A Study of the Malpighian Tubules of the Pill Millipede, Glomeris Marginata (Villers)

Isolated Malpighian tubules of Glomeris produce fluid which is identical to the bathing media with respect to ion concentrations and osmotic pressure. The tubules secrete normally in a potassium-free Ringer, but they cease to function in a sodium-free Ringer. The rate of fluid production is dependent on the availability of sodium ions, but it is also inhibited by higher concentrations (25 mM/1) of potassium ions. Throughout a wide variation in concentrations of sodium and potassium ions in the bathing Ringer, the concentrations of these ions in the tubule fluid are the same as their concentrations in the Ringer. Fluid production by the tubules of Glomeris is inhibited by ouabain at a concentration of 5 × 10−6 M/1 or greater. The rate of fluid production varies inversely with the osmotic pressure, and the secreted fluid remains approximately isosmotic to the bathing media. No electrical potential difference could be recorded across the tubule wall.