocked I(Kr). In contrast to E-4031 risperidone markedly reduced the force of contraction due to the partial blocking of I(Ca,L) in the same concentration range as required for block of I(Kr). Reduction of the repolarization reserve by the blocking of I(Ks) cannot exacerbate the APD-prolonging effect of risperidone. However, even incomplete concomitant blocking of I(Ca,L) attenuates the APD-prolonging effect of the complete blocking of I(Kr). This behaviour may explain the small APD-prolonging effect of risperidone despite the drug's robust blocking of I(Kr).

Silencing the cardiac potassium channel Kv4.3 by RNA interference in a CHO expression system

RNA interference (RNAi) is a powerful technique for gene silencing, in which the downregulation of mRNA is triggered by short RNAs complementary to a target mRNA sequence, with consequent reduction of the encoded protein. The aim of this study was to test the effects of silencing the expression of the cardiac potassium channel Kv4.3 in a heterologous expression system, in order to investigate the effect of RNAi on channel properties. A Chinese hamster ovary cell line stably expressing Kv4.3 and the accessory beta-subunit KChIP2 was transfected with small-interfering RNAs (siRNAs) targeting Kv4.3. Effects of RNAi were monitored at the mRNA, protein, and functional levels. Real-time PCR and immunofluorescence staining revealed significant reduction of Kv4.3 mRNA and protein expression. These results were confirmed by functional patch-clamp measurements of the transient outward current (I(to)) which was reduced up to 80% by RNAi. We conclude that the use of siRNAs reagents for post-transcriptional gene silencing is a new effective method for the reduction of the expression and function of different ionic channels which may be adapted for studying their role also in native cells.

Comparing the global mRNA expression profile of human atrial and ventricular myocardium with high-density oligonucleotide arrays

OBJECTIVES

The knowledge of chamber-specific gene expression in human atrial and ventricular myocardium is essential for the understanding of myocardial function and the basis for the identification of putative therapeutic targets in the treatment of cardiac arrhythmia and heart failure. In this study the gene expression pattern of human left atrial and ventricular myocardium was analyzed.

METHODS

Global mRNA expression patterns with high-density oligonucleotide arrays between left atrial and left ventricular myocardium of 6 patients with heart failure undergoing heart transplantation were compared. Clustering of microarray data confirmed chamber-specific gene expression profiles. Genes similarly expressed in all patients were further analyzed, and data were confirmed by means of real-time polymerase chain reaction and Western blot analysis.

RESULTS

Of 22,215 genes examined, 7115 transcripts were found to be expressed in all 12 human myocardial samples. One hundred twenty-five genes were differentially expressed between left atrial and left ventricular specimens in all patients examined. Novel genes preferentially expressed in human atria were identified. Interestingly, several potassium channels of subfamily K are more highly expressed in atria than in ventricles. Members of the potassium inwardly rectifying channel of subfamily J were found to be more highly expressed in human ventricular myocardium. Finally, chronic atrial fibrillation was associated with reduced atrial expression of the potassium channel TWIK-1, suggesting potential contribution of the corresponding current to electrical remodeling.

CONCLUSIONS

Human atria and ventricles show specific gene expression profiles. Our data provide the basis of a comprehensive understanding of chamber-specific gene expression in diseased human hearts and will support the identification of therapeutic targets in the treatment of arrhythmia and heart failure.

CHOLINERGIC AND PURINERGIC RESPONSES IN ISOLATED HUMAN DETRUSOR IN RELATION TO AGE

PURPOSE

We investigated whether the contractility of isolated human detrusor muscle, responsiveness to commonly used spasmolytic drugs, and expression of selected muscarinic and purinergic (P2X) receptor subtypes (M2, M3, P2X1 and P2X3) change with age.

MATERIALS AND METHODS

Tissues were taken from 63 patients 37 to 84 years old undergoing radical cystectomy. Specimens from 49 patients were used for contractility studies and those from 50 were used for mRNA analysis.

RESULTS

Propiverine, oxybutynin, tolterodine and atropine decreased contractions evoked by electric field stimulation to different degrees. However, neither the efficacy nor potency of the drugs showed age related changes. Since human detrusor muscle shows atropine resistant noncholinergic responses, we also studied the putative age dependence of concentration-response curves to the muscarinic agonist carbachol, and the purinergic agonists adenosine triphosphate (ATP) and alpha-beta-methylene-ATP. Sensitivity to alpha-beta-methylene-ATP increased with age, while the efficacy and potency of spasmolytic drugs did not depend on age. In addition, mRNA detected for M2, M3, P2X1 and P2X3 receptors did not change with age.

CONCLUSIONS

Our results do not provide evidence for age related contractile deterioration in human detrusor muscle strips, nor do they suggest that responses to anticholinergic spasmolytic drugs change substantially with age.

