Calcium-activated Cl(-) current contributes to delayed afterdepolarizations in single Purkinje and ventricular myocytes.

BACKGROUND: The ionic mechanism underlying the transient inward current (I(ti)), the current responsible for delayed afterdepolarizations (DADs), appears to be different in ventricular myocytes and Purkinje fibers. In ventricular myocytes, I(ti) was ascribed to a Na(+)-Ca(2+) exchange current, whereas in Purkinje fibers, it was additionally ascribed to a Cl(-) current and a nonselective cation current. If Cl(-) current contributes to I(ti) and thus to DADs, Cl(-) current blockade may be potentially antiarrhythmogenic. In this study, we investigated the ionic nature of I(ti) in single sheep Purkinje and ventricular myocytes and the effects of Cl(-) current blockade on DADs. METHODS AND RESULTS: In whole-cell patch-clamp experiments, I(ti) was induced by repetitive depolarizations from -93 to +37 mV in the presence of 1 micromol/L norepinephrine. In both Purkinje and ventricular myocytes, I(ti) was inward at negative potentials and outward at positive potentials. The anion blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) blocked outward I(ti) completely but inward I(ti) only slightly. The DIDS-sensitive component of I(ti) was outwardly rectifying, with a reversal close to the reversal potential of Cl(-) currents. Blockade of Na(+)-Ca(2+) exchange by substitution of extracellular Na(+) by equimolar Li(+) abolished the DIDS-insensitive component of I(ti). DIDS reduced both DAD amplitude and triggered activity based on DADs. Conclusions-In both Purkinje and ventricular myocytes, I(ti) consists of 2 ionic mechanisms: a Cl(-) current and a Na(+)-Ca(2+) exchange current. Blockade of the Cl(-) current may be potentially antiarrhythmogenic by lowering DAD amplitude and triggered activity based on DADs.

Pacemaker synchronization of electrically coupled rabbit sinoatrial node cells.

The effects of intercellular coupling conductance on the activity of two electrically coupled isolated rabbit sinoatrial nodal cells were investigated. A computer-controlled version of the "coupling clamp" technique was used in which isolated sinoatrial nodal cells, not physically in contact with each other, were electrically coupled at various values of ohmic coupling conductance, mimicking the effects of mutual interaction by electrical coupling through gap junctional channels. We demonstrate the existence of four types of electrical behavior of coupled spontaneously active cells. As the coupling conductance is progressively increased, the cells exhibit: (a) independent pacemaking at low coupling conductances, (b) complex dynamics of activity with mutual interactions, (c) entrainment of action potential frequency at a 1:1 ratio with different action potential waveforms, and (d) entrainment of action potentials at the same frequency of activation and virtually identical action potential waveforms. The critical value of coupling conductance required for 1:1 frequency entrainment was <0.5 nS in each of the five cell pairs studied. The common interbeat interval at a relatively high coupling conductance (10 nS), which is sufficient to produce entrainment of frequency and also identical action potential waveforms, is determined most by the intrinsically faster pacemaker cell and it can be predicted from the diastolic depolarization times of both cells. Evidence is provided that, at low coupling conductances, mutual pacemaker synchronization results mainly from the phase-resetting effects of the action potential of one cell on the depolarization phase of the other. At high coupling conductances, the tonic, diastolic interactions become more important.

The cellular basis of intrinsic sinus node recovery time.

STUDY OBJECTIVE: The aim was to investigate the mechanism of the intrinsic sinus node recovery time. DESIGN: The effect of 2 min periods of 20% and 50% overdrive on the electrical activity of fibres in the sinoatrial node was studied in isolated atria of rabbits under complete autonomic blockade (atropine 3 x 10(-6) M and propranolol 3 x 10(-7) M). EXPERIMENTAL MATERIAL: Rabbits (New Zealand white) of either sex up to 3 kg weight were used. MEASUREMENTS AND MAIN RESULTS: The first returning cycle after overdrive is prolonged not only by the time needed for retrograde plus antegrade conduction but also by a delay in impulse formation (overdrive suppression). During pacing, action potential duration, amplitude, maximum diastolic potential (only in primary pacemaker fibres), and diastolic depolarisation rate were all decreased. Action potential duration, amplitude and maximum diastolic potential returned to control value during the first cycle following a period of overdrive, but diastolic depolarisation remained depressed during many consecutive cycles. In primary pacemaker fibres, diastolic depolarisation appeared to be depressed throughout diastole. In latent pacemaker fibres diastolic depolarisation was depressed only in the second part of the diastole. CONCLUSIONS: Sinus node recovery time has two components: (1) a conduction component of both retrograde and antegrade conduction, and (2) a depression of the automaticity (= overdrive suppression), which is only due to a slowing of diastolic depolarisation.

