Informatics United

Objectives: Medical informatics, neuroinformatics and bioinformatics provide a wide spectrum of research. Here, we show the great potential of synergies between these research areas on the basis of four exemplary studies where techniques are transferred from one of the disciplines to the other.

Methods: Reviewing and analyzing exemplary and specific projects at the intersection of medical informatics, neuroinformatics, and bioinformatics from our experience in an interdisciplinary research group. Results: Synergy emerges when techniques and solutions from medical informatics, bioinformatics, or neuroinformatics are successfully applied in one of the other disciplines. Synergy was found in 1. the modeling of neurophysiological systems for medical therapy development, 2. the use of image processing techniques from medical computer vision for the analysis of the dynamics of cell nuclei, and 3. the application of neuroinformatics tools for data mining in bioinformatics and as classifiers in clinical oncology. Conclusions: Each of the three different disciplines have delivered technologies that are readily applicable in the other disciplines. The mutual transfer of knowledge and techniques proved to increase efficiency and accuracy in a manifold of applications. In particular, we expect that clinical decision support systems based on techniques derived from neuro- and bioinformatics have the potential to improve medical diagnostics and will finally lead to a personalized delivery of healthcare.

Nutritional impact on the plasma metabolome of rats

Metabolite profiling (metabolomics) elucidates changes in biochemical pathways under various conditions, e.g., different nutrition scenarios or compound administration. BASF and metanomics have obtained plasma metabolic profiles of approximately 500 compounds (agrochemicals, chemicals and pharmaceuticals) from 28-day rat studies. With these profiles the establishment of a database (MetaMap(®)Tox) containing specific metabolic patterns associated with many toxicological modes of action was achieved. To evaluate confounding factors influencing metabolome patterns, the effect of fasting vs. non-fasting prior to blood sampling, the influence of high caloric diet and caloric restriction as well as the administration of corn oil and olive oil was studied for its influence on the metabolome. All mentioned treatments had distinct effects: triacylglycerol, phospholipids and their degradation product levels (fatty acids, glycerol, lysophosphatidylcholine) were often altered depending on the nutritional status. Also some amino acid and related compounds were changed. Some metabolites derived from food (e.g. alpha-tocopherol, ascorbic acid, beta-sitosterol, campesterol) were biomarkers related to food consumption, whereas others indicated a changed energy metabolism (e.g. hydroxybutyrate, pyruvate). Strikingly, there was a profound difference in the metabolite responses to diet restriction in male and female rats. Consequently, when evaluating the metabolic profile of a compound, the effect of nutritional status should be taken into account.

The use of metabolomics for the discovery of new biomarkers of effect

Will metabolomics have a greater chance of success in toxicology and biomarker assessment than genomics and proteomics? Metabolomics has the advantage that (1) it analyses the last step in a series of changes following a toxic insult, (2) many of the metabolites have a known function and (3) changes are detectable in blood. If the analysis of a great number of individual organs can be replaced by one matrix then this will provide significant advantages (less invasive method, no need to kill animals, time course analysis possible). We have chosen to perform the analysis of blood metabolites in such a way as to minimize the risk of artifacts and to have a high number of known metabolites. We have also reduced the amount of variation in the biological system as well as during analysis. In a series of proof of concept studies it could be demonstrated that (1) the metabolome of control animals was stable of a period of nearly 1 year, with a remarkable differentiation between males and females, (2) a dose response relationship in metabolome changes was induced by phenobarbital and that (3) different modes of action could be distinguished by blood metabolome analysis. To investigate the potential of metabolomics to find biomarkers or specific patterns of change we have analyzed the blood metabolome of rats treated with HPPD inhibitors, a novel class of herbicides. The results demonstrated that a single metabolite, tyrosine, can be used as a biomarker. In addition to tyrosine we also found a specific pattern of change that involved nine metabolites. Though the extent of change was less than for tyrosine the consistent change of these metabolites is diagnostic for this (toxicological) mode of action.

Informatics united: exemplary studies combining medical informatics, neuroinformatics and bioinformatics

OBJECTIVES

Medical informatics, neuroinformatics and bioinformatics provide a wide spectrum of research. Here, we show the great potential of synergies between these research areas on the basis of four exemplary studies where techniques are transferred from one of the disciplines to the other.

METHODS

Reviewing and analyzing exemplary and specific projects at the intersection of medical informatics, neuroinformatics, and bioinformatics from our experience in an interdisciplinary research group.

