The exchange of calcium in larvae of the mosquito Aedes aegypti

Dietary calcium (Ca2+) impacts Ca2+ content and molecular expression of Ca2+-transporters in Malpighian tubules of the yellow fever mosquito, Aedes aegypti

The renal (Malpighian) tubules of insects play important roles in hemolymph Ca2+ regulation. Here we investigated how dietary Ca2+ loads from sucrose or blood meals affect the Ca2+ content and mRNA expression of Ca2+ transporters in Malpighian tubules of adult female mosquitoes. Using the yellow fever mosquito Aedes aegypti we found that feeding females 10% sucrose with elevated Ca2+ concentration ad libitum for 6 days led to increased Ca2+ content in Malpighian tubules. The increases of Ca2+ content correlated with up-regulations of mRNAs encoding intracellular Ca2+-ATPases (SERCA and SPCA), a plasma membrane Ca2+-ATPase (PMCA), and a K+-dependent Na+/Ca2+ exchanger (NCKX1). We also found that when adult females were fed blood, tubule Ca2+ content changed dynamically over the next 72 h in a manner consistent with redistribution of tubule Ca2+ stores to other tissues (e.g., ovaries). The changes in tubule Ca2+ were correlated with dynamic changes in mRNA abundances of SERCA, SPCA, PMCA, and NCKX1. Our results are the first to demonstrate that Malpighian tubules of adult female mosquitoes have a remarkable capacity to handle high dietary Ca2+ loads, most likely through the combination of storing excess Ca2+ within intracellular compartments and secreting it into the tubule lumen for excretion. Our results also suggest that the Malpighian tubules play key roles in supplying Ca2+ to other tissues during the processing of blood meals.

Toxicological perspective on the osmoregulation and ionoregulation physiology of major ions by freshwater animals: Teleost fish, crustacea, aquatic insects, and Mollusca

Anthropogenic sources increase freshwater salinity and produce differences in constituent ions compared with natural waters. Moreover, ions differ in physiological roles and concentrations in intracellular and extracellular fluids. Four freshwater taxa groups are compared, to investigate similarities and differences in ion transport processes and what ion transport mechanisms suggest about the toxicity of these or other ions in freshwater. Although differences exist, many ion transporters are functionally similar and may belong to evolutionarily conserved protein families. For example, the Na+ /H+ -exchanger in teleost fish differs from the H+ /2Na+ (or Ca2+ )-exchanger in crustaceans. In osmoregulation, Na+ and Cl- predominate. Stenohaline freshwater animals hyperregulate until they are no longer able to maintain hypertonic extracellular Na+ and Cl- concentrations with increasing salinity and become isotonic. Toxic effects of K+ are related to ionoregulation and volume regulation. The ionic balance between intracellular and extracellular fluids is maintained by Na+ /K+ -adenosine triphosphatase (ATPase), but details are lacking on apical K+ transporters. Elevated H+ affects the maintenance of internal Na+ by Na+ /H+ exchange; elevated HCO3- inhibits Cl- uptake. The uptake of Mg2+ occurs by the gills or intestine, but details are lacking on Mg2+ transporters. In unionid gills, SO42- is actively transported, but most epithelia are generally impermeant to SO42- . Transporters of Ca2+ maintain homeostasis of dissolved Ca2+ . More integration of physiology with toxicology is needed to fully understand freshwater ion effects. Environ Toxicol Chem 2017;36:576-600. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Calcium uptake in aquatic insects: Influences of phylogeny and metals (Cd and Zn)

Calcium sequestration in the hypo-osmotic freshwater environment is imperative in maintaining calcium homeostasis in freshwater aquatic organisms. This uptake process is reported to have the unintended consequence of potentially toxic heavy metal (Cd, Zn) uptake in a variety of aquatic species. However, calcium uptake remains poorly understood in aquatic insects, the dominant invertebrate faunal group on most freshwater ecosystems. Here we examined Ca uptake and interactions with heavy metals (Cd, Zn) at low ambient Ca levels (12.5 μmol l-1) in 12 aquatic insect species within Ephemerellidae (mayfly) and Hydropsychidae (caddis fly), two families differentially responsive to trace metal pollution. We found Ca uptake varied 70-fold across the 12 species studied. Body weight and clade (family) were found to significantly influence both Ca uptake and adsorption (p < 0.05). Zn and Cd uptake rate constants (kus) exhibited a strong correlation (r = 0.96, p < 0.0001), suggesting a shared transport system. Ca uptake failed to significantly correlate with either Zn or Cd kus. Further, neither Zn nor Cd exhibited inhibitory effects toward Ca uptake. In fact we saw evidence of modest stimulation of Ca uptake rates in some metal treatments. This work suggests that insects generally differ from other freshwater taxa in that aqueous Ca uptake does not appear to be compromised by Cd or Zn exposure. It is important to understand the trace metal and major ion physiology of aquatic insects due to their ecological importance and widespread use as ecological indicators.

