Effects of continuous haemofiltration vs intermittent haemodialysis on systemic haemodynamics and splanchnic regional perfusion in septic shock patients: a prospective, randomized clinical trial.

BACKGROUND: Parameters of splanchnic regional perfusion, like intramucosal pH (pHi) and pCO(2) (pCO(2)i), may predict outcome in septic shock patients. Continuous venovenous haemofiltration (CVVH) has been considered beneficial in haemodynamically unstable septic shock patients. In a prospective, randomized, clinical study, we investigated whether CVVH, in comparison to intermittent haemodialysis (IHD), is able to improve splanchnic regional perfusion in critically ill patients. METHODS: Thirty septic shock patients with acute renal failure were randomized to either CVVH (n=20) or IHD (n=10) groups for renal replacement therapy. Patient characteristics at baseline were not different in terms of severity of illness (APACHE II scores), haemodynamics, and pHi/pCO(2)i values. Systemic haemodynamics, oxygen transport variables, and splanchnic regional perfusion parameters were measured at 0.5, 2, 4 and 24 h after initiation of renal replacement therapy. There were no major changes in vasopressor support throughout the 24-h study period. RESULTS: In contrast to IHD, CVVH caused a decrease in heart rate (-3+/-11 vs +9+/-8/min, P<0.01) and an increase in systolic blood pressure (+12+/-1 vs -5+/-17 mmHg, P<0.05) after 2 h. After 24 h, increased systemic vascular resistance was found in the CVVH group in comparison with the IHD group (+312+/-755 vs -29+/-89 dyne/cm(5), P<0.05) and was accompanied by a decrease in cardiac output (-1.54+/-1.4 vs -0.25+/-0.9 l/min, P<0.01). However pHi values remained constant throughout the 24-h study period in both groups and were not different between the groups (CVVH 7.19+/-0.1 vs IHD 7.19+/-0.1, n.s.) as did the pCO(2)i values (CVVH +7+/-17 vs IHD 0+/-15 mmHg, n.s.) and pCO(2) gap values (CVVH +6+/-15 vs IHD +5+/-12 mmHg, n.s.). CONCLUSIONS: Despite different changes of systemic haemodynamics between CVVH and IHD, CVVH did not improve parameters of splanchnic regional perfusion like pHi, pCO(2)i or pCO(2) gap in septic shock patients.

Increased bioavailability of nitric oxide after lipid-lowering therapy in hypercholesterolemic patients: a randomized, placebo-controlled, double-blind study.

BACKGROUND: Impaired endothelium-dependent vasodilation is an early sign of atherosclerosis in hypercholesterolemic patients. We hypothesized that lipid-lowering therapy can improve endothelial function and that this effect is mainly mediated by increased bioavailability of nitric oxide (NO). METHODS AND RESULTS: In a randomized, double-blind, placebo-controlled trial, we studied 29 patients (age, 50+/-12 years) with hypercholesterolemia (LDL cholesterol > or = 160 mg/dL) randomly assigned to receive either fluvastatin (40 mg twice daily; 17 patients) or placebo (12 patients). Forearm blood flow was measured by plethysmography before and after 24 weeks of treatment. Endothelium-dependent vasodilation was assessed by intra-arterial infusion of acetylcholine (ACh; 3, 12, 24, and 48 microg/min) and basal NO synthesis rate by intra-arterial infusion of NG-monomethyl-L-arginine (L-NMMA; 1, 2, and 4 micromol/min). Simultaneous intra-arterial infusion of L-NMMA (4 micromol/min) and ACh (12, 24, and 48 microg/min) was used to test whether any increase in endothelium-dependent vasodilation after lipid-lowering therapy could be blocked by this NO synthase inhibitor. Endothelium-dependent vasodilation improved significantly after 24 weeks of lipid-lowering therapy compared with before therapy (ACh 24 microg/min: 240+/-34% before versus 347+/-50% after therapy; P< or =0.01) and placebo (changes between after and before therapy with ACh 24 microg/min: 108+/-39% for fluvastatin versus -26+/-32% for placebo; P< or =0.05). This improvement in endothelium-dependent vasodilation could be blocked by simultaneous administration of L-NMMA (ACh 24 microg/min plus L-NMMA 4 micromol/min: 170+/-69% before versus 219+/-47% after treatment; P=NS). CONCLUSIONS: Lipid-lowering therapy with fluvastatin can improve disturbed endothelial function in hypercholesterolemic patients compared with placebo. This improvement is mediated by increased bioavailability of NO.