RESULTS

Synergy emerges when techniques and solutions from medical informatics, bioinformatics, or neuroinformatics are successfully applied in one of the other disciplines. Synergy was found in 1. the modeling of neurophysiological systems for medical therapy development, 2. the use of image processing techniques from medical computer vision for the analysis of the dynamics of cell nuclei, and 3. the application of neuroinformatics tools for data mining in bioinformatics and as classifiers in clinical oncology.

CONCLUSIONS

Each of the three different disciplines have delivered technologies that are readily applicable in the other disciplines. The mutual transfer of knowledge and techniques proved to increase efficiency and accuracy in a manifold of applications. In particular, we expect that clinical decision support systems based on techniques derived from neuro- and bioinformatics have the potential to improve medical diagnostics and will finally lead to a personalized delivery of healthcare.

Haemodialysis monocytopenia: differential sequestration kinetics of CD14+CD16+ and CD14++ blood monocyte subsets

In peripheral blood the majority of circulating monocytes present a CD14highCD16- (CD14++) phenotype, while a subpopulation shows a CD14lowCD16+ (CD14+CD16+) surface expression. During haemodialysis (HD) using cellulosic membranes transient leukopenia occurs. In contrast, synthetic biocompatible membranes do not induce this effect. We compared the sequestration kinetics for the CD14+CD16+ and CD14++ monocyte subsets during haemodialysis using biocompatible dialysers. Significant monocytopenia, as measured by the leucocyte count, occurred only during the first 30 min. However, remarkable differences were observed between the different monocyte subsets. CD14++ monocyte numbers dropped to 77 +/- 13% of the predialysis level after 15 min, increasing to > or = 93% after 60 min. In contrast, the CD14+CD16+ subset decreased to 33 +/- 15% at 30 min and remained suppressed for the course of dialysis (67 +/- 11% at 240 min). Approximately 6 h after the end of HD the CD14+CD16+ cells returned to basal levels. Interestingly, the CD14+CD16+ monocytes did not show rebound monocytosis while a slight monocytosis of CD14++ monocytes was occasionally observed during HD. A decline in CD11c surface density paralleled the sequestration of CD14+CD16+ monocytes. Basal surface densities of important adhesion receptors differed significantly between the CD14+CD16+ and CD14++ subsets. In conclusion, during HD the CD14+CD16+ subset revealed different sequestration kinetics, with a more pronounced and longer disappearance from the blood circulation, compared with CD14++ monocytes. This sequestration kinetics may be due to a distinct surface expression of major adhesion receptors which facilitate leucocyte-leucocyte, as well as leucocyte-endothelial, interactions.

Learning cortical topography from spatiotemporal stimuli

Stimulus representation is a functional interpretation of early sensory cortices. Early sensory cortices are subject to stimulus-induced modifications. Common models for stimulus-induced learning within topographic representations are based on the stimuli's spatial structure and probability distribution. Furthermore, we argue that average temporal stimulus distances reflect the stimuli's relatedness. As topographic representations reflect the stimuli's relatedness, the temporal structure of incoming stimuli is important for the learning in cortical maps. Motivated by recent neurobiological findings, we present an approach of cortical self-organization that additionally takes temporal stimulus aspects into account. The proposed model transforms average interstimulus intervals into representational distances. Thereby, neural topography is related to stimulus dynamics. This offers a new time-based interpretation of cortical maps. Our approach is based on a wave-like spread of cortical activity. Interactions between dynamics and feedforward activations lead to shifts of neural activity. The psychophysical saltation phenomenon may represent an analogue to the shifts proposed here. With regard to cortical plasticity, we offer an explanation for neurobiological findings that other models cannot explain. Moreover, we predict cortical reorganizations under new experimental, spatiotemporal conditions. With regard to psychophysics, we relate the saltation phenomenon to dynamics and interaction in early sensory cortices and predict further effects in the perception of spatiotemporal stimuli.

Self-organizing maps for visual feature representation based on natural binocular stimuli

We model the stimulus-induced development of the topography of the primary visual cortex. The analysis uses a self-organizing Kohonen model based on high-dimensional coding. It allows us to obtain an arbitrary number of feature maps by defining different operators. Using natural binocular stimuli, we concentrate on discussing the orientation, ocular dominance, and disparity maps. We obtain orientation and ocular dominance maps that agree with essential aspects of biological findings. In contrast to orientation and ocular dominance, not much is known about the cortical representation of disparity. As a result of numerical simulations, we predict substructures of orientation and ocular dominance maps that correspond to disparity maps. In regions of constant orientation, we find a wide range of horizontal disparities to be represented. This points to geometrical relations between orientation, ocular dominance, and disparity maps that might be tested in experiments.

