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