Impact of dietary sodium intake on left ventricular diastolic filling in early essential hypertension.

AIMS: Dietary sodium intake modulates left ventricular hypertrophy in established essential hypertension independent of blood pressure level. We conducted this study to elucidate the relationship between sodium intake and left ventricular structural or functional changes in early essential hypertension. METHODS: Forty-four young male patients (age 25.9 +/- 2.6 years) with mild essential hypertension that had never been treated and 45 normotensive male control subjects of similar age were examined. Dietary sodium intake was measured from 24 h urinary sodium excretion, blood pressure from 24 h ambulatory monitoring (SpaceLabs 90207), left ventricular structure from 2-D guided M-mode echocardiography, and diastolic filling of the left ventricle (as the main compound of diastolic function in a young population) by pulse-wave Doppler sonography. RESULTS: In hypertensive patients, daily sodium excretion correlated with the ratio of late (A) to early (E) maximum velocity (A/E; r = + 0.27, P = 0.07), velocity time integrals (A/E; r = + 0.54, P < 0.001) as well as atrial contribution, as a percent of left ventricular filling (VH ATCO; r = + 0.52, P < 0.001) independent of heart rate, whereas the opposite correlations were observed in normotensive (all P < 0.001). Stepwise multiple regression analysis confirmed these results. Sodium excretion emerged as the strongest independent determinant of impaired diastolic filling in hypertensive patients (velocity time integrals A/E: R(2) = 0.49, beta = 0.57, P = 0.0001; VH ATCO: R(2) = 0.48, beta = + 0.56, P < 0.0001; Vmax A/E: ns). In normotensive subjects, sodium excretion was a similar strong, but inverse determinant of diastolic filling (velocity time integrals A/E: R(2) = 0.40, beta = -0.43, P = 0.0028). Heart rate was a strong determinant of diastolic filling in hypertensive patients (beta = +0.55, P = 0.0002) and in normotensive subjects (beta = + 0.34, P = 0.011). Left ventricular mass and end-diastolic volume index were not related to diastolic filling in either group. CONCLUSION: In early essential hypertension, sodium excretion is correlated with impaired left ventricular diastolic filling independent of left ventricular mass. The renin-angiotensin-aldosterone-aldosterone system might be a mediator of the observed correlation.

Is endothelial function of the radial artery altered in human essential hypertension?

There is controversy over whether endothelial function is impaired in human essential hypertension. All studies to date have used measurements of forearm blood flow by plethysmography to assess endothelium-dependent vasodilation and endothelial function. In contrast to these studies, which have focused on resistance vessels, we have determined what effects the endothelium has on underlying smooth muscle cells in conduit arteries by measuring arterial compliance of the radial arteries (change in diameter of radial artery over pressure for each arterial pulse). In 13 normotensive healthy subjects and 11 young patients with essential hypertension, arterial compliance of the radial artery was assessed directly with a new high-precision ultrasonic device (NIUS 02) after infusion of acetylcholine (endothelium-dependent response) or sodium nitroprusside (endothelium-independent response). Arterial compliance of the radial artery was similar at baseline and with increasing doses of acetylcholine and sodium nitroprusside in normotensive and in hypertensive subjects. The increase in arterial compliance from baseline at each individual concentration of acetylcholine and sodium nitroprusside was the same in both normotensive and hypertensive subjects. However, after a single oral dose of a combination of the angiotensin converting enzyme inhibitor spirapril and the calcium entry blocker isradipine, the increase in arterial compliance in response to the maximum dose of intraarterial acetylcholine was enhanced in normotensives (0.38 +/- 1.23 to 0.76 +/- 1.01 mm2/mm Hg x 10(-3), P < .05), but not in hypertensives (+0.41 +/- 1.26 to 0.36 +/- 1.31 mm2/mm Hg x 10(-3), not significant), and differed significantly between normotensive and hypertensive subjects (P < .05). Thus, pharmacologic stimulation disclosed a blunted response of endothelium-dependent action in the arterial compliance of the conduit arteries in hypertensive subjects. This suggests an impaired endothelial function reserve in persons with essential hypertension.

