Hypertrophy of the denervated hemidiaphragm.

The phenomenon of hypertrophy of the hemidiaphragm following denervation was found to occur in the mouse, rabbit and in the right hemidiaphragm of the rat. The peak of the hypertrophy occurred between 5 and 8 days in the mouse, at which time increases of 2437% in wet weight of the hemidiaphragm were observed. In the rabbit the maximum hypertrophy observed was 29.5% at IO days after denervation. The response of the right hemidiaphragm to denervation was quantitatively very similar to the response of the left hemidiaphragm in the rat. A slight increase in relative amount of water was observed in the hemidiaphragms of both the mouse and rat which does not account for the observed increases in wet weight. T HE HEMIDIAPHRAGM of the rat responds to denervation in an unusual fashion ( I, 2). Approximately 7 days after section of the phrenic nerve the wet weight of the left hemidiaphragm increased by 25-55 %. The right hemidiaphragm increased about the same amount after sectioning the right phrenic nerve. Since the wet-dry weight ratios of the denervated hemidiaphragms were only slightly altered, the response was designated hypertrophy. This was a temporary condition and in about 30 days the weight was back to normal, following which further losses in weight occurred. These experiments were repeated in another laboratory with quite different results (3, 4). No change in weight of the right hemidiaphragm was observed until the 28th day, when loss of weight began. The left hemidiaphragm, on the other hand, showed a transient increase in weight but this was attributed entirely to an increased water content and was not considered a true hypertrophy. Inasmuch as further research has confirmed the increases in both wet and dry weights of the denervated left hemidiaphragm and has demonstrated an increase in the absolute amounts Received for publication November 2, 1955. 1 Supported in part by Initiative 171 Funds for Research in Biology and Medicine of the State of Washington. of sarcoplasmic and contractile proteins, it would appear that the response obtained bY Sola and Martin must represent hypertroPhY W* If the phenomenon is of general significance it should be possible to demonstrate its occurrence in species other than the rat. The present investigation has shown the hypertrophy to occur in two additional species, the mouse and rabbit. In addition, the hypertrophic response of the right hemidiaphragm of the rat has been confirmed.