Tubuloglomerular feedback and salt-sensitive hypertension

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Hypertension affects over 25% of the adults and is a major cardiovascular risk to our population. More than half of hypertensive humans are salt-sensitive and have significant blood pressure fluctuations when salt intake is altered. However, the mechanisms for salt-sensitivity are not clear. Increases in glomerular filtration rate (GFR) play a vital role in the rapid elimination of sodium following acute volume expansion associated with ingestion of a sodium load, thereby contributing to restoration of sodium and water balance which maintains normal blood pressure. This GFR response is blunted in humans and in animal models with salt-sensitive hypertension. Tubuloglomerular feedback (TGF) is an essential regulator of GFR. Increasing tubular flow initiates a TGF response mediated by raising NaCl delivery to the macula densa (MD), which triggers signals that enhance the tone of the afferent arterioles and thereby reduces GFR. This fall in GFR helps restore MD flow rate toward normal and prevents marked changes in NaCl excretion. However, in persistent situations such as experimental or postprandial volume expansion, intrinsic mechanisms reset TGF, which shifts the operating point to a higher flow rate thus allowing GFR to rise. TGF resetting could facilitate the excretion of salt and water via mechanisms that may be dependent on suppression of angiotensin II and increased activity of the nitric oxide (NO) system. NO derived from the MD has been shown to dilate the afferent arteriole and blunt TGF. This NO is mainly produced by neuronal NO synthase (nNOS), which is abundantly expressed in the MD. However, the roles of the MD-delivered NO and TGF in regulation of volume homeostasis are only assumptions from these experiments. We still do not know whether NO from the MD and TGF play any roles in control of salt-water balance and blood pressure, which is the focus of this proposal. In this proposal, we will test the hypothesis that nNOS¿ in the MD is a salt sensitive isoform, which contributes to enhanced NO generation by the MD during high salt intake. Enhanced nNOS¿ activity blunts TGF and increases GFR, a mechanism which is essential in rapid elimination of a salt load and restoration of salt-water balance. Inadequate NO generation by the MD induces salt sensitive hypertension.