Angiotensin II related to sodium excretion modulates left ventricular structure in human essential hypertension.

BACKGROUND: Urinary sodium excretion and angiotensin II (Ang II), which are linked in a physiological feedback mechanism, have both been described to be blood pressure-independent determinants of left ventricular hypertrophy in essential hypertension. We conducted a study to investigate the interaction of sodium excretion with Ang II and its potential impact on myocardial hypertrophy. METHODS AND RESULTS: Sixty-eight patients (46 men and 22 women; mean age, 52 +/- 10 years) with untreated World Health Organization stage I to II essential hypertension were examined in a cross-sectional study. Left ventricular structure and function (two-dimensionally guided M-mode echocardiography), dietary sodium intake (as estimated by 24-hour urinary sodium excretion), and noninvasive ambulatory blood pressure over 24 hours (Spacelab 90207) were determined in parallel with plasma renin activity and plasma Ang II and serum aldosterone concentrations (radioimmunoassay). Twenty-four-hour urinary sodium excretion emerged as a strong correlate of relative wall thickness independent of 24-hour ambulatory blood pressure (partial r = .49, P < .001). Ang II concentrations were weakly correlated with septal wall thickness (r = .27, P < .05) and left ventricular mass (r = .25, P < .05). Patients with high Ang II concentrations in relation to sodium excretion had a greater left ventricular mass (318 +/- 77 versus 257 +/- 54 g, P < .02), posterior wall thickness (11.8 +/- 1.9 versus 10.5 +/- 0.8 mm, P < .02), and septal wall thickness (13.6 +/- 1.8 versus 11.9 +/- 1.3 mm, P < .01) than those with "relatively low" Ang II levels in relation to sodium excretion. CONCLUSIONS: Impaired suppression of the renin-Ang II system appeared to act as a stimulus for myocardial hypertrophy in hypertensive patients.

How should renal hemodynamic data be indexed in obesity?

Clearance data are customarily indexed to body surface area of 1.73 m2. This study examined whether this standard procedure gives correct values for renal perfusion in obese subjects. In 215 subjects who varied in age, gender, height, weight, obesity, and mean arterial blood pressure, RPF was determined by measuring the clearance of (131I)para-aminohippuric acid. Multiple regression analysis of the whole study group revealed that age (beta = -0.44, P < 0.001), height (beta = +0.25, P < 0.01), and arterial blood pressure (beta = -0.19, P < 0.01) were independent predictors of RPF, but that weight or body mass index was not. When related to body surface area, RPF appeared to decline with increasing obesity as follows: normal weight, 609 +/- 153 mL/min per 1.73 m2; overweight, 572 +/- 149 mL/min per 1.73 m2; severely overweight, 530 +/- 145 mL/min per 1.73 m2 (P < 0.012). In contrast, RPF related to height reflected a pattern concordant with the multiple regression analysis: normal weight, 3.76 +/- 0.9 mL/min per meter; overweight, 3.86 +/- 1.0 mL/min per meter; and severely overweight, 3.86 +/- 1.0 mL/min per meter (not significant). A separate repetition of the whole analysis for both normotensive (N = 55) and hypertensive subjects (N = 160) revealed a result similar to that found for the whole group. Thus, our results show that obesity was not a determinant of RPF, and when related to body surface area, inappropriately low values of RPF were calculated for obese patients. It was concluded that RPF values correlate with height and not with surface area in obese subjects.

Renal hemodynamic response to stress is influenced by ACE-inhibitors.

Cardiovascular responses to new antihypertensive agents have been studied extensively under resting and stress conditions. However, the effects of antihypertensive agents on renal hemodynamics have only been investigated at rest. To test whether antihypertensive therapy leads to disparate renal hemodynamic effects during exposure to mental stress as opposed to resting conditions, we performed a double-blind, placebo-controlled crossover study. In 17 normotensive healthy men, glomerular filtration rate (GFR; inulin clearance) and renal plasma flow (RPF; para-aminohippuric acid clearance) were measured at rest and after a standardized mental stress test, with or without one week's treatment with a new, long-acting ACE-inhibitor cilazapril 2.5 mg/day. ACE-inhibition did not change resting blood pressure, RPF, GFR, or derived parameters, such as filtration fraction, renal vascular resistance and renal fraction of cardiac output. Mental stress caused an acute increase in GFR (p < 0.001) and filtration fraction with both treatments (0.001). However, the increase in GFR and filtration with mental stress was greater (p < 0.05) with cilazapril treatment than with placebo. Thus, renal hemodynamic responses to mental stress were influenced by ACE inhibitors. We conclude that the impact of antihypertensive agents on renal hemodynamics during stress cannot a priori be delineated from measurements at rest.

