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

Ca2+ 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 Ca2+ and to play a role, therefore, in calcium homeostasis and elimination of excess dietary Ca2+. Approximately 85% of Ca2+ which enters the tubule is sequestered, and ∼15% is secreted in soluble form into the tubule lumen. Tubules secreting fluid at maximal rates can remove an amount of Ca2+ equal to the whole animal calcium content in ∼9 h. Distal segments of the pair of anterior MTs can sequester the same amount of Ca2+ in <2 h. Functional advantages of high Ca2+ turnover rates are discussed. Transepithelial Ca2+ 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 Ca2+ channels are an important component of sequestration. The contribution of some form of apical Ca2+ pump is evaluated.