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

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Abstract
Unstimulated fluid transport by everted locus 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 (NKa+ + K+ - Cl-) observed under unstimulated conditions. Stimulation caused large decreases in absorbate HCO3- 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 cotnrol of Na+ reabsorption in insect excretory systems.