The renal cGMP-gated cation channel: Its molecular structure and physiological role

Cyclic nucleotide-gated cation channels, which are permeable to monovalent and divalent cations, are expressed in a number of tissues. cDNAs encoding cGMP-gated cation channel subunits have been cloned in retinal rods, cones, olfactory neuroepithelium, pineal gland, aorta, testis, heart, and most recently kidney. Patch clamp studies have identified and characterized cGMP-gated cation channels in the cortical collecting duct (CCD) and inner medullary collecting duct (IMCD). cGMP-gated cation channels in kidney share many biophysical and molecular properties with the retinal rod cGMP-gated channel. However, unlike the retinal rod channel, the cGMP-gated cation channel in kidney is inhibited by cGMP and stimulated by increased calcium levels. In the IMCD the cGMP-gated cation channel mediates electrogenic sodium absorption which is inhibited by ANP via cGMP. Recently, cGMP-gated cation channel poly(A⁺) RNA has been identified in other nephron segments by RT-PCR and in situ PCR hybridization. Furthermore, cGMP-gated cation channel protein has also been identified in all nephron segments by Western blot analysis. These observations suggest that cGMP-gated cation channels, or closely related gene products, may play an important physiological role in all nephron segments. Hormones that increase intracellular cGMP may regulate sodium, and perhaps calcium, uptake in nephron segments proximal to the IMCD. Increases in cell sodium and calcium may regulate other transport and signaling pathways.