Albert J.C. Mackaay

Sinus node and atrium cells from the rabbit heart: A quantitative electron microscopic description after electrophysiological localization

Abstract Electrophysiologically identified cell groups in the sinus node from the rabbit have been compared with atrial fibers with the electron microscope. The point counting method has been used to estimate the volume density of the following structures: nucleus, mitochondria, myofilaments, sarcoplasmic reticulum tubules and subsarcolemmal vesicles. These data were collected in leading pacemaker cells, latent pacemaker cells and atrium cells from the crista terminalis. It has been found that organized structures in leading pacemaker cells occupy about 50% of the cell volume, as compared with over 90% in atrial fibers. Leading pacemaker cells consequently appear extremely “empty”. It has also been found that the group of cells which show the characteristic features of leading pacemaker cells at the ultrastructural level as observed in the correlated experiments is larger than the leading center found in electrophysiology and thus it seems impossible with the actual observation methods to delineate the leading pacemaker center using only cytological criteria.

Asymmetry of the sino-atrial conduction in the rabbit heart

Abstract In isolated right atria of the rabbit heart the impulse conduction from the sinus node to the interatrial septum was studied, using the microelectrode technique. After the electrophysiological experiments several preparations were subjected to a correlative morphological investigation, using light or electron microscopy. By making incisions in the preparation we could demonstrate that the pacemaker region was bordered at the medial (septal) side by a zone in which conduction was blocked completely; the entering wavefronts died out gradually in both antegrade and retrograde directions. The double component action potentials that can be recorded in this region in the spontaneously beating intact preparation, appeared to be an almost passive summation of the wavefronts that entered this zone from the pacemaker side (first component) and from the septal side (second component). The tissue at the septal side was activated by a front that encircled the conduction block. Microscopically the area of the block was not uniform: it showed transition in cell type and tissue architecture, as was observed in a previous study also at the lateral (cristal) border of the node, where the pacemaker impulse is conducted with increasing speed toward the atrium. Neither did we find a difference in spread of electrotonus between the medial and lateral border of the sinus node. The block is therefore thought to be caused by the electrophysiological properties of the cells, most likely their low excitability.

Ultrastructural and Functional Aspects of the Rabbit Sinoatrial Node

The heart beat originates within a certain group of cells of the sinoatrial node (SA node), and from this location, the excitation is propagated through the entire node, the right atrium and the other parts of the heart. Using intracellular recordings of electrical activity, West showed already in 1955 that within small areas of the SA node large differences in the shape of the recorded action potentials could be found. In addition it is known that the leading pacemaker site is small (for review see Brooks and Lu, 1972) and that its location within the node is variable even in animals from the same species (Sano and Yamagishi, 1965; Bleeker et al., 1978). Also morphologically the SA node is not homogeneous while a gradual transition between different territories is observed within the node. So it is difficult to localize precisely the leading pacemaker center. Nevertheless, among the numerous studies dealing with the ultrastructure of the SA node of several mammalian species: mole (Kikuchi, 1976), rat (Bompiani et al., 1959; Viregh, and Porte, 1960; Cheng, 1971), rabbit (Tori, 1962; Trautwein and Uchizono, 1963; Challice, 1966; Tranum-Jensen, 1976), dog (Kawamura, 1961; James et al., 1966; Hayashi, 1971), cow (Rhodin et al., 1961; Hayashi, 1962), monkey (Colborn and Carsey, 1972; Viragh and Porte, 1973) and man (James et al., 1966), only that of Trautwein and Uchizono (1963) has been made after electrophysiological identification, but they selected rather large pieces (1 mm2) for morphological examination.

Temperature dependence of the chronotropic action of calcium: Functional inhomogeneity of the rabbit sinus node

Abstract The chronotropic effect of variation of the external calcium concentration was investigated in the isolated rabbit sinus node at different temperatures. This Ca effect was temperature dependent; at 38°C excess calcium induced an acceleration of the sinus node whereas at 30°C there was on average no significant chronotropic Ca effect. This temperature dependency reconciles the different observations about the chronotropic action of calcium reported by others. With intracellular micro-electrodes the Ca effects were investigated in pacemaking cells of the sinus node. Maps of the activation pattern within the node at different Ca concentrations showed a pacemaker shift from the cranial towards the caudal portion of the node induced by a decrease of the Ca concentration from 2.2 to 1.1 m m . In a concomitant morphological study the intranodal location of the pacemaker centres was confirmed. In the centre leading at 2.2 m m Ca (and higher Ca concentrations) excess calcium increased the rate of diastolic depolarization whereas in the caudal pacemaker centre, leading at 1.1 m m Ca, excess calcium did not change the diastolic depolarization phase. Lowering the temperature induced a pacemaker shift towards the caudal portion of the node also. At 30°C no Ca effect on diastolic depolarization was observed in the leading pacemaker cells. We conclude that the diminished chronotropic action of calcium at low temperature is explained by a temperature dependent pacemaker shift towards cells which are less sensitive to variation of the Ca concentration.

Interaction of Adrenaline and Acetylcholine on Sinus Node Function

Already 25 years ago T.C West (1955) described the first microelectrode study of the sinus node. Since then many investigators have been concerned with sinus node function (Brooks and Lu, 1972; Bonke, 1978). Although highly sophisticated techniques have been applied, it looks today that the definite words about the sinus node cannot yet be written. West’s own observations gave already an indication of some of the problems involved. Describing the electrical activity recorded from single nodal cells, he defined “the true pacemaker” as the site which “depolarizes earlier than any other region during any one cardiac cycle”. He established that this “true pacemaker activity” was not confined to a fixed site, he could easily induce shifts of pacemaker dominance by adding drugs like acetylcholine and adrenaline. Those observations indicated that the sinus node is functionally inhomogeneous, at least in its responses to autonomic mediators. Several pacemakers are capable to drive the sinus node and those pacemakers show different responses. Thus, one single pacemaker cell is not representative for the sinus node as a whole. Sinus nodal cells are coupled electrically (Bonke, 1973; Masson-Pevet et al., 1979; Bukauskas et al., 1981), so the configuration of the electrical activity recorded from single cells depends not only on their own activity, but also on changes of the activity of surrounding cells (Bonke, 1978; Mackaay et al., 1980a).

On the Intrinsic Cardiac Rhythm

Among writers of physiology text books it is common use to present a conceptual model to explain the mode of action of the two divisions of the autonomic nervous system in their control of heart rate. This model states that there is an intrinsic heart rate being the frequency of the undisturbed sinus node; an increase of frequency above this intrinsic rate will be the consequence of adrenergic activity; a fall in heart rate below the intrinsic value will be caused by cholinergic action on the sinus node.

On the Ultrastructural Identification of Pacemaker Cell Types within the Sinus Node

The heart beat originates within a small group of cells in the sinus node. In the rabbit this site of earliest discharge or leading pacemaker center, is located in the intercaval region, 0.5 – 2 mm away from the medial border of the crista terminalis. It contains typical nodal cells, and is part of a much larger group of cells that all show diastolic depolarization. This latter group of cells, which under standard conditions (see Bleeker et al., 1980) follow the discharge of the leading pacemaker cells and which never reach threshold spontaneously are called transitional cells. From its site of origin the excitation wave propagates through the sinus node preferentially in an oblique cranial direction toward the crista terminalis. In all other directions, the spread of excitation is considerably slower; in the direction of the interatrial septum conduction fails (Fig. 1).