Electrophysiological effects of alinidine (ST 567) on sinoatrial node fibres in the rabbit heart.

In a study of the electrophysiological effects of alinidine a concentration of 0.7-14.3 mumol X litre-1 decreased the rate of diastolic depolarisation and prolonged especially the terminal part of the action potential in the rabbit sinoatrial node. It did not induce pacemaker shifts since the effects were not restricted to the primary pacemaker or the central nodal area but were evident in the more peripheral nodal region. The substitution of chlorine ions by other anions did not prevent the decrease in the rate of diastolic depolarisation due to alinidine but did prevent the effect on the action potential duration. The decreased chronotropic action of alinidine in low chlorine Tyrode solution was, however, caused by a shift of pacemaker dominance towards an atrial pacemaker. This pacemaker shift concealed the response of the primary pacemaker to alinidine in low chlorine Tyrode. Blockade of the pacemaker current of if by caesium prevented neither the alinidine effect on the diastolic depolarisation completely nor its effect on the action potential duration, but blockade of if probably was one of the determinants of the action of alinidine. It cannot be excluded that alinidine interferes with still another current than if. Alinidine decreased the chronotropic responses to adrenaline and to acetylcholine and also prevented pacemaker shifts due to these substances.

Effect of transmural vagal stimulation on electrotonic current spread in the rabbit sinoatrial node.

OBJECTIVE: The effect of vagal stimulation on the decay of electrotonic potential caused by intracellular current injection and on input resistance was measured in the sinoatrial node of isolated rabbit right atria. METHODS: Studies were performed on New Zealand White rabbits weighing approximately 2-3 kg. Vagal stimulation was achieved by transmural stimulation of intramural nerve fibres in the presence of propranolol. A K+ perfused suction electrode was used to inject hyperpolarising current pulses; input resistance was measured by means of a double barrel microelectrode. RESULTS: Vagal stimulation which caused a 14-20% increase of cycle length diminished electronic potential significantly by a decrease of membrane resistance. The input resistance of the sinoatrial node was not affected. Space constant values calculated by using either a one or a two dimensional model of electrotonic current spread were decreased on average by 13% and 14% respectively. CONCLUSIONS: The results from this study show that vagal stimulation which gave rise to a moderate negative chronotropic effect and marked changes in action potential configuration of nodal fibres affects the electrotonic interaction within the sinoatrial node. This may have consequences for the electrical activity and synchronisation of the sinoatrial nodal fibres.

Differences between rabbit sinoatrial pacemakers in their response to Mg, Ca and temperature.

The rabbit sinoatrial node is functionally inhomogeneous with respect to its response to changes in Mg concentration (0.6 to 6.0 mmol X litre-1) and in Ca concentration (1.1 to 2.2 mmol X litre-1) and to changes in experimental temperature (30 to 38 degrees C). High Mg (6.0 mmol X litre-1) stabilises the position of the leading pacemaker. This pacemaker decelerates under high Mg, but the subsidiary ones decelerate even more. Consequently when a subsidiary pacemaker turns dominant--eg under low Ca or at low temperature--an enhanced chronotropic response to high Mg is observed. The superior (cranial) part of the rabbit sinoatrial node is more responsive to changes in Ca concentration than the inferior (caudal) part. The same holds true for changes in temperature.

Electrophysiological features of the mouse sinoatrial node in relation to connexin distribution.

