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Am J Physiol Regul Integr Comp Physiol 275: R1703-R1711, 1998;
0363-6119/98 $5.00
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Vol. 275, Issue 5, R1703-R1711, November 1998

Plasma hyperosmolality and arterial pressure regulation during heating in dehydrated and awake rats

Yasufumi Nakajima, Hiroshi Nose, and Akira Takamata

Departments of Anesthesiology and Physiology, Kyoto Prefectural University of Medicine, Kyoto 602-0841; and Department of Sports Medicine, Shinshu University School of Medicine, Matsumoto 390-8621, Japan

To gain better insights into the effect of dehydration on thermal and cardiovascular regulation during hyperthermia, we examined these regulatory responses during body heating in rats under isosmotic hypovolemia and hyperosmotic hypovolemia. Rats were divided into four groups: normovolemic and isosmotic (C), hypovolemic and isosmotic [L, plasma volume loss (Delta PV) = -20% of control], hypovolemic and less hyperosmotic [HL1, increase in plasma osmolality (Delta Posm) = 23 mosmol/kgH2O, Delta PV = -16%], and hypovolemic and more hyperosmotic (HL2, Delta Posm = 52 mosmol/kgH2O, Delta PV = -17%). Hyperosmolality was attained by subcutaneous injection of hypertonic saline and hypovolemia by intra-arterial injection of furosemide before heating. Then rats were placed in a thermocontrolled box (35°C air temperature, ~20% relative humidity) for 1-2 h until rectal temperatures (Tre) reached 40.0°C. Mean arterial pressure in L decreased with rise in Tre (P < 0.001), whereas mean arterial pressure remained constant in the other groups. Maximal tail skin blood flow in L, HL1, and HL2 was decreased to ~30% of that in C (P < 0.001). Tre threshold for tail skin vasodilation (TVD) was not changed in L, whereas the threshold shifted higher in the HL groups. Tre threshold for TVD was highly correlated with Posm (r = 0.94, P < 0.001). Heart rate in the HL groups increased with rise in Tre (P < 0.001), whereas it remained unchanged in C and L. Cardiovascular responses to heating were not influenced by V1 antagonist in C, L, and HL2. Thus isotonic hypovolemia attenuates maximal tail skin blood flow, whereas hypertonic hypovolemia causes an upward shift of Tre threshold for TVD and an increase in heart rate during hyperthermia. These results suggest that plasma hyperosmolality stimulates pressor responses in the hypovolemic condition that subsequently contribute to arterial pressure regulation during heat stress.

plasma osmolality; thermal dehydration; cardiovascular control; tail skin blood flow


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