Expression and function of dipeptidyl-aminopeptidase-like protein 6 as a putative beta-subunit of human cardiac transient outward current encoded by Kv4.3

Dipeptidyl-aminopeptidase-like protein 6 (DPPX) was recently shown in the brain to modulate the kinetics of transient A-type currents by accelerating inactivation and recovery from inactivation. Since the kinetics of human cardiac transient outward current (I(to)) are not mimicked by coexpression of the alpha-subunit Kv4.3 with its known beta-subunit KChIP2, we have tested the hypothesis that DPPX may serve as an additional beta-subunit in the human heart. With quantitative real-time RT-PCR strong mRNA expression of DPPX was detected in human ventricles and was verified at the protein level in human but not in rat heart by a DPPX-specific antibody. Co-expression of DPPX with Kv4.3 in Chinese hamster ovary cells produced I(to)-like currents, but compared with expression of KChIP2a and Kv4.3, the time constant of inactivation was faster, the potential of half-maximum steady-state inactivation was more negative and recovery from inactivation was delayed. Co-expression of DPPX in addition to Kv4.3 and KChIP2a produced similar current kinetics as in human ventricular myocytes. We therefore propose that DPPX is an essential component of the native cardiac I(to) channel complex in human heart.

Effect of Local Anaesthesia on Neuronal c-fos Expression in the Spinal Dorsal Horn and Hypothalamic Paraventricular Nucleus after Surgery in Rats

The surgical stress response is the neurophysiologic reflex response to surgery, which involves activation of the hypothalamic-pituitary-adrenal axis and is regulated by the hypothalamic paraventricular nucleus. The effect of pre-operative use of local anaesthetics on activation of neurones in the paraventricular nucleus during surgery was studied by quantification of the neuronal expression of the c-fos-gene after a standardized plantar incision in rats. Furthermore, c-fos expression in the spinal dorsal horn was used as a measure of spinal nociception. Six halothane-anaesthetized animals underwent surgery following infiltration with lidocaine and bupivacaine, six animals were operated without local anaesthetics, and six control animals were subjected to the anaesthetic procedures. After two hours, the animals were perfused with 4% formaldehyde and the spinal cords and brains were collected and processed by immunohistochemistry for stereological quantification of the number of neurones with Fos-like immunoreactivity. Furthermore, brain and spinal cord were sampled from nine control animals right after induction of halothane anaesthesia. Surgery without local anaesthetics caused a significant increased number of neurones with Fos-like immunoreactivity in the spinal cord (4258+/-1710; mean+/-S.D.; P<0.01) compared to the anaesthesia control group (1204+/-436). Local anaesthetics reduced this number to 2029+/-919 (P<0.05), which was not significantly different from the anaesthesia control group. After surgery, the number of neurones with Fos-like immunoreactivity in paraventricular nucleus increased from 2948+/-1365 in the anaesthetized control group to 5550+/-3875 and 5191+/-1558 in the surgery and local anaesthetics plus surgery group, respectively, although significance was only reached for the group receiving local anaesthetics (P<0.05). In conclusion, preoperative local anaesthetic infiltration did not reduce the surgery-induced c-fos expression in paraventricular nucleus after paw surgery in rats, although spinal nociception was reduced.

Defective Cardiac Ryanodine Receptor Regulation During Atrial Fibrillation

Background— Ca 2+ leak from the sarcoplasmic reticulum (SR) may play an important role in triggering and/or maintaining atrial arrhythmias, including atrial fibrillation (AF). Protein kinase A (PKA) hyperphosphorylation of the cardiac ryanodine receptor (RyR2) resulting in dissociation of the channel-stabilizing subunit calstabin2 (FK506-binding protein or FKBP12.6) causes SR Ca 2+ leak in failing hearts and can trigger fatal ventricular arrhythmias. Little is known about the role of RyR2 dysfunction in AF, however.

Methods and Results— Left and right atrial tissue was obtained from dogs with AF induced by rapid right atrial pacing (n=6 for left atrial, n=4 for right atrial) and sham instrumented controls (n=6 for left atrial, n=4 for right atrial). Right atrial tissue was also collected from humans with AF (n=10) and sinus rhythm (n=10) and normal cardiac function. PKA phosphorylation of immunoprecipitated RyR2 was determined by back-phosphorylation and by immunoblotting with a phosphospecific antibody. The amount of calstabin2 bound to RyR2 was determined by coimmunoprecipitation. RyR2 channel currents were measured in planar lipid bilayers. Atrial tissue from both the AF dogs and humans with chronic AF showed a significant increase in PKA phosphorylation of RyR2, with a corresponding decrease in calstabin2 binding to the channel. Channels isolated from dogs with AF exhibited increased open probability under conditions simulating diastole compared with channels from control hearts, suggesting that these AF channels could predispose to a diastolic SR Ca 2+ leak.

Conclusions— SR Ca 2+ leak due to RyR2 PKA hyperphosphorylation may play a role in initiation and/or maintenance of AF.

Recording atrial monophasic action potentials using standard pacemaker leads: an alternative way to study electrophysiology properties of the human atrium in vivo?

AF leads to electrophysiological changes, but it is not known if similar alterations also appear before the onset of the first episode of AF because invasive electrophysiological studies are not justified in otherwise symptom-free patients. To address this question requires a safe method of obtaining atrial electrophysiological parameters at no extra risk or discomfort for the patient. The aim of this study was to test if recording of monophasic action potentials (MAPs) is feasible during pacemaker implantation. The study included 22 patients undergoing pacemaker implantation for symptomatic bradycardia without any history of AF. Using a custommade amplifier and a minor modification of the routine procedure for intraoperatively measured P waves, atrial electrograms could be recorded using a standard active pacemaker lead. MAP-like electrograms were obtained in 15 patients. MAP amplitude was 2.6 +/- 0.3 mV, mean action potential duration was 316 +/- 12 ms at a spontaneous heart rate of 67.2 +/- 3.2 beats/min. MAP duration was decreased when atria were stimulated at shorter cycle lengths (249 +/- 12 ms at 150 beats/min, P <0.05 vs sinus rhythm). In about two thirds of patients undergoing pacemaker implantation, recording of MAP-like electrograms was feasible with only minor modification of the atrial electrogram recording technique. The method should allow screening patients for electrophysiological alterations even before the onset of AF.

