Nitric oxide, atrial natriuretic factor, and dynamic renal autoregulation

Inhibition of nitric oxide (NO) synthase by Nω-nitro-L-arginine methyl ester (L-NAME) increases arterial pressure (PA) and profoundly reduces renal blood flow (RBF). Here we report that L-NAME causes changes in the PA-RBF transfer function which suggest augmentation of the approximately 0.2 Hz autoregulatory mechanism. Attenuation of PA fluctuations from 0.06 to 0.11 Hz was enhanced, indicating increased efficacy of autoregulation. Also, the rate of gain reduction between 0.1 and 0.2 Hz increased while the associated phase peak became >= π/2 radians, indicating emergence of a substantial rate-sensitive component in this system so that autoregulatory responses to rapid PA changes become more vigorous. Infusion of L-arginine partly reversed the pressor response to L-NAME, but not the renal vasoconstriction or the changes in the transfer function. The ability of atrial natriuretic factor (ANF), which also acts via cGMP, to replace NO was assessed. ANF dose dependently reversed but did not prevent the pressor response to L-NAME, indicating additive responses. ANF did not restore RBF or reverse the changes in the transfer function induced by L-NAME. The rate-sensitive component that was enhanced by L-NAME remained prominent, suggesting that either ANF did not adequately replace cGMP or provision of a basal level of cGMP was not able to replace cGMP generated in response to NO. It is concluded that NO synthase inhibition changes RBF dynamics with the most notable change being increased contribution by a rate-sensitive component of the myogenic system.Key words: Nω-nitro-L-arginine methyl ester (L-NAME), renal blood flow, rat, blood pressure, transfer function.

Nitric oxide, atrial natriuretic factor, and dynamic renal autoregulation

Inhibition of nitric oxide (NO) synthase by N(omega)-nitro-L-arginine methyl ester (L-NAME) increases arterial pressure (PA) and profoundly reduces renal blood flow (RBF). Here we report that L-NAME causes changes in the PA-RBF transfer function which suggest augmentation of the approximately 0.2 Hz autoregulatory mechanism. Attenuation of PA fluctuations from 0.06 to 0.11 Hz was enhanced, indicating increased efficacy of autoregulation. Also, the rate of gain reduction between 0.1 and 0.2 Hz increased while the associated phase peak became > or = pi/2 radians, indicating emergence of a substantial rate-sensitive component in this system so that autoregulatory responses to rapid PA changes become more vigorous. Infusion of L-arginine partly reversed the pressor response to L-NAME, but not the renal vasoconstriction or the changes in the transfer function. The ability of atrial natriuretic factor (ANF), which also acts via cGMP, to replace NO was assessed. ANF dose dependently reversed but did not prevent the pressor response to L-NAME, indicating additive responses. ANF did not restore RBF or reverse the changes in the transfer function induced by L-NAME. The rate-sensitive component that was enhanced by L-NAME remained prominent, suggesting that either ANF did not adequately replace cGMP or provision of a basal level of cGMP was not able to replace cGMP generated in response to NO. It is concluded that NO synthase inhibition changes RBF dynamics with the most notable change being increased contribution by a rate-sensitive component of the myogenic system.

Incommensurate frequencies of major vascular regulatory mechanisms

The dynamic relationship among three major vascular control mechanisms that operate on large fractions of cardiac output: arterial baroreflex and renal and mesenteric autoregulation, was investigated in conscious rats. Wistar and spontaneously hypertensive rats were studied in their home cages 10 days after implantation of pulsed Doppler flow probes. There was an oscillation of blood pressure centered at 0.45 Hz that is associated with operation of arterial baroreflexes. Hindquarters blood flow displayed a featureless, "1/f " power spectrum, in which no autoregulatory or baroreflex signatures could be discerned, although active control of resistance over a wide range of frequencies was evident. The renal pressure - flow transfer function was dominated by an autoregulatory mechanism with a resonance peak at 0.25 ± 0.01 Hz. In the mesenteric circulation an autoregulatory mechanism was seen with a resonance peak at 0.15 ± 0.01 Hz and another active mechanism was seen above 0.2 Hz that appeared from its negative admittance phase to be a baroreflex. The center frequencies of mesenteric and renal autoregulation and of the arterial baroreflex were related in a ratio of 1 : 1.7 ± 0.1 : 3.0 ± 0.2 (approximately 4:7:12). Such relatively high order ratios can be expected to minimize the possibility of phase locking and (or) entrainment among the various control mechanisms.Key words: flow, pressure, blood, dynamics, Wistar, spontaneously hypertensive rat (SHR).