Synergistic effects of angiotensin and adenosine in the renal microvasculature.

Experiments were performed in the in situ kidney of rats and in isolated afferent arterioles of the rabbit to study the interaction between angiotensin II and adenosine in producing afferent arteriolar vasoconstriction. In the intact rat kidney, we observed that 1) peritubular infusion of the adenosine1-receptor blocker 8-cyclopentyl-1,3-dipropylxanthine (CPX, 10(-4) M) blocked the fall in stop-flow pressure (PSF) induced by angiotensin II (5 x 10(-7) M) but not that caused by vasopressin (10(-5) M) or norepinephrine (10(-5) M), 2) peritubular infusion of saralasin (5 x 10(-5) M) attenuated the fall in PSF caused by N6-cyclohexyladenosine (CHA, 10(-5) M) 3) the fall in PSF following luminal application of CHA (10(-5) M) was reduced in angiotensin II-depleted states (volume expansion and converting enzyme inhibition) and this could be reversed by infusion of low doses of angiotensin II, and 4) the reduction in PSF in response to luminal infusion of CHA was augmented in a greater-than-additive fashion when angiotensin was simultaneously administered at low intravenous infusion rates. In isolated afferent arterioles of the rabbit, we noted that 1) addition of 10(-5) M CPX to the bath significantly blunted the constrictor effect of angiotensin II (10(-8) M), 2) presence of the converting enzyme inhibitor quinaprilate (10(-7) M) caused a right shift in the constrictor-response curve to increasing concentrations of CHA (10(-9)-10(-5) M) but 10(-5) M saralasin did not affect vasoconstriction caused by 10(-6) M adenosine, 3) simultaneous administration of submaximal constrictor doses of angiotensin II (10(-10) M) and adenosine (10(-6) M) produced vasoconstriction that was greater than additive, and 4) 10(-5) M CHA and angiotensin II (10(-7) M) constricted afferent arterioles after removal of the glomerulus, whereas either agent alone had no constrictor effect in this preparation. Our results suggest that adenosine and angiotensin cooperate in a mutually dependent and synergistic fashion in producing afferent arteriolar vasoconstriction.

Effects of adenosine and angiotensin on macula densa-stimulated renin secretion.

The present studies were performed to assess, in the isolated perfused juxtaglomerular apparatus of the rabbit kidney, the effect of exogenous adenosine on renin secretion stimulated by a low NaCl concentration at the macula densa. Addition of adenosine to the bath resulted in a change of renin secretion from 30.4 to 23.9 nGU/min at an adenosine concentration of 10(-6) M (n = 7; P = NS), from 38.6 to 17.9 nGU/min at a concentration of 10(-4) M (n = 7; P = 0.038), and from 18.4 to 5.8 nGU/min at 10(-2) M (P = 0.0053). Addition of the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine at 10(-5) M fully reversed the effect of adenosine at 10(-4) M, but not at 10(-2) M. Inhibition of adenosine breakdown by the adenosine deaminase inhibitor pentostatin (10(-6) M) enhanced the inhibitory effect of adenosine with renin secretion falling from 61.7 to 19.5 nGU/min at 10(-6) M adenosine (P = 0.035) and from 44.7 to 13.5 nGU/min at 10(-4) M adenosine (n = 0.027). A marked inhibition of NaCl-dependent renin secretion was caused by both angiotensin II (P = 0.011) and angiotensin III (P = 0.006), both at 10(-8) M. These results show that adenosine is capable of reducing macula densa-mediated renin secretion, but that this effect, even at very high concentrations or during adenosine deaminase blockade, does not fully mimic the inhibitory potency of increasing luminal NaCl concentration. Because the marked effect caused by angiotensins establishes the potential of this preparation to demonstrate inhibitory hormonal influences, it is concluded that adenosine does not appear to be the sole paracrine factor responsible for the NaCl-induced reduction of renin secretion.