Beta adrenergic receptor-mediated atrial specific up-regulation of RGS5

Previous investigations had suggested that signaling from the overexpressed beta(2) adrenergic in the heart of transgenic TG4 mice was dampened in the atria. Using an RT-PCR based strategy, we have identified Regulator of G-protein Signaling 5 (RGS5) as being up-regulated in the atria of TG4 mice. Northern blot analysis demonstrated that RGS5 levels were 3 fold higher in the atria of TG4 mice. Western blot analysis of a panel of rat tissues demonstrated that basal expression of RGS5 protein was confined to the heart and skeletal muscle. Furthermore, RGS5 protein was detected in skeletal muscle C2C12 and cardiomyocyte HL-1 cultured cell lines. As observed for RGS5 mRNA levels in TG4 mice, RGS5 protein levels were increased in the atria of rats that were administered the beta adrenergic agonist isoproterenol during a 14 day period. Taken together, these results indicate that RGS5 is a housekeeping RGS in the heart and in skeletal muscle while its beta adrenergic-mediated induction in the atrium suggests that it also has a highly specialized function.

An aqueous extract of the marine sponge Ectyoplasia ferox stimulates L-type Ca2+-current by direct interaction with the Cav1.2 subunit

Marine organisms have attracted much attention as a source of pharmacological tools or potential drugs. We have produced and screened a library of sponge extracts in search of biologically active compounds that may contain useful pharmaceutical lead structures. Sponges were collected from various locations and their aqueous extracts were freeze dried. Murine right and left atria were used to screen 75 extracts for putative cardiac effects. Among seven extracts with a positive inotropic and chronotropic effect the extract C47 from Ectyoplasia ferox proved to be the most active and was chosen for further analysis. C47 also produced a beta-adrenoceptor-independent, propranolol-resistant positive inotropic effect in human atrial trabeculae. To elucidate one possible mode of action the effects of C47 on L-type Ca(2+) current (I(Ca,L)) were measured with a standard patch-clamp technique. In isolated human atrial myocytes exposure to C47 increased peak amplitude of I(Ca,L) in a concentration-dependent manner. The threshold concentration was 15 microg/ml. In addition, voltage dependency of activation and steady-state inactivation were shifted to more negative potentials. C47 slowed the initial phase of time-dependent current inactivation and the recovery from inactivation. In cell-attached patches of HEK 293 cells expressing human Ca(v)1.2 addition of C47 to the bath solution did not affect gating properties, whereas inclusion of the extract into the pipette solution strongly increased single-channel activity, suggesting a direct effect on the pore-forming channel subunit. Despite its robust effect on I(Ca,L) C47 enhanced cardiac force of contraction by only a fraction of the maximum increase caused by high extracellular concentrations of Ca(2+) and failed to increase vascular tone. These findings suggest that the effect of C47 is restricted to the Ca(2+) channel.

Pharmacological modulation of ion channels and transporters

Ion channels and transporter proteins are prerequisites for formation and conduction of cardiac electrical impulses. Acting in concert, these proteins maintain cellular Na(+) and Ca(2+) homeostasis. Since intracellular Ca(2+) concentration determines contractile activation, we expect the majority of agents that modulate activity of ion channels and transporters not only to influence cellular action potentials but also contractile force. Drugs which block ion channels usually possess antiarrhythmic properties, those inhibiting the Na(+) pump have predominantly inotropic effects and those affecting Na(+),Ca(2+)- or Na(+),H(+)-exchanger protect against ischaemic cell damage. However, irrespective of their primary indication, all compounds targeted against ion channels and transporter proteins possess potential proarrhythmic activity.

Role of I Kur in Controlling Action Potential Shape and Contractility in the Human Atrium

Background— The ultrarapid outward current I Kur is a major repolarizing current in human atrium and a potential target for treating atrial arrhythmias. The effects of selective block of I Kur by low concentrations of 4-aminopyridine or the biphenyl derivative AVE 0118 were investigated on right atrial action potentials (APs) in trabeculae from patients in sinus rhythm (SR) or chronic atrial fibrillation (AF).

Methods and Results— AP duration at 90% repolarization (APD 90 ) was shorter in AF than in SR (300±16 ms, n=6, versus 414±10 ms, n=15), whereas APD 20 was longer (35±9 ms in AF versus 5±2 ms in SR, P <0.05). 4-Aminopyridine (5 μmol/L) elevated the plateau to more positive potentials from −21±3 to −6±3 mV in SR and 0±3 to +12±3 mV in AF. 4-Aminopyridine reversibly shortened APD 90 from 414±10 to 350±10 ms in SR but prolonged APD 90 from 300±16 to 320±13 ms in AF. Similar results were obtained with AVE 0118 (6 μmol/L). Computer simulations of I Kur block in human atrial APs predicted secondary increases in I Ca,L and in the outward rectifiers I Kr and I Ks , with smaller changes in AF than SR. The indirect increase in I Ca,L was supported by a positive inotropic effect of 4-aminopyridine without direct effects on I Ca,L in atrial but not ventricular preparations. In accordance with the model predictions, block of I Kr with E-4031 converted APD shortening effects of I Kur block in SR into AP prolongation.

