Use of Rb+ and Br- as tracers for investigating ion transport by X-ray microanalysis in the Malpighian tubules of the black field cricket Teleogryllus oceanicus

Substitution of Rb+ for K+ in the incubation saline for in vitro preparatio ns of Malpighian tubules had little effect on tubule function. Secretion ra tes increased by 10% for whole tubules, 9% for distal segments and 10% for main segments. In the secreted fluids Rb+ almost completely replaced K+. Wi thin the cells of the main segment of the tubules Rb replaced the majority of the intracellular K. Treatment by ouabain in Rb saline resulted in a con siderable increase in intracellular Na and Cl concentrations but no change in Rb concentration. This suggests that Rb+ did not enter the cell via Na K ATPase and that the latter was not directly involved in Rb+ secretion and by inference K+ secretion. Substitution of Br- for Cl- in the incubation sa line resulted in a 30% reduction in secretion rate from the distal segments but only a 10% reduction for the main segment. Cl- was almost completely r eplaced by Br- in fluid from both main and distal segments. In cells of the main segment the intracellular concentration of Br- did not exceed 30 mmol kg(-1) dry weight and the Cl- concentration was unchanged in the apical re gion of the cell and increased in the basal region. These data suggest that Br- was transported across the tubule epithelium by a paracellular route a nd that the basal cell membrane is relatively impermeable to Cl-. By infere nce Cl- may also be transported by a paracellular route. (C) 1999 Elsevier Science Ltd. All rights reserved.