Effect of Norepinephrine on Heat Balancc in Cold-Adapted and Repeatedly Norepinephrine-Treated Conscious Rats
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Metabolic and heart rate responses to norepinephrine (NE: 0.4mg·kg<SUP>-1</SUP>) injected subcutaneously in conscious cold-adapted (CA: reared at 4 ± 1°C for more than 4 weeks), NE-treated (NE: daily injections of norepinephrine, 0.4mg·kg<SUP>-1</SUP> in oil, sc., for more then 4 weeks at 22 ± 2°C) and control (CT: reared at 22 ± 2°C for more than 4 weeks) rats were measured with a direct calorimeter of which wall temperatures (Tw) were 14°C and 25°C. At Tw 14°C, heat production (M), heat loss (HL) and colonic temperature (Tcol) at rest were higher in CA rats than in CT rats. Heart rate (HR) was, however, significantly lower in CA rats. An injection of NE produced an increase in M and Tcol in CA rats. The same dose of NE produced, however, a marked decrease in M and Tcol in CT rats. NE caused a significant fall in CT rats, but not in CA animals. NE produced an insignificant fall in HR and no significant change in M and Tcol in NE rats. At Tw 25°C, an injection of NE produced a significant increase in M and Tcol and an insignificant decrease in HR in both CA and CT rats. At this Tw, HR at rest was significantly lower than that observed at Tw 14°C in both CA and CT rats. From these data the followings were concluded ; 1) cardiac acceleration was not produced by norepinephrine in conscious rats particularly at lower ambient temperatures, 2) cold acclimation greatly reduced the bradycardiac response to this hormone, and 3) heart rate as well as cardiac output could be a limiting factor for the enhancement of calorigenic effects of norepinephrine in rats. Daily injections of norepinephrine seemed to reduce the pressor response to this hormone. The less bradycardiac response to norepinephrine in CA rats may be attributed to the decreased pressor response to this hormone, and the less cardio-inhibition elicited by the baroreceptor reflexes.
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