Conclusions— Whether inhibition of I Kur prolongs or shortens APD depends on the disease status of the atria and is determined by the level of electrical remodeling.

L-type Ca2+ current downregulation in chronic human atrial fibrillation is associated with increased activity of protein phosphatases

BACKGROUND

Although downregulation of L-type Ca2+ current (I(Ca,L)) in chronic atrial fibrillation (AF) is an important determinant of electrical remodeling, the molecular mechanisms are not fully understood. Here, we tested whether reduced I(Ca,L) in AF is associated with alterations in phosphorylation-dependent channel regulation.

METHODS AND RESULTS

We used whole-cell voltage-clamp technique and biochemical assays to study regulation and expression of I(Ca,L) in myocytes and atrial tissue from 148 patients with sinus rhythm (SR) and chronic AF. Basal I(Ca,L) at +10 mV was smaller in AF than in SR (-3.8+/-0.3 pA/pF, n=138/37 [myocytes/patients] and -7.6+/-0.4 pA/pF, n=276/86, respectively; P<0.001), though protein levels of the pore-forming alpha1c and regulatory beta2a channel subunits were not different. In both groups, norepinephrine (0.01 to 10 micromol/L) increased I(Ca,L) with a similar maximum effect and comparable potency. Selective blockers of kinases revealed that basal I(Ca,L) was enhanced by Ca2+/calmodulin-dependent protein kinase II in SR but not in AF. Norepinephrine-activated I(Ca,L) was larger with protein kinase C block in SR only, suggesting decreased channel phosphorylation in AF. The type 1 and type 2A phosphatase inhibitor okadaic acid increased basal I(Ca,L) more effectively in AF than in SR, which was compatible with increased type 2A phosphatase but not type 1 phosphatase protein expression and higher phosphatase activity in AF.

CONCLUSIONS

In AF, increased protein phosphatase activity contributes to impaired basal I(Ca,L). We propose that protein phosphatases may be potential therapeutic targets for AF treatment.

Decreased ATP-sensitive K(+) current density during chronic human atrial fibrillation

Chronic atrial fibrillation (AF) is associated with shortening of action potential duration (APD), which involves modified activity of atrial ion currents. However, little is known about the activity of ATP-sensitive K(+) channels (I(K,ATP)) during chronic AF. An AF-related increase in the activity of I(K,ATP) would reduce APD and could contribute to initiation and/or perpetuation of AF. Here, we studied the activity of I(K,ATP) in atrial myocytes from patients with sinus rhythm (SR) and chronic AF. Human atrial myocytes were isolated from atrial tissue obtained from patients undergoing open-heart surgery. Inward rectifier currents were measured with the whole-cell patch-clamp technique by applying a depolarizing ramp pulse (1245 ms) from -100 to +40 mV (0.5 Hz). I(K,ATP) was activated with the I(K,ATP) channel opener rilmakalim. The inward rectifier I(K1) and I(K,ATP) were identified by their sensitivity to 1 mM Ba(2+). Density of I(K1) did not differ between cells from patients with AF (at -100 mV: -14.8 +/- 1.3 pA/pF, n = 38/10 (cells/patients)) and SR (-13.8 +/- 1.5 pA/pF, n = 33/16). In both types of cells, rilmakalim stimulated I(K,ATP) (defined as rilmakalim-inducible current) in a concentration-dependent manner (0.3-10 microM). However, maximum activation of I(K,ATP) with 10 microM rilmakalim was smaller in AF than in SR cells (at -100 mV: -5.3 +/- 0.8 pA/pF, n = 22/7 vs. -11.2 +/- 2.9 pA/pF, n = 19/9; at +40 mV: +9.6 +/- 2.1 pA/pF, n = 22/7 vs. +23.7 +/- 3.4 pA/pF, n = 19/9 for AF and SR, respectively; P < 0.05). Only aortic valve disease and pulmonary hypertension were found to be independent contributors to I(K,ATP) current density. We provide evidence that chronic AF is associated with a downregulation of ATP-sensitive K(+) currents. These changes may provide an additional molecular mechanism for electrical remodeling in chronic AF.

Epinephrine activates both Gs and Gi pathways, but norepinephrine activates only the Gs pathway through human beta2-adrenoceptors overexpressed in mouse heart

