Prereplicative changes in the soluble calmodulin of isoproterenol‐activated rat parotid glands

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Abstract
Cells of rat parotid glands were maximally stimulated to initiate DNA synthesis by injecting into the animal a single dose of 25 to 150 mg of isoproterenol/ kg of body weight. During the 18‐ to 21‐hr prereplicative period following injection of the highest dose of the drug, there were two predominant and transient redistributions of calmodulin from the bound to the soluble form, which tripled the level of soluble calmodulin at 3 hr and again at 18 hr just before the initiation of DNA synthesis. A small (50%) increase in total calmodulin was observed only during the early (3‐h) prereplicative surge of soluble calmodulin. The late, pre‐DNA‐synthetic surge of soluble camodulin and the initiation of DNA synthesis were both prevented in rats that lacked their parathyroid‐thyroid gland complex and had been hypocalcemic for 48 or 72 hr. Unlike the effect of high doses of isoproterenol, low doses (e.g., 25 mg/kg body weight) of the β‐adrenergic drug could maximally stimulate DNA synthetic activity without the later pre‐DNA‐synthetic surge of soluble calmodulin, suggesting that any apparent correlation between the level of calmodulin and DNA synthesis may be spurious and that an actual increase in the level of soluble calmodulin just before the onset of DNA synthesis was not a prerequisite for DNA synthetic activity in parotid cells.