Special K: testing the potassium link between radioactive rubidium (86Rb) turnover and metabolic rate

Abstract

Summary The measurement of 86Rb turnover recently has been suggested as a useful method of measuring field metabolic rate in small animals. We investigated a proposed mechanism of 86Rb turnover, its analogy for K+, by comparing the turnover of 86Rb in a model insect, the rhinoceros beetle Xylotrupes gideon, fed diets of plum jam, or plum jam enriched with K+ or Rb+. The turnover of 86Rb in the beetles on the K+ and the Rb+ diets was higher than on the Jam diet (F2, 311 = 32.4; p = 1.58 × 10-13). We also exposed the beetles to different ambient temperatures to induce differences in metabolic rate (VCO2) while feeding them the Jam and K+ diets. VCO2 was higher at higher Ta for both Jam (F1,11 = 14.56; p = 0.003) and K+ (F1,8 = 15.39; p = 0.004) dietary groups, and the turnover of 86Rb was higher at higher Ta for both Jam (F1,11 = 10.80; p = 0.007) and K+ (F1,8 = 12.34; p = 0.008) dietary groups. There was a significant relationship between 86Rb turnover and VCO2 for both the Jam (F1,11 = 35.00; p = 1.0× 10-3) and the K+ (F1,8 = 64.33; p = 4.3 × 10-5) diets, but the relationship differed between the diets (F1,19 = 14.07; p = 0.001), with a higher 86Rb turnover on the K+-enriched than the Jam diet at all Ta. We conclude that 86Rb turnover is related to K+ metabolism, and that this is the mechanism of the relationship between 86Rb turnover and VCO2. Studies relating the 86Rb turnover to VCO2 should maintain dietary [K+] as close as possible to natural diets for the most accurate calibrations for free-ranging animals.