Vasoconstriction during volume expansion is independent of central control

Role of the kidney in the pathogenesis of hypertension: time for a neo-Guytonian paradigm or a paradigm shift?

The "Guytonian paradigm" places the direct effect of arterial pressure, on renal excretion of salt and water, at the center of long-term control of blood pressure, and thus the pathogenesis of hypertension. It originated in the sixties and remains influential within the field of hypertension research. However, the concept of one central long-term feedback loop, through which arterial pressure is maintained by its influence on renal function, has been questioned. Furthermore, some concepts in the paradigm are undermined by experimental observations. For example, volume retention and increased cardiac output induced by high salt intake do not necessarily lead to increased arterial pressure. Indeed, in multiple models of salt-sensitive hypertension the major abnormality appears to be failure of the vasodilator response to increased cardiac output, seen in salt-resistant animals, rather than an increase in cardiac output itself. There is also evidence that renal control of extracellular fluid volume is driven chiefly by volume-dependent neurohumoral control mechanisms rather than through direct or indirect effects of changes in arterial pressure, compatible with the concept that renal sodium excretion is controlled by parallel actions of different feedback systems, including hormones, reflexes, and renal arterial pressure. Moreover, we still do not fully understand the sequence of events underlying the phenomenon of "whole body autoregulation." Thus the events by which volume retention may develop to hypertension characterized by increased peripheral resistance remain enigmatic. Finally, by definition, animal models of hypertension are not "essential hypertension;" progress in our understanding of essential hypertension depends on new results on system functions in patients.

Preoptic recess lesions reduce right atrial pressure responses to volume expansion

Ablation of the periventricular tissue surrounding the anteroventral portion of the third cerebral ventricle (AV3V-X) abolishes natriuresis and diuresis during volume expansion. Although deficits in several efferent mechanisms have been identified, effects of AV3V-X on afferent input to the volume reflex have not been investigated. Therefore, these experiments measured right atrial pressure (RAP) in conscious AV3V-X and control-operated (CONT) rats before, during, and following acute (60 s) or continuous (60 min) infusion of isotonic saline, or vascular volume expansion with whole blood (15 min). Changes in RAP were significantly smaller in AV3V-X rats than CONT animals during acute isotonic saline infusion and during whole-blood expansion. Treatment with hexamethonium abolished the difference in RAP during acute volume infusion. However, there was no significant difference in RAP between groups during continuous isotonic saline expansion. These data suggest that AV3V-X reduces afferent input from cardiopulmonary stretch receptors during acute volume expansion, which may contribute to diminished natriuresis and diuresis observed in these animals.

Salt-sensitive hypertension in (mREN-2)27 transgenic rats

The (mREN-2)27 transgenic model of hypertension was developed to investigate the effect of genetic over activity of angiotensin II systems as a contributing factor in the development of arterial hypertension. In this model, transgene-positive rats demonstrate elevated renin-angiotensin system activity not only in the circulatory system but also in adrenal gland, reproductive organs, and brain. Since evidence indicates that angiotensin peptides and osmotic stimuli interact synergistically to produce exaggerated behavioral, endocrine, and cardiovascular effects, we examined the effect of salt consumption on arterial pressure, plasma vasopressin, and body fluid balance in male (mREN-2)27 transgene-positive and -negative rats. Four days of drinking 2% NaCl increased mean arterial pressure from 165 +/- 10 to 199 +/- 7 mm Hg in transgene-positive rats. In contrast, transgene-negative rats showed no change in arterial pressure (126 +/- 5 to 128 +/- 3 mm Hg). Plasma vasopressin levels were significantly elevated only in transgene-positive rats, whereas pituitary levels of vasopressin were significantly lower in transgene-positive rats compared with transgene-negative controls (18 +/- 3 and 118 +/- 14 ng, respectively). Although transgene-positive rats consumed significantly more 2% NaCl than did transgene-negative rats, during this period 24-hour sodium balance did not differ between the groups. Since fluid and electrolyte balance is similar between the two groups of rats, the data suggest that transgene-positive rats may be more sensitive to the effects of increased NaCl intake in terms of both endocrine and cardiovascular responses.

