Segment-specific Ca2+ transport by isolated Malpighian tubules of Drosophila melanogaster: A comparison of larval and adult stages

Abstract

Haemolymph calcium homeostasis in insects is achieved through the regulation of calcium excretion by Malpighian tubules in two ways: (1) sequestration of calcium within biomineralized granules and (2) secretion of calcium in soluble form within the primary urine. Using the scanning ion-selective electrode technique (SIET), basolateral Ca2+ transport was measured at the distal, transitional, main and proximal tubular segments of anterior tubules isolated from both 3rd instar larvae and adults of the fruit fly Drosophila melanogaster. Basolateral Ca2+ transport exceeded transepithelial secretion by 800-fold and 11-fold in anterior tubules of larvae and adults, respectively. The magnitude of Ca2+ fluxes across the distal tubule of larvae and adults were larger than fluxes across the downstream segments by 10 and 40 times, respectively, indicating a dominant role for the distal segment in whole animal Ca2+ regulation. Basolateral Ca2+ transport across distal tubules of Drosophila varied throughout the life cycle; Ca2+ was released by distal tubules of larvae, taken up by distal tubules of young adults and was released once again by tubules of adults ⩾168h post-eclosion. In adults and larvae, SIET measurements revealed sites of both Ca2+ uptake and Ca2+ release across the basolateral surface of the distal segment of the same tubule, indicating that Ca2+ transport is bidirectional. Ca2+ uptake across the distal segment of tubules of young adults and Ca2+ release across the distal segment of tubules of older adults was also suggestive of reversible Ca2+ storage. Our results suggest that the distal tubules of D. melanogaster are dynamic calcium stores which allow efficient haemolymph calcium regulation through active Ca2+ sequestration during periods of high dietary calcium intake and passive Ca2+ release during periods of calcium deficiency.