Colon/chemistry; Cyclic AMP/analogs & derivatives; Enzyme Inhibitors/pharmacology; Intracellular Signaling Peptides and Proteins; Sodium Channels/genetics; Thionucleotides/pharmacology; Amiloride/pharmacology; Animals; Carrier Proteins/pharmacology; Cloning, Molecular; Colon/enzymology; Cyclic AMP/pharmacology; Cyclic AMP-Dependent Protein Kinases/antagonists & inhibitors; Diuretics/pharmacology; Epithelial Cells/chemistry; Epithelial Cells/enzymology; Epithelial Sodium Channels; Gene Expression/physiology; Guinea Pigs; Ion Channel Gating/drug effects; Ion Channel Gating/genetics; Molecular Sequence Data; Oocytes/physiology; Patch-Clamp Techniques; Peptide Fragments/pharmacology; RNA, Complementary/pharmacology; Rats; Sequence Homology, Amino Acid; Sodium/metabolism; Xenopus laevis