Incommensurate frequencies of major vascular regulatory mechanisms

The dynamic relationship among three major vascular control mechanisms that operate on large fractions of cardiac output: arterial baroreflex and renal and mesenteric autoregulation, was investigated in conscious rats. Wistar and spontaneously hypertensive rats were studied in their home cages 10 days after implantation of pulsed Doppler flow probes. There was an oscillation of blood pressure centered at 0.45 Hz that is associated with operation of arterial baroreflexes. Hindquarters blood flow displayed a featureless, "1/f' power spectrum, in which no autoregulatory or baroreflex signatures could be discerned, although active control of resistance over a wide range of frequencies was evident. The renal pressure - flow transfer function was dominated by an autoregulatory mechanism with a resonance peak at 0.25 +/- 0.01 Hz. In the mesenteric circulation an autoregulatory mechanism was seen with a resonance peak at 0.15 +/- 0.01 Hz and another active mechanism was seen above 0.2 Hz that appeared from its negative admittance phase to be a baroreflex. The center frequencies of mesenteric and renal autoregulation and of the arterial baroreflex were related in a ratio of 1 : 1.7 +/- 0.1 : 3.0 +/- 0.2 (approximately 4:7:12). Such relatively high order ratios can be expected to minimize the possibility of phase locking and (or) entrainment among the various control mechanisms.

Responses of mesenteric and renal blood flow dynamics to acute denervation in anesthetized rats

Previous studies have shown that renal autoregulation dynamically stabilizes renal blood flow (RBF). The role of renal nerves, particularly of a baroreflex component, in dynamic regulation of RBF remains unclear. The relative roles of autoregulation and mesenteric nerves in dynamic regulation of blood flow in the superior mesenteric artery (MBF) are similarly unclear. In this study, transfer function analysis was used to identify autoregulatory and baroreflex components in the dynamic regulation of RBF and MBF in Wistar rats and young spontaneously hypertensive rats (SHR) anesthetized with isoflurane or halothane. Wistar rats showed effective dynamic autoregulation of both MBF and RBF, as did SHR. Autoregulation was faster in the kidney (0.22 +/- 0.01 Hz) than in the gut (0.13 +/- 0.01 Hz). In the mesenteric, but not the renal bed, the admittance phase was significantly negative between 0.25 and 0. 7 Hz, and the negative phase was abrogated by mesenteric denervation, indicating the presence of an arterial baroreflex. The baroreflex was faster than autoregulation in either bed. The presence of sympathetic effects unrelated to blood pressure was inferred in both vascular beds and appeared to be stronger in the SHR than in the Wistar rats. It is concluded that a physiologically significant baroreflex operates on the mesenteric, but not the renal circulation and that blood flow in both beds is effectively stabilized by autoregulation.

Dynamic autoregulation in the in vitro perfused hydronephrotic rat kidney

Renal autoregulation is mediated by tubuloglomerular feedback, operating at 0.03-0.05 Hz, and a faster system, operating at 0.1-0.2 Hz, that has been attributed by exclusion to myogenic vasoconstriction. In this study, we examined dynamic autoregulation in the hydronephrotic rat kidney, which lacks tubuloglomerular feedback but exhibits pressure-induced afferent arteriolar vasoconstriction. Kidneys were harvested under anesthesia from Sprague-Dawley rats and perfused in vitro using defined, colloid-free medium. Renal perfusate flow was assessed during forced pressure fluctuations at mean pressures of 60-140 mmHg. Transfer function analysis revealed passive behavior at 60 mmHg and active, pressure-dependent responses at higher pressures. In all cases, coherence was high (0.89 +/- 0.03 between 0.01 and 0.9 Hz). There was a resonance peak in admittance gain at approximately 0.3 Hz and an associated broad peak in phase angle. Below this frequency, gain declined progressively. The minimum gain achieved at 0.01-0.05 Hz was pressure sensitive, being 1.08 +/- 0.02 at 60 mmHg and 0.71 +/- 0.04 at 140 mmHg. These findings are consistent with in vivo results and with model-based predictions of the dynamics of myogenic autoregulation, supporting the postulate that the rapid component of autoregulation reflects operation of a myogenic mechanism.

