Immunolocalization of hyperpolarization-activated cationic HCN1 and HCN3 channels in the rat nephron: regulation of HCN3 by potassium diets.

Hyperpolarization-activated cationic and cyclic nucleotide-gated channels (HCN) comprise four homologous subunits (HCN1-HCN4). HCN channels are found in excitable and non-excitable tissues in mammals. We have previously shown that HCN2 may transport ammonium (NH4 (+)), besides sodium (Na(+)), in the rat distal nephron. In the present work, we identified HCN1 and HCN3 in the proximal tubule (PT) and HCN3 in the thick ascending limb of Henle (TALH) of the rat kidney. Immunoblot assays detected HCN1 (130 kDa) and HCN3 (90 KDa) and their truncated proteins C-terminal HCN1 (93 KDa) and N-terminal HCN3 (65 KDa) in enriched plasma membranes from cortex (CX) and outer medulla (OM), as well as in brush-border membrane vesicles. Immunofluorescence assays confirmed apical localization of HCN1 and HCN3 in the PT. HCN3 was also found at the basolateral membrane of TALH. We evaluated chronic changes in mineral dietary on HCN3 protein abundance. Animals were fed with three different diets: sodium-deficient (SD) diet, potassium-deficient (KD) diet, and high-potassium (HK) diet. Up-regulation of HCN3 was observed in OM by KD and in CX and OM by HK; the opposite effect occurred with the N-terminal truncated HCN3 in CX (KD) and OM (HK). SD diet did not produce any change. Since HCN channels activate with membrane hyperpolarization, our results suggest that HCN channels may play a role in the Na(+)-K(+)-ATPase activity, contributing to Na(+), K(+), and acid-base homeostasis in the rat kidney.