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Ammonia transport by the turtle urinary bladder
The American Journal of Physiology
|May 1, 1984
Summary
Turtle bladders transport ammonia via both uncharged ammonia (NH3) and charged ammonium (NH4+). Different pH conditions reveal distinct transport mechanisms and permeabilities for NH3 and NH4+ across the bladder wall.
Area of Science:
- Physiology
- Comparative Anatomy
- Renal Transport Mechanisms
Background:
- Ammonia is a key waste product that requires efficient excretion.
- The turtle bladder serves as a model for studying urinary concentrating and ammonia transport mechanisms.
- Understanding the differential transport of ammonia species (NH3 and NH4+) is crucial for comprehending nitrogenous waste handling in reptiles.
Purpose of the Study:
- To investigate the mechanisms and characteristics of ammonia transport across the turtle bladder.
- To determine the relative contributions and permeabilities of uncharged ammonia (NH3) and ammonium ions (NH4+) to transcellular transport.
- To explore the influence of pH and solute concentration on ammonia flux and identify potential transport pathways.
Main Methods:
- Ammonia transport was studied by altering ammonium chloride (NH4Cl) concentrations and pH in serosal (S) and mucosal (M) solutions bathing the turtle bladder.
- Transepithelial ammonia flow was measured under varying conditions to assess permeability and transport rates.
- Short-circuit current measurements were used to evaluate the electrogenic nature of ammonia transport.
Main Results:
- Ammonia transport occurs via both NH3 and NH4+; NH3 permeability (S-M) is significantly higher than NH4+ permeability.
- Transport direction and magnitude are pH-dependent, with different permeabilities observed for NH4+ across serosal and mucosal membranes.
- Ammonia transport exhibits both linear and saturation kinetics depending on NH4Cl concentration and pH, and competitive inhibition by methylamine suggests a common carrier system.
Conclusions:
- The turtle bladder actively transports ammonia, utilizing distinct pathways for NH3 and NH4+.
- Ammonia transport can be electrogenic, particularly for NH4+ at lower pH, indicating the involvement of ion channels or carriers.
- The findings highlight the complex and adaptable nature of ammonia excretion in reptiles, influenced by physiological conditions.