Modulation of vascular calcium channel activity in response to acute volume expansion in rats

The mechanisms of the increased resistance in hypertension are still unclear. Several studies have indicated that the potential-sensitive Ca2+ channels (PSC) are altered in arteries isolated from hypertensive patients or animals. An expansion of body fluid volume may trigger local autoregulatory responses or may induce the release of humoral factors, either of which could increase systemic vascular resistance and cause volume-dependent forms of hypertension. We tested the hypothesis that volume expansion per se may cause the alterations of PSC similar to those seen in hypertension. For this, we examined the alterations of PSC in aortas from volume-expanded rats with the use of dihydropyridine-type Ca2+ channel activator, BayK 8644, in parallel with the changes in endothelium-dependent relaxation. Volume expansion was produced by a rapid intravenous infusion of saline (10% of body weight) over 30 min in rats. At the end of infusion, rats were killed and aorta removed for in vitro measurement of isometric tension. Relaxation to acetylcholine (10(-7)-10(-5) mol/L, % relaxation to 10(-7) mmol/L norepinephrine contraction) was not significantly changed. In contrast, contractile response to BayK 8644 (10(-9)-10(-6) mol/L, % response to 50 mmol/L KCl) was significantly enhanced in rats with volume expansion (12 control rats: 11.6 +/- 4.9%; 18 volume-expanded rats: 40.9 +/- 10.4% at 10(-6) mol/L, p < 0.05). These findings suggest that acute volume expansion could induce a similar enhanced vascular Ca2+ channel activity to that seen in hypertension in rats.

Inhibited breathing decreases renal sodium excretion

This study tested the hypothesis that breathing at the upper end of the normal range of end tidal CO2 decreases renal sodium excretion. Normotensive human subjects learned to self-regulate end tidal CO2 using a respiratory gas monitor and feedback procedure. Urine flow rates were increased by a standardized water drinking regimen. Urinary volume and sodium and potassium excretion were decreased during 30 minutes of inhibited (i.e. high normal end tidal CO2) breathing, compared with levels preceding and after task performance. Blood pressure, but not heart rate, increased during task performance. Plasma volume increase under these conditions is indicated by the observation that urinary excretion of an endogenous digoxin-like factor was increased. The physiological mechanism by which inhibited breathing elicits renal sodium retention remains to be determined. This breathing pattern could mediate the role of behavioral stress in some forms of hypertension.

Contribution of regional vascular responses to whole body autoregulation in conscious areflexic rats

We designed studies to evaluate the autoregulation response during volume expansion in three major circulation regions (intestine, kidney, and hind limb) during simultaneous determination of whole body autoregulation in conscious areflexic rats. Cardiac output was measured with chronically implanted electromagnetic flow probes on the ascending aorta. Regional blood flow velocity was measured with pulsed Doppler flow probes on the superior mesenteric (n = 7), left renal (n = 7), and right iliac (n = 7) arteries. Doppler flow probes were calibrated in situ in each rat to determine regional blood flow values. Neurohumoral reflex control of pressure was removed pharmacologically, and blood pressure and cardiac output were returned to resting control values with intravenous norepinephrine infusion, which was maintained at that constant level throughout the study. Hemodynamic changes were measured in response to blood volume expansion with infusion of 0.9 ml blood over 6 minutes. This small change in blood volume resulted in significant increases in vascular resistance of 15% in the whole body, 8% in the intestine, 18% in the kidney, and 15% in the hind limb. The pressure-flow slope, used as an index of autoregulation (slope = 0, perfect autoregulation; slope = 1, rigid vasculature), averaged 0.34 in the whole body, 0.52 in the intestine, 0.19 in the kidney, and 0.39 in the hind limb. When compared with the whole body, blood flow autoregulation was less in the intestine, greater in the kidney, and the same in the hind limb.