Chemical basis for the [H+] gradient across frog skin

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Abstract
Frog skin can establish an [H+] gradient which is always such that the inside is alkaline and the outside acidic. The pH range is about 8-6. The present study shows that the operation of the HCO3-/H2CO3 system is adequate to explain this. Acidification is the result of H2CO3 formation from CO2 for which the skin and not the lung is the chief excretory organ according to the literature. Alkalinization is the result of the high level of blood-borne HCO3-in corium and mucous gland (30 mM/kg). Acid-base titrations on intact frogs (Rana pipiens: anus tied off) in pH-stat controlled water showed that NaOH titrant at pH >6 and HCl titrant at pH <6 were consumed. This suggests that the frog tends to prevent the lowering of the skin surface pH below 6 by releasing NaHCO3 from the skin. Chemical estimations on skin secretions collected from frogs in air showed that the secretions are rich in Na (orK) HCO3, have a pH of 7.3-7.8, and contain NaCl and KCl. Subcutaneous injections of epinephrine, norepinephrine, or Pitressin, but not secretin or histamine, stimulated production of alkaline secretions. The epinephrine effect did not show a consistent relationship to changes in skin potential difference. For frogs sitting in NaCl solutions there was no indication of an Na+H+ exchange. Addition of sugars to the water bath containing the frog stimulated NaOH titrant consumption at pH [image]7 only if spontaneous, temporary cessation of alkaline titrant consumption had occurred prior to the addition of the sugars.