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Metabolic studies on the locust rectum

University of British Columbia

The in vitro, short-circuited locust rectum actively transports chloride. This electrogenic transport is energized by aerobic metabolism, based primarily upon proline oxidation. Isolated rectal mito-chondrialoxidize a variety of substrates, but the highest state 3 rate of 0₂ consumption occurs when proline is the stubstrate. An enzyme profile of the rectal tissue indicates that the rectum can oxidize amino acids and carbohydrates but has a limited capacity for lipid oxidation. Analysis of the two fluids which bathe the rectum in vivo, the hemolymph and the Malpighian tubule fluid, reveals that proline is present in bothethe hemolymph (12-15 mM) and tubule fluid (38-43 mM). Although glycine is also found in high concentration in the hemolymph (13-17 mM), far lower concentrations of this and other amino acids are found in the Malpighian tubule fluid. Glucose is also found in the hemolymph (2 mM) and Malpighian tubule fluid (4 mM). The high concentraion of proline in the tubule fluid indicated that this amino acid may be actively transported by tubules. This prediction was confirmed by experiments with in vitro tubules. This is the first evidence of aetive secretion of a metabolically useful compound by insect Malpighian tubules. Experiments involving the measurement of short-circuit current (Isc) across the locust rectum were also performed. The results of these experiments indicate that high levels (50 mM) of proline stimulate the Isc of substrate-depleted recta better than high levels of any other substrate tested. Physiological levels of proline also cause a large increase in the Isc of substrate-depleted recta, while physiological levels of glucose produce a much smaller stimulation. Over 90% of the Isc stimulation can be produced by adding proline (15 mM) solely to the lumen side of the tissue. Other studies were performed to estimate the metabolic cost of actively transporting chloride. The oxygen consumption of chloride-depleted recta were measured before and after reintroduction of chloride. From these data a Cl⁻/ATP ratio of 3-4 was obtained. Further calculations indicated that tissue proline oxidation is sufficient to energize active chloride transport. The results outlined here suggest that in vivo, the rectal lumen is bathed with a high concentration of proline which can be readily oxidized by the rectal mitochondria to support the work of actively transporting chloride.

Text

2010-04-13

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http://hdl.handle.net/2429/23462

Zoology

University of British Columbia

Graduate