OBJECTIVE: The sinoatrial (SA) node consists of a relatively small number of poorly coupled cells. It is not well understood how these pacemaker cells drive the surrounding atrium and at the same time are protected from its hyperpolarizing influence. To explore this issue on a small tissue scale we studied the activation pattern of the mouse SA node region and correlated this pattern with the distribution of different gap junction proteins, connexin (Cx)37, Cx40, Cx43 and Cx45. METHODS AND RESULTS: The mouse SA node was electrophysiologically mapped using a conventional microelectrode technique. The primary pacemaker area was located in the corner between the lateral and medial limb of the crista terminalis. Unifocal pacemaking occurred in a group of pacemaking fibers consisting of 450 cells. In the nodal area transitions of nodal and atrial waveform were observed over small distances ( approximately 100 microm). Correlation between the activation pattern and connexin distribution revealed extensive labeling by anti-Cx45 in the primary and secondary pacemaker area. Within these nodal areas no gradient in Cx45 labeling was found. A sharp transition was found between Cx40- and Cx43-expressing myocytes of the crista terminalis and the Cx45-expressing myocytes of the node. In addition, strands of myocytes labeled for Cx43 and Cx40 protrude into the nodal area. Cx37 labeling was only present between endothelial cells. Furthermore, a band of connective tissue largely separates the nodal from the atrial tissue. CONCLUSIONS: Our results demonstrate strands of Cx43 and Cx40 positive atrial cells protruding into the Cx45 positive nodal area and a band of connective tissue largely separating the nodal and atrial tissue. This organization of the mouse SA node provides a structural substrate that both shields the nodal area from the hyperpolarizing influence of the atrium and allows fast action potential conduction from the nodal area into the surrounding atrium.

The graduate training in medical information sciences in the Academic Medical Centre at the University of Amsterdam.

Since its inception in 1987, the 4-year Medical Information Sciences (MIS) curriculum at the Academic Medical Centre (AMC), Amsterdam has gone through several major changes. The present curriculum started in 1994. The course takes 4 years, the first 3 years are programmed in integrated modules of 7 weeks in duration each. In these modules much attention is given to interactive teaching, problem based learning and private study. Typical for the Amsterdam curriculum is a strong emphasis on the role and significance of data and information in health care and its management. The authors see information technology per se as auxiliary to this orientation. Presently, about 150 students follow the courses.

[Medical education of tomorrow: a glance into the (near) future]. / Medisch onderwijs van morgen: een blik in de (naaste) toekomst.

Educational reform is a topical subject in Dutch medical schools. Public visitation reports were issued in 1992 and 1997. In 1994 an 'outline plan' was presented, setting forth communal training requirements (final results). An important consequence will be reorganization of (assistant) housemanships. Options are to start already in the 3rd- and 4th-year with a few (assistant) housemanships, to reduce the number but increase the duration of the (assistant) housemanships and to introduce a more comprehensive training (including elements from several disciplines). Other areas for special attention are the insufficient professionalization of teachers in medicine, the bureaucracy weighing down the teaching and the continuing need of educational research. Regarding the future doctors' image of their profession, emphasis should be placed on the international trend toward more attention for prevention and cost control, apart from the physician's traditional curative task. With respects to curriculum reorganization, six items should be considered: the need of a basic philosophy (per faculty), the priority to be given to the process of medical problem solving over that of acquiring knowledge and information, the desirability of increasing orientation to practice, the necessity of an integrated approach of medical teaching (teaching elements to be deduced from the final terms, instead of vice versa), development of a professional attitude and the realization of a master-apprentice relationship in medical education.

Electrophysiology of the ageing rabbit and cat sinoatrial node--a comparative study.

Intrinsic properties of the human sinoatrial (SA) node have been shown to decline with age. In the present study we aimed at investigating the underlying mechanisms of age-dependent changes in intrinsic cycle length and sinoatrial conduction time. To this end, the cycle length and transmembrane potentials of the SA nodes of rabbits (2 days-5.6 years) and cats (6 weeks-18 years) were recorded and nodal conduction was reconstructed. The size of the SA nodes was measured in Sirius Red stained sections. Cycle length increases with age in both the rabbit and cat SA node, and in both species cycle length is dependent on diastolic depolarization rate and action potential duration. Nodal action potential duration increases with age in both rabbit and cat, whereas diastolic depolarization rate decreases in the cat only. The location of the primary pacemaker is not related to age. With age, sinoatrial conduction time increases in both species as a result of an enlargement of the area with low phase 0 upstroke velocities. The size of the SA node of adult animals does not increase with age.

Cardiac acceleration in man elicited by a muscle-heart reflex.

