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Exchanges of Calcium with the Environment and between Body Compartments in Amphibia

Amphibians are a diverse class of vertebrates that made the first highly successful transition from the aquatic to the terrestrial environment. Diversity in calcium ion regulation plays a significant role in their general diversity. Amphibians' extracellular [Ca²⁺] varies from species to species with terrestrial amphibians tending toward higher [Ca²⁺]. Another aspect of amphibian Ca²⁺ regulatory diversity is expressed in the variety of Ca²⁺ regulatory sites or potential regulatory sites. Skin, gill, kidney, urinary bladder, intestine, bone, and endolymphatic sacs have all been suggested as Ca²⁺ exchange sites. This article will review data pertaining to these Ca²⁺ exchange sites and present new data that suggest a regulated, active transport system for Ca²⁺ in the skin of amphibians. In Rana pipiens Ca²⁺ influx varies with external [Ca²⁺], is saturable, and occurs against the electrochemical gradient. Hypocalcemia stimulates this influx, as do parathyroid hormone and calcitonin. Xenopus laevis and Ambystoma tigrinum have similar [Ca²⁺]-related, saturable Ca²⁺ cutaneous influxes oriented against electrochemical gradients.