[When lipids increase in dialysis: the role of heparin! Standard heparin increases, low-molecular-weight heparin lowers triglycerides]

Dyslipoproteinemia in patients on hemodialysis is characterized by a decrease in high density lipoprotein (HDL), cholesterol, hypertriglyceridemia, increased triglyceride-rich lipoproteins, such as very low density lipoprotein (VLDL) and intermediate density lipoprotein (IDL), a higher proportion of the small dense low density lipoprotein (small dense LDL) subfraction, and higher lipoprotein(a) concentration. The reason for the changes in triglyceride metabolism is an increase in the production of apolipoprotein B, and a decrease in the metabolism of VLDL as a consequence of decreased endothelial cell delipidation. The endothelial lipoprotein lipase, which plays a major role in this process, is released by heparin, which is essential for the function of the enzyme. Repeated administration of heparin for anticoagulation during hemodialysis apparently leads to an LPL depletion in the endothelium. This results in further exhaustion of lipolysis. Clinical studies in hemodialysis patients with high triglyceride and cholesterol levels indicate that a change from standard heparin to low-molecular-weight heparin improves the lipid profile by lowering triglycerides and cholesterol.

Extracellular matrix structure after heart transplantation

Following heart transplantation remodeling of the donor heart causes changes in the extracellular myocardial matrix. We investigated 20 right ventricular endomyocardial biopsies taken 17+/-4 days (group I, n=9) and 63+/-13 days (group II, n=11) after heart transplantation from 16 patients transplanted for end-stage cardiomyopathy (15 dilated/1 ischemic). Immunohistochemical staining for collagen I, collagen III, collagen IV, and fibronectin was used. Evaluation was performed at a magnification of 400x using a computer-assisted image analyzing system measuring the relative area stained by the immunoperoxidase method, the number of cells in the given area, and the total area. Collagen I per cell was 13.9+/-5.9 microm2 in group I and increased significantly 66+/-13 days after heart transplantation in the perimysium around the myocardial cells as well as in the endocardium to 49.9+/-15.1 microm2 (P<0.05). No quantitative change in collagen III was noted (75.7+/-12.4 versus 75.5+/-16.0 microm2 n.s.). Collagen IV was found in the perimysial, in the capillary bed and in the vascular network. Significant quantitative change in the amount of collagen IV was not found (64.1+/-12.6 versus 61.0+/-8.9 microm2). Fibronectin was found in the entire perimysial extracellular matrix and in the endocardium in relationship with collagen I and III. An increased amount of fibronectin from 87.09+/-9.9 microm2 (group I) to 140.8+/-17.9 microm2 (group II, P<0.05) was found. The cell area and cell diameters were not significantly different (group I; cell area 772+/-227 microm2, diameter 31.3 microm; group II; cell area 776+/-224 microm2, diameter 31.4 microm). It is concluded that remodeling of the donor heart after transplantation is characterized by a specific increase in collagen I and fibronectin, whereas a change in other collagen subtypes was not observed.

Variations of segmental endothelium-dependent and endothelium-independent vasomotor tone after cardiac transplantation (qualitative changes in endothelial function)

Endothelial dysfunction is a common phenomenon after cardiac transplantation. However, qualitative differences in endothelial vasoregulation at different coronary segments and at different postoperative times have rarely been explored. To uncover the functional variations of endothelium responses we infused the endothelium-dependent vasodilator acetyl-choline (50 micrograms) followed by the endothelium-independent vasodilator 3-morpholinosydnonimine (SIN-1) (1 mg; 16 patients) or nitroglycerin (0.3 mg; 14 patients) sequentially into the left coronary artery. We investigated the responses of 120 nonstenotic coronary segments (proximal and distal left anterior descending and right circumflex coronary arteries) in 30 patients with quantitative angiography (group 1: 13 patients, 12 +/- 1 months after cardiac transplantation; group 2: 17 patients, 55 +/- 3 months after cardiac transplantation). Continuous-flow measurement was performed to exclude significant reduction of microvascular response influencing epicardial dilation. Five responses to acetylcholine administration followed by nitrates were observed. On the one end of the spectrum, segments dilate to acetylcholine administration with no further dilation to exogenous nitric oxide, indicating completely preserved endothelial function. On the other end, segments constrict to acetylcholine with no change after endogenous nitric oxide, reflecting a defective endothelial and defective smooth muscle function. The different patterns of coronary vasomotor lone responses to endogenous nitric oxide followed by exogenous nitric oxide represent different degrees of endothelial function after cardiac transplantation. In addition, the functional assessment of endothelial integrity shows qualitative time-dependent differences between proximal and distal coronary parts. The existence of coronary segments with functioning endothelium indicates that the latter is not diffusely disturbed in all cardiac transplant recipients and that the endothelial damage is perhaps not irreversibly lost.

