Elevated renal perfusion pressure does not contribute to natriuresis induced by isotonic saline infusion in freely moving dogs

Brief early life angiotensin-converting enzyme inhibition attenuates the diuretic response to saline loading in sheep with solitary functioning kidney

A solitary functioning kidney (SFK) from birth predisposes to hypertension and kidney dysfunction, and this may be associated with impaired fluid and sodium homeostasis. Brief and early angiotensin-converting enzyme inhibition (ACEi) in a sheep model of SFK delays onset of kidney dysfunction. We hypothesized that modulation of the renin-angiotensin system via brief postnatal ACEi in SFK would reprogram renal sodium and water handling. Here, blood pressure (BP), kidney haemodynamics and kidney excretory function were examined in response to an isotonic saline load (0.13 ml/kg/min, 180 min) at 20 months of age in SFK (fetal unilateral nephrectomy at 100 days gestation; term 150 days), sham and SFK+ACEi sheep (ACEi in SFK 4-8 weeks of age). Basal BP was higher in SFK than sham (∼13 mmHg), and similar between SFK and SFK+ACEi groups. Saline loading caused a small increase in BP (∼3-4 mmHg) the first 2 h in SFK and sham sheep but not SFK+ACEi sheep. Glomerular filtration rate did not change in response to saline loading. Total sodium excretion was similar between groups. Total urine excretion was similar between SFK and sham animals but was ∼40% less in SFK+ACEi animals compared with SFK animals. In conclusion, the present study indicates that water homeostasis in response to a physiological challenge is attenuated at 20 months of age by brief early life ACEi in SFK. Further studies are required to determine if ACEi in early life in children with SFK could compromise fluid homeostasis later in life.

A mathematical model of salt-sensitive hypertension: the neurogenic hypothesis

Salt sensitivity of arterial pressure (salt-sensitive hypertension) is a serious global health issue. The causes of salt-sensitive hypertension are extremely complex and mathematical models can elucidate potential mechanisms that are experimentally inaccessible. Until recently, the only mathematical model for long-term control of arterial pressure was the model of Guyton and Coleman; referred to as the G-C model. The core of this model is the assumption that sodium excretion is driven by renal perfusion pressure, the so-called 'renal function curve'. Thus, the G-C model dictates that all forms of hypertension are due to a primary shift of the renal function curve to a higher operating pressure. However, several recent experimental studies in a model of hypertension produced by the combination of a high salt intake and administration of angiotensin II, the AngII-salt model, are inconsistent with the G-C model. We developed a new mathematical model that does not limit the cause of salt-sensitive hypertension solely to primary renal dysfunction. The model is the first known mathematical counterexample to the assumption that all salt-sensitive forms of hypertension require a primary shift of renal function: we show that in at least one salt-sensitive form of hypertension the requirement is not necessary. We will refer to this computational model as the 'neurogenic model'. In this Symposium Review we discuss how, despite fundamental differences between the G-C model and the neurogenic model regarding mechanisms regulating sodium excretion and vascular resistance, they generate similar haemodynamic profiles of AngII-salt hypertension. In addition, the steady-state relationships between arterial pressure and sodium excretion, a correlation that is often erroneously presented as the 'renal function curve', are also similar in both models. Our findings suggest that salt-sensitive hypertension is not due solely to renal dysfunction, as predicted by the G-C model, but may also result from neurogenic dysfunction.

A new conceptual paradigm for the haemodynamics of salt-sensitive hypertension: a mathematical modelling approach

