Dietary fructose enhances angiotensin II-stimulated Na+transport via activation of PKC-α in renal proximal tubules

Abstract

Angiotensin II (ANG II) stimulates proximal nephron transport via activation of classical protein kinase C (PKC) isoforms. Acute fructose treatment stimulates PKC and dietary fructose enhances ANG II's ability to stimulate Na+transport, but the mechanisms are unclear. We hypothesized that dietary fructose enhances ANG II's ability to stimulate renal proximal tubule Na+reabsorption by augmenting PKC-α activation and increases in intracellular Ca2+. We measured total and isoform-specific PKC activity, basal and ANG II-stimulated oxygen consumption, a surrogate of Na+reabsorption, and intracellular Ca2+in proximal tubules from rats given either 20% fructose in their drinking water (fructose group) or tap water (control group). Total PKC activity was measured by ELISA. PKC-α, PKC-β, and PKC-γ activities were assessed by measuring particulate-to-soluble ratios. Intracelluar Ca2+was measured using fura 2. ANG II stimulated total PKC activity by 53 ± 15% in the fructose group but not in the control group (-15 ± 11%, P < 0.002). ANG II stimulated PKC-γ by 0.134 ± 0.026 but not in the control group (-0.002 ± 0.020, P < 0.002). ANG II increased PKC-β activity by 0.008 ± 0.003 in the fructose group but not in the control group (P < 0.046). ANG II did not stimulate PKC-β in either group. ANG II increased Na+transport by 454 ± 87 nmol·min-1·mg protein-1in fructose group, and the PKC-αβ inhibitor Gö6976 blocked this increase (-96 ± 205 nmol·min-1·mg protein-1, P < 0.045). ANG II increased intracellular Ca2+by 148 ± 53 nM in the fructose group but only by 43 ± 10 nM in the control group (P < 0.035). The intracellular Ca2+chelator BAPTA blocked the ANG II-induced increase in Na+transport in the fructose group. We concluded that dietary fructose enhances ANG II's ability to stimulate renal proximal tubule Na+reabsorption by augmenting PKC-α activation via elevated increases in intacellular Ca2+