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 short-circuit current (Isc) 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 (iNa). iNa-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 micron-2 (n = 5) to 0.49 +/- 0.17 micron-2 (n = 5) in the amiloride experiments. In our studies with CDPC we obtained smaller values for M in control (0.12 +/- 0.04 micron-2; n = 6) as well as during cAMP treatment (0.19 +/- 0.06 micron-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.