The shortening of the R-R interval in response to voluntary and electrically induced isometric muscle contractions of short duration was investigated in 15 volunteers. In some of those experiments the effect of vagal blockade was also studied. The results show: 1) a lag time between the start of the contraction and the following decrease in R-R interval duration of 550 milliseconds; 2) a similar R-R interval response due to voluntary and electrically induced contractions of the same force; 3) no shortening of the R-R interval when the skin is stimulated without ensuing muscular contraction; 4) a complete disappearance of the response to isometric contractions during vagal blockade. A difference in lag time between the onset of arm contraction and cardiac acceleration could not be demonstrated. Most of the results give strong evidence to the existence of a muscle-heart reflex in man, involved in the instantaneous cardiac acceleration at the onset of exercise, that has its origin in the muscles and the vagal nerves as its efferent pathway.

Reentry in the atrium.

Studies with isolated atrial preparations of the rabbit showed that the occurrence of a single early premature beat may cause reentry not only in nodal tissue (SA node and AV node) but also in working myocardial tissue. In the SA node an early premature beat will cause a reentrant activation of the atrium only when the SA node is driven by an ectopic pacemaker. If the SA node is discharging spontaneously, no reentry could be demonstrated. In this situation the early impulse can not reach the center of the SA node because of a sinoatrial entrance block. Since the AV node fibers normally do not discharge spontaneously, an atrial premature beat may find an alternative route through the node and reenter the atrium. Such a reentrant beat or echo beat can start a tachycardia based on a circus movement of the impulse through the AV node. A supraventricular tachycardia can be started too by an early premature beat in the isolated left atrium, containing only working myocardial fibers and no slow conducting fibers as the nodal fibers are. By careful mapping the spread of activation during the premature beat and the subsequent beats of the tachycardia, a unidirectional block of the impulse of the premature beat was demonstrated. The impulse then turned around and invaded the blocked area retrogradely and reentered the area where it originated. This circus movement of the premature impulse was maintained during the subsequent tachycardial beats, showing that even in a small area of atrial muscle, containing no anatomical obstacle, a circus tachycardia can take place. To describe this kind of circus movement a new model (the "leading circle" concept) is introduced and briefly discussed.

Age-related changes in structure and relative collagen content of the human and feline sinoatrial node. A comparative study.

Age-related changes in the structure and size of the human and cat sinoatrial nodes were studied by light microscopy, with emphasis on changes in relative collagen volume. Sinoatrial nodes from 41 humans (aged 0-94 years) and 21 cats (aged 6 weeks-18 years) were used. It was found that there were no changes in the dimensions of the sinoatrial node during adult life in either species. In Sirius Red F3 Ba stained sections, the relative volume of collagen was measured using an interactive image analysis system. The relative volume of collagen in the human sinoatrial node increases from 38% during childhood to 70% during adulthood. Once adulthood is reached, there are no further changes in the relative volume of collagen. In the cat sinoatrial node the relative volume of collagen is only 27% and does not change with age. The organisation of collagen in the sinoatrial node, however, demonstrates an age-dependent change in both humans and cats. From coarse strands between clusters of nodal cells it gradually changes into a fine network of isolated collagen fibres which surround individual nodal cells. This process is more pronounced in humans. It is concluded that age-related changes in sinoatrial node function are not related to an increase in collagen content in the sinoatrial node.

Short-latency tachycardia evoked by stimulation of muscle and cutaneous afferents.

The short-latency effect on heart rate of peripheral nerve stimulation was studied in decerebrate cats. Selective activation (17-40 microA, 100 Hz, 1 s long) of low-threshold fibers in the nerves to the triceps surae muscle yielded isometric contractions of maximal force that were accompanied by a cardiac cycle length shortening within 0.4 s from the start of stimulation. This effect was abolished by pharmacologically induced neuromuscular blockade. The cardiac cycle length shortening during paralysis reappeared after a 6- to 10-fold increase of the stimulation strength. Cutaneous (sural) nerve stimulation (15-25 microA, 100 Hz, 1 s long) elicited reflex contractions in the stimulated limb, which were also accompanied by a cardiac acceleration with similar latency. Paralysis prevented the reflex contractions and reduced the cardiac response in some cats and abolished it in others. The response reappeared in either case after a 5- to 10-fold increase of the stimulus strength. It is concluded that muscle nerve and cutaneous nerve activity both cause a similar cardiac acceleration with a latency of less than 0.4 s. The response to muscle nerve stimulation is elicited by activity in group III afferents. It is excluded that the cardiac response to nerve stimulation is secondary to a change in the respiratory pattern.