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Biomedical subjects

L N Bouman

Publications and source records attributed to L N Bouman.

17 recordsLinked to original sources
PubMed1992

Spatial and functional relationship between myocytes and fibroblasts in the rabbit sinoatrial node.

In an attempt to understand better the directional differences in conduction velocity in the rabbit sinoatrial node, a possible conductive role of the abundant connective tissue surrounding the myocytes has been investigated. In particular, starting from the finding of communicating junctions between heart muscle cells and fibroblasts in tissue culture, heterologous gap junctions were searched for in thin sections of the rabbit sinoatrial node. Within and at the edge of nodal cell clusters, fibroblasts often show thin sheet-like extensions parallel to the surface of myocytes. In contrast to the intimately contacting myocytes, fibroblast extensions are kept separated from the myocytes by the basement membrane of the latter. Besides some rare undefined membrane appositions a single tiny gap junction-like structure was found between a fibroblast and a myocyte in a tissue area in which the calculated number of gap junctions between myocytes amounts from 1.10(4) to 3.10(4). Yet, fibroblasts are linked together regularly by small gap junctions containing a wider gap than the junctions between the myocytes (1.4 +/- 0.4 nm vs. 1.0 +/- 0.4 nm, resp., P less than 0.05). As an alternative to direct electrical coupling, the possibility of interaction between fibroblasts and nodal cells by capacitive coupling has been considered. Model calculations based on the reconstruction of some fibroblast extensions parallel to nodal cells show that the current which can be transmitted from discharging nodal cells to fibroblasts is negligible. It is concluded that fibroblasts do not participate in the impulse conduction within the sinoatrial node. The origin of the directional differences in conduction velocity in the sinoatrial node must be found in the spatial arrangement of the myocytes and the distribution of the gap junctions between these cells only.

Animals↗
PubMed1992

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.

Action Potentials↗
PubMed1991

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.

Action Potentials↗
PubMed1990

Structure and function of the simian sinoatrial node (Macaca fascicularis).

In isolated preparations of the simian (Macaca fascicularis) heart, we studied the activation pattern within the sinoatrial node, using the conventional microelectrode technique. After electrophysiological experiments we subjected three preparations to a correlative light microscopical and two to an electron microscopical investigation. The sinoatrial node of the Macaca fascicularis is characterized by unifocal impulse generation. The impulse is propagated preferentially in an oblique direction towards the inferior vena cava, which is a unique direction compared to all other mammals studied so far. Possible consequences for A-V nodal input are discussed. Conduction block was seen in an oblique superior direction towards the atrial septum. In this small zone of blocked conduction double component action potentials could be recorded. The morphology of the sinoatrial node of the Macaca fascicularis is essentially the same as found in other mammals.

Animals↗
PubMed1990

Phase dependency of electrotonic spread of hyperpolarizing current pulses in the rabbit sinoatrial node.

Electrotonic current spread in the SA node of the rabbit was measured by means of hyperpolarizing current pulses (1 to 10 microA, 60 ms), which were injected intracellularly through a K(+)-perfused suction electrode. The pulses were applied at the beginning, middle or end of the diastolic depolarization phase. The resulting membrane potential change of nodal fibers was measured with microelectrodes. Space constants were calculated by fitting single exponential curves to the data. The input resistance (Rin) of fibers at different sites in the SA node was measured by means of a double barrel microelectrode (current pulses 5.5 to 11 nA, 60 ms) to detect a change in the internal resistance during the diastolic depolarization phase. During diastole the average electrotonic potential increased by 30% (P less than 0.001), the increase of the space constant ranged from 9 to 183% (P less than 0.05). Rin however, did not change during diastole. It is concluded that the electrotonic spread increased phase dependently, due to an increase of membrane resistance; the internal resistance was not phase dependent.

Animals↗
PubMed1989

Anisotropy of electrotonus in the sinoatrial node of the rabbit heart.

In fibers of the sinoatrial node of isolated right atria of rabbits the decay of the electrotonic potential caused by intracellular current injection was measured in two directions: parallel to the crista terminalis and perpendicular to it. A K+-perfused extracellular suction electrode was used to apply current pulses (10(-5) A, 60 ms) to fibers located in the primary center of the SA node every fourth cardiac cycle at a fixed moment during diastole. The decay of the electrotonic spread was measured in a series of impalements on a straight line from the current source. Space constants were calculated by fitting single exponential curves to the data. Considerable regional differences in space constant values were found in either direction. Parallel to the crista terminalis the mean value was 529 +/- 446 microns (S.D., n = 7), perpendicular to it 306 +/- 295 microns (n = 12); the difference was not significant (P less than 0.2). However, a significant anisotropy (P less than 0.05) of the electrotonic spread was found when measurements were taken from small areas of the node. Large abrupt changes in the electrotonic potential within 200 microns were observed in the center of the node. These data indicate a non-uniformity of electrotonic spread in this part of the SA node.

