REGRESSION of the MOUSE PAROTID GLAND INDUCED WITH ISOPROTERENOL

Acinar cells in rodent salivary glands are induced into one cycle of cell proliferation by a single injection of isoproterenol (IPR) (Baserga, 1970). Repeated injections of animals with IPR lead these cells to a few cycles of cell proliferation and then hypertrophy, when IPR stimulated proliferative response is lost and enlargement of the glands occurs. Without further IPR injections the enlarged glands regress to a normal size (Barka, 1965; van den Brenk, Sparrow & Moore, 1970). the present communication concerns the regression, or disappearance and reappearance, of the acinar cells which respond to IPR treatment. The parotid responds more readily to IPR in respect of cell proliferation and gland enlargement than the submaxillary gland (Baserga, 1970) and so was used in all studies. to produce full cell proliferation in the parotid, male SWR mice aged 3 months were given four injections at 48 hr intervals of DL‐isoproterenol‐HC1 (Sigma, St Louis, Missouri) at the dose of 7.5 mg per 30 g body weight. In the following description day zero refers to the date of fourth IPR injection. the parotid was then dissected out from animals at various days, and carefully freed of lymph nodes and fat tissues under a dissecting microscope. DNA content per gland (extracted by the method of Scott, Fraccastoro & Taft, 1956; Sasaki & Toda, 1972) increasea with the number of IPR injections, reaching a maximum after the fourth (Furuno, Iwasaki & Matsu‐daira, 1974), and then reverted to a control level in 15 days (Fig. 1). to follow the acinar cells induced into DNA synthesis, 20 hr after a second IPR treatment animals received single injections of methyl‐ 3 H‐thymidine (The Radiochemical Centre, Arnersham, England) ( 3 H‐TdR) by the amount of 20 μCi per 30 g body weight and then were killed at various intervals. Measurements were made on the amount of parotid DNA and its radioactivity in each animal, from which the specific activity of parotid DNA was calculated. the specific activity decreased about seven‐fold to a constant level in 15 days (Fig. 1). the DNA synthesis stimulated by a third and a fourth IPR treatment would account for the decrease in specific activity between day 3 and day 1. an autoradiographic study gave some estimate of cell turnover in the period day 1 to day 10. the IPR‐treated mice were given seventeen injections of 3 H‐TdR at 12 hr intervals from 20.00 hours on day 2 to 20.00 hours on day 10 and then