Effects of Potassium Supplementation and Kir7.1 Knockout on Renal Function During the Progression of Salt‐Sensitive Hypertension

Abstract

The inward‐rectifying potassium channel (Kir) family has been shown to play an important role in the kidney’s ability to maintain electrolyte homeostasis as well as blood pressure. Kir7.1 (encoded by the Kcnj13 gene) is expressed in the kidney, though its role in renal function is rudimentary. To examine the role of Kir7.1 in renal electrolyte handling and the development of salt‐induced hypertension, we generated a knockout of Kcnj13 on the Dahl salt‐sensitive (SS) rat background. Rats with the knockout of Kcnj13 were embryonically lethal; therefore, to study the Kir7.1 channel, we used heterozygous (SSKcnj13+/‐) rats. Immunohistochemistry staining for Kir7.1 revealed basolateral staining in the cortical sections of the kidneys in wild‐type littermates (WT), while SSKcnj13+/‐ rats showed little to no staining. In addition, immunofluorescence analysis confirmed that Kir7.1 was localized on the basolateral side in tubules and revealed that Kir7.1 co‐localized with aquaporin 2 and sodium‐chloride cotransporter (NCC). This data indicates that Kir7.1 is expressed in the cortical collecting duct (CCD) and the distal convoluted tubule (DCT). SSKcnj13+/‐ rats were further used to measure blood pressure and electrolyte handling changes during the development of salt‐induced hypertension. DSI telemeters were implanted in animals at 8 weeks of age; after a week to recover from surgery, animals were fed a high salt 4% NaCl diet (HS) for three weeks. Both WT and SSKcnj13+/‐rats become hypertensive after three weeks of HS; no difference between mean arterial pressures (153.1 ± 5.7 vs. 159.2 ± 6.9 mmHg for WT and SSKcnj13+/‐rats, respectively) was identified. Serum electrolytes were measured and also showed no difference in the amount of potassium (3.4 ± 0.1 vs. 3.5 ± 0.1 mmol/L) or sodium (142 ± 0.8 vs. 142 ± 0.4 mmol/L). Furthermore, urinary electrolyte excretion was measured and revealed no differences between WT and SSKcnj13+/‐ animals for potassium (14.0 ± 0.6 vs. 15.6 ± 1.9 K+/Cre) or sodium excretion (91 ± 11 vs. 117 ± 16 Na+/Cre). Potassium supplementation has previously been shown to augment renal Kir7.1 expression. Thus, we repeated the previous protocol but instead using a high salt and high potassium 4% NaCl and 2% KCl (HSHK) diet. After three weeks on this diet, animals exhibited no difference in blood pressure (154.2 ± 9.1 vs 145.6 ± 4.1 mmHg) and no differences in plasma potassium (4.0 ± 0.1 vs 4.0 ± 0.1 mmol/L) or sodium (139 ± 1 vs 139 ± 1 mmol/L). Urinary electrolyte excretion showed no differences in potassium excretion (60.9 ± 3.3 vs 54.8 ± 1.1 K+/Cre), but SSKcnj13+/‐ animals did have a significant decrease in sodium excretion (130 ± 8 vs 103 ± 3 Na+/Cre). Altogether, we have confirmed the presence of Kir7.1 in the CCD and DCT, found that heterozygous knockout of Kir7.1 does not affect the development of blood pressure on a HS or HSHK diet. However, we do see a reduction in sodium excretion in SSKcnj13+/‐rats on the HSHK diet, suggesting that Kir7.1’s role may be more pronounced under conditions of higher potassium.