Signaling mechanisms that link salt retention to hypertension: endogenous ouabain, the Na(+) pump, the Na(+)/Ca(2+) exchanger and TRPC proteins

Abstract

Salt retention as a result of chronic, excessive dietary salt intake, is widely accepted as one of the most common causes of hypertension. In a small minority of cases, enhanced Na(+) reabsorption by the kidney can be traced to specific genetic defects of salt transport, or pathological conditions of the kidney, adrenal cortex, or pituitary. Far more frequently, however, salt retention may be the result of minor renal injury or small genetic variation in renal salt transport mechanisms. How salt retention actually leads to the increase in peripheral vascular resistance (the hallmark of hypertension) and the elevation of blood pressure remains an enigma. Here we review the evidence that endogenous ouabain (an adrenocortical hormone), arterial smooth muscle α2 Na(+) pumps, type-1 Na/Ca exchangers, and receptor- and store-operated Ca(2+) channels play key roles in the pathway that links salt to hypertension. We discuss cardenolide structure-function relationships in an effort to understand why prolonged administration of ouabain, but not digoxin, induces hypertension, and why digoxin is actually anti-hypertensive. Finally, we summarize recent observations which indicate that ouabain upregulates arterial myocyte Ca(2+) signaling mechanisms that promote vasoconstriction, while simultaneously downregulating endothelial vasodilator mechanisms. In sum, the reports reviewed here provide novel insight into the molecular mechanisms by which salt retention leads to hypertension.

Fig. 1

Model of the plasma membrane-junctional sarco-/endoplasmic reticulum (PM-jS/ER) region, the PLasmERosome, showing the location of key transport proteins involved in local control of jS/ER Ca²⁺ stores and Ca²⁺ signaling. The PLasmERosome consists of a PM microdomain, the adjacent jS/ER (with SERCA, IP₃R and ryanodine receptors, RYR), and the intervening ‘diffusion-restricted’ junctional space (J). The PM microdomain contains agonist receptors, ARs (GPCRs), ROCs and SOCs (composed of various transient receptor potential channel proteins or TRPCs), α2 (in smooth muscle) or α3 Na⁺ pumps, and NCX. Activation of GPCRs and release of G-proteins (GPs) stimulates phospholipase C (PLC) to produce IP₃ and diacylglycerol (DAG). DAG may activate ROCs directly. Na⁺ may enter locally, through ROCs, SOCs or, perhaps, SACs (not shown) to promote Ca²⁺ entry via NCX. Shading indicates relative Na⁺ and/or Ca²⁺ concentrations. Other regions of the PM contain α1 Na⁺ pumps and PMCA. Other abbreviations are defined in the text. Reprinted with permission [184].

Fig. 2

Oubain, but not digoxin, induces hypertension; digoxin and digitoxin reverse ouabain-induced hypertension. Rats were infused with vehicle (■), ouabain, 15 μg/kg/day (▼), or digoxin, 30 μg/kg/day (▲), for 42 days. From days 35 to 42, three groups of 8 ouabain-infused rats received a secondary infusion of digoxin, 30 μg/kg/day (●), digitoxin, 30 μg/kg/day (○), or vehicle (▼). Mean blood pressures (MBP) were obtained by tail cuff at weekly intervals or as indicated. *P<0.05 vs ouabain; ***P<0.001 vs ouabain; #P<0.005 vs vehicle; **P<0.001 vs digoxin. Reprinted with permission [99]. Effects of Ouabain, Digoxin and Digitoxin on BP

Fig. 3

Prototypical cardenolide steroid skeleton. The primary feature is a steroid skeleton with the rings fused in a cis-trans-cis arrangement. The cardenolides discussed here have a 14βOH, an unsaturated lactone ring attached via C17 in the β configuration, and a methyl group at C18. When present, sugars are attached via the steroid 3βOH group. See Table 1 for the list of substituents in ouabain, ouabagenin, digoxin, digitoxin and Rostafuroxin. Reprinted with permission [99].

Fig. 4

Relative blood pressures of mice with genetically-engineered α2 Na⁺ pumps and NCX1. The data from several sources, are normalized to the BPs of the respective control wild type (WT) mice. Mice with a null mutation in one α2 Na⁺ pump allele (α2^+/-) [36] or smooth muscle-specific α2 knockdown (α2^SM/DN) (Song, Chen, Zhang, Lee, Kotlikoff and Blaustein, unpublished), or increased smooth muscle-specific NCX1 overexpression (NCX1^SM/Tg) [166], had significantly elevated BP. A high salt diet augmented the elevated BP in α2^+/- mice (4% NaCl × 2 weeks) and NCX1^SM/Tg mice (8% NaCl + 1% NaCl in tap water × 4 weeks). Smooth muscle-specific overexpression of α2 Na⁺ pumps (α2^SM/Tg)[187] or knockdown of NCX1 (NCX1^SM-/-) [198] significantly reduced BP. * = P < 0.05, ** = P < 0.01 vs WT or the respective genotypes on a normal (0.5%) salt diet. Reprinted with permission [65].