Noise analysis of cAMP-stimulated Na current in frog colon

The effects of oxytocin and cAMP on the electrogenic Na⁺-transport in the short-circuited epithelium of the frog colon (Rana esculenta, Rana temporaria) were investigated. Oxytocin (100 mU · ml^–1) elevated the shortcircuit current (I_sc) transiently by 70% whereas cAMP (1 mmol · l^–1) elicited a comparable sustained response. The mechanism of the natriferic action of cAMP was studied by analysing current fluctuations through apical Na⁺-channels induced by amiloride or CDPC (6-chloro-3,5-diaminopyrazine-2-carboxamid). The noise data were used to calculate Na⁺-channel density (M) and single apical Na⁺-current (i_Na).i_Na-Values obtained with amiloride and CDPC were 1.0±0.1 pA (n=5) and 1.1±0.2 pA (n=6) respectively and unaffected by cAMP. On the other hand, cAMP caused a significant increase in M from 0.23±0.08 m^–2 (n=5) to 0.49±0.17 m^–2 (n=5) in the amiloride experiments. In our studies with CDPC we obtained smaller values for M in control (0.12±0.04 m^–2;n=6) as well as during cAMP treatment (0.19±0.06 m^–2;n=6). However, the cAMP-induced increase in M was also significant. We conclude that cAMP stimulates Na⁺-transport across the frog colon by activating silent apical Na⁺-channels. Thus, the mechanism of regulation of colonic Na-transport in frogs differs considerably from that in other vertebrates as mammals and birds.