Mechanisms of acid-base regulation in the African lungfish Protopterus annectens.

African lungfish Protopterus annectens utilized both respiratory and metabolic compensation to restore arterial pH to control levels following the imposition of a metabolic acidosis or alkalosis. Acid infusion (3 mmol kg^-1 NH₄Cl) to lower arterial pH by 0.24 units increased both pulmonary (by 1.8-fold) and branchial (by 1.7-fold) ventilation frequencies significantly, contributing to 4.8-fold and 1.9- fold increases in, respectively, aerial and aquatic CO₂ excretion. This respiratory compensation appeared to be the main mechanism behind the restoration of arterial pH, because even though net acid excretion (J_netH⁺) increased following acid infusion in 7 of 11 fish, the mean increase in net acid excretion, 184.5±1185 µmol H⁺ kg^-1 h^-1 (mean ± s.e.m., N=11), was not significantly different from zero. Base infusion (3 mmol kg^-1 NaHCO₃) to increase arterial pH by 0.29 units halved branchial ventilation frequency, although pulmonary ventilation frequency was unaffected. Correspondingly, aquatic CO₂ excretion also fell significantly (by 3.7-fold) while aerial CO₂ excretion was unaffected. Metabolic compensation consisting of negative net acid excretion (net base excretion) accompanied this respiratory compensation, with J_netH⁺ decreasing from 88.5±75.6 to -337.9±199.4 µmol H⁺ kg^-1 h^-1 (N=8). Partitioning of net acid excretion into renal and extra-renal (assumed to be branchial and/or cutaneous) components revealed that under control conditions, net acid excretion occurred primarily by extra-renal routes. Finally, several genes that are involved in the exchange of acid-base equivalents between the animal and its environment (carbonic anhydrase, V-type H⁺-ATPase and Na⁺/HCO₃-cotransporter) were cloned, and their branchial and renal mRNA expressions were examined prior to and following acid or base infusion. In no case was mRNA expression significantly altered by metabolic acid-base disturbance. These findings suggest that lungfish, like tetrapods, alter ventilation to compensate for metabolic acid-base disturbances, a mechanism that is not employed by water-breathing fish. Like fish and amphibians, however, extra-renal routes play a key role in metabolic compensation. [ABSTRACT FROM AUTHOR]