The effect of epinephrine on the myocardium of the embryonic chick.

The myocardium of the embryonic chick heart, before it has been innervated, responds to concentrations of epinephrine as dilute as 1 part in 20,000,000 by an abrupt acceleration in its inherent rate of pulsation. The more slowly the heart beats initially, the more marked is the accelerating effect of epinephrine. Usually the initial acceleration is followed by a period during which the heart slows to a rate intermediate between the initial and the maximal. This deceleration is not due to a decrease in the stimulating effect of the drug applied. T HE FOLLOWING report is concerned with the moot question as to whether or not the sympathomimetic substance, epinephrine, is effective in accelerating the rate of pulsation of the myocardium of the chick embryo prior to the establishment of its in-nervation (i.e., before the fifth day of incubation , according to His5). In 1927, Fujii3 reported that in chick embryos up to the third day of incubation, adrenalin had no effect on the rate of cardiac pulsation except in "large doses," and that the first characteristic response was obtained in chicks during the fourth day of incubation. In 1931, Nordmann and Rlither9 stated that the rate of pulsation of the pacemaking centers in ex-plants of the myocardium of chick embryos of various ages was not influenced by adrenalin. In the same year, MarkowNitz8 examined the effect of epinephrine on explanted cardiac tissue from chick embryos. This work assumed that no change in rate less than 25 per cent, was significant. Using this criterion, it was found that out of 38 tested, ten explants from chick embryos of 2 to 3 days of age reacted to epi-nephrine; and that explants from older embryos yielded a higher percentage of positive reactions. All explants showed "typical physiologic reactions" when they were taken from embryos older than 6 days of incubation. It, was assumed from the foregoing observation that a substance within the myocardium was responsible for the "typical response" of cardiac tissue to epinephrine. This substance appeared to be absent from most 72-hour embryos, but was present in increasing concentrations in older embryos. Two years later Hsu6 reported that epineph-rine was effective in accelerating the rate of intact embryonic chick hearts between the ages of 53 and 103 hours, and of fragments from hearts between the ages of 37 and 480 hours of incubation. The greatest acceleration obtained for any fragment of the …