Temporal relationships of blood pressure, heart rate, baroreflex function, and body temperature change over a hibernation bout in Syrian hamsters

Hibernating mammals undergo torpor during which blood pressure (BP), heart rate (HR), metabolic rate, and core temperature (T_C) dramatically decrease, conserving energy. While the cardiovascular system remains functional, temporal changes in BP, HR, and baroreceptor-HR reflex sensitivity (BRS) over complete hibernation bouts and their relation to TC are unknown. We implanted BP/temperature telemetry transmitters into Syrian hamsters to test three hypotheses: H-1) BP, HR, and BRS decrease concurrently during entry into hibernation and increase concurrently during arousal; H-2) these changes occur before changes in T_C; and H-3) the pattern of changes is consistent over successive bouts. We found: 1) upon hibernation entry, BP and HR declined before T_C and BRS, suggesting baroreflex control of HR continues to regulate BP as the BP set point decreases; 2) during the later phase of entry, BRS decreased rapidly whereas BP and T_C fell gradually, suggesting the importance of T_C in further BP declines; 3) during torpor, BP slowly increased (but remained relatively low) without changes in HR or BRS or increased T_C, suggesting minimal baroreflex or temperature influence; 4) during arousal, increased T_C and BRS significantly lagged increases in BP and HR, consistent with establishment of tissue perfusion before increased T_C/metabolism; and 5) the temporal pattern of these changes was similar over successive bouts in all hamsters. These results negate H-1, support H-2 with respect to BP and HR, support H-3, and indicate that the baroreflex contributes to cardiovascular regulation over a hibernation bout, albeit operating in a fundamentally different manner during entry vs. arousal.