[Multiplicity of clinical symptoms and manifestations of unruptured aneurysms of the sinus of Valsalva--3 case reports]

Sinus Valsalva aneurysms belong to the less common congenital or acquired structural cardiac anomalies. However, in patients with known cardiac anomalies and uncertain or uncharacteristic cardiac symptoms the existence of a sinus Valsalva aneurysm must be taken into consideration. A sinus Valsalva aneurysm can be clinically silent as in the case of the 56-year-old patient with an accompanying bacterial endocarditis. An increasing aortic regurgitation after dilatation of a coarctation of the aorta can also proceed with an ecstasy of the ascending aorta and an aneurysm of the sinus Valsalva (case 2). Furthermore, a rapid dilatation of a non-ruptured sinus Valsalva aneurysm can cause a severe compression of coronary arteries with subsequent myocardial infarction, as in the 27-year-old patient with congenital aortic stenosis and acute endocarditis in case 3.

Biochemical and immunological properties of urinary angiotensinase A and dipeptidylaminopeptidase IV. Their use as markers in patients with renal cell injury

Dipeptidyl peptidase IV (EC 3.4.14.5) and angiotensinase A (EC 4.4.11.7) were purified to homogeneity from pooled urine concentrate of patients with renal damage, using ultrafiltration, ammonium sulphate precipitation, lectin affinity chromatography, FPLC-ion-exchange(Mono-Q-)chromatography, and FPLC-gel filtration (Superdex). Based on the specific enzyme activity of the starting material, dipeptidyl peptidase IV was enriched 1629 fold, angiotensinase A 1183 fold. The relative molecular masses, Michaelis constants and isoelectric points were determined. Negative staining of the purified enzymes revealed globular proteins (5-7 nm). Antisera raised against dipeptidyl peptidase IV and angiotensinase A reacted specifically with tubular and, in the case of anti-angiotensinase A sera, with tubular and glomerular structures. In addition, urinary membrane vesicles of proximal tubule origin were eluted with the void volume (Superdex-gel filtration), indicating heavy epithelial cell disintegration. Both soluble tissue enzymes (dipeptidyl peptidase IV, angiotensinase A) and vacuolar blebs shed from epithelia contribute to proteinuria, as was shown in patients with glomerulonephritis, interstitial nephritis, diabetic nephropathy and, for angiotensinase A, in patients with essential arterial hypertension.

Excitation-contraction coupling in voltage-clamped cardiac myocytes

Myocytes isolated from guinea pig ventricles were voltage-clamped using patch pipettes in the whole-cell configuration. For proper voltage control fast Na+ current was blocked by TTX or inactivated by an appropriate prepulse. Zero-load cell shortening was monitored by a photoelectric device. The mechanical response to a short depolarizing clamp was mainly a phasic (transient) contraction. Long-lasting depolarizations caused a tonic (sustained) shortening of a cell. Different clamp patterns were used to study the mode of activation of phasic contraction. 1) With a constant Ca2+ preload established by a train of conditioning pulses, the shortening-voltage relation measured with test pulses of varying height was a bell-shaped curve reflecting the slow inward current (ICa)-voltage relation. The test pulse had a striking influence on the first contraction of the following conditioning series, resulting in an S-shaped relation between post-test contraction and test potential. 2) With series of identical clamps of varying height, steady-state contraction was maximal around 40 mV and not in proportion to ICa. In these measurements Ca2+ preload was likely to increase with increasing potential. It is concluded that ICa initiates phasic contraction by inducing a release of Ca2+ from internal stores while replenishment of the stores is largely determined by an electrogenic transsarcolemmal Na+-Ca2+ exchange. The data suggest that Na+-Ca2+ exchange is not only involved in long-term changes of cardiac contractility but also in beat-to-beat regulation.

