Indices of protein synthesis and RNA translating activities in the major salivary glands of rat and comparison to synthetic rates in liver

To date there have been no published studies which have reliably compared the general protein synthesis rates of the three major salivary glands either between themselves or with reference to other exocrine tissues. The main shortcoming of previous work has been the failure of the experimentalists to recognize the need to define the specific radioactivity of the precursor pool at the site of protein synthesis i.e. aminoacyl-tRNA [l]. By flooding the intracellular pool of phenylalanine with tritiated amino-add we have accurately measured the fractional rates of protein synthesis in the salivary glands and compared data with that from liver in this study. Mde wstar rats weighing approximately 150 g were purchased from B. and K. Universal Ltd (Hull, UK) and housed in a temperature controlled. humidified animal houseon a 12 hour light: 12 hour dark cycle. The rats were maintained on a commercial peileted chow and water ad libitum. On attaining 200-220 g body weight the rats were divided into 3 groups. Food, but not water, was withdrawn from al the animals a14 pm the day before the experiment. On the day of the experimentation rats were either injected i.p. with 10 ml/ kg ethanol (Ethanol-treated Group) or saline (Control Group) or allowed to refeed (Refed Group) in a darkened room.Two and a half hours later the rates of protein synthesis were measured by the 'flooding dose' technique as described by Garlidc et d 121. At 2 min and 10 min afler an i.v. dose of L-[4 3H]-phenylalanine the rats were decapitated and salivary glands and livers rapidly dissected out and plunged into liquid nitrogen. Processing of tissues for phenylalanine specific radioactivities was as described previously [2] and modified later to include the addition of protease inhibitors during the initial processing steps [3]. The fractional rate of protein synthesis, k, , was defined as the percentage of the protein pool renewed each day, and is calculated from the formula