Calcium Regulation in Drosophila Melanogaster and Mechanisms of Malpighian Tubule Calcium Transport

Abstract

Most studies of insect Malpighian tubules (MTs) have examined transport of monovalent ions (K^+, Na^+, Cl^-). Isolated Drosophila melanogaster MTs also transport Ca^2+ from bath to lumen and transport is stimulated by cAMP. The lower segment of the MTs transports Ca^2+ at a higher rate per unit length than does the main segment known to produce the primary urine. This study examines both whole animal calcium regulation in larvae, pupae and adults and the mechanisms of Ca^2+ transport by isolated MTs. Drosophila melanogaster appears to regulate its calcium content and haemolymph calcium level. Calcium content of the whole fly only increased 10% with a 6.2-fold increase in dietary calcium. Anterior MTs can contain as much as 50% of the whole animal calcium content. The difference in MTs accumulation is due primarily to the enlarged initial segment of the anterior MTs. This segment, absent from the posterior MT, contains calcium-containing concretions. Whole fly calcium content does not increase continuously with the age implying that calcium is eventually being excreted. Haemolymph calcium concentrations do not change in response to changes in dietary calcium, suggesting that calcium concentration is regulated either by the rate of absorption or by the rate of excretion. The midgut and the enlarged initial segment of the anterior MTs may play important roles in haemolymph calcium regulation. Isolated MTs show sensitivity to both Ca^2+ channel blockers and Ca^2+ -ATPAse inhibitors on the basolateral and apical membranes respectively. Voltage-gated calcium channels appear to mediate calcium movement from bath to cell. A ruthenium red sensitive Ca^2+ -ATPAse may be used to transport calcium against the electrochemical gradient from cell to lumen. Lastly, the dissolution of luminal concretions plays a large role in net calcium secretion.