ABSTRACT
Adult Ambystoma tigrinum were cannulated non-occlusively in the truncus arteriosus and subjected to 24 h of hypercapnia in 3% CO2. Adults showed the typical compensatory pattern, shared by larvae and many other amphibians, of partial compensation (44%) for the induced respiratory acidosis. Adults whose urinary bladders were ligated to allow the urine to bypass the bladder compensated as well as shams, indicating that the urinary bladder of this species is not necessary for compensation. Radioisotopic measurements of net and unidirectional fluxes of Na+ or Cl- in whole animals showed no effects of hypercapnia. Partitioning of acid-base responses showed that 75–80 % of the regulation takes place across the skin. The rest is accomplished by the kidneys. This did not change during hypercapnia and there was no evidence of renal involvement in the compensation. Total ammonia (NH3+NH4+) comprised only about one-sixth of the total cutaneous acid excretion. The charge associated with the cutaneous excretion of H+ equivalents was balanced by both Na+ uptake and Cl- loss. In contrast to larvae, whose cutaneous electrical potential difference (PD) increases during hypercapnia, adults decrease their PD. This could mean that acidosis stimulates an electrogenic H+ secretion and/or that cutaneous Na+ and Cl-permeabilities change. Both possibilities are consistent with the data.
