Studies on large mobile protein appearing in submandibular glands of isoproterenol-treated rats. V. Further studies on some aspects of its formation.

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Abstract
Isoproterenol (IPR), a .beta.-adrenergic drug, accelerates protein secretion from and stimulates protein synthesis in the salivary glands of rats. Experiments on in vivo (3H)-Lys incorporation into rat submandibular glands after IPR treatment were performed. In the IPR-treated group, incorporation of radioactivity into the trichloroacetic acid (TCA)-insoluble protein fraction was 42% higher than in the control group. The specific radioactivity of large mobile (LM) protein was 4.5-fold higher than that of the TCA-insoluble protein fraction. The stimulatory effect of IPR was markedly suppressed by the pretreatment of rats with actinomycin D. Actinomycin D inhibited the increase of LM protein concentration in submandibular glands by more than 90%, and markedly suppressed the incorporations of radioactivity into TCA-insoluble protein fraction and LM protein, indicating that the LM protein was not derived from other proteins, but was newly biosynthesized after IPR administration. The amount of LM protein in submandibular glands reached 27.6% of total soluble protein, and the steady-state level was achieved at the 11th day upon chronic IPR administration. The rate constant of disappearance of LM protein was estimated to be 0.23/d [day]. The half-life and the rate constant for its biosynthesis were calculated to be 3.0 d and 410 .mu.g .cntdot. 100 mg gland-1 .cntdot. day-1, respectively.