Action of polyvalent cations on sodium transport across skin of larval and adult Rana catesbeiana

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Abstract
The actions of alkaline earth (AE) and transition element (TE) cations on Na+ transport across skin of larval and adult Rana catesbeiana were compared. Bathed on the outside by Ca+2‐free Ringer's, both larval and adult skins maintained a stable short‐circuit current (3–4 μAmps cm−2 for larval skin and 20–30 μAmps cm−2 for adult skin). Addition of Ca+2 to the external bath reduced the SCC; maximal inhibition was about 36% for larval skin and 22% for adult skin. Other AE divalent cations were also inhibitory. The order of effectiveness was: Ba+2 = Ca+2 > Sr+2 > Mg+2 for larval skin and Ba+2 > Ca+2 = Mg+2 for adult skin. Sodium influx was markedly elevated when Ca+2 was removed from the external medium. Current‐voltage analysis indicated that Ca+2 increases the resistance of the active pathway without affecting the shunt resistance or the electromotive force of Na+ transport (ENa) in larval and adult skins. The SCC across adult skin was stimulated by TE cations (Co+2, Cd+2, La+3). These ions were inhibitory on larval skin. The transition in the response occurred at stage XXI. The inhibitory effect of TE on larvel skin resembles that seen in response to AE cations and we postulate a common mechanism. Since larval skin lacks the selective Na+ channels found in apical membranes of adult skin, we infer that the mechanism of inhibition by AE cations is not on these channels. A more general phenomenon such as change in surface charge at the apical membrane seems more reasonable. The stimulatory action of TE cations in adult skin is discussed in terms of interference with the Na+‐self‐inhibition response. Apparently, larval skin lacks some component of the molecular mechanism underlying this response.