Effects of dietary fat, NaCl, and fructose on renal sodium and water transporter abundances and systemic blood pressure

Abstract

Dietary fructose, NaCl, and/or saturated fat have been correlated with mean arterial pressure (MAP) rises in sensitive strains of rats. Dysregulation of sodium and/or water reabsorption by the kidney may contribute. Using radiotelemetry and parallel semiquantitative immunoblotting, we examined the effects of various diets on MAP and the regulation of abundance of the major renal sodium and water transport proteins in male Sprague-Dawley rats. In study 1, rats ( approximately 275 g) were fed one of four diets for 4 wk (n = 6/group): 1) control, 2) 65% fructose, 3) control + added NaCl (2.59%), or 4) fructose + NaCl. In study 2, 5% butter (fat) was added to the above four diets. Both fat and NaCl, but not fructose, caused modest rises in MAP (5-10 mmHg) and increased the day-to-night ratio in diastolic blood pressure. NaCl or fructose increased kidney size. Creatinine clearance was increased by salt or fat, and fractional excretion of sodium was decreased by fat. In study 1, high NaCl markedly reduced plasma renin and aldosterone and its regulated proteins in whole kidney, i.e., the thiazide-sensitive Na-Cl cotransporter and the alpha- and gamma (70-kDa band)-subunits of the epithelial sodium channel. These effects were blunted by fat. Fructose increased the abundance of the sodium phosphate cotransporter, whereas it decreased the bumetanide-sensitive Na-K-2Cl cotransporter and aquaporin-2. Overall, doubling of dietary fat appeared to impair dietary sodium adaptation, i.e., blunt the downregulation of aldosterone-mediated effects, thus allowing blood pressure to rise at an accelerated rate.

Reduced Expression of Insulin Receptors in the Kidneys of Insulin-Resistant Rats

“…23 Furthermore, unlike glucose, fructose does not stimulate insulin release from the pancreatic ␤ cells. Moreover, it is interesting to note that in our hands, fructose feeding to Sprague-Dawley rats did not result in a significant increase in BP (as measured by radiotelemetry), 24 whereas increased BP was found in obese Zucker rats 18 and in high-fat-fed rats. 19 Our findings suggest that the mechanism for decreased IR protein in the kidney of insulin-resistant rats is not due to hyperinsulinemia, per se, because long-term insulin infusion into insulin-sensitive Sprague-Dawley rats did not reduce either IR-␣ or IR-␤ in any region of the kidney.…”

Regulation of blood pressure, the epithelial sodium channel (ENaC), and other key renal sodium transporters by chronic insulin infusion in rats

“…Immunoperoxidase labeling of ␣-ENaC. ␣-ENaC was localized on tissue with our own specific antibody against the ␣-ENaC subunit (45,46). Shown are examples of staining in collecting duct principal cells in (top to bottom) cortex (CTX), outer medulla (OM), and inner medulla (IM; ϫ1,000 magnification) from a control (left), insulin-infused (middle), and glucose-drinking rat (right).…”

“…For immunoblotting, 5-30 g of protein from each sample were loaded into individual lanes of precast minigels of 7, 10, or 12% polyacrylamide (Bio-Rad, Hercules, CA). Our immunoblotting protocol and the production, affinity purification, and characterization of the polyclonal antibodies against NCC, ␣-, ␤-, and ␥-ENaC have been previously described (45,46). WNK4 was a rabbit anti-human polyclonal antibody obtained from Alpha Diagnostic (San Antonio, TX), and SGK-1 was a sheep polyclonal obtained from Upstate Biotechnology (Lake Placid, NY).…”

Regulation of the renal thiazide-sensitive Na-Cl cotransporter, blood pressure, and natriuresis in obese Zucker rats treated with rosiglitazone

“…For NCC immunoblotting and immunoperoxidase labeling, we used either our own (25,27) or Dr. M. A. Knepper's (15) rabbit peptide-derived polyclonal antibody. For immunofluorescence, for NCC we used guinea pig peptide-derived polyclonal antibody (7), and for calbindin-D we used commercial mouse monoclonal antibody (Sigma).…”