Pharmacological modulation of spontaneous renal blood flow dynamics

Two mechanisms contribute to renal autoregulation. The faster system, which is thought to be myogenic, operates at 0.1-0.2 Hz (i.e., 5-10 s/cycle), while the slower one, tubuloglomerular feedback, operates at 0.03-0.05 Hz (i.e., 20-30 s/cycle). Both attenuate spontaneous or induced fluctuations of blood pressure, but it has proven difficult to separate their individual contributions because there is potential for interaction between the two. The present study was designed to examine the dynamics of the faster system during pharmacological blockade of tubuloglomerular feedback. Normotensive and hypertensive rats were studied under isoflurane or halothane anesthesia. Administration of the loop diuretic furosemide plus the angiotensin II (ANGII) AT1 receptor antagonist losartan caused a 10-fold or greater natriuresis, indicating profound inhibition of ascending limb salt transport, and also produced characteristic changes in the transfer function relating blood pressure (input) to renal blood flow (output). Operation of the 0.1-0.2 Hz mechanism was essentially unaltered, as shown by the presence of a peak in phase angle at 0.1-0.2 Hz and reduction of gain at frequencies slower than 0.15 Hz. The 0.03-0.05 Hz mechanism was markedly inhibited, as shown by loss of the second phase angle peak at 0.03-0.05 Hz, loss of the local maximum in gain at 0.05 Hz, and loss of the second gain reduction below 0.05 Hz. Both during control and after inhibition of tubuloglomerular feedback, the 0.1-0.2 Hz system attenuated = 50% of the effects of spontaneous blood pressure fluctuations, suggesting that this mechanism, operating alone, can significantly stabilize renal blood flow in the face of spontaneous fluctuations of blood pressure.

Spontaneous blood pressure fluctuations and renal blood flow dynamics

Two mechanisms operating at 0.03-0.05 and 0.1-0.2 Hz are involved in autoregulation of renal blood flow (RBF). To examine the behavior of the faster system, the response of RBF to spontaneous fluctuations of arterial pressure was assessed in Sprague-Dawley rats anesthetized by isoflurane or halothane. During halothane anesthesia, autonomous oscillation of total RBF was observed at 0.10-0.15 Hz, and normalized admittance gain became negative at 0.11 +/- 0.01 Hz. During isoflurane anesthesia, there was autonomous power in blood flow in a broad peak between 0.15 and 0.25 Hz, and gain became negative at 0.15 +/- 0.01 Hz. Increasing inspired isoflurane concentration from 1.4 +/- 0.1% to 2.2 +/- 0.1% reduced pressure by 22 +/- 2 mmHg but did not alter blood flow or the transfer function, indicating that the operating frequency was not changed. In another experiment, changing from isoflurane to halothane increased peak power in the autonomous blood flow oscillation fivefold and reduced its frequency from 0.18 +/- 0.01 to 0.14 +/- 0.01 Hz. Gain became negative at a higher frequency (0.16 +/- 0.01 Hz) during isoflurane than halothane anesthesia (0.12 +/- 0.01 Hz). The results show that the 0.1-0.2 Hz system is reliably detected under unforced conditions and provides modest attenuation of pressure fluctuations at < or = 0.1 Hz. Its operating frequency under isoflurane anesthesia is consistent with previous estimates from barbiturate-anesthetized rats, whereas it operates significantly slower under halothane anesthesia.