Isoproterenol increases and decreases contractile force at low and high concentrations, respectively, through beta(2)-adrenoceptors overexpressed in transgenic mouse heart (TG4), consistent with activation of both G(s) and G(i) proteins. Using TG4 hearts, we demonstrated that epinephrine behaves like isoproterenol, but norepinephrine does not. Epinephrine both increased (-log EC(50)M = 9.4) and decreased (-log EC(50)M = 6.5) left atrial force. Pertussis toxin (PTX) abolished the negative inotropic effects of epinephrine, consistent with mediation through G(i) protein. Norepinephrine only increased contractile force (-log EC(50)M = 7.5). Norepinephrine (10-100 microM) prevented the positive inotropic effects but hardly affected the negative inotropic effects of epinephrine. Cardiodepressive epinephrine concentrations (1-10 microM) antagonized the positive inotropic effects of norepinephrine. In the free wall of TG4 right ventricle, norepinephrine and low epinephrine concentrations caused positive inotropic effects, and high epinephrine concentrations caused PTX-sensitive negative inotropic effects, as observed in the left atrium. Epinephrine (10 nM), a concentration causing maximum increase in contractile force, and norepinephrine (1 and 100 microM) increased cAMP-dependent protein kinase activity in TG4 left ventricle. Cardiodepressive concentrations of epinephrine (1 and 100 microM) did not increase cAMP-dependent protein kinase activity. The inotropic results were simulated with a model of two beta(2)-adrenoceptor sites. For one site involved in receptor coupling to G(s), both epinephrine and norepinephrine compete. The other site, recognized by epinephrine but not by norepinephrine, leads to receptor G(i) coupling.

Electrophysiological properties of human mesenchymal stem cells

Human mesenchymal stem cells (hMSC) have gained considerable interest due to their potential use for cell replacement therapy and tissue engineering. One strategy is to differentiate these bone marrow stem cells in vitro into cardiomyocytes prior to implantation. In this context ion channels can be important functional markers of cardiac differentiation. At present there is little information about the electrophysiological behaviour of the undifferentiated hMSC. We therefore investigated mRNA expression of 26 ion channel subunits using semiquantitative RT-PCR and recorded transmembrane ion currents with the whole-cell voltage clamp technique. Bone marrow hMSC were obtained from healthy donors. The cells revealed a distinct pattern of ion channel mRNA with high expression levels for some channel subunits (e.g. Kv4.2, Kv4.3, MaxiK, HCN2, and alpha1C of the L-type calcium channel). Outward currents were recorded in almost all cells. The most abundant outward current rapidly activated at potentials positive to +20 mV. This current was identified as a large-conductance voltage- and Ca(2+)-activated K(+) current, conducted by MaxiK channels, due to its high sensitivity to tetraethylammonium (IC(50)= 340 microm) and its inhibition by 100 nm iberiotoxin. A large fraction of cells also demonstrated a more slowly activating current at potentials positive to -30 mV. This current was selectively inhibited by clofilium (IC(50)= 0.8 microm). Ba(2+) inward currents, stimulated by 1 microm BayK 8644 were found in a few cells, indicating the expression of functional L-type Ca(2+) channels. Other inward currents such as sodium currents or inward rectifier currents were absent. We conclude that undifferentiated hMSC express a distinct pattern of ion channel mRNA and functional ion channels that might contribute to physiological cell function.

Cardiac mechano-electric feedback: past, present, and prospect

Mechanical effects on heart rhythm have been known to the clinical community for well over a century, and documented cases include both arrhythmogenic and pro-rhythmic consequences of mechanical stimulation. The intracardiac pathway that leads from changes in the cardiac mechanical environment to altered electrical activity is referred to as mechano-electric feedback (MEF). Fundamental research into the mechanisms underlying cardiac MEF is 'engineering-intensive', and much of the current insight would have been impossible without the introduction of novel techniques for the study of isolated cardiac cells. Clinical and basic research into MEF have developed over different time scales, often uninformed of each other, and utilizing disparate concepts and terminology. Bridging the gap between the two domains is not straightforward, as physicians and scientists tend to publish in different journals and attend different meetings. There is, however, a growing interest in 're-uniting' the clinic and basic MEF research, as witnessed by an increasing number of dedicated journal issues and international meetings, including events hosted by major European and American professional organisations such as the ESC and NASPE. Last year alone saw an international workshop on Cardiac MEF & Arrhythmias at Oxford, as well as dedicated sessions at NASPE's 23rd annual meeting in San Diego, CardioStim 2002 in Nice, and the UK Physiological Society meeting in Leeds. This volume of Progress in Biophysics and Molecular Biology incorporates clinical and basic science results, and it is fitting that its publication coincides with a special session on cardiac MEF at the 2003 meeting of NASPE.

Remodeling of cardiomyocyte ion channels in human atrial fibrillation

This review is focused on electrical adaptational processes in patients with chronic AF. Cellular electrical remodeling includes shortening of action potential duration and effective refractory period that can be explained by concomitant alterations in ion channel activity. While most currents studied are reduced or unaffected, the inward rectifier I(K1) is increased in amplitude. The time courses of these changes and the putative molecular mechanisms suggest that electrical remodeling in chronically fibrillating human atria are adaptive processes. New therapeutic options could consist of supporting rather than reversing the adaptive mechanisms.

Mechano-electric feedback and arrhythmias

The mechanical state of the heart feeds back to modify cardiac rate and rhythm. Mechanical stretch of myocardial tissue causes immediate and chronic responses that lead to the common end point of arrhythmia. This review provides a brief summary of the author's personal choice of contributions that she considers have fostered our understanding of the role of mechano-electric feedback in arrhythmogenesis. Acute mechanical stretch reversibly depolarises the cell membrane and shortens the action potential duration. These electrophysiological changes are related to the activation of mechano-sensitive ion channels. Several different ion channels are involved in the sensing of stretch, among them K(+)-selective, Cl(-)-selective, non-selective, and ATP-sensitive K(+) channels. Sodium and Ca(2+) entering the cells via non-selective ion channels are thought to contribute to the genesis of stretch-induced arrhythmia. Mechano-sensitive channels have been cloned from non-vertebrate and vertebrate species. Chronic stress on the heart activates gene expression in cardiomyocytes and non-myocytes. The signal transduction involves atrial natriuretic peptides and growth factors that initiate remodelling processes leading to hypertrophy which in turn may contribute to the electrical instability of the heart by increasing the responsiveness of mechano-sensitive channels. Selective block of these channels could provide some new form of treatment of mechanically induced arrhythmias, although at present there are no drugs available with sufficient selectivity. Detailed understanding of how mechanical strain on myocardial cells is translated into channel activation will allow to identify new targets for putative antiarrhythmic drugs.

