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beta-Adrenergic receptor (bAR) agonists have been reported to modulate growth in several mammalian and avian species, and bAR agonists presumably exert their physiological action on skeletal muscle cells through this receptor. Because of the importance of bAR regulation on muscle protein metabolism in muscle cells, the objectives of this study were to determine the developmental expression pattern of the bAR population in C2C12 skeletal muscle cells, and to analyze changes in both the quantity and isoform expression of the major muscle protein, myosin. The number of bAR in mononucleated C2C12 cells was approximately 8,000 bAR per cell, which is comparable with the population reported in several other nonmuscle cell types. However, the bar population increased after myoblast fusion to greater than 50,000 bAR per muscle cell equivalent. The reasons for this apparent over-expression of bAR in C2C12 cells is not known. The quantity of myosin also increased after C2C12 myoblast fusion, but the quantity of myosin was less than that reported in primary muscle cell cultures. Finally, at least five different isoforms of myosin heavy chain could be resolved in C2C12 cells, and three of these exhibited either increased or decreased developmental regulation relative to the others. Thus, C2C12 myoblasts undergo developmental regulation of bAR population and myosin heavy chain isoform expression.

beta-adrenergic receptor (bAR) agonists presumably exert their physiological action on skeletal muscle cells through the bAR. Since the signal generated by the bAR is cyclic AMP (cAMP), experiments were initiated in primary chicken muscle cell cultures to determine if artificial elevation of intracellular cAMP by treatment with forskolin would alter the population of bAR expressed on the surface of muscle cells. Chicken skeletal muscle cells after 7 days in culture were employed for the experiments because muscle cells have attained a steady state with respect to muscle protein metabolism at this stage. Cells were treated with 0-10 uM forskolin for a total of three days. At the end of the 1, 2, and 3 day treatment intervals, the concentration of cAMP and the bAR population were measured. Receptor population was measured in intact muscle cell cultures as the difference between total binding of [H-3]CGP-12177 and non-specific binding of [H-3]CGP-12177 in the presence of 1 uM propranolol. Intracellular cAMP concentration was measured by radioimmunoassay. The concentration of cAMP in forskolin-treated cells increased up to 10-fold in a dose dependent manner. Increasing concentrations of forskolin also led to an increase in (beta)AR population, with a maximum increase of approximately 50% at 10 uM. This increase in (beta)AR population was apparent after only 1 day of treatment, and the pattern of increase was maintained for all 3 days of the treatment period. Thus, increasing the intracellular concentration of cAMP leads to up-regulation of (beta)AR population. Clenbuterol and isoproterenol gave similar effects on bAR population. The effect of forskolin on the quantity and apparent synthesis rate of the heavy chain of myosin (mhc) were also investigated. A maximum increase of 50% in the quantity of mhc was observed at 0.2 UM forskolin, but higher concentrations of forskolin reduced the quantity of mhc back to control levels.

Analogies of epinephrine are known to cause hypertrophy of skeletal muscle when fed to animals. These compounds presumably exert their physiological action through interaction with the P-adrenergic receptor. Since the intracellular signal generated by the Beta-adrenergic receptor is cyclic AMP (cAMP), experiments were initiated in cell culture to determine if artificial elevation of cAMP by treatment with forskolin would alter muscle protein metabolism and P-adrenergic receptor expression. Chicken skeletal muscle cells after 7 days in culture were treated with 0.2-30 micrometers forskolin for a total of three days. At the end of the treatment period, both the concentration of cAMP and the quantity of myosin heavy chain (MHC) were measured. Concentration of cAMP in forskolin-treated cells increased up to 10-fold in a dose dependent manner. In contrast, the quantity of MHC was increased approximately 50% above control cells at 0.2 micrometers forskolin, but exhibited a gradual decline at higher levels of forskolin so that the quantity of MHC in cells treated with 30 micrometers forskolin was not significantly different from controls. Curiously, the intracellular concentration of cAMP which elicited the maximum increase in the quantity of MHC was only 40% higher than cAMP concentration in control cells.

Skeletal muscle hypertrophy is promoted by in vivo administration of beta-adrenergic receptor (betaAR) agonists. These compounds presumably exert their physiological action through the betaAR, and alterations in the population of betaAR could potentially change the ability of the cell to respond to the betaAR agonists. Since the intracellular chemical signal generated by the betaAR is cyclic AMP (cAMP), experiments were initiated in primary chicken muscle cell cultures to determine if artificial elevation of intracellular cAMP by treatment with forskolin would alter the population of functional betaAR expressed on the surface of muscle cells. Chicken skeletal muscle cells after 7 days in culture were employed for the experiments because muscle cells have attained a steady state with respect to muscle protein metabolism at this stage. Cells were treated with 0-10 microM forskolin for a total of three days. At the end of the 1, 2, and 3 day treatment intervals, the concentration of cAMP and the betaAR population were measured. Receptor population was measured in intact muscle cell cultures as the difference between total binding of [H-3]CGP-12177 and non-specific binding of [H-3]CGP-12177 in the presence of 1 microM propranolol. Intracellular cAMP concentration was measured by radioimmunoassay. The concentration of cAMP in forskolin-treated cells increased up to 10-fold in a dose dependent manner. Increasing concentrations of forskolin also led to an increase in betaAR population, with a maximum increase of approximately 50% at 10 microM. This increase in PAR population was apparent after only 1 day of treatment, and the pattern of increase was maintained for all 3 days of the treatment period. Thus, increasing the intracellular concentration of cAMP leads to up-regulation of betaAR population. The effect of forskolin on the quantity and apparent synthesis rate of the heavy chain of myosin (mhc) were also investigated. A maximum increase of 50% in the quantity of mhc was observed at 0.2 microM forskolin, but higher concentrations of forskolin reduced the quantity of mhc back to control levels.