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.