Hypophyseal influence on function and composition of cardiac muscle.

• Relationships between cardiac and hypophyseal functions have been recognized since early clinical observations of cardiac enlargement and failure in acromegaly. Experimental attention has been directed at hypophyseal influences on cardiac size," cardiac output,' and the ability of the heart to hypertrophy." The relationship of cardiac changes to peripheral circulatory events following hypophysectomy has been investigated. Although hypophyseal hormones are known to have widespread effects on growth and metabolism, the functional characteristics of the myocardium following modification of hypophyseal activity have been barely studied. Beznak found that the response of the intact heart to an increased input or resistance load was not abnormal in hypophysectomized rats. Direct studies on cardiac muscle are lacking. Such studies would seem to be highly desirable, since the investigations of Plattener and Reed, Gray and Young, and Bigland and Jehring have raised questions regarding the functional significance of hypophyseal hormone effects on skeletal muscle. Moreover, since Scow reported effects of hypophyseal growth hormone on various protein fractions of skeletal muscle, it would be of interest to have comparable data, particularly with regard to contractile protein concentrations, for cardiac muscle. It was the object of this study to determine the mechanical properties of isolated cardiac muscle of hypophysectomized and growth hormone-treated rats and to relate these to the