A major component of our research in the past has been determining how the renal-body fluid control system participates in the long-term regulation of body fluid volumes and arterial pressure. A key part of this control system is the renal-pressure natriuresis mechanism by which increases in renal perfusion pressure increase renal sodium excretion. Both experimental and mathematical modeling analyses indicate that renal pressure natriuresis is abnormal in all forms of hypertension studied thus far. Hypertension in humans is a major cardiovascular risk factor, causing stroke, heart failure and kidney failure. The arterial pressure of some human hypertensives is very sensitive to changes in sodium intake, and they have been classified as "salt-sensitive", but the cause of the salt-sensitivity is not known. Recent studies have indicated that nitric oxide (NO) production in salt-sensitive essential hypertensives is decreased. However, little is known about the importance of NO in salt-sensitive hypertension and specifically the relative importance of the various isoforms of NO synthase (NOS) in the kidney in causing salt-sensitivity. The model we will use to study these important unanswered questions is the Dahl salt-sensitive (S) rat, since it has many characteristics in common with salt-sensitive humans, such as decreased NO production and a suppressed renin-angiotensin system. The importance of the kidney in Dahl salt-sensitive hypertension was demonstrated by Lewis Dahl who showed that transplanting Dahl S rat kidneys into Dahl salt-resistant (R) rats caused hypertension in the R recipients. However, the cause of the renal abnormality that leads to salt-sensitive hypertension in the Dahl S rat and in essential hypertension is not clear. Therefore, insights from studies of Dahl S rats may be very helpful in understanding and treating salt-sensitive essential hypertension.
Our laboratory has shown that long-term decreases in NO synthesis, by iv infusion of a NO synthase inhibitor, causes sustained hypertension associated with decreases in pressure natriuresis, renal plasma flow and glomerular filtration rate. We also showed that net NO production was decreased in the Dahl S rat resulting in a blunted pressure natriuresis. When L-arginine was infused iv in Dahl S rats for 16 days, NO production increased, and the attenuation in pressure natriuresis was prevented; thus, the rats did not develop salt-sensitive increases in arterial pressure. Therefore, the arterial pressure of the Dahl S rat is highly dependent on the amount of NO present in the body. These data support the concept that a deficiency in NO in the Dahl S rat may be partly responsible for the salt-sensitivity of its arterial pressure. We have recently been studying the role on NO produced by nNOS in the macula densa in the prevention of salt-sensitive hypertension. Macula densa cells serve as a distal nephron sensor that detects changes in tubular fluid composition and transmits information to afferent arteriolar smooth muscle cells [tubuloglomerular feedback (TGF)] and renin-containing granular cells. Nitric oxide (NO) is one of the most important factors that regulate TGF. This NO, which sets the sensitivity of the TGF system, is mainly generated by neuronal NO synthase (nNOS) that is abundantly expressed in the macula densa cells.
Expression of nNOS in the macula densa is modulated by salt intake; a high-salt diet decreases nNOS expression, whereas a low-salt diet increases it. However, this pattern of expression of nNOS is contrary to what one would expect, because NO activity is increased, rather than decreased, during a high-salt diet. Indeed, increasing either salt intake or delivery to the macula densa elevates macula densa NO levels and attenuates TGF in vivo and in vitro. Although the reasons for this discrepancy between NO activity and expression of nNOS are not known, several possibilities exist including 1) increased activity of the nNOS enzyme, 2) an alternative source of NO, and 3) the presence of distinct splice variants of nNOS that might not all be detected by the methods used in the previous studies. Alternate splicing can produce several nNOS mRNA variants and protein isoforms, such as nNOS-a, nNOS-b, nNOS-g, and nNOS-m (the latter is only expressed in myocytes). nNOS-b, which normally has a low level in the kidney, becomes more abundantly expressed during renal injury.
Because nNOS-b has similar activity to that of nNOS-a (it has 82% the activity of nNOS-a), net nNOS activity will depend on the sum of the two isoforms, and thus an increase in nNOS-b can significantly increase NOS activity, even in the setting of decreased nNOS-a. Given that the antibodies used in several of the previous studies to assess nNOS expression may have only detected nNOS-a and not the other isoforms, it is possible that the discrepancy between nNOS activity and level during high-salt intake may be due to an increase in nNOS-b isoforms. Therefore, it is important to determine the level of the different nNOS splice variants in the kidney and macula densa, and their changes during diverse physiologic conditions such as during changes in sodium intake, to understand their role in regulating TGF, hypertension and kidney function. We hypothesize that dietary salt causes differential level of nNOS splice variants in the macula densa, which in turn modulates TGF. Specifically, we will test whether a high-salt diet increases the nNOS-b, which contributes to the enhanced NO production and blunted TGF observed during salt loading.
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