ACTH and growth hormone in myocardial LDH adaptation to hypoxia in rats

The role of hypophysis in cardiac growth, and in myocardial lactic dehydrogenase (LDH) isoenzyme adaptations to hypobaric hypoxia (9.5% O2) during 3 weeks was studied in four groups of hypophysectomized rats: one group received no hormone, and the second, third, and fourth groups received, respectively, growth hormone, ACTH, and growth hormone+ACTH. One hypophysectomized and one non-surgical group maintained at sea-level and one non-surgical group exposed to hypoxia served as controls. Exposure of hypophysectomized rats to hypoxia produced a proportional loss of body and heart weight with an equal decrease in both LDH subunits, H (heart) and M (muscle). Growth hormone and/or ACTH did not reverse cardiac atrophy. ACTH given alone or with growth hormone enhanced M-subunit synthesis (2- to 4-fold) and corrected the H/M ratio to the level of the non-surgical controls exposed to hypoxia, but at the expense of a lower total level of LDH. The adaptative changes in LDH isoenzymes induced by hypoxia could be abolished with hypophysectomy, and partly restored by giving growth hormone and ACTH, but this restoration was not coupled with cardiac growth.