Animals↗
PubMed1989

Mechanisms of impulse generation in isolated cells from the rabbit sinoatrial node.

The contribution to spontaneous activity of three currents INa, If, and Isi was investigated in isolated spontaneously active SA nodal cells. It was demonstrated that isolated cells have electrophysiological properties similar to those of cells in the intact node. Evidence for the contributory role of INa to the upstroke of the action potential was obtained from membrane responsiveness curves and from the observation that Vmax was strongly reduced after the addition of 9 microM TTX to the bathing solution. The relative role of If and Isi as depolarizing diastolic membrane currents was investigated by relating the maximal density of each of these currents to the corresponding DDR during spontaneous activity. Both currents If and Isi-peak appear to be linearly related to DDR, indicating their contributory role to diastolic depolarization. Although the cells studied were heterogeneous with regard to the density of If and Isi we found no evidence for their separation into distinct groups of pacemaker cell types.

Action Potentials↗
PubMed1987

Functional morphology of the pig sinoatrial node.

The porcine sinoatrial node in an isolated right atrium preparation is characterized by unifocal impulse generation. It has a rather elongated shape and the larger part of its volume is taken up by collagen and fibroblasts. The impulse appears to emerge from a site where the percentage of myofilaments is relatively low. The impulse is propagated faster towards the crista terminalis than to the interatrial septum with preference for the oblique-upward direction. A very large zone of cells with low excitability is located at the interatrial septal side of the node.

Actin Cytoskeleton↗
PubMed1985

Aerobic energy expenditure of handicapped children after training.

The effect is reported of a 10-week physical training program, consisting of three sessions with a total duration of two hours weekly, on the physical work capacity and efficiency of physically handicapped children aged 8 to 14 years. The program for the experimental group (n = 6) was an intensification of the usual school physical education activities. The control group (n = 5) received the usual physical education. The intensity of training was measured by heart rate recording. In the experimental group attempts were made to achieve heart rate values higher than 160 beats/min as long as possible. The relationship of oxygen uptake (VO2) to heart rate and to workload was determined before and after the end of the training program by submaximal bicycle ergometer tests. After the training program a significant decrease in VO2 at different workloads was found (delta oxygen uptake/delta workload remained unchanged). No effect of the training program on the relationship of oxygen uptake and heart rate was found. The implication of this study is that the children can perform the same amount of external work after training as before training but with a lower expenditure of aerobic energy. The decrease of the oxygen uptakes for the workloads used could be induced by enhanced coordination of movement.

Adolescent↗
PubMed1976

The effect of heart rate on the membrane responsiveness of rabbit atrial muscle.

The maximum rate of rise of action potentials in myocardial fibers of the rabbit atrium decreases with an increase in heart rate. This decrease of the dV/dt max is accompanied by a decrease of the diastolic transmembrane potential prior to the moment of activation (take-off potential). Comparison of the membrane responsiveness curve (relation between dV/dt max and take-off potential) as measured by varying the extracellular potassium concentration at a fixed rate of stimulation, with the effect of changes in the frequency of stimulation on dV/dt max and take-off potential made clear that the fall in dV/dt max after a sudden increase in heart rate was stronger than could be explained by the concomitant decrease of the take-off potential alone. This implicates that the membrane responsiveness itself is heart rate dependent. A possible explanation for this observation is that when heart rate is increased the active Na/K pump is not able to maintain the intracellular concentration of Na and K at the original level. Acceleration of the heart will lead to an intracellular loss of potassium and a gain of sodium. The first causes a diminishment of the diastolic membrane potential which according to the membrane responsiveness curve is attended with a decrease of the dV/dt max. The second results in a decrease of the sodium concentration gradient and therefore in a further reduction of the dV/dt max. This hypothesis was confirmed by experiments with ouabain added to the perfusion fluid. Ouabain, which is known to inhibit the Na/K pump, caused a decrease of both the take-off potential and dV/dt max that was completely comparable with the effects of an increase of the frequency of stimulation. In addition, observation of the time course of the changes in dV/dt max and membrane "resting" potential after a sudden change in the rate of stimulation, gave support to the electrogenic concept of the active Na/K pump in cardiac muscle.

Action Potentials↗
PubMed1975

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.

Adolescent↗
PubMed1975

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.

Atrial Function↗
PubMedDate not supplied

Chronotropic effects of calcium and magnesium ions at different temperatures.

In the isolated right atrium of rabbit hearts, excess calcium causes an increase of heart rate at 38 degrees C; this effect is absent or even reversed at a temperature of 30 degrees C. From microelectrode studies it seems that when the calcium concentration is increased the pacemaker shifts within the sinoatrial node to cells in which excess calcium causes an acceleration of diastolic depolarization (type A). In fibers where the impulse originates at low calcium (type B), excess calcium causes a deceleration of diastolic depolarization. At low temperature, the effects on type A fibers are completely absent. Excess magnesium has a negative chronotropic effect at both temperatures, mainly by a deceleration of diastolic depolarization in type B fibers.

Animals↗