Sympathetic α-adrenergic regulation of blood flow and volume in hamsters arousing from hibernation

Abstract

Mammals arousing from hibernation display pronounced regional heterothermy, where the thoracic and head regions warm faster than the abdominal and hindlimb regions. We used laser-Doppler flowmetry to measure peripheral hind foot blood flow during hibernation and arousal and gamma imaging of technetium-labeled albumin to measure whole blood volume distribution in hamsters arousing from hibernation. It was discovered that the hibernating hamster responds to physical but not to sound or hypercapnic stimulation with rapid, 73% reduction of hind foot blood flow. Hind foot blood flow vasoconstriction was maintained from the onset of arousal until late in arousal when rectal temperature was rapidly increased. ␣-Adrenergic blockade early in arousal increased hind foot blood flow by 700%, suggesting that vasoconstriction was mediated by activation of sympathetic tone. Gamma imaging revealed that, by the early phase of arousal from hibernation, the blood volume of the body below the liver is greatly reduced, whereas blood volumes of the thorax and head are much greater than corresponding volumes in anesthetized hamsters. As arousal progresses and cardiac activity increases and regional heterothermy develops, this regional blood volume distribution is largely maintained; however, blood volume slowly decreases in the thoracic region and slowly increases in the shoulder and head regions. The rapid increase in rectal temperature, characteristic of mid-to latearousal phases, is probably mediated, in part, by reduction of adrenergic tone on abdominal and hindlimb vasculature. Warm blood then moves into the hind body, produces an increase in temperature, blood flow, and blood volume in the hind body and compensatory reductions of blood volume in the neck, head, and thoracic regions.laser-Doppler flowmetry; gamma imaging; torpor; vasoconstriction THE TRANSITION FROM THE low body temperatures of classical hibernation to cenothermia is associated with probably the most remarkable changes in mammalian circulatory physiology, and yet this has been the subject of few in vivo investigations and remains poorly understood. For small mammals in natural hibernation, body temperature, metabolism (20, 24), heart rate (HR), blood pressure (1, 16), and cerebral blood flow (6,20) are greatly reduced relative to cenothermia. However, despite these enormous reductions in physiological parameters, in vivo studies demonstrate that blood pressure and HR during hibernation remain sensitive to pharmacological activation of ␣-and ␤-adrenergic receptors, respectively (16). Evidence for parasympathetic influences on HR and respiration during hibernation is more equivocal and may only be evident in some species that exhibit episodic breathing (10). Respiratory chemosensitivity to hypercapnic gas is retained at low body temperatures during hibernation (14, 18). Collectively, these results are consistent with the functional conservation of some aspects of cenothermic autonomic regulation during hibernation in hamsters, sciurids, and possibly all other small...

No oxygen? No problem! Intrinsic brain tolerance to hypoxia in vertebrates

“…When hibernating, animals may spend from a few days to several weeks at a time in a highly regulated and reversible state of prolonged torpor during which whole body metabolic rate, core T b , heart rate and blood flow plummet to levels seemingly inconsistent with life support. When T b falls below 30°C, prolonged periods of torpor are interrupted by brief intervals of euthermy during which animals spontaneously return to high, euthermic T b of 35-37°C (Dausmann et al, 2004;Geiser and Ruf, 1995) and blood flow returns to vital organs in a heterogeneous manner (Osborne et al, 2005).…”

Temporal and temperature effects on the maximum rate of rewarming from hibernation

“…Regional blood flow is regulated during arousal from torpor. Blood flow to the hind foot is markedly reduced during early arousal in hibernators (Osborne et al, 2005). Blood flow posterior to the diaphragm is restricted in arousing thirteen-lined ground squirrels until the thoracic temperature is ∼ 25° C (Bullard and Funkhouser, 1962).…”

Brain ECF antioxidant interactions in hamsters during arousal from hibernation

“…However, the demonstration that XOR generated superoxide increases basal cortical blood flow (CBF) and impairs cerebral autoregulation [44] may have relevance to the process by which cerebral hemodynamic compliance can be increased to preserve non-pathological integrity during the enormous increase in CBF and loss of autoregulation that hamsters experience late in arousal from hibernation [30] when blood is shunted for warming into the anterior half of the body [33].…”

“…Mammalian hibernation is a naturally induced metabolic depression and re-warming from the low body temperatures of hibernation, typically 4-5 •C, to cenothermia (IUPS term replacing euthermia, typically 36-37 •C) involves the endogenous generation of heat by metabolic activity [30,39] and the induction and later abolition of regional body heterothermy via vasoconstrictive regulation of regional blood flow [33]. This combination of increasing metabolic activity coincident with large fluxes in blood flow and temperature that occurs during arousal from hibernation is similar in some respects to postischemic re-perfusion.…”