Divalent metal (Ca, Cd, Mn, Zn) uptake and interactions in the aquatic insect Hydropsyche sparna

Despite their ecological importance and prevalent use as ecological indicators, the trace element physiology of aquatic insects remains poorly studied. Understanding divalent metal transport processes at the water–insect interface is important because these metals may be essential (e.g. Ca), essential and potentially toxic (e.g. Zn) or non-essential and toxic (e.g. Cd). We measured accumulation kinetics of Zn and Cd across dissolved concentrations ranging 4 orders of magnitude and examined interactions with Ca and Mn in the caddisfly Hydropsyche sparna. Here, we provide evidence for at least two transport systems for both Zn and Cd, the first of which operates at concentrations below 0.8 μmol l–1 (and is fully saturable for Zn). We observed no signs of saturation of a second lower affinity transport system at concentrations up to 8.9 μmol l–1 Cd and 15.3 μmol l–1 Zn. In competition studies at 0.6 μmol l–1 Zn and Cd, the presence of Cd slowed Zn accumulation by 35% while Cd was unaffected by Zn. At extreme concentrations (listed above), Cd accumulation was unaffected by the presence of Zn whereas Zn accumulation rates were reduced by 58%. Increasing Ca from 31.1 μmol l–1 to 1.35 mmol l–1 resulted in only modest decreases in Cd and Zn uptake. Mn decreased adsorption of Cd and Zn to the integument but not internalization. The L-type Ca2+ channel blockers verapamil and nifedipine and the plasma membrane Ca2+-ATPase inhibitor carboxyeosin had no influence on Ca, Cd or Zn accumulation rates, while Ruthenium Red, a Ca2+-ATPase inhibitor, significantly decreased the accumulation of all three in a concentration-dependent manner.

Analysis of Na+, Cl-, K+, H+and NH4+ concentration gradients adjacent to the surface of anal papillae of the mosquitoAedes aegypti: application of self-referencing ion-selective microelectrodes

Ion concentration gradients adjacent to the surface of the anal papillae of larvae of the mosquito Aedes aegypti were measured using self-referencing ion-selective microelectrodes. The gradients were used to calculate estimates of ion fluxes into and out of the papillae. There was a net influx of Na+, Cl- and K+ from the bathing medium and a net efflux of acid and NH4+. No Ca2+ gradients were detectable. Na+ and Cl-influx occurred against a concentration gradient suggesting active transport. Although Na+, Cl- and NH4+gradients were uniform along the length of the papillae, the proximal regions of the papillae in vivo revealed significantly higher H+and K+ gradients compared with distal regions. The calculated ion fluxes at the papillae are sufficient for complete Na+,K+ and Cl- haemolymph replacement in ∼4 h with external ion concentrations of 5 mmol l-1. Ion gradients were also detected adjacent to the surface of isolated papillae; however, Na+and H+ gradients were higher, and Cl- gradients were lower relative to papillae in vivo. The results support previous findings that the anal papillae of mosquito larvae are important structures for ion regulation, and suggest that these structures may be used for the excretion of nitrogenous waste.

Calcium transport by isolated anterior and posterior Malpighian tubules of Drosophila melanogaster: roles of sequestration and secretion

Ca(2+) transport was examined in isolated Malpighian tubules (MTs) of adult Drosophila melanogaster. All segments of both anterior and posterior MTs have substantial capacity to transport Ca(2+) and to play a role, therefore, in calcium homeostasis and elimination of excess dietary Ca(2+). Approximately 85% of Ca(2+) which enters the tubule is sequestered, and approximately 15% is secreted in soluble form into the tubule lumen. Tubules secreting fluid at maximal rates can remove an amount of Ca(2+) equal to the whole animal calcium content in approximately 9 h. Distal segments of the pair of anterior MTs can sequester the same amount of Ca(2+) in <2 h. Functional advantages of high Ca(2+) turnover rates are discussed. Transepithelial Ca(2+) secretion is increased by treatments which depolarize the transepithelial potential (thapsigargin, high K(+)), or acidify the secreted fluids (bicarbonate-free salines). The effects of pharmacological reagents and variations in bathing saline ionic composition indicate that the processes of secretion and sequestration are controlled independently, and that diltiazem-sensitive Ca(2+) channels are an important component of sequestration. The contribution of some form of apical Ca(2+) pump is evaluated.