THE ACTION OF YOHIMBINE ON THE HEART, WITH SPECIAL REFERENCE TO TOXIC HEART‐BLOCK

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Abstract
This communication consists of a special and of a general part, of which the latter is the more important. The special part deals with the action of yohimbine on the heart. The general part arises out of the special, and includes a discussion on three of the four cardinal properties of heart tissue recognised by Engelmann, viz. conductivity, irritability, and contractility.A. Special:—(1) Yohimbine does not alter the absolutely refractory phase of the heart, or, to be more precise, it does not affect the ratio A/S, where A = duration of absolutely refractory phase and S = time of systole.(2) In sufficient concentration it greatly prolongs the relatively refractory phase.(3) It depresses the irritability of the tissue to induced currents and probably to chemical stimulation.(4) In concentration sufficient to bring the heart to a standstill it has no depressing action on contractility.(5) Under yohimbinisation the conducting mechanism as distinct from the contracting mechanism of the heart is readily fatigued.B. General:—Evidence is adduced in support of the following conclusions:—(1) Irritability and conductivity in the heart, just as in the case of nerve, are essentially the same property. No change in irritability can occur without a corresponding change in conductivity and vice versa. When Engelmann attempted to separate the two properties he was led astray by a wrong assumption.(2) It follows from the above that the internal mechanism responsible for the existence of irritability and conductivity is a single thing. We may for convenience call it the excito‐conducting mechanism.(3) It also follows, provided the distinction initiated by Engelmann between irritability and contractility is upheld, that the heart muscle is capable of excitation only through the excito‐conducting mechanism.(4) The excito‐conducting mechanism is in more close functional and possibly structural relationship with the contractile mechanism than are the nerve fibres with the contractile mechanism in a skeletal muscle. After activity the cardiac excito‐conducting mechanism undergoes repair at a much slower rate than ordinary nerve fibres.(5) Toxic heart‐block, which has received different explanations at the hands of different observers, is due to a depressing action on the excitoconducting mechanism. The individual explanations advanced to account for the condition are all aspects of one complex change, each feature of which is present at the same time, while no single one can be present without the others.The expenses of this research have been defrayed by a grant from the Carnegie Trust. For my supply of yohimbine I am indebted to Drs Hillringhaus and Heilmann, Chemische Fabrik, Güstrow.

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