Cardiostimulant and cardiodepressant effects through overexpressed human beta2-adrenoceptors in murine heart: regional differences and functional role of beta1-adrenoceptors

(-)-Isoprenaline enhances cardiac contractility through beta-adrenoceptors. However, in cardiac tissue from transgenic mice with a 200-400-fold cardiac overexpression of the human beta(2)-adrenoceptor (TG4) we observed a pronounced cardiodepression at high (-)-isoprenaline concentrations. Here, we investigated the functional role of the coexisting beta(1)-, beta(2)-, and beta(3)-adrenoceptor subtypes in several regions of the TG4 heart, and in particular their contribution to the negative inotropic effect. In paced TG4 left atria, (-)-isoprenaline produced bell-shaped concentration-effect curves increasing (-logEC(50)M=9.0) and decreasing (-logIC(50)M=6.4) contractile force. These effects were unaffected by the beta(1)-selective CGP 20712A (300 nM). The beta(2)-selective inverse agonist ICI 118,551 (30-1,000 nM) antagonised in surmountable manner both the positive and negative inotropic effects of (-)-isoprenaline with similar concentration-dependence, consistent with an exclusive mediation through beta(2)-adrenoceptors. The beta(3)-adrenoceptor-selective agonist BRL37344 (1 nM-10 microM) failed to produce significant inotropic effects in TG4 left atria. Subsequently, we measured left atrial action potentials accompanying the inotropic changes induced by (-)-isoprenaline. Action potentials tended to have shorter duration in left atria from TG4 mice than from non-transgenic littermate mice. However, (-)-isoprenaline prolonged the duration of 30% repolarisation in atria from non-transgenic littermate but not from TG4 mice, while 90% repolarisation was abbreviated in both groups of atria. Negative inotropic effects of (-)-isoprenaline were also observed in right ventricular preparations. Pertussis toxin-treatment of the mice abolished the negative inotropic effects in left atria and reduced cardiodepression in right ventricle, indicating an involvement of beta(2)-adrenoceptor coupling to PTX-sensitive G-proteins. In additional experiments, designed to study the native murine beta(1)-adrenoceptor function, we used the physiological beta(1)-adrenoceptor agonist (-)-noradrenaline. In the presence of 600 nM ICI 118,551 we failed to find a functional role of the beta(1)-adrenoceptors in left atria, and detected only a marginal contribution to the positive chronotropic effect in right atria. We also investigated the effects of the non-conventional partial agonist (-)-CGP 12177 (0.2 nM-6 microM), which in wild-type mice causes tachycardia through beta(1)-adrenoceptors. In TG4 right atria, however, (-)-CGP 12177-evoked tachycardia was resistant to blockade by CGP 20712A but antagonised by ICI 118,551, consistent with mediation through human beta(2)-adrenoceptors. The results from TG4 mice suggest that the positive and negative inotropic effects of (-)-isoprenaline are mediated through human overexpressed beta(2)-adrenoceptors coupled to G(s) protein and G(i) protein, respectively. The (-)-isoprenaline-evoked shortening of the atrial action potential combined with reduced responses of L-type Ca(2+) current may contribute to the negative inotropic effects. The function of murine cardiac beta(1)-adrenoceptors is suppressed by overexpressed human beta(2)-adrenoceptors.

Identification and characterization of G beta 3s2, a novel splice variant of the G-protein beta 3 subunit

The T-allele of a polymorphism (C825T) in the gene for the G-protein beta 3 subunit (GNB3) is associated with cardiovascular and metabolic disorders, distinct cellular features and altered drug responses. The molecular mechanisms that give rise to this complex phenotype have been linked to the occurrence of G beta 3s, a splice variant of GNB3. G beta 3s is predominantly expressed in cells with the 825T-allele. In the present study we describe the identification and characterization of an additional G beta 3 splice variant referred to as G beta 3s2. Its mRNA is expressed in heart, blood cells and tumour tissue, and its expression is also tightly associated with the GNB3 825T-allele. G beta 3s2 is generated by alternative splicing using non-canonical splice sites. G beta subunits belong to the family of propeller proteins and consist of seven regular propeller blades. Transcripts for G beta 3s2 are lacking 129 bp of the coding sequence of the wild-type G beta 3 protein. Thus the predicted structure consists of only six propeller blades, which resembles the structure of G beta 3s. Co-immunoprecipitation analyses indicated that G beta 3s2 dimerizes with different G gamma subunits, e.g. G gamma 5, G gamma 8(C) and G gamma 12. In Sf9 insect cells, expression of G beta 3s2 together with G gamma 12 enhances receptor-stimulated activation of G alpha(i2). Expression of G beta 3s2 in mammalian cells activated the mitogen-activated protein kinase cascade. Together, these results suggest that G beta 3s2 is a biologically active G beta variant which may play a role in the manifestation of the complex phenotype associated with the 825T-allele.

