Abstract
The effects of a somatomedin analog, Temin's multiplication stimulating activity (MSA), on amino acid transport into muscle cells have been characterized in a series of experiments on myoblasts and myotubes in culture. Addition of MSA to serum‐starved L6 myoblasts increased the rate of aminoisobutyrate (AIB) uptake 50‐150% within five hours. This early effect on transport was followed by increases in cell number, protein content and ^3^H‐thymidine incorporation. Kinetic analyses indicated that MSA increased the maximal velocity of AIB uptake but had no effect on the K~M~ for AIB. When myoblasts were allowed to fuse (and dividing cells eliminated by addition of 10^−4^ M cytosine arabinoside) the AIB transport system(s) remained similarly responsive to MSA. In myoblasts and in myotubes, both the basal and MSA‐stimulated rate of AIB uptake were sodium‐dependent processes; little stimulation occurred if sodium was absent from the labeling medium. Further suggesting the involvment of cations in response to hormone, MSA stimulated uptake of the potassium analog, ^86^Rb^+^, and increased net intracellular potassium in both myoblasts and myotubes. MSA was active at concentrations equivalent to in vivo levels of somatomedins; neither insulin nor growth hormone had any effect at or near physiological concentrations.