J. Ushiyama

Intracellular stimulation and recording from single cardiac cells in the rabbit atrium

Summary The mechanism of pacemaker action in an isolated rabbit sinoatrial node preparation was studied by means of intracellular stimulation and recording through a single-barrelled microelectrode. This technique permitted not only stimulation of individual cells of the various types found in and around the pacemaker tissue but also the recording of nonpropagated activity in cells stimulated or in those in the immediate vicinity. It was found that: 1) The myocardial atrial fiber has a threshold of the order of 10−6 A. when stimulated intracellulary with a 15 msec duration pulse. This threshold was constant throughout diastole, except during its very early phase. The resulting response was of an all-or-none type and propagated over the tissue. 2) Thresholds for latent pacemaker cells were different from cell to cell; within a cell, it also varied slightly with the degree of diastolic depolarization attained. The response initiated by intracellular stimulation was also of an all-or-none type and, in most cases, it was propagated to the atrium. When a response was induced at an early phase in diastole, a conduction failure was sometimes observed. When such a nonpropagated response occurred in a latent pacemaker cell, the duration of diastole was increased. 3) Most pacemaker cells tested did not respond to intracellular stimulation. In a few cells, however, a graded response proportional to stimulus intensity, was obtained. Such response did not propagate to the atrium. These graded responses differed from the local responses which occur in other types of cell. It was concluded that a synchronization of pacemaker cell activity is required for effective pacemaker action.

Intracellular stimulation and recording from single cardiac cells

Abstract A method was devised for stimulating and recording from single cardiac cells using a single intracellular microelectrode and bridge circuit. Purkinje fibers were found to have a much lower threshold but a greater latency of response than trabecular muscle fibers of the same dog heart. Threshold intensities for rectangular pulses of different duration were determined for muscle, and it was found that prolonging pulse duration beyond 10 milliseconds did not permit further lowering of intensity. Application of brief pulses of inward current flow during the terminal phases of trabecular muscle action potential revealed a period, shortly after the plateau phase, during which break of the current resulted in excitation. Earlier or later application of the pulse had no effect. A technic was used to determine the safety factor for conduction in cardiac tissue. The safety factor was found to have a value of approximately 2 in trabecular muscle and 4 in Purkinje fibers.

Interaction of oscillatory processes: the effects of subthreshold A.C. current on sinoatrial nodal rhythm.

In this study of interactions occurring within the heart, isolated superfused strips of rabbit atria, containing the sinoatrial (s.a.) node, were subjected to sinusoidal subthreshold current pulses of varied frequencies and intensities. A.C. current from an R.C. oscillator was applied through a Grass stimulation isolation unit, push-pull connection, and non-polarizing (Ag-AgCl-KCl-Tyrode Agar-Agar) electrodes. A Grass polygraph and tachometer were used to record the applied pulses and nodal firing rates; simultaneous magnetic tape recordings were obtained and used for data analysis. Suction electrode recordings and oscilloscope displays were used to determine how the cyclic impulses affected cellular activity. The s.a. nodal rhythm was modified by subthreshold A.C. current applications; when frequencies were low, firing rates of the node were modulated by the A.C. and mean rates were reduced. As frequencies were progressively increased, slow waxings and wanings in heart rate were produced. These periodic fluctuations were not readily correlated with either the A.C. frequency nor the intrinsic nodal rate, but were representative of the difference between the two. As applied current frequency neared the pacemakers intrinsic rate, a synchronization occurred and the discharges locked in at a specific phase of the applied current alternation. This synchronization maintained during slight further increases in A.C. frequency but above a critical rate this relationship broke down and the waxing and wanings in frequency of pacemaker rate again developed. It was concluded that pacemaker action of the s.a. node is effected by integration of cellular activity through electron coupling.