Heterogeneous distribution of elemental contents in the larval Malpighian tubules of Drosophila hydei: X-ray microanalysis of freeze-dried cryosections.

The elements Na, Cl and K are distributed heterogeneously in the larval cells of the four Malpighian tubules of Drosophila hydei. Elemental gradients are steeper when they are related to the dry weight in comparison to the weight of water. The high contents of Na, Cl and K in the cytoplasm close to the basal plasma membrane differ significantly from the low values found in the neighboring basal "labyrinth" and in the intermediate cytoplasm. Elemental contents (K, Cl) increase from the intermediate cytoplasm to the apical microvilli and further to the lumen. The K/Na-ratio changes across the cells from 0.9 in the hemolymph to 2 in the basal cytoplasm close to the plasma membrane, to 27 in the microvilli, and 89 in the fluid phase of the lumen. In the lumen, K is accumulated in proteoglycan containing type-II concretions. The water content increases from the distal to the proximal segments of anterior and posterior tubules.