Effect of a reduction in sodium intake on cold-induced elevation of blood pressure in the rat.

Chronic exposure of rats to cold (5 degrees C) induces hypertension within 3 weeks. The objective of this study was to determine the effect of treatment with graded levels of dietary NaCl on the induction of hypertension during chronic exposure to cold. Four groups of male rats were used. The first, given a commercial sodium-deficient diet containing 0.30% NaCl, served as the warm-adapted control group. The second, third, and fourth groups were given the same diet containing 0.075%, 0.15%, and 0.30% NaCl, respectively. Because cold-exposed rats ingest approximately twice as much food as warm-adapted controls, this represented half, the same, and twice the amount of NaCl ingested by the control group. The latter three groups were placed in cold air (5 degrees C). All cold-treated groups had an elevation of systolic blood pressure that was proportional to the concentration of NaCl in the diet by the seventeenth week of exposure to cold. Cardiac hypertrophy occurred to the same extent in all cold-exposed groups and was thus unaffected by the NaCl content of the diet or by the extent of elevation of blood pressure. Hence, cardiac hypertrophy during chronic exposure to cold is supported by other factors, possibly by the increased concentration of either norepinephrine or triiodothyronine, or both, which occurs characteristically in rats under these conditions. The results of this experiment suggest that the amount of NaCl ingested daily plays a role in the cold-induced elevation of blood pressure observed in rats.