Reproducibility of dorsal hand vein responses to phenylephrine and prostaglandin F2 alpha using the dorsal hand vein compliance method

Assessment of drug-induced venodilation by the dorsal hand vein compliance method requires stable constriction of the vein. This study was designed to investigate intra- and intersubject reproducibility of the venous preconstriction technique in response to phenylephrine and prostaglandin F2 alpha and to determine the influence of basal vein size. Twelve healthy male nonsmokers participated in a prospective cross-over study. Inter- and intrasubject variability was tested in response to phenylephrine and PGF2 alpha on different study days in the same hand vein. The dose of the respective constrictor causing approximately 80% constriction of the vein (ED80) was determined and infused for another 100 minutes. Actual vein size was measured every 5 minutes. Coefficient of variation and regression analyses were performed to analyze influence of vessel size on ED80 of the respective constrictor. Adjusted constriction levels were stable and well reproducible in all subjects. The intersubject coefficient of variation of ED80 ranged from 0.9% to 6.7% for phenylephrine and from 0.9% to 6.9% for PGF2 alpha. Whereas responses to phenylephrine were independent of basal vein diameter, there was a positive correlation between ED80 of PGF2 alpha and basal vein size. Thus, the hand vein compliance method is a suitable method to study dilatory responses in phenylephrine- or PGF2 alpha-constricted veins with considerable interindividual but small intraindividual variability. However, in such studies, phenylephrine appears to be a more reliable tool than PGF2 alpha.

Therapeutically relevant concentrations of neomycin selectively inhibit P-type Ca2+ channels in rat striatum

The effects of neomycin on voltage-activated Ca(2+) channels (VACCs) were studied by Ca(2+)-dependent K(+)- and veratridine-evoked [3H]dopamine release from rat striatal slices. Neomycin (0.01-1 mM) concentration dependently reduced K(+)-evoked [3H]dopamine release (IC(50) approximately 25 microM), producing approximately 98% inhibition at 1 mM. Contribution of N-, P- and Q-type Ca(2+) channels to this neomycin-sensitive [3H]dopamine release was tested by the combined application of 100 microM neomycin and selective Ca(2+) channel blockers. The effects of neomycin combined with 1 microM of omega-conotoxin GVIA (N-type Ca(2+) channels) or with 100 nM of omega-conotoxin MVIIC (Q-type Ca(2+) channels) were additive, excluding involvement of N- and Q-type Ca(2+) channels. However, the combined effects of neomycin with 30 nM of omega-agatoxin-IVA (P-type Ca(2+) channels) were not additive, suggesting involvement of P-type Ca(2+) channels in neomycin-induced inhibition of [3H]dopamine release. On the other hand, veratridine-evoked [3H]dopamine release was shown to be mediated by Q-type Ca(2+) channels only. In addition, neither the inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase thapsigargin (500 nM) nor the blocker of sarcoplasmic reticulum ryanodine Ca(2+) channels ryanodine (30 microM) modulate veratridine-evoked [3H]dopamine release, suggesting no contribution of intracellular Ca(2+) stores. Neomycin (up to 100 microM) did not affect veratridine-evoked [3H]dopamine release, suggesting that intracellular Ca(2+) stores are not a prerequisite for the action of neomycin. Lack of inhibitory effect of neomycin is taken as additional indirect evidence for the involvement of P-type Ca(2+) channels. In conclusion, therapeutically relevant concentrations of neomycin preferentially block P-type Ca(2+) channels which regulate dopamine release in rat striatum. This block could be responsible for aminoglycoside-induced toxicity.

Changes in morphology and inward rectifier currents in human atrial myocytes depend on culture conditions

Human atrial myocytes were cultured under systematically varied conditions in order to obtain stable cells for future gene manipulation. Transient (I(to)) and sustained outward current (I(so)), and voltage- and muscarinic receptor-activated inward rectifier K(+) currents (I(K1), I(K,ACh)) were measured in freshly isolated cells and after 5 days in culture. Myocytes were grown on polylysin or laminin in medium with or without 10 % serum (medium+S, medium-S). Cultured myocytes dedifferentiated to a greater extent in medium+S than medium-S, but independent of the chemical nature of the adherence surface. Apparent surface area increased in medium+S, whereas membrane capacitance declined under all culture conditions. I(to) of myocytes cultured in medium-S was increased. Myocytes grown on polylysin and laminin exhibited reduced I(K1) current density. Under all culture conditions, I(K,ACh) was attenuated with carbachol but hardly affected with sphingosine-1-phosphate as agonists. In conclusion, morphological and electrophysiological changes depended on serum in the culture medium rather than on adherence surface being coated with laminin or polylysin.

