Cellular dynamics of rats' submaxillary gland during heat acclimatization.

Male laboratory rats (Rattus norvegicus) ranging between 130 and 160 g were subjected to ambient temperatures of 34.5 degrees C from 0 to 28 days. The submaxillary gland, liver, spleen, and kidney were weighed. The acini size and the mitotic index of the acinar cells after intraperitoneal injection of vinblastine sulfate and colchicine were recorded for the submaxillary and parotid glands. The submaxillary glands showed increased growth rate over the initial 5 days, whereas the growth rate of the other organs decreased. The submaxillary gland enlargement was due to an initial hyperplasia of the acini cells reaching a maximum on the 2nd day. This was followed by hypertrophy of the Acini reaching a maximum on the l0th day. The parotid acini at control values throughout the experiment. It is postulated that the gland enlargement is effected to increase evaporative heat loss during the early stages of acclimatization prior to the lowering of heat production resulting from biochemical adaptation.