Vasomotor effects of purinergic agonists in isolated rabbit afferent arterioles.

To examine the vasomotor effects of purinergic agonists, experiments were performed in isolated afferent arterioles of rabbit kidneys dissected together with their glomerulus and perfused with a pressure head of 120 cmH2O. Changes in vascular diameter were taken as measure of changes in vasomotor tone. Adenosine caused a dose-dependent and persistent decrease in vascular diameter along the entire afferent arteriole with significant changes being detectable at 10(-8) M. Constrictor effects were more pronounced in the glomerular entrance segment of the arteriole where adenosine caused a progressive diameter reduction with maximum contraction at 10(-4) M. Similar monophasic diameter reductions of the distal afferent arteriole were seen with increasing bath concentrations of 2-chloroadenosine (2-ClAdo), cyclohexyladenosine (CHA), and 5'-(N-ethylcarboxamido)adenosine (NECA). Concentrations to achieve half-maximum responses were 92.5 nM for 2-ClAdo, 39 nM for CHA, and 107 nM for NECA. The A2-receptor agonist N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine had no effect on vessel diameter. Increasing bath concentrations of ATP caused significant diameter reductions in both the proximal and distal parts of the afferent arterioles. Addition of the A1-receptor blocker, 8-cyclopentyl-1,3-dipropylxanthine, eliminated the effect of ATP in the proximal region of the arteriole, but a significant diameter reduction was still seen in the glomerular entrance segment. The ATP analogue, beta gamma-methylene-ATP, caused a significant diameter reduction in this segment. These results are consistent with an essentially exclusive presence of A1 receptors in the glomerular entrance segment of the afferent arteriole, whereas in more proximal regions A2 receptors appear to also be expressed in low density. These studies also provide functional evidence for the presence of P2x receptors in renal afferent arterioles.

Vasoconstrictor effect of angiotensin and vasopressin in isolated rabbit afferent arterioles.

The present studies were performed to examine the vasoconstrictor effect of angiotensin II (ANG II), angiotensin III (ANG III), and vasopressin in isolated afferent arterioles of the rabbit kidney. Afferent arterioles were dissected together with their glomerulus and perfused with a pressure head of 120 cmH2O. Changes in vasomotor tone were assessed as diameter changes on videotaped recordings. Afferent arterioles responded to the angiotensins and vasopressin with dose-dependent reductions in vascular diameters with half-maximum responses being observed at concentrations between 10(-9) and 10(-8) M. Responses to ANG II and III were inhibited by saralasin. Contractile responses to ANG II and vasopressin were not altered by prior occlusion of the efferent arteriole, suggesting that afferent vasoconstriction does not represent a myogenic reaction to an increase in efferent resistance. The vasoconstrictor response to ANG II was largely eliminated by removal of the glomerulus and the distal-most portion of the afferent arteriole, whereas the response to vasopressin remained intact. Our data are consistent with the notion that the juxtaglomerular apparatus (JGA) and/or glomerulus may control proximal afferent arteriolar contractility by electrotonic or myogenic coupling mechanisms or by producing cofactors that modulate vasomotor responses.

Macula densa control of renin secretion.

The technique of perfusing isolated tubular segments from rabbit kidneys was used to study macula densa (MD) control of renin secretion. Renin secretion was found to be markedly and reversibly increased when perfusate NaCl concentration was reduced. This response of renin secretion occurred within the physiological concentration range, i.e. between 80 and 20 mM. The stimulatory effect of reduced NaCl was seen even when total solute concentration was kept constant. In this preparation a reduction of NaCl concentration was a stronger stimulus than a fall in NaCl load. Loop diuretics were found to stimulate renin secretion. Suppression of renin secretion was seen when most of the Na was replaced by Rb or choline, but not when Cl was replaced with isethionate or acetate. Activation of adenosine-1-receptors inhibited MD-stimulated renin secretion, an effect that was blocked by an A1-antagonist. This agent partially blocked NaCl-induced inhibition of renin release, suggesting a possible role of adenosine in MD control of renin secretion.