A conceptually novel mathematical model of neurogenic angiotensin II-salt hypertension is developed and analysed. The model consists of a lumped parameter circulatory model with two parallel vascular beds; two distinct control mechanisms for both natriuresis and arterial resistances can be implemented, resulting in four versions of the model. In contrast with the classical Guyton-Coleman model (GC model) of hypertension, in the standard version of our new model natriuresis is assumed to be independent of arterial pressure and instead driven solely by sodium intake; arterial resistances are driven by increased sympathetic nervous system activity in response to the elevated plasma angiotensin II and increased salt intake (AngII-salt). We compare the standard version of our new model against a simplified Guyton-Coleman model in which natriuresis is a function of arterial pressure via the pressure-natriuresis mechanism, and arterial resistances are controlled via the whole-body autoregulation mechanism. We show that the simplified GC model and the new model correctly predict haemodynamic and renal excretory responses to induced changes in angiotensin II and sodium inputs. Importantly, the new model reproduces the pressure-natriuresis relationship--the correlation between arterial pressure and sodium excretion--despite the assumption of pressure-independent natriuresis. These results show that our model provides a conceptually new alternative to Guyton's theory without contradicting observed haemodynamic changes or pressure-natriuresis relationships. Furthermore, the new model supports the view that hypertension need not necessarily have a renal aetiology and that long-term arterial pressure could be determined by sympathetic nervous system activity without involving the renal sympathetic nerves.

Lowering of blood pressure by chronic suppression of central sympathetic outflow: insight from prolonged baroreflex activation

Device-based therapy for resistant hypertension by electrical activation of the carotid baroreflex is currently undergoing active clinical investigation, and initial findings from clinical trials have been published. The purpose of this mini-review is to summarize the experimental studies that have provided a conceptual understanding of the mechanisms that account for the long-term lowering of arterial pressure with baroreflex activation. The well established mechanisms mediating the role of the baroreflex in short-term regulation of arterial pressure by rapid changes in peripheral resistance and cardiac function are often extended to long-term pressure control, and the more sluggish actions of the baroreflex on renal excretory function are often not taken into consideration. However, because clinical, experimental, and theoretical evidence indicates that the kidneys play a dominant role in long-term control of arterial pressure, this review focuses on the mechanisms that link baroreflex-mediated reductions in central sympathetic outflow with increases in renal excretory function that lead to sustained reductions in arterial pressure.

Blood volume, blood pressure and total body sodium: internal signalling and output control

Total body sodium and arterial blood pressure (ABP) are mutually dependent variables regulated by complex control systems. This review addresses the role of ABP in the normal control of sodium excretion (NaEx), and the physiological control of renin secretion. NaEx is a pivotal determinant of ABP, and under experimental conditions, ABP is a powerful, independent controller of NaEx. Blood volume is a function of dietary salt intake; however, ABP is not, at least not in steady states. A transient increase in ABP after a step-up in sodium intake could provide a causal relationship between ABP and the regulation of NaEx via a hypothetical integrative control system. However, recent data show that subtle sodium loading (simulating salty meals) causes robust natriuresis without changes in ABP. Changes in ABP are not necessary for natriuresis. Normal sodium excretion is not regulated by pressure. Plasma renin is log-linearly related to salt intake, and normally, decreases in renin secretion are a precondition of natriuresis after increases in total body sodium. Renin secretion is controlled by renal ABP, renal nerve activity and the tubular chloride concentrations at the macula densa (MD). Renal nerve activity is related to blood volume, also at constant ABP, and elevates renin secretion by means of beta(1)-adrenoceptors. Recent results indicate that renal denervation reduces ABP and renin activity, and that sodium loading may decrease renin without changes in ABP, glomerular filtration rate or beta(1)-mediated nerve activity. The latter indicates an essential role of the MD mechanism and/or a fourth mediator of the physiological control of renin secretion.

Chronic activation of plasma renin is log-linearly related to dietary sodium and eliminates natriuresis in response to a pulse change in total body sodium

Responses to acute sodium loading depend on the load and on the level of chronic sodium intake. To test the hypothesis that an acute step increase in total body sodium (TBS) elicits a natriuretic response, which is dependent on the chronic level of TBS, we measured the effects of a bolus of NaCl during different low-sodium diets spanning a 25-fold change in sodium intake on elements of the renin-angiotensin-aldosterone system (RAAS) and on natriuresis. To custom-made, low-sodium chow (0.003%), NaCl was added to provide four levels of intake, 0.03–0.75 mmol·kg−1·day−1for 7 days. Acute NaCl administration increased PV (+6.3–8.9%) and plasma sodium concentration (∼2%) and decreased plasma protein concentration (−6.4–8.1%). Plasma ANG II and aldosterone concentrations decreased transiently. Potassium excretion increased substantially. Sodium excretion, arterial blood pressure, glomerular filtration rate, urine flow, plasma potassium, and plasma renin activity did not change. The results indicate that sodium excretion is controlled by neurohumoral mechanisms that are quite resistant to acute changes in plasma volume and colloid osmotic pressure and are not down-regulated within 2 h. With previous data, we demonstrate that RAAS variables are log-linearly related to sodium intake over a >250-fold range in sodium intake, defining dietary sodium function lines that are simple measures of the sodium sensitivity of the RAAS. The dietary function line for plasma ANG II concentration increases from theoretical zero at a daily sodium intake of 17 mmol Na/kg (intercept) with a slope of 16 pM increase per decade of decrease in dietary sodium intake.

