Metabolic Support of Chloride-dependent Short-circuit Current across Locust Rectum*

1. Recta of desert locusts were short-circuited and depleted of endogenous substrates by exposing them to saline containing cyclic AMP but no metabolites. Individual substrates were then added to substrate-depleted recta and the change in short-circuit current (/sc) monitored. 2. Proline or glucose (50 mil) caused by far the largest increase in 780 of all substrates tested. Stimulation of the 7SC by proline was not dependent upon external sodium, but did require external chloride. 3. Physiological levels of proline also caused a large increase in 7SC, while physiological levels of glucose produced a much smaller stimulation. Over 90% of the proline-dependent 7SC stimulation can be produced by adding 15 mM proline solely to the lumen side of the tissue. 4. These results are discussed with regard to rectal oxidative metabolism and availability of metabolic substrates in vivo. High levels of proline in Malpighian tubule fluid are probably the major substrate source for rectal Cl~ transport.