A Model for Fluid Secretion in Rhodnius Upper Malpighian Tubules (UMT)

We have measured fluid secretion rate in Rhodnius prolixus upper Malpighian tubules (UMT) stimulated to secrete with 5-OH-tryptamine. We used double perfusions in order to have access separately to the basolateral and to the apical cell membranes. Thirteen pharmacological agents were applied: ouabain, Bafilomycin A₁, furosemide, bumetanide, DIOA, Probenecid, SITS, acetazolamide, amiloride, DPC, BaCl₂, pCMBS and DTT. These agents are known to block different ion transport functions, namely ATPases, co- and/or counter-transporters and ion and water channels. The basic assumption is that water movement changes reflect changes in ion transport mechanisms, which we localize as follows: (i) At the basolateral cell membrane, fundamental are a Na⁺-K⁺-2Cl^– cotransporter and a Cl^–-HCO₃^– exchanger; of intermediate importance are the Na⁺-K⁺-ATPase, Cl^– channels and Rp-MIP water channels; K⁺ channels play a lesser role: (ii) At the apical cell membrane, most important are a K⁺-Cl^– cotransport that is being located for the first time, a V-H⁺-ATPase; and a Na⁺-H⁺ exchanger; a urate-anion exchanger and K⁺ channels are less important, while Cl^– channels are not important at all. A tentative model for the function of the UMT cell is presented.Symbols and abbreviations:ACTZ, acetazolamide; cAMP, cyclic adenosine-mono-phosphate; DIOA, [(dihydroindenyl)oxy] alkanoic acid; DPC, diphenylamine-2-carboxylate; DTT, dithiothreitol; 5-HT, 5-hydroxy-tryptamine; IR, Insects Ringer; J_v, secretion rate [nl/cm².s]; pCMBS, parachloro-mercuri-benzene-sulphonate; Rp-MIP, Rhodnius prolixus water channels; SITS, 4-acetamido-4-isothiocyanatostilbene -2,2-disulfonic Acid; UMT, upper malpighian tubules.