Ion and water transport by the malpighian tubules of Locusta
The mechanism by which diuretic hormone augments the rate of fluid secretion by the Malpighien tubules of Locusta migratoria miqratorioides has been investigated. Ion-substitution experiments were used to examine the ionic dependences of fluid secretion. With < C20 mM K+ in the bathing medium, the rate of fluid secretion is profoundly affected by small changes in [K+]; also K+ is secreted into the urine against a considerable concentration gradient supporting the contention that K+ is the "prime-mover" involved in fluid secretion. By contrast the [Na+] in the urine is low relative to the bathing medium; yet reduction of the [Na+] in the medium abolishes the stimulatory response, despite the presence of high levels of potassium (80-115 mM). This indicates a sodium dependent component of DH stimulated fluid secretion.The electrical events accompanying the secretory process were studied by the "paraffin-liquid gap" method and by the use of intracellular microelectrodes on single cells. The mean membrane potential across the basal membrane (Em) is ca. -40 mV, and the tronsepithelial potential (Tm) ca. +4.4 mV. On addition of DH no change in Em or Tm occurred, suggesting coupled pumps to be operative. Inhibitors of ion transport (amiloride, ethacrynic acid, ouabain and DNP) were tested for their effects on both Em and the fluid secretory process.Ion-selective microelectrodes were used to establish the ionic gradients across both the basal and apical membranes. This data together with that for the electrical gradients has been integrated in the form of a model for fluid secretion. Basal secretion can almost totally be accounted for by passively coupled NaCl and KC1 movements, whereas active transport must be invoked for DH stimulated secretion.