PubMed Health⌕ Search

Biomedical subjects

A J Mackaay

Publications and source records attributed to A J Mackaay.

24 records · Page 2Linked to original sources
PubMed1980

Functional and morphological organization of the rabbit sinus node.

In isolated right atria of the rabbit heart, we studied the activation pattern within the sinus node, using the microelectrode technique. After the electrophysiological experiments, the preparations were subjected to a correlative morphological investigation, using light or electron microscopy. Different criteria for defining the dominant pacemaker were compared. A group of at least 5000 cells, located within the central part of the node where the most characteristic tissue architecture was found, was considered to be responsible for generation of the impulse. At the ultrastructural level, this leading cell group appeared to be part of a larger uniform cell group. The number of gap junctions observed suggests that all nodal cells are coupled by these structures. Toward the periphery, the excitation wave was propagated preferentially in an oblique cranial direction toward the crista terminalis. Neither morphologically nor electrophysiologically specific pathways were found for the conduction, but the preferential direction could be explained by the tissue architecture.

Action Potentials↗
PubMed1980

Dependence of the chronotropic effects of calcium, magnesium and sodium on temperature and cycle length in isolated rabbit atria.

Ca (1.1--5.5 mM) acts positive chronotropic on isolated right atria of the rabbit at 38 degrees C. At 34 degrees C, the chronotropic effect was also positive but smaller than at 38 degrees C. At 30 degrees C, the chronotropic effect is variable. Negative and positive chronotrpic effects were observed. At all three temperatures, the magnitude of the effect depended on the cycle length. Mg (0.6--6.0 mM) has a negative chronotropic effect. This effect was not dependent on the cycle length. The relationship between prolongation of the cycle length and the Mg concentration turned out to be linear. The chronotropic effects of Ca and Mg were simply algebraically additive. On the average, Na (77.4--154.8 mM) has no significant chronotropic effect. However, it acts positive chronotropic on fast preparations, although it acts negative chronotropic on slow preparations. So the chronotropic effect of Na depends on the cycle length also.

Animals↗
PubMed1980

Interaction of adrenaline and acetylcholine on cardiac pacemaker function. Functional inhomogeneity of the rabbit sinus node.

In the isolated rabbit sinus node, adrenaline induced a pacemaker shift toward the inferior part of the node (distance, 2.5 mm). Acetylcholine gave a pacemaker shift either toward the inferior part of the node (distance, 1.2 mm) or, at higher acetylcholine concentrations, toward cells near the upper part of the crista terminalis. After severing the superior from the inferior part of the preparation, we compared the chronotropic responses in the two parts. The node appeared to be functionally inhomogeneous: in the inferior part the adrenaline-induced acceleration was larger and the acetylcholine-induced deceleration was smaller than in the superior part. In the total preparation and the isolated inferior part, the acceleration due to adrenaline was suppressed by acetylcholine and the deceleration due to acetylcholine was enhanced by adrenaline. Such an interference could not be established in the isolated superior part. The acetylcholine-induced pacemaker also dominated during the simultaneous addition of the two neurotransmitters. We conclude therefore that the functional inhomogeneity of the sinus node explains the predominance of the effect of acetylcholine over that of adrenaline.

Acetylcholine↗
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↗