Mechanisms of Calcium Sequestration by Isolated Malpighian Tubules of the House Cricket Acheta Domesticus

Hemolymph calcium homeostasis in insects is achieved by the Malpighian tubules, primarily by sequestering excess Ca 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 Ca fluxes across the Malpighian tubules of an Orthopteran insect, the house cricket Acheta domesticus. Ca transport was specific to midtubule segments, where 97% of the Ca 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 Ca influx ≥fivefold in adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated tubules, suggesting basolateral Ca influx is facilitated by voltage-gated Ca channels. Increasing fluid secretion through manipulation of intracellular levels of cAMP or Ca had opposite effects on tubule Ca transport. The adenylyl cyclase-cAMP-PKA pathway promotes Ca 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 Ca sequestration through stimulatory (cAMP) and inhibitory (Ca ) regulatory pathways.