Exchangeable sodium pool size and turnover in relation to diet in reindeer

Abstract

Sodium pool size and transfer rate was estimated in reindeer (Rangifer tarandus L.) using intravenous and intraluminal injections of ²²NaCl. Body water pool size and water transfer rate was estimated with ³HOH. Reindeer were consuming diets high in sodium (Na) and potassium (K) (commercial livestock ration), low in Na and high in K (summer grazing), and low in Na and K (lichens). The exchangeable Na pool size (milliequivalents per kilogram body weight) in reindeer given the high sodium diet (61 ± 2) was similar to other mammalian species. A marked decline in the pool (to 36 ± 3) was noted when reindeer were grazing summer vegetation in central Alaska and the Na pool was also low (46 ± 5) when they were fed lichens. Concentrations of Na in plasma, saliva, urine, and feces were directly correlated with the exchangeable Na pool size. Sodium transfer rates for reindeer given the high Na diet or grazing were two- to three-fold higher than daily Na intake rates. Loss of Na in feces and urine were of roughly equal importance in reindeer fed lichens or at pasture. Signs of Na insufficiency indicated by a salivary Na:K ratio of less than 2:1 were noted when the Na pool size declined below 40 mequiv./kg body weight. Evidence is presented for an extremely rapid replenishment of the Na pool following Na supplement of the diet.