Atrial natriuretic factor, angiotensin II, and the slow component of renal autoregulation

Autoregulation of renal blood flow is highly efficient and is mediated partly by tubuloglomerular feedback (TGF), which couples regulation of blood flow to that of sodium excretion. Atrial natriuretic factor (ANF) dilates preglomerular resistance vessels, in which autoregulation occurs, and has been reported to inhibit TGF. This study addressed potential actions of ANF on the slow, TGF-mediated, component of autoregulation. Renal blood flow was measured by an electromagnetic flow probe in Sprague-Dawley rats anesthetized by halothane or isoflurane while renal perfusion pressure was manipulated by a servo-controlled clamp placed on the aorta between the renal arteries. Progressive reduction of perfusion pressure to 60 mmHg (1 mmHg = 133.3 Pa) induced resetting of autoregulation to operate at the reduced pressure and to defend lower renal blood flow. Infusion of ANF at a dose shown to reliably increase sodium excretion did not affect autoregulation or its resetting. Because resetting is angiotensin II dependent, the converting enzyme inhibitor Enalaprilat was used to provide angiotensin II blockade. As expected, autoregulation did not reset to operate at reduced perfusion pressure. Again ANF was without effect. In a third experiment, relaxation of resistance was assessed in response to repeated steps of perfusion pressure between 65 and 75 mmHg. Time constants of constriction and dilatation were recovered by fitting to a single exponential before and during ANF infusion. Time constants ranged form 0.045 to 0.055 Hz, were consistent with operation of TGF, were not different for constriction or dilatation, and were unaltered by ANF; nor did ANF affect the magnitude of constriction or dilatation.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 2-adrenergic mediation of the effects of angiotensin II on rat renal artery in vitro

Angiotensin II (ANG II) is a major influence on renal blood flow, acting directly on the renal vasculature and upon other controllers. In vivo observations suggest that ANG II affects renal artery resistance, although explicit in vitro studies have produced negative results. To resolve this issue, potential interactive effects of ANG II on renal artery in vitro were tested. Renal arteries were harvested from ketamine-anesthetized Sprague-Dawley rats and perfused in vitro at constant flow. Resistance was determined from the axial pressure drop while downstream pressure was held constant at approximately 80 mmHg (1 mmHg = 133.3 Pa). ANG II, per se, had only a trivial effect on arterial diameter (-5.5 +/- 1.5% at 10(-7) M ANG II) and failed to affect resistance at concentrations ranging from 10(-11) to 10(-7) M. Norepinephrine caused strong concentration-dependent constriction, increasing resistance from 0.32 +/- 0.04 to 1.86 +/- 0.73 mmHg.mL-1.min at 10(-6) M and 3.27 +/- 0.88 mmHg.mL-1.min at 10(-5) M. In the presence of 10(-8) M ANG II, these responses were significantly increased to 3.31 +/- 1.00 and 5.02 +/- 1.22 mmHg.mL-1.min, respectively. Similarly, in the presence of 10(-6) M norepinephrine, ANG II caused significant, concentration-dependent constriction of renal artery. In a separate experiment, 10(-7) M yohimbine, a relatively specific antagonist of alpha 2-adrenergic receptors, reversed the resistance increment due to ANG II, but not that due to norepinephrine. When yohimbine was applied before norepinephrine and ANG II, it did not affect the response to norepinephrine, but again blocked potentiation of the response by ANG II.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro response of rat renal artery to perfusion pressure

Both tubuloglomerular feedback and a myogenic response contribute to autoregulation of renal blood flow. Vascular interaction initiated by tubuloglomerular feedback has been described and prevents definition, in vivo, of the contribution of myogenic responses to autoregulation. Segments of rat renal artery were perfused in vitro at constant flow while upstream and downstream pressures were measured on-line, allowing determination of resistance. Transmural pressure was governed by a downstream resistor. Outside diameter was measured by an ocular micrometer. The segments were bathed in bicarbonate Ringer solution and perfused with Ringer containing 50 g/L bovine serum albumin. Potassium depolarization reduced the diameter and made it more sensitive to perfusion pressure. Serosal norepinephrine, 10(-7)-10(-5) M, caused graded constriction and increased the axial pressure drop due to vessel resistance. Addition to the perfusate of rat red blood cells to hematocrit approximately 33% significantly reduced arterial diameter and enhanced the increased axial pressure drop induced by 10(-6) M norepinephrine. Sequential elevation of perfusion pressure from 50 to 100 mmHg (1 mmHg = 133.3 Pa) increased the diameter significantly. Red cells reduced the slope of the diameter-pressure relationship. In another experiment, norepinephrine reduced the slope of diameter versus perfusion pressure, while 10(-4) M papaverine plus norepinephrine increased the slope, compared with norepinephrine alone. Norepinephrine caused a sizable axial pressure drop (15.7 +/- 3.7 mmHg), which decayed as perfusion pressure increased; the decay was accentuated by papaverine. The changes in axial pressure drop were linearly related to the inverse 4th power of diameter, indicating that both measurements assessed the same behavior. Several different maneuvers thus affect the relationship between arterial diameter and perfusion pressure, and the relationship between axial pressure drop and perfusion pressure. The results indicate the presence of a myogenic response, which is, however, not strong enough to defend vessel diameter when pressure rises.

