Composition of fluid transported by locust ileum: influence of natural stimulants and luminal ion ratios

Abstract

Unstimulated fluid transport by everted locust ileal sacs in vitro was supported at 50% of control levels by the presence externally of any one of Na+, K+, or Cl−, whereas removal of all but trace levels of these ions reduced fluid transport to 25% of control transport rates. Stimulation of fluid transport by corpus cardiacum or fifth ventral ganglion extracts did not occur unless Cl− was present. The presence of Na+ or K+ was also required for maximum stimulation of fluid transport by these factors, the greatest stimulation occurring when the Na+:K+ ratio was 1:1. Cyclic AMP, and corpus cardiacum and fifth ventral ganglion extracts all stimulated Na+, Cl−, and K+ absorption across everted ileal sacs. Stimulation of fluid transport by these factors largely eliminated the anion deficit (Na+ + K+−Cl−) observed under unstimulated conditions. Stimulation caused large decreases in absorbate [Formula: see text] concentrations and pH concurrent with the increased absorbate Cl− levels. These results indicate a switch from low-capacity NaHCO3 plus NaCl transport under unstimulated conditions to high-capacity NaCl transport under stimulated conditions. Stimulation of fluid transport also causes a 3-fold increase in transepithelial potential (hemocoel negative), indicating stimulation of electrogenic anion (Cl−) movement to the hemocoel. These results provide the first direct evidence for hormonal control of Na+ reabsorption in insect excretory systems.