Mechanisms of calcium sequestration by isolated Malpighian tubules of the house cricketAcheta domesticus
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Hemolymph calcium homeostasis in insects is achieved by the Malpighian tubules, primarily by sequestering excess Ca2+ within internal calcium stores (Ca-rich granules) most often located within type I (principal) tubule cells. Using both the scanning ion-selective electrode technique and the Ramsay secretion assay this study provides the first measurements of basolateral and transepithelial Ca2+ fluxes across the Malpighian tubules of an Orthopteran insect, the house cricket Acheta domesticus. Ca2+ transport was specific to midtubule segments, where 97% of the Ca2+ entering the tubule is sequestered within intracellular calcium stores and the remaining 3% is secreted into the lumen. Antagonists of voltage-gated (L-type) calcium channels decreased Ca2+ influx ≥fivefold in adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated tubules, suggesting basolateral Ca2+ influx is facilitated by voltage-gated Ca2+ channels. Increasing fluid secretion through manipulation of intracellular levels of cAMP or Ca2+ had opposite effects on tubule Ca2+ transport. The adenylyl cyclase-cAMP-PKA pathway promotes Ca2+ sequestration whereas both 5-hydroxytryptamine and thapsigargin inhibited sequestration. Our results suggest that the midtubules of Acheta domesticus are dynamic calcium stores, which maintain hemolymph calcium concentration by manipulating rates of Ca2+ sequestration through stimulatory (cAMP) and inhibitory (Ca2+ ) regulatory pathways.
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