Different responses to drugs against overactive bladder in detrusor muscle of pig, guinea pig and mouse

Direct comparison of experimental data for drugs commonly used in the treatment of overactive bladder is difficult because of possible species differences. In this study, we compare the effects of atropine, propiverine, oxybutynin and tolterodine in strips of pig, guinea pig and mouse detrusor muscle. In the three species, we observed slight differences in potency of carbachol-induced biphasic contractile responses between the species (guinea pig>pig>mouse). Cumulative concentration-response curves for carbachol were shifted to the right by atropine, propiverine, oxybutynin and tolterodine. However, at higher concentrations of the latter three antagonists, the maximum response to carbachol was also reduced. Therefore, propiverine, oxybutynin and tolterodine must have additional pharmacological actions beyond competitive antagonism at muscarinic receptors. Electric field stimulation (30 Hz) of detrusor strips led to contraction amplitudes, which remained constant over time (210 min) in pig, decreased by 17+/-5% in guinea pig, and increased by 28+/-9% in mouse detrusor muscle. Electric field stimulation-evoked contractions were suppressed to 18% of pre-drug control by high concentrations of atropine (10 microM) in pig, but to a much lesser extent in guinea pig and mouse (to 46% and 70%, respectively). In all three species, a myogenic component of contraction was observed in the presence of tetrodotoxin (1 microM). Compared to atropine, the bladder spasmolytic agents propiverine, oxybutynin and tolterodine also reduced electrically evoked contractions in the three species, though higher concentrations were required. The differences in the reported effects of the spasmolytic agents commonly used for treating overactive bladder suggest that drug action is strongly dependent on the species. Thus, a comparison of drug effects is only feasible in the same animal model and the results cannot easily be transferred to humans.

Microvascular endothelial cells from human omentum lack an inward rectifier K+ current

In most macrovascular endothelial cell (EC) preparations, resting membrane potential is determined by the inwardly rectifying K+ current (I(K1)), whereas in microvascular EC the presence of I(K1) varies markedly. Cultured microvascular EC from small vessels of human omentum were examined by means of the voltage-clamp technique to elucidate the putative role of I(K1) in maintaining resting membrane potential. Macrovascular EC from human iliac artery and bovine aorta served as reference. Human omentum EC showed an outwardly rectifying current-voltage relation. Inward current was hardly sensitive to variations of extracellular [K+] and Ba2+ block suggesting lack of I(K1). However, substitution of extracellular [Na+] and/or [Cl-] affected the current-voltage relation indicating that Na+ and Cl- contribute to basal current. Furthermore, outward current was reduced by tetraethylammonium (10 mM), and cell-attached recordings suggested the presence of a Ca2+-activated K+ current. In contrast to human omentum EC, EC from human iliac artery and bovine aorta possessed inwardly rectifying currents which were sensitive to variations of extracellular [K+] and blocked by Ba2+. Thus, the lack of I(K1) in human omentum EC suggests that resting membrane potential is determined by Na+ and Cl- currents in addition to K+ outward currents.

Rate-adaptive pacing using intracardiac impedance shows no evidence for positive feedback during dobutamine stress test

BACKGROUND

The Inos2 DDDR pacemaker senses unipolar intracardiac impedance signals for adapting heart rate to meet the haemodynamic needs of physical activity. Theoretically, such pacing devices could be limited by positive feedback since increase in beating frequency per se enhances contractility. We have addressed this problem in patients with chronotropic incompetence who were subjected to a pharmacological stress test.

METHODS AND RESULTS

Twelve patients with chronically implanted Inos2 DDDR pacemakers were studied using the standard protocol of stress echocardiography. Most of the patients reached the programmable maximum closed-loop rate during the incremental dobutamine challenge. The time courses for increase in as well as for recovery of heart rate were not different from control patients receiving routine diagnostic stress echocardiography.

CONCLUSIONS

In patients implanted with the Inos2 DDDR pacemaker acute stimulation with intravenous dobutamine leads to a robust increase in paced heart rate without any evidence of positive feedback.

What is a good tutorial from the student's point of view? Evaluation of tutorials in a newly established PBL block course "Basics of Drug Therapy"

Problem-based learning (PBL) within a tutorial is the core element in a 6-week block course "Basics in Drug Therapy" recently introduced into the medical curriculum of an eastern German university. Under the supervision of one tutor a group of 6-9 students solved a total of six clinical cases related to drugs used in general anaesthetics; to analgetic/antiphlogistic, antihypertensive, cardiovascular and lipid-lowering drugs; to drugs acting on the central nervous system; and to intoxicating agents. By answering questionnaires students and tutors extensively evaluated each individual case as well as the whole block course. Factors evaluated also included students' and tutors' behaviour during sessions. The purpose of the analysis was to identify factors associated with the students' perception of tutorials with the aim of improving future courses. The ratings which tutorials, cases and tutor performance received by the students differed significantly between 14 groups that could be analysed. A ranking order of groups was established according to the scores they gave for the cases and the three groups with top ratings were further compared with the three groups with lowest ratings. We found significant differences between the two groups in their assessment of tutor performance and perception of team-work during tutorials. The tutors of the groups giving top ratings were considered to have more effectively supported the students' work during tutorials. Furthermore, self-assessment of the tutors judging their own ability to facilitate PBL correlated with their familiarity with case content. The marks achieved in the first national exam (i.e. "Physikum", equivalent to medical school entrance exams), performance in triple jump examination and multiple choice test at the end of the course, or marks obtained in the first state board exam after the course, did not differ between the two groups. From students' point of view, the main characteristics for a good tutorial as far as tutors are concerned consist of allowing enough time for discussion, accepting students as partners, refraining from interference and having expertise. As a consequence we will increase our efforts to optimise case quality as well as support and training of tutors in order to improve our new PBL course.