Cutaneous Exchange of Ions in Lower Vertebrates

SYNOPSIS. Cutaneous ion transport has long been studied for its role in osmotic and ionic regulation in amphibians. In addition to this role, it is becoming clear that cutaneous ion exchange influences, and is influenced by, a number of other physiological and morphological factors. The exchange of Na$ and Cl− across the skin of larval Ambystoma tigrinum is clearly involved in acid-base balance. The animals require NaCl in their bathing media in order to compensate for a respiratory acidosis or to recover normally from a metabolic acidosis. Conversely, Na$ influx is stimulated and Cl− influx is inhibited during hypercapnia. Cutaneous ion transport in larval A. tigrinum appears to be at least partially influenced by cutaneous circulation as immersion in isosmotic media to reduce skin blood flow also slows Na$ influx. Allometric analysis of Na$ influx yielded a slope of 0.52, which is significantly less than the slope predicted for surface area. The lower slope for ion transport may reflect changes in capillary recruitment, skin thickness or distribution of ion transporters over the skin. In addition to the well known cutaneous ion transport of amphibians, a number of fish and reptilian species have also been discovered to exchange ions across skin in ways which may have homeostatic consequences.