title: Acid-base regulation in response to environmental hypercapnia in two aquatic salamanders, Siren lacertina and Amphiuma means.
abstract: The partial pressure of CO2 (PCO2) in certain areas of the aquatic habitat of the salamanders Siren lacertina and Amphiuma means frequently rises to values of up to 60 mm Hg. This ambient hypercapnia occurs due to hindrance of gas exchange between water and air caused by dense water-surface vegetation. In order to investigate the acid-base regulation in response to the respiratory acidosis, which wound be expected to result from the high CO2 conductance of the amphibian skin, specimens of both species were subjected to water PCO2 of 47 mm Hg while having free access to normocapnic air in a closed water recirculation system. Arterial PCO2 rose considerably from 12 to 35 mm Hg in Siren and from 17 to 36 mm Hg in Amphiuma. The resultant fall in plasma pH remained uncompensated, whereas intracellular pH of white muscle and heart muscle of Siren were little affected owing to elevated intracellular bicarbonate concentrations. The bicarbonate accumulated in the intracellular compartments was in part produced by intracellular and extracellular nonbicarbonate buffering, and in part gained from the environment in exchange for Cl- ions. Elevated water bicarbonate concentration or bicarbonate infusion into Siren had no effect on the acid-base regulation. These data suggest that the availability of bicarbonate is not a limiting factor for extracellular compensation of increased PCO2, but that the threshold of the bicarbonate-regulating structures is simply not readjusted in hypercapnia. This type of regulation may have evolved as a result of the specific environmental conditions of these animals and may be considered as an energetically efficient way of maintaining a constant milieu for the pH-sensitive intracellular structures.
mesh_terms: D000136:Acid-Base Equilibrium; D000818:Animals; D001158:Arteries; D001639:Bicarbonates / Q000494:pharmacology / Q000652:urine; D002245:Carbon Dioxide; D004777:Environment; D006863:Hydrogen-Ion Concentration; D006935:Hypercapnia / Q000503:physiopathology*; D010100:Oxygen / Q000097:blood; D010313:Partial Pressure; D014562:Caudata / Q000502:physiology*; D014867:Water
chemical_list: D001639:Bicarbonates; D014867:Water; D002245:Carbon Dioxide; D010100:Oxygen