Since it had been demonstrated that cold swelling of tissues at low temperatures in vitro is caused by a failure of the "sodium pump, " and since tissues of hibernating mammals have been shown to be resistant to cold in vitro with regard to membrane function and metabolism, it was of interest to determine whether tissues of hibernating mammals were resistant to cold swelling. This was particularly so because it had been reported that ground squirrel tissues were not.
A new type of freezing point depression osmometer is described which utilizes stream-mixing as a mechanism of temperature control. In vitro weight change experiments on tissues of warm room and hibernating hamsters and of hibernating ground squirrels indicate that swelling does occur in these species as in rats. The in vivo water content of tissues of hibernating ground squirrels and hamsters is never greater and frequently is less than that of tissues from awake animals.
In hamsters, resistance to cold swelling is abolished as the individual arouses from hibernation and is absent during at least part of the subsequent period of normothermia. Loss of solids from incubated diaphragms is less at 6° C than at 37° C in vitro in all groups except awake ground squirrels. In awake hamster incubated diaphragms, solid loss is increased at 6° C by the presence of NaF. Diaphragms of hibernating hamsters and ground squirrels, however, resist this effect. Saturated fat in the diet of awake hamsters tends to abolish the resistance of their diaphragms to cold swelling.
Freezing point determinations of plasma indicated that hibernating hamsters have slightly but significantly more concentrated plasma than awake hamsters. The concentration of hibernating hamster plasma is considerably higher than Krebs' medium. The concentration of awake ground squirrel plasma is very high. The plasma concentration of hibernating ground squirrels is lower, though it is as concentrated as hibernating hamster plasma. Comparison of weight change and hydration change of incubated diaphragtns in this study indicated that weight change curves are a poor measure of swelling because of solid loss during incubation. The mechanism of in vivo swelling is discussed. It is concluded that, although other factors surely play a role, in vivo swelling may occur by the same process as that of in vitro swelling. Possible mechanisms of resistance to cold swelling are discussed.