Norepinephrine (NE) and epinephrine (Epi) concentrations in arterial plasma and in skin tissue were measured chromatographically before and after external cooling. Urethan-anesthetized rats were cooled either slowly (0.004–0.006°C/s) or rapidly (0.03– 0.05°C/s). Blood samples were drawn three times from each animal: 1) before cooling and at a rectal temperature decreased 2) by 0.5°C and 3) by 3–4°C. Skin samples were taken from controls and from rapidly or slowly cooled rats at a rectal temperature lowered by 0.5°C. The resting mean values were 36.7 ± 0.3°C for rectal temperature, 0.62 ± 0.079 and 1.09 ± 0.203 ng/ml for plasma NE and Epi, and 85.6 ± 4.1 and 137.6 ± 34.3 ng/g for skin NE and Epi. A decrease in rectal temperature by 0.5°C at rapid cooling produced a 2.6-fold increase of NE and a 2.8-fold increase of Epi in plasma. Concomitantly, there was a significant decrease in skin NE concentration by 28% and Epi by 86%. At a rectal temperature decreased by 0.5°C after slow cooling, plasma catecholamines did not change; at unaltered skin NE concentration, there was a reduction in skin Epi concentration (60%). When rectal temperature was lowered by 3–4°C, the increase in plasma NE was virtually the same at both cooling rates and only plasma Epi increased more after deep rapid cooling than slow cooling. Thus the sympathoadrenal system may be differently activated depending on cooling rate. Rapid cooling, when the dynamic activity of the skin cold receptors is involved in the cold response, may provide conditions for an earlier activation of the sympathoadrenal system. This may evidence the functional significance of the dynamic activity of the skin cold receptors in the formation of the cold defense responses.

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