Hemolymph calcium homeostasis in insects is achieved by the McDonald, & O'Donnell, 2000b;Maddrell et al., 1991;O'Donnell & Maddrell, 1995). Hemolymph Ca 2+ concentrations of flies increase < 1.5-fold when fed on diets with > sixfold higher calcium concentrations, demonstrating that Dipterans have the capacity for hemolymph calcium regulation (Dube et al., 2000b;Taylor, 1985). Hemolymph Ca 2+ concentrations are altered by changes in the rates of Ca 2+ absorption (input) or Ca 2+ excretion (removal). Rates of 45 Ca 2+ absorption across isolated midguts of the blowfly, Calliphora vicina, are unaffected by increases in the calcium content of the diet (from 0 to 12.5 mM CaCl 2 ), leading Taylor (1985) to conclude that absorption of dietary Ca 2+ across the midgut is unregulated and therefore modulation of rates of Ca 2+ excretion is the primary means by which hemolymph calcium regulation is achieved in insects.The Malpighian (renal) tubules and hindgut together form the functional "kidney" in insects (Maddrell, 1972).Malpighian tubules generate primary urine by transporting ions and osmotically-obliged water from the hemolymph into the tubule lumen. Several lines of evidence suggest that the tubules play a major role in excreting excess Ca 2+ from the hemolymph, either by secretion or sequestration. Secretion refers to the transport of Ca 2+ in soluble form into the primary urine, whereas sequestration refers to the transport of Ca 2+ into tubule calcium stores. In tubules of adult Drosophila melanogaster, ≥85% of the Ca 2+ which enters the tubules is sequestered and the remaining ≤15% is secreted into the lumen (Browne & O'Donnell, 2016;Dube, McDonald, & O'Donnell, 2000a). A reliance on Ca 2+ sequestration may be an effective strategy for eliminating large quantities of concentrated calcium (and counter ions) with relatively little water loss. Measurements of the calcium content of the Malpighian tubules indicate that they are also sites of physiologically relevant calcium storage. All 4 tubules isolated from larvae of Drosophila hydei contain approximately 88% of the calcium content of the entire flies . The majority of the calcium content within insect Malpighian tubules is correlated with the presence of numerous calcium-rich granules found in the cells and/or lumen (Brown, 1982).In crickets, these granules are spherical, relatively small (0.2 to 5 m in diameter) and are composed of concen-
Summary Ammonia is a toxic nitrogenous waste product of amino acid metabolism that may accumulate to high levels in the medium ingested by larvae of the fruit fly, Drosophila melanogaster. Here we report measurements of haemolymph NH4+ concentration and the secretion of NH4+ by the Malpighian (renal) tubules. Measurement of NH4+ concentrations in secreted droplets is complicated either by the requirement for large sample volumes for enzymatic assays or by the inadequate selectivity of NH4+-selective microelectrodes based on nonactin. We have developed a novel liquid membrane NH4+-selective microelectrode based on a 19-membered crown compound (TD19C6), which has been used previously in ammonium-selective macroelectrodes. In conjunction with an improved technique for correcting for interference of potassium, NH4+-selective microelectrodes based on TD19C6 permit accurate measurement of ammonium concentration in haemolymph samples and nanoliter droplets of fluid secreted by the Malpighian tubules of Drosophila melanogaster. The results indicate that active secretion of ammonium into the Malpighian tubule lumen is sufficient to maintain concentrations of approximately 1 mmol l-1 ammonium in the haemolymph of larvae reared on diets containing 100 mmol l-1 ammonium chloride.
The Malpighian tubules play a major role in haemolymph calcium homeostasis in insects by sequestering excess Ca2+ within the biomineralized granules that often accumulate in the tubule cells and/or lumen. Using the scanning ion‐selective microelectrode technique, measurements of basolateral Ca2+ transport are determined at several sites along the length of the Malpighian tubules isolated from the eight insects representing seven orders: Drosophila melanogaster (Diptera), Aedes aegypti (Diptera), Tenebrio molitor (Coleoptera), Acheta domesticus (Orthoptera), Trichoplusia ni (Lepidoptera), Periplaneta americana (Blattodea), Halyomorpha halys (Hemiptera) and Pogonomyrmex occidentalis (Hymenoptera). Ca2+ transport is specific to tubule segments containing Ca‐rich granules in D. melanogaster and A. aegypti, whereas Ca2+ transport is relatively uniform along the length of whole tubules in the remaining species. Generally, manipulation of second messenger pathways using cAMP and thapsigargin has little effect on rates of basolateral Ca2+ transport, suggesting that previous effects observed across midtubules of A. domesticus are unique to this species. In addition, the present study is the first to provide measurements of basolateral Ca2+ across single principal and secondary tubule cells, where Ca2+ uptake occurs only across principal cells. Estimated times for all tubules to eliminate the entire haemolymph Ca2+ content in each insect range from 6 min (D. melanogaster) to 19 h (H. halys) or more, indicating that rates of Ca2+ uptake by the Malpighian tubules are not always rapid. The results of the present study suggest that the principal cells of the Malpighian tubules contribute to haemolymph calcium homeostasis by sequestering excess Ca2+, often within specific tubule segments.
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