The 'body fluid pressure control system' relies on the Renin-Angiotensin-aldosterone system: balance studies in freely moving dogs

1. The physiological role of the 'renal body fluid pressure control system', including the intrarenal mechanism of 'pressure natriuresis', is uncertain. 2. Balance studies in freely moving dogs address the following questions: (i) what is the physiological contribution of pressure natriuresis to the control of total body sodium (TBS); (ii) to what extent is long-term mean arterial blood pressure (MABP) determined by TBS and total body water (TBW); and (iii) during Na accumulation, is Na stored in an osmotically inactive form? 3. Diurnal time-courses of Na excretion (U(Na)V) and MABP reveal no correlation. Spontaneous MABP changes do not affect U(Na)V. The long-term 20% reduction of renal perfusion pressure (RPP) results in Na retention via pressure-dependent stimulation of the renin-angiotensin-aldosterone system (RAAS), not via a pressure natriuresis mechanism. Prevention of pressure natriuresis does not result in ongoing Na retention when the RAAS is operative. The long-term 20% elevation of RPP induced by sustained TBS elevation facilitates Na excretion via pressure natriuresis, but does not restore TBS to normal. 4. Changes in TBW correlate well with changes in TBS (r(2) = 0.79). This correlation is even closer when concomitant changes in total body potassium are also considered (r(2) = 0.91). 5. With normal or elevated TBW, long-term MABP changes correlate well with TBW changes (r(2) = 0.69). At lowered TBW, no correlation is found. 6. In conclusion, the physiological role of pressure natriuresis is limited. Pressure natriuresis does not appear to be operative when RPP is changed from -20 to +10% and neurohumoral control of U(Na)V is unimpeded. Within this range, pressure-dependent changes in the RAAS mediate the effects of changes in RPP on U(Na)V. Pressure natriuresis may constitute a compensating mechanism under pathophysiological conditions of substantial elevation of RPP. A large portion of the long-term changes in MABP are attributable to changes in TBW. The notion of osmotically inactive Na storage during Na accumulation appears to be invalid.

Are large amounts of sodium stored in an osmotically inactive form during sodium retention? Balance studies in freely moving dogs

Alterations in total body sodium (TBSodium) that covered the range from moderate deficit to large surplus were induced by 10 experimental protocols in 66 dogs to study whether large amounts of Na+ are stored in an osmotically inactive form during Na+ retention. Changes in TBSodium, total body potassium (TBPotassium), and total body water (TBWater) were determined by 4-day balance studies. A rather close correlation was found between individual changes in TBSodium and those in TBWater (r2 = 0.83). Changes in TBSodium were often accompanied by changes in TBPotassium. Taking changes of both TBSodium and TBPotassium into account, the correlation with TBWater changes became very close (r2 = 0.93). The sum of changes in TBSodium and TBPotassium was accompanied by osmotically adequate TBWater changes, and plasma osmolality remained unchanged. Calculations reveal that even moderate TBSodium changes often included substantial Na+/K+ exchanges between extracellular and cellular space. The results support the theory that osmocontrol effectively adjusts TBWater to the body's present content of the major cations, Na+ and K+, and do not support the notion that, during Na+ retention, large portions of Na+ are stored in an osmotically inactive form. Furthermore, the finding that TBSodium changes are often accompanied by TBPotassium changes and also include Na+/K+ redistributions between fluid compartments suggests that cells may serve as readily available Na+ store. This Na+ storage, however, is osmotically active, since osmotical equilibration is achieved by opposite redistribution of K+.