Hypertension is the leading cause of “loss of health” worldwide. angiotensin II is a key regulator of blood pressure primarily through actions on the kidney, including the proximal tubule. This nephron segment reabsorbs ~70% of the filtered Na via Na/H exchanger type 3 (NHE3) and Na/K ATPase. Angiotensin II stimulates transport via activation of angiotensin II type 1 (AT1) receptors and protein kinase C (PKC), specifically classic isoforms (α, β, γ). When animals are on normal salt physiological concentrations of angiotensin II nearly maximally stimulate proximal tubule Na reabsorption. Usually when dietary salt is elevated, angiotensin II levels fall by ~70-80%, urinary Na excretion (UNaV) increases and salt is eliminated. However, if the effects of angiotensin II don’t decline with a high salt, salt is retained and hypertension results.
More than 15 million Americans consume more than 20% of their calories as fructose. Dietary fructose causes hypertension and cardiovascular disease in humans. It causes hypertension in rats fed comparable amounts of fructose to those consumed by people. We reported that when rats consume 20% of their calories as fructose (20% fructose diet) they develop salt-sensitive hypertension, and that blood pressure begins to increase 1-2 days after starting a high-salt diet. Blood pressure increases rapidly for about 1 week and remains elevated for at least 4 weeks. A 20% glucose+high-salt diet does not alter blood pressure. Angiotensin II AT1 receptor blockers prevent hypertension in fructose-fed rats; however, the proximal tubule’s role and the mechanisms by which fructose causes salt-sensitive hypertension are unknown.
Our data show that fructose activates either PKCα or β in isolated, perfused proximal tubules. We show that a 20% fructose+high-salt diet enables low concentrations of angiotensin II to stimulate proximal tubule transport measured in vitro, but does not enhance the maximum effect. Whether the effects of dietary fructose on the proximal tubule contribute to fructose-induced salt-sensitive hypertension and the mechanisms involved are unknown. We hypothesize that a 20% fructose diet blunts salt-induced natriuresis and causes salt-sensitive hypertension by activating PKCα thereby enabling low concentrations of angiotensin II (induced by a high-salt diet) to stimulate proximal tubule Na reabsorption. If this is correct, many with high blood pressure may benefit by reducing fructose intake or taking Angiotensin II AT1 receptor blockers rather than angiotensin converting enzyme inhibitors.
To test this hypothesis we will measure the effect of angiotensin II on Na reabsorption and NHE3 and Na/K ATPase activities in proximal tubules from rats fed either: 1) a normal; 2) a 20% fructose; 3) a high-salt (4% NaCl); or 4) a 20% fructose+high-salt diet, and the effects of the AT1 and angiotensin II type 2 (AT2) receptor antagonists on proximal tubule Na reabsorption in vivo in these groups. We will also measure NHE3 and Na/K ATPase total expression and the amount in the plasma membrane before and after angiotensin II-treatment. Next we will test whether PKCα mediates the fructose-induced increase in proximal tubule Na reabsorption in response to angiotensin II, and the isoform involved. Finally we will measure Na balance (intake – excretion) and blood pressure in rats fed either: 1) a normal; 2) a 20% fructose; 3) a high-salt diet (4% NaCl); or 4) a 20% fructose+high-salt diet for 3, 7 and 14 days; and the chronic effects of an AT1 receptor antagonist and various dominant-negative PKC isoforms.
