Sequestration of haemolymph sodium and potassium by fat body in the water-stressed cockroach Periplaneta americana

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Cockroaches that were dehydrated for 8 days lost 50% of the body water, with haemolymph volume decreasing from 18.9% to 9.3% of total wet weight. Haemolymph osmotic pressure. Na + and K + corcentrations were regulated within narrow limits (O.P. 413–459 mOs/Kg; Na + 185 – 210 mM; K + 16.0–19.5 mM). Rehydration of dehydrated insects restored the haemolymph osmalality and Na + and K + concentrations to near original levels within 24 hr. During dehydration 1219 μg Na + (expressed as NaCl) and 158 μg K + (expressed as KCl) were removed from the haemolymph. Only 4% (approximately) of these amounts were excreted. Na + and K + ingestion and excretion being virtually in balance. The Na + and K + removed from the blood was entirely accounted for by the observed increase in Na + and K + concentrations in the fat body (Na + 11.02–18.10 mg/g: K + 31.85–43.78 mg/g expressed as chlorides during dehydration. The increase in K + in the fat body at day 8 was approximately eight times the amount of K + removed from the haemolymph. It is suggested that this is due to a transfer of intracellular K + from other tissues. The rate of uptake of these cations appears to show saturation kinetics. It is suggested that K + and possibly Na + are sequestered as urates in the fat body by a reversible process.