Abstract
The question arises whether the marked increase in rate of heart heat which develops in hyperthyroid and in thyroid fed animals will appear in isolated hearts treated with thyroxin if a sufficient time is allowed for the action of the hormone. Lewis and Mc-Eachern 1 have shown that hearts removed from thyroid fed, or thyroxin treated rabbits maintain a 50% increase in rate for 8 to 10 hours, and that normal excised rabbits' hearts show no immediate effect of thyroxin.
We have kept excised turtle hearts at 20°C. ± .02° for 60 hours, recording automatically the rate during this period by means of the Loomis chronograph. 2 The heart was tied by the tips of the auricles to a heart lever and suspended in a modified Ringer's solution, containing 0.1% glucose and phosphate buffer to pH = 7.3, oxygenated by pure O2 gas. Addition of thyroxin 20 , dissolved in a minimal amount of alkali as a stock solution of 1:1000, does not change the pH of the solution. The details of transmitting the heart beat to the chronograph are described in a previous paper. 2
The hearts beat about 10 times in 20 seconds at first, gradually slowing to 10 beats in 30 seconds in 2 hours when the thyroxin, 1:105, is added. No immediate change in rate occurs, the heart gradually slowing, with some rhythmic variation in rate, over a period of 2 days, when the rate may be 10 beats in 80 seconds. The 4 hearts studied showed no tendency whatever to increase in rate and behaved in every way like normal hearts, which likewise continually slow over long time periods.
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