Photoelectric recording of mechanical responses of cardiac myocytes

A method to monitor contraction of isolated myocytes by transmicroscopic photometry is illustrated. Two photodiodes are mounted inside an inverse microscope used for visual control of a cell. Illumination of one diode varies in proportion to changes in cell length. The contraction signal is amplified in a comparator circuit. Spatial resolution of the device is in the order of 1 micron which corresponds to about 5% of cell shortening in the fully activated state of contraction. The method was tested on isolated myocytes from guinea-pig ventricle. Optical records of contraction in response to action potentials or during voltage clamp compare well with the contractile behavior of multicellular preparations.

The response of the calcium dependent myocardial contractility to the new cardiotonic agent sulmazole

In vitro and in vivo animal experiments to test the positive inotropism of 2-[(2-methoxy-4-methylsulfinyl)-phenyl]-1H-imidazo[4,5-b]pyridine (sulmazole, AR-L 115 BS) have shown increases of contractility of up to 220% in the mmol/l concentration range. Comparative studies designed to attribute to sulmazole mechanisms established for other positive inotropic drugs have been negative for digitalis and beta-adrenoceptor agonists but showed slight similarities to xanthine derivatives. To explore the yet unknown mode of action of sulmazole we investigated the interaction between sulmazole and non-humoral/non-drug interventions for positive inotropism in vitro. Peak tension obtained in solutions varying in calcium content between 0.36 and 7.2 mmol/l Ca2+ were normalized with respect to the tension measured at standard 1.8 mmol/l Ca2+ for control and sulmazole treated preparations (sheep interventricularis cordis muscle and guinea pig papillary muscle). Double logarithmic plots of normalized tension versus calcium concentration were linear and superimposable for control, 10(-4), and 10(-3) mol/l sulmazole. In depleted sodium solutions (choline substitution) normalized peak tension of control and 10(-4) mol/l sulmazole containing solutions did not significantly differ. In length-tension plots sulmazole behaved as did increased extracellular calcium. We assume that the positive inotropism of sulmazole is primarily due to a displacement of calcium at non-specific ligand sites within the protein matrix of the cytosol which effectively raises the apparent intracellular calcium activity.

Voltage clamp study of the action of the new cardiotonic compound acrihellin on fast and slow inward currents in cardiac Purkinje fibres

The action of 3 beta,5,14-trihydroxy-19-oxo-5 beta-bufa-20,22-dienolide-3-(3-methylcrotonate) (D 12316, acrihellin) on the fast and slow inward currents of sheep cardiac Purkinje fibres was analyzed by means of a conventional double microelectrode voltage clamp technique. Acrihellin diminished action potential duration and the plateau phase. Effective refractory periods were shortened in relation to the decrease in action potential duration. The upstroke velocity was slightly decreased; a systematic study of dV/dtmax, however, did not reveal any specific local anaesthetic side-effect of the drug. The decrease in the plateau phase of the action potential was due to a reduction in time-dependent slow inward current. The kinetics and the reversal potential of the slow inward current was not affected by acrihellin, but only the current voltage relationship, which was decreased.

Tl+-ions: influence on cardiac contractility

Attention has recently focussed on the heavy metal thallium as an environmental contaminant of increasing importance. From accidental or suicidal ingestions of thallium it has been known for many years that cardiovascular disorders regularly emerge, and for this reason, a variety of investigations of cardiological interest have been conducted. Amongst these, the effects of thallium on the contractile force of isolated myocardial tissues have been studied. Previous experiments were all carried out at concentrations far beyond those encountered during intoxication and yielded controversal data. We therefore reinvestigated the effects of thallium on myocardial tissue at levels between l0(-8) and 10(-3) M, thus covering the range of thallium concentrations encountered after uptake from a polluted environment through those seen after unintentional or intentional ingestion to levels at which previous studies were performed. Sheep interventricular cordis muscles were used at a stimulation frequency of 0.4 Hz showing three types of responses to thallium exposure. From a total of 32 experiments in 15% of all cases thallium caused a persistent increase in contractility which tended to decrease with time and thallium concentration but always remained greater than the control value. 50% of the experiments showed a progressive loss of contractile force with time and thallium concentration, despite transient increases in contractility which lasted for only 2-5 min after the application of each new thallium concentration. A combination of these types of reaction was observed in the remaining experiments in that a low thallium concentrations myocardial contractility increased considerably but then decreased progressively with time and thallium concentration. Guinea pig papillary muscles were used to test one thallium concentration only for up to 75 min. At 10(-8) M there was no effect, at 10(-7), 10(-6), 10(-5) M Tl+ there were positive inotropic transients followed by an inotropic decay; at 10(-4) M Tl+ only a progressive decrease of contractility was observed. The relationships between time and thallium concentration at different rates of stimulation were examined in two series of experiments at 0.1, 0.2, 0.4, 1.0, and 2.0 Hz. The effects of thallium were accelerated with increased beating rate and the decay of contraction also proceeded to markedly lower levels. In the rested state, thallium was also very effectual; this was illustrated in two series of experiments in which after 10 min intervals of quiescency 15 or more test stimuli were applied at different beating rates (0.1 to 2.0 Hz). The configurations of the resulting staircase phenomena were analyzed with respect to control behavior for each frequency of the test stimuli and for each thallium concentration. These results suggested an involvement of the slow inward current. The steady state values after quiescency showed a pronounced thallium-induced decay similar to that obtained at high constant stimulation rates...

