Electrophysiological and cable parameters of perfused beetle malpighian tubules

Isolated perfused Malpighian tubules of the desert beetle Onymacris plana (Coleoptera: Tenebrionidae) have been subjected to cable analysis under the following conditions: control, adenosine 3',5'-cyclic monophosphate (cAMP), corpora cardiaca homogenate (CCH), and high ambient K (130 mM). In addition, we investigated possible effects of perfusate composition on proximal transtubular potential (Vo) by reducing K, Na, or Cl or by adding ouabain, furosemide, or dinitrophenol. The effects of cAMP, CCH, and high K on Vo and cable parameters were consistent with increased fluid secretion, i.e., diminished input and core resistances and increased virtual short-circuit current, length constant, and luminal diameter. They differed in that CCH had variable effects on Vo and high K did not reduce transepithelial resistance. In terms of their effects on the parameters of a simple equivalent electrical circuit, the responses to cAMP, CCH, and a high ambient K concentration appear to be mediated by different mechanisms. Alterations in perfusate composition were almost without effect.