Temperature and the Responsive Mechanism of Cardiac Muscle: II. Temperature and the Rate of the Rhythm of the Isolated Sinus

The isolated right sinus of Pseudyms elegans immersed in modified Ringer solution is used throughout. The heart is carefully excised by an incision through the left superior vena cava, the inferior vena cava close to the liver and the right superior vena cava where it leaves the pericardial chamber. The heart is immersed immediately in modified Ringer solution, to which is added 1/200,000 adrenalin to stop the tonic smooth muscle contractions. The auricles, with sinus attached, are severed from the ventricle by an incision passing around the sino-auricular junction. The pink right sinus is dissected from the white left sinus by an incision along the well-defined line of separation, care being taken to remove all traces of the white tissue of the left sinus. The tubular right sinus is opened to form a rectangular strip by an incision through the mid-posterior region to the superior vena cava. Any auricular tissue remaining about the sino-auricular opening is carefully removed.

In preparing the sinus strip it is necessary that all adhering tissue of the auricles or left sinus be removed, and that the incision serving to open the right sinus be in the exact position indicated. Failure to adequately perform these tasks results in the preparation producing a succession of beats irregular in magnitude and rhythm. In the case of adhering left sinus or auricular tissue, the irregularity is due to the response of these portions occurring either before, during or after the response of the right sinus. A displaced incision interferes with the normal conducting path in the sinus and a creeping contraction results.

The Ringer solution used is similar to that already described, with the exception that no adrenalin or magnesium chloride was present during the experiment. The methods of recording and temperature control were similar in every respect to those already described (Brown, 1930).

After 15 minutes exposure at the desired temperature, a continuous photographic record was taken of two contractions, and the time for ten beats also measured by a stop-watch.

In analysing the records, the duration of contraction was taken as the average of that for the two contractions, while the time between beats was taken as the time between these recorded contractions. At very low temperatures the heart rate was so slow that it was not feasible to obtain the continuous record. In that case two successive contractions were recorded and the time between beats taken as the average time of ten beats as determined by a stop-watch.

The data in all cases was expressed in terms of the Arrhenius equation.

After several preliminary attempts seven experiments were performed. The data of a typical one of these is given in Table I and plotted in Fig. 1. These data show that neither the logarithm of the velocity of the rhythm nor the logarithm of the velocity of contraction is a linear function of the temperature over the whole temperature range.¹

In all experiments the general shape of the curves was similar to those shown in Fig. 1, although the average slope varied from preparation to preparation.

The values for both the velocity of the rhythm and the velocity of contraction, obtained following “deterioration “caused by brief exposure to the higher temperatures or prolonged exposure to the low temperatures, are lower than those obtained previous to such exposure. Moreover, the deterioration affects the rhythmical process to a greater extent than the contractile process.

The velocity-temperature relation for the rate of the sinus rhythm described above agrees in every respect with the results of previous investigations on the heart rhythm obtained under similar experimental conditions (see Crozier, 1926), and it is quite certain that this relation describes the response of a rhythmically contracting heart muscle cell. It is questionable, however, whether the rate of the rhythmic process within the muscle cell varies as a function of temperature alone, or whether other conditions in the cell dependent on temperature also modify the process. If the latter be true, the rhythmic process at any one temperature would be occurring under one set of conditions, while at another temperature a new set of conditions would prevail. The results obtained in this investigation show that this is the case ; for, from the results obtained in a previous investigation of the temperature-velocity relation in the non-beating auricular muscle strip, it is certain that if temperature alone were the independent variable the logarithm of the velocity of contraction of the sinus would be a linear function of the temperature from o° to 20° with a μ value of 14,000 ± 500. The deviation from this linear relation in all of the experiments indicates that other factors beside temperature are modifying the contractile process. Moreover, since the deviation from linearity is such that the values for the duration of contraction vary in the same way with respect to temperature as the rate of the rhythm, it is reasonably certain that the rate of stimulation at different temperatures is a major factor in modifying the contractile process¹.

Although these results show that altered intracellular conditions at each temperature modify the velocity-temperature relation for the contractile process, it still remains to be shown that these same intracellular conditions can modify the velocity-temperature relation for the sinus rhythm. The evidence that changes in the intracellular conditions produced by the rate of stimulation can modify the subsequent rate of the rhythm depends upon two observations. (1) The rhythmical process is associated with the process determining the refractory period, initiation of a second response being brought about only when a certain critical condition in the refractory process is attained. This relation is shown by the phenomenon of “the compensatory pause,” resulting from interposed stimuli during the cycle of the rhythmically beating heart. (2) The duration of the refractory process is modified as the magnitude of the response is modified, as shown by the experiment of determining the refractory period of a second response resulting from a stimulus applied during the relative refractory period of a first response. The nature of the responsive mechanism is accordingly such that with alterations in the magnitude and duration of the response due to the rate of stimulation there must be associated alterations in the subsequent rate of the rhythm. It may be concluded, therefore, that both the velocity of the rhythm and the velocity of contraction vary not only as a function of temperature but as a function of cellular conditions, which, since they depend primarily upon the rate of stimulation, also vary with the temperature.

It is certain, therefore, that these results do not represent the velocity of the rhythmic process in the heart cell as a function of temperature alone, and for that reason can yield little information as to the nature of the process unless amplified by a more detailed experimental analysis. The results, however, are of importance in describing the velocity-temperature relation for the rate of the rhythmically beating heart cell considered as a functional unit.

1. In a rhythmically beating sinus strip neither the logarithm of the velocity of the rhythm nor the logarithm of the velocity of contraction is a linear function of the temperature over the entire physiological temperature range.

2. The velocity of the rhythm and the velocity of contraction of the sinus vary not only as a function of temperature, but also as a function of intracellular conditions, determined in this case primarily by the rate of stimulation which likewise varies as a function of temperature.

3. The significance of the velocity-temperature relation in the interpretation of protoplasmic, processes of this type is discussed.