The afferent arteriole--the target for macula densa-generated signals.

The contractile characteristics of the afferent arteriole revealed in the present series of experiments lead one to predict that TGF-induced vasoconstriction should be dependent on both A-II and adenosine. In fact, previous evidence has suggested a role for both adenosine and angiotensin in the TGF mechanism. Acceleration of adenosine deamination as well as adenosine receptor blockade markedly reduced the effect of distal NaCl concentration on SNGFR or PSF. Conversely, inhibition of adenosine breakdown or cellular adenosine uptake, two interventions which are likely to increase interstitial adenosine levels, augmented TGF responses. The same effect was seen when adenosine1-receptor analogs were locally applied by microinfusion. The administration of A-II-converting-enzyme blockers or A-II-receptor antagonists reduced TGF responses, while peritubular or intravenous administration of A-II augmented them. Furthermore, the inhibition of TGF responses caused by volume expansion-induced reductions in plasma A-II concentrations could be restored to normal by A-II infusion. The present results raise the possibility that normal TGF responsiveness depends upon the availability of both adenosine and A-II in sufficiently high concentrations. Figure 1 outlines a mechanism of this mutual dependency. One may assume that adenosine is generated as a consequence of NaCl-dependent changes in NaCl transport by macula densa cells or by TALH cells in the immediate vicinity of the macula densa. The absence of capillaries in the juxtaglomerular interstitium could permit an accumulation of the autacoid to an extent not possible in other regions of the renal interstitium.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct vasoconstriction as a possible cause for amphotericin B-induced nephrotoxicity in rats.

In anesthetized rats we tested the hypothesis that amphotericin B (AmB) reduces glomerular filtration rate (GFR) by activating the tubuloglomerular feedback (TGF) mechanism. Infusion of 1 mg/kg AmB over 50 min was followed by a reduction in kidney GFR (from 0.47 +/- 0.03 to 0.39 +/- 0.02 ml/min per 100 g body wt during the second hour after infusion; P less than 0.05) and by an increase in urine flow and urinary chloride excretion. Single-nephron GFR (SNGFR) measured in proximal (TGF interrupted) or distal tubules (TGF intact) decreased to a similar degree from 33.4 +/- 1.8 and 30.6 +/- 1.2 nl/min in the control period to 19.7 +/- 1.9 and 21.2 +/- 1.6 nl/min during the second hour after AmB infusion (P less than 0.05). Distal chloride concentrations and TGF responses to changes in loop of Henle flow rate were not significantly altered by AmB. AmB at 10(-5) M reduced the diameter of isolated perfused afferent arterioles from rabbit kidneys. In isometrically contracting rings of rabbit aorta and renal artery in vitro AmB produced endothelium-independent constriction, with half-maximal contraction (EC50) being achieved by 1.8 x 10(-6) and 2.6 x 10(-6) M in intact vessels and 1.3 x 10(-6) and 1.7 x 10(-6) M in endothelium-denuded vessels respectively. Tension development did not occur in Ca-free media or in the presence of Ca channel blockers. Pretreatment with ouabain or Bay K 8644 potentiated the effect of AmB. The vasoconstrictive effect of AmB was counteracted by aminophylline and atrial natriuretic peptide. We conclude that the AmB-induced reduction in GFR is not caused by TGF activation and that AmB has a direct vasoconstrictor effect that is probably initiated by depolarization-induced opening of Ca channels. This effect may be an important component of the nephrotoxic actions of AmB.

Renin release from isolated juxtaglomerular apparatus depends on macula densa chloride transport.

