Electrical and mechanical responses of the auricle of the rat under massive stimulation

Abstract

Electrical responses from the atria of the rat with intracellular microelectrodes were recorded simultaneously with the mechanogram; a 5734 RCA being the transducer. Square waves of different strengths were applied between two large surface electrodes, one 011 each side of the muscle, which was stimulated through the Tyrode solution.

The monophasic action potential of the atria cells is similar to but shorter than those of other mammalian atria; there is a hump in the repolarization and this may end with an afterpotential which lengthens the duration to 100 ms. If there is no afterpotential the wave lasts about 50 ms.
1 The latency of the action potential is shorter when the stimuli are stronger, until it becomes zero latency with superposition of the stimulus artefact and depolarization.

Minimal mechanical latency is very constant, from 10 to 14 ms, and independent of the strength of the stimulus. Thislatency ought to be measured from the depolarization only if the response is massive, but if this is not massive the measurement must be from the stimulus artefact.
The mechanogram has a very symmetrical bellshape in the superior portions, but this is unsettled for the lowest part near the resting level. Durations of the mechanical wave at dif­ferent relative tensions are very constant and independent of the absolute tension; it was about 60 ms at half height and 90 ms at 25 % of the height, the total duration amounting to about 140 ms. The latency of the peak of the mechanogram that averages about 83 ms for the threshold, becomes about 70 ms for the strongest stimuli, this could be explained by the simul- taneous shortening in the electrical latency as a manifestation of the increased synchronization ; the stronger stimulus pro­ duces a faster rate of rise on the onset of the mechanical res­ ponse.

Massive electrodes made it possible to apply different strengths of stimulation; this is not possible in the classical preparations where the physiological stimulus running through the muscle is nearly constant, at about 100 mV. For the threshold stimulus the response starts at some point of the atria near the cathode and diffuses from there through the whole muscle. The area of direct action increases with stronger stimuli and the path for the action potential is proportionally shortened, the latency decreasing to the same degree. When the latency is zero the area of direct response ineludes at least the muscle from the cathode to the microelectrode. A series of degrees in the response related to the strength of stimuli can be obtained; this must be kept in mind in those experiments with the possibility of spreading of current over the tissues. In dealing with experiments on the refractory period of little muscles this fact may play a role; further the effect of the stimulation does not depend exclusively on the stimulating current, but also on the excitability.

Long periods of strong stimulation have additional effects 6n the mechanical responses. Commonly there is a reduction in the height of contractions of the atria and sometimes, but rarely, a potentiating action may be seen, both of delayed and accumulative character. The highest portions of the mechanogram do not change, except in their absolute values, but the duration and symmetry remain the same. It seems also that there is some direct relation between the height of the mechanogram and the latency of its peak.

The increase of strength of stimuli improves the synchronization, and the reduction of mechanical amplitude persists throughout the strongest stimulation; by diminishing the sti­mulating current a potentiating effect appears and the contraction grows up gradually. This suggests activation of some po­ tentiating mechanism by the high current, but the mechanical potentiation does not appear because it is counteracted by an inhibition that develops simultaneously.

Whether the increase in the strength of stimulus is fast or slow the height of the mechanical response is reduced to the same level, this being lower for the larger stimulating current. On decreasing the stimulus the depression disaopears and recovery is gradual with or without temporal potentiation. The potentiating action evoked by strong stimuli lasts for several minutes and can be hidden if the rate of diminution of stimula­ tion is slower than the rate of disappearance of the potentiating action. Potentiation in atria of the rat generally needs the previous high stimulation, but it was not possible to establish if the depression is a necessary condition. In every case depres­ sion and potentiation seem to be independent of the changes in synchronization.

The accumulative character of responses after strong stimu­ lation suggested to Whalen two different substances, namely epinephrine as potentiator and acetylcholine as depressor, but the literature about these substances is very far from being conclusive. The theory of significance of intervals, earlier proposed by the author, admits a potentiating substance, whose production might be increased by the strong stimulation, as seems to occur in the staircase phenomenen.