Formation and volatilization of ammonia gas by terrestrial snails

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Abstract
The snails, Otala.(= Helix) lactea and H. aspersa were shown to evolve NH3 gas. The average rate of NH3 formation measured for O. lactea would account for the elimination of 9.6 mg NH3-NAg tissue per 24 hr and for H. aspersa, 5.5 mg NH3-N/kg tissue/24 hr. A major metabolic source of the evolved NH3 was shown to be from urea: urea injected into O. lactea caused an increased rate of NH3 formation and injected urea-l,3-15N gave rise to 15NH3 O. lactea urease, the enzyme responsible for the conversion of urea to NH3, could be distinguished from bacterial urease by its more alkaline pH optimum. A mechanism for the exchange of NH3 between the blood and lung is proposed on the basis of the observed alkalinization (pH 8.6-8.8) of the blood in vitro and the location of carbonic anhydrase in the lung tissue. This mechanism is one of "diffusion trapping" and is similar to that proposed for mammalian kidney. The alkalinization of the blood at the lung to facilitate the removal of NH3 is consistent with the function of the hemocyanin in these snails. This hemocyanin is known to have a maximum affinity for O2 at pH values from 8.6 to 9.0. Because the major route of exit of NH3 from the snails is through the shell, it is probable that the formation and volatilization of NH3 is related to shell CaCO3 deposition. NH3 elaborated into the extrapallial fluid may serve to increase the concentration of CO3= from the reaction, HCO3H+ + CO3=, by shifting the pH of the fluid toward the alkaline pKa for HCO3-dissociation.