Transport inhibitor and ion substitution studies were performed using perfused, superfused preparations of the isolated rabbit juxtaglomerular apparatus to investigate transport dependency of macula densa-mediated renin secretion. In the first experimental series, tubular perfusion with a high-NaCl solution containing 10(-6) M bumetanide increased renin secretion compared with perfusion with high NaCl alone from 8.7 to 24.6 nano-Goldblatt hog units (nGU)/min. Bath addition of 10(-6) M bumetanide had no effect on renin release. The second series tested ability of luminal addition of 54 mmol/l Na or Cl salts to inhibit renin secretion, starting from a stimulated value produced by low-NaCl perfusion. Perfusion with a high-NaCl solution decreased renin secretion from 58.9 to 14.8 nGU/min, which served as a positive control. Addition of choline chloride decreased renin secretion from 42.7 to 16.6 nGU/min, and RbCl decreased renin secretion from 54.9 to 17.0 nGU/min. In contrast, addition of two different Na salts had no effect on renin release (from 41.7 to 31.6 nGU/min with sodium isethionate and from 14.1 to 13.5 nGU/min with sodium acetate). Also, in the presence of 26 mmol/l Cl, addition of 54 mmol/l Na had no effect on renin secretion (29.9-36.8 nGU/min). These data demonstrate that renin secretion is directly stimulated by luminal application of transport blockers and can be inhibited by increases in Cl concentration at the macula densa but not by changes in Na concentration. These results support the hypothesis that the initiating signal for macula densa control of renin secretion is an inverse change in transport rate via the luminal Na(+)-K(+)-2Cl- cotransporter.

Characterization of the macula densa stimulus for renin secretion.

These studies utilize the isolated perfused rabbit juxtaglomerular apparatus (JGA) to study the macula densa signal for renin secretion in the absence of the confounding influences of intravascular pressure and renal nerve activity. In the first experimental series, JGAs were perfused alternately with high- and low-NaCl solutions to determine the reversibility of the renin response to changes in NaCl concentration. Compared with high-NaCl controls, perfusion with a low-NaCl solution resulted in a fivefold increase in renin secretion rate (RSR) [2.1-10.0 nano-Goldblatt hog units (nGU)/min], and this response was largely reversible. When the solutions were presented in the reverse order, a similar inhibition by high NaCl was observed. In the second series, JGAs were perfused with high-, medium-, and low-NaCl solutions to determine the sensitive range of the renin response to NaCl concentration changes. The full renin response (3.2-16.6 nGU/min), similar in magnitude to that seen in series 1, was found to occur between 80 and 24 mM for Na+ and 61 and 7 mM for Cl-. In the third series, the NaCl concentration and flow rate of the perfusate were altered independently to separate the effects of flow rate, NaCl delivery, and NaCl concentration on RSR. Although a decrease in perfusate flow rate slightly increased RSR (3.4-8.1 nGU/min), a comparable decrease in NaCl concentration resulted in a much higher RSR (26.3 nGU/min). We conclude that in this preparation 1) RSR responds equally to both increases and decreases in macula densa NaCl concentration, and these changes are rapid and largely reversible, 2) the full renin response occurs within the concentration range normally occurring at the macula densa, i.e., below 80 mM Na+ and 61 mM Cl-, and 3) RSR responds with a larger change to alterations in NaCl concentration than in NaCl delivery or fluid flow rate.

Effect of adenosine1-receptor blockade on renin release from rabbit isolated perfused juxtaglomerular apparatus.

Adenosine has been proposed to act within the juxtaglomerular apparatus (JGA) as a mediator of the inhibition of renin secretion produced by a high NaCl concentration at the macula densa. To test this hypothesis, we studied the effects of the adenosine1 (A1)-receptor blocker 8-cyclopentyl-1,3-dipropylxanthine (CPX) on renin release from single isolated rabbit JGAs with macula densa perfused. The A1-receptor agonist, N6-cyclohexyladenosine (CHA), applied in the bathing solution at 10(-7) M, was found to inhibit renin secretion, an effect that was completely blocked by adding CPX (10(-5) M) to the bath. Applied to the lumen, 10(-5) M CPX produced a modest stimulation of renin secretion rates suppressed by a high NaCl concentration at the macula densa (P less than 0.05). The effect of changing luminal NaCl concentration on renin secretion rate was examined in the presence of CPX (10(-7) and 10(-5) M) in the bathing solution and in vehicle control experiments. The control response to increasing luminal NaCl concentration was a marked suppression of renin secretion, that was maintained as long as luminal NaCl concentration was high and was promptly reversible when concentration was lowered. CPX did not alter renin release when luminal NaCl was low, but diminished the reduction caused by high NaCl (P less than 0.01). It is concluded that A1-receptors are located within the JGA, and that A1-receptor activation inhibits renin release. A high NaCl concentration at the macula densa appears to influence A1-receptor activation, but a low NaCl concentration does not. The findings support participation of adenosine in macula densa control of renin secretion.