Elevated growth hormone increases the Ca2+ sensitivity of slow- and fast-twitch skeletal muscle of female rats.

To determine the effect of plasma growth hormone (GH) on skeletal muscle function, we measured the free Ca2+concentration-tension relationship of slow-twitch (soleus) and fast-twitch (peroneus longus) muscles isolated from rats undergoing acromegaly in response to implanted, GH-secreting tumors. Muscles from adult (9 mo) and aged rats (24 mo) were studied after the tumor-bearing rats weighed over 50% more than their age-matched controls. Ca2+-activated isometric tension was recorded from skinned muscle fibers. For soleus muscles, the free Ca2+ concentration producing 50% of maximal tension ([Ca2+]50) was 2.0 μM for rats with tumors and 3.4-3.6 μM for controls. For peroneus longus fibers, [Ca2+]50shifted from 6.1-6.7 μM in controls to 3.5 μM after tumors were introduced into either adult or aged rats. Soleus muscle fibers from neonatal rats (14 days) were less sensitive to Ca2+ than those isolated from adult rats, having a [Ca2+]50of 7.3 μM. The Ca2+ sensitivity of peroneus longus fibers did not change with age. We conclude that significant increases in myofibrillar Ca2+ sensitivity occur in skeletal muscles undergoing rapid growth induced by GH-secreting tumors.