Mechanisms of cell volume regulation in the proximal segment of the Malpighian tubule of Rhodnius neglectus.

The cell volume regulation of the lower segment cells of the Malpighian tubule of Rhodnius neglectus in anisosmotic media was evaluated by using video-optic techniques. When the medium osmolality was increased with addition of 100 mM mannitol the cells shrank to a minimum of 16.84 +/- 2.62% and subsequently swelled towards their initial volume undergoing a typical regulatory volume increase (RVI). Replacement of either K+ or Cl- or HCO(3-) by Na+, gluconate and phosphate, respectively, abolished the RVI response. Furthermore, the substitution of Na+ by tetramethylammonium (TMA+) in isosmotic conditions led to cellular swelling and death. Addition of either amiloride 10(-4) M, anthracene-9-COOH 5 x 10(-4) M, furosemide 5 x 10(-4) M or ethacrynic acid 5 x 10(-5) M, also abolished RVI. On the other hand, addition of either Ba2+ 10(-3) M, SITS 5 x 10(-4) M, ouabain 10(-3) M or vanadate 10(-3) M, did not change the RVI response. When the tubules were incubated in hyperosmotic media with EGTA 2 mM or verapamil 10(-6) M, the RVI response was abolished. In contrast, a decrease of NaCl concentration from 129 to 79 mM induced a cell swelling to a maximum of 33.11 +/- 1.73%, but the cells maintained swollen, only partially regulating their volume. These results show that the proximal cells of Malpighian tubule of R. neglectus are able to regulate their volume in hyperosmotic but only partially regulating in hyposmotic solutions. The mechanisms in RVI involve Na+, K+, Cl-, Ca2+ and HCO(3-) transport pathways and a ouabain-insensitive ATPase stimulated by Na+.