Properties and concentrations of somatomedin A in various rat tissues

The concentration of somatomedin A (SMA) in various rat tissues was determined directly by the radioreceptor assay in supernates of tissue homogenates at 100,000 x g for 60 min. The SMA concentrations in pancreas, kidney, lung, liver, spleen, testis, brain and muscle were 2.56, 2.40, 2.28, 2.16, 1.56, 1.44, 0.60 and 0.30 U/g of wet tissue, respectively. The SMA content in rat serum was 6.66 U/ml. The SMA concentration in tissues never exceeded that in serum. When the 100,000 x g pellet of liver was treated with a hypotonic solution, the SMA content in the hypotonic solution was about 35% of that in the initial supernate. This result indicates that about 30% of the SMA in liver was stored in organellae. SMA in tissues decreased with incubation at 37 degrees C, though SMA in serum did not. This decrease could be precluded by adding 0.75 mM HgCl2 to the liver homogenate. The SMA content of the liver homogenate was dependent on growth hormone in experiments both in vivo and in vitro.