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Substrate utilization and ammonia secretion in the locust ileum

University of British Columbia

Increases in chloride-dependent short-circuit current (ΔIsc; an indicator of the major energy-requiring membrane transport process) were used to determine which external substrates support aerobic respiration in locust ileum stimulated by cAMP and theophylline in vitro. At high bilateral concentrations, only glucose and 5 amino acids (alanine, asparagine, glutamine, proline and serine) sustained ileal ΔIsc, while trehalose, lipid (diolein) and all other amino acids normally found in locust haemolymph were not utilized. When individual substrates were provided unilaterally at physiological concentrations, the predominant source of substrates supporting ileal Isc was the luminal fluid, which is mostly derived in situ from Malpighian tubule secretion. Only proline was used almost equally well from both the lumen or haemolymph sides. This situation contrasts with locust rectum where luminal proline is by far the predominant source of respiratory substrate. An inhibitor of amino transferases, amino-oxyacetate, largely abolished ileal ΔIsc sustained by alanine, but not that sustained by either proline or glutamine. The 5 amino acids that caused an increase in chloride-dependent Isc were assayed for their effect on luminal secretion of ammonia (Jamm), with and without cAMP. A saline with alanine, asparagine, glutamine, proline and serine supported ileal Jamm not significantly different from with Jamm complete saline. Each of these 5 amino acids, when applied individually, resulted in a Jamm significantly above a substrate-free control. The remaining amino acids found in a complete physiological saline (arginine, glycine, histidine, lysine, and valine) that do not stimulate ileal Isc were combined in one saline and gave a Jamm value insignificant from the substrate-free control. Asparagine and glutamine have the greatest effect on Jamm with both bilateral and luminal presentation. The addition of cAMP caused an increase in Jamm when either the complete saline or the 5 amino acid saline were present bilaterally, but no significant effect on Jamm caused by a luminal addition of individual amino acids. Jamm was unaffected by changes in luminal pH from 4.5 to 7.5, Na⁺ substitution and amiloride addition, indicating that luminal Jamm is primarily occurring by NH₄⁺ transport and not by diffusion trapping (NH₃).

Text

2010-12-02

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http://hdl.handle.net/2429/30270

Zoology

University of British Columbia

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