Tl+-ions: effects on the automaticity of sinoatrial tissue and dV/dtmax and iK2 of cardiac Purkinje fibres

The disrhythmic effects of thallium were investigated in various cardiac tissues to determine the primary site of intoxication with respect to ensuing arrhythmias. In isolated cardiac tissue Lameijer and van Zwieten [1] had contended that arrhythmias arise from the sinus node after thallium poisoning. To test this hypothesis we administered concentrations of Tl+ between 10(-7) and 10(-4) M to guinea pig sinoatrial preparations, to guinea pig papillary muscles and to sheep cardiac Purkinje fibres. In sinoartial preparations thallium provoked increases and decreases of spontaneous beat frequency which were not linked to corresponding changes in contractile force. In conductive tissue, Purkinje fibres, the inactivation kinetics of the fast sodium current and the pacemaker current iK2 were investigated by voltage clamp experiments. Here, thallium was seen to be essentially without toxic effects which could account for arrhythmias. In ventricular muscle actions potentials and contractile force were recorded simultaneously. Here again, ventricular arrhythmias are not to be expected from thallium intoxication in rather high concentrations. The findings support the view that arrhythmogenic effects of thallium are restricted to the sinus node.

Tl+-ions: comparison on the effects on the slow inward current and contractility of ventricular tissues

To conclude our investigation of thallium effects on cardiac tissues, we studied the slow inward current of sheep cardiac Purkinje fibres exposed to 10-7 to 10-5 ᴍ Tl+ for extended periods of up to 80 min. Our previous results had suggested a possible involvement of the slow inward current during thallium intoxication: a) the modification of contractility staircases observed during thallium exposure, b) action potential recordings of ventricular muscle, c) changes in spontaneous beating in sino-atrial preparations. The thallium levels chosen were between those yielding strong positive inotropic transients and those producing a marked long­term decay of contraction force.

The slow inward current was measured using a conventional two-microelectrode-technique and the standard voltage clamp protocol for this current system. The experimental work was restricted to the determination of d∞, the kinetics of activation of the slow inward current and of īsi, the current voltage relation of the current system. This was necessary since the effects of thallium were known to be short-lived and therefore frequent repeat runs of the voltage clamp program had to be performed in order to obtain the time courses of possible transient changes.

The results showed that the slow inward current was first increased and then declined at the low concentration of 10-7 ᴍ Tl+. At 10-5 m Tl+ the initial increase was smaller, whereas the decay of the slow inward current proceeded to lower values. Comparison with contractility measure­ments at the same concentrations of thallium showed a distinct parallelism between changes of the slow inward current and myocardial contractility. Despite this apparent relationship, we do not conclude that the contractile events are primarily a result of changes of the slow inward current, since thallium does not seem to specifically alter the parameters of the slow inward current at the membrane level.

A voltage clamp study of the effects of AR-L 115 BS on the pacemaker current of cardiac Purkinje fibres

The new cardiotonic agent AR-L 115 BS was investigated by means of the double-micro-electrode voltage clamp technique on sheep cardiac Purkinje fibres. Clinical and pharmacological studies show that AR-L 115 BS increases heart rate as a side effect at medium to high therapeutic doses. The classical analysis of the pacemaker current was therefore performed to study the possible mechanism of this effect at a cellular level. The kinetic parameter s infinity and the reversal potential of the pacemaker current were shifted in the depolarizing direction after exposure to AR-L 115 BS. Peak values of the fully activated pacemaker current were either increased or diminished, while potassium leakage was slightly increased. These results are not related to the action of AR-L 115 BS on beta-adrenergic receptors but possibly due to enhanced intracellular calcium (see third paper in this series). Despite its tendency to increase heart rate, high concentrations of AR-L 115 BS should not be expected to promote arrhythmias in the Purkinje system since the electrophysiological effects tend to counteract each other.