Effects of pharmacological manipulation of the renin-angiotensin system on the hibernation cycle of the 13-lined ground squirrel (Spermophilus tridecemlineatus).

Adult 13-lined ground squirrels were monitored for body temperature and state of torpor or arousal in a cold room at photoperiod LD 2:22 after receiving intraperitoneal injections of 0.9% saline vehicle or with the vehicle plus various test drugs. Saline injections (200 microliter) caused arousal of over 70% of the squirrels within 6 hours. Concomitant injections with Asp1 Val5 Angiotensin II (10 micrograms/kg, IP), however, significantly delayed arousal. This delaying effect was eliminated by simultaneous injections of the angiotensin II receptor antagonist, sarlasin (Sar1-Ala8-Ang II; 50 micrograms/kg, IP), In addition, 30 hours after induced arousal, the group receiving nonapeptide angiotensin I converting enzyme inhibitor (SQ 20, 881; 3 mg/kg) exhibited a significantly enhanced reentry into torpor. These data suggest that arousal from the reentry into torpor may be involved with renal function and the renin-angiotensin system. However, there appears to be temporal changes in the effect of angiotensin on torpor. Possible mechanisms, including the antagonistic effect of melatonin on angiotensin are discussed.