Search PubMed⌕ Search

Biomedical subjects

Ursula Ravens

Publications and source records attributed to Ursula Ravens.

At least 19 recordsLinked to original sources
PubMed2004

Role of IKur in controlling action potential shape and contractility in the human atrium: influence of chronic atrial fibrillation.

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 (APD90) was shorter in AF than in SR (300+/-16 ms, n=6, versus 414+/-10 ms, n=15), whereas APD20 was longer (35+/-9 ms in AF versus 5+/-2 ms in SR, P<0.05). 4-Aminopyridine (5 micromol/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 APD90 from 414+/-10 to 350+/-10 ms in SR but prolonged APD90 from 300+/-16 to 320+/-13 ms in AF. Similar results were obtained with AVE 0118 (6 micromol/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.

4-Aminopyridine↗
PubMed2004

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.

Animals↗
PubMed2004

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.

Animals↗
PubMed2003

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.

Bone Marrow↗
PubMed2003

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.

Alleles↗
PubMed2003

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.

Action Potentials↗
PubMed2003

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.

Animals↗
PubMed2003

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.

Adaptation, Physiological↗
PubMed2003

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.

Action Potentials↗
PubMed2002

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.

Animals↗
PubMed2002

Specific beta(2)AR blocker ICI 118,551 actively decreases contraction through a G(i)-coupled form of the beta(2)AR in myocytes from failing human heart.

BACKGROUND: We have observed direct (noncatecholamine-blocking) negative inotropic effects of the selective beta(2)-adrenoceptor (AR) antagonist ICI 118,551 in myocytes from failing human ventricle. In this study we characterize the effect in parallel in human myocytes and in myocytes from animal models where beta(2)ARs or G(i) proteins are overexpressed. METHODS AND RESULTS: Enzymatically isolated, superfused ventricular myocytes were exposed to betaAR agonists and antagonists/inverse agonists, and contraction amplitude was measured. ICI 118,551 decreased contraction in ventricular myocytes from failing human hearts by 45.3+/-4.1% (n=20 hearts/31 myocytes, P<0.001) but had little effect in nonfailing hearts (4.9+/-4%, n=5 myocytes/3 hearts). Effects were significantly larger in patients classified as end-stage. Transgenic mice with high beta(2)AR number and increased G(i) levels had normal basal contractility but showed a similar negative inotropic response to ICI 118,551. Overexpression of human beta(2)AR in rabbit myocytes using adenovirus potentiated the negative inotropic effect of ICI 118,551. In human, rabbit, and mouse myocytes, the negative inotropic effects were blocked after treatment of cells with pertussis toxin to inactivate G(i), and overexpression of G(i)alpha(2) induced the effect de novo in normal rat myocytes. CONCLUSIONS: We hypothesize that ICI 118,551 binding directs the beta(2)AR to a G(i)-coupled form and away from the G(s)-coupled form (ligand-directed trafficking). ICI 118,551 effectively acts as an agonist at the G(i)-coupled beta(2)AR, producing a direct negative inotropic effect. Conditions where beta(2)ARs are present and G(i) is raised (failing human heart, TGbeta(2) mouse heart) predispose to the appearance of the negative inotropic effect.

Adrenergic beta-2 Receptor Antagonists↗
PubMed2002

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.

Education, Medical, Undergraduate↗
PubMed2002

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.

Acetylcholine↗