Angiotensin II conditions the slow component of autoregulation of renal blood flow

Release of a suprarenal aortic clamp results in angiotensin-dependent, arterial pressure-mediated renal vasoconstriction. The experiments reported here were designed to show whether this represents operation of autoregulation and whether the slow component of autoregulation is affected by angiotensin II (ANG II). They were performed using halothane-anesthetized Sprague-Dawley rats. In the 1st experiment renal perfusion pressure (RPP) was reduced in steps from spontaneous level to 45 mmHg and then returned in steps to the spontaneous level. The autoregulatory plateau was left-shifted some 20-30 mmHg, with the lower limit of autoregulation reduced from approximately 85 mmHg on the downward leg to approximately 60 mmHg on the upward leg. This resetting was blocked by captopril. Two experiments examined low pressure autoregulation in more detail. After RPP was reduced, three pairs of steps between 65 and 75 mmHg were performed. Significant renal vasodilatation was observed after downward pressure steps in both experiments. Time constants (tau) of resistance adjustment were recovered from most steps by curve fitting. In both experiments tau down = 0.07 +/- 0.01 Hz was faster than tau up = 0.04 +/- 0.01 Hz. Blockade of ANG II by enalaprilat or by the AT1-receptor blocker losartan potassium significantly inhibited regulatory vasodilatation and vasoconstriction at low RPP. Also, tau down = 0.04 +/- 0.01 Hz collapsed to the value of tau up = 0.04 +/- 0.01 Hz. These results demonstrate a significant role for ANG II in renal autoregulation. They show that ANG II is necessary for autoregulation to reset to operate at reduced arterial pressure and to defend a lower blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Model of TGF-proximal tubule interactions in renal autoregulation

Previous models, assuming constant reabsorption in the proximal tubule, have shown that tubuloglomerular feedback (TGF) can explain only a fraction of glomerular filtration rate (GFR) and renal blood flow autoregulation. Increased arterial pressure inhibits proximal tubule fluid reabsorption, an effect that should increase the efficacy of TGF because of the resulting increased flow rate in the loop of Henle. Models describing pressure and flow in a glomerulus and a nephron were derived to test this prediction. The models were coupled by a TGF function with tubular flow rate at the end of the proximal tubule (superficial nephron) or at the macula densa (juxtamedullary nephron) as input and with afferent arteriolar resistance as output. In agreement with others, the model predicted that TGF alone could account for about one-half of autoregulation. Pressure-dependent inhibition of proximal reabsorption increased the ability of TGF to account for autoregulation, providing compensation for increases in arterial pressure comparable to published whole kidney values. The inclusion of an approximation of an effect of arterial pressure on TGF marginally improved predicted autoregulation. Although the results suggest that the proximal tubule-TGF interaction can provide a quantitatively adequate explanation for autoregulation, they also indicate that the effect of the interaction is spent at arterial pressures greater than 130 mmHg. Additional mechanisms are required to extend this range.

Autoregulation of blood flow in renal medulla of the rat: no role for angiotensin II

Autoregulation of blood flow was assessed by a dual-slit technique in descending and ascending vasa recta of the exposed renal papillae of antidiuretic rats. There was complete autoregulation of blood flow in descending vasa recta. The lower limit of autoregulation was approximately 85 mmHg (1 mmHg = 133.3 Pa) and the upper limit was greater then 160 mmHg. Autoregulation in ascending vasa recta was also good. To test the role of angiotensin II in this autoregulation, the converting enzyme inhibitor captopril was infused. Captopril had no effect on autoregulation of blood flow in either descending or ascending vasa recta. We conclude that blood flow in vasa recta of renal medulla is efficiently autoregulated and that this autoregulation is independent of angiotensin II.

Angiotensin II and prostaglandins in control of vasa recta blood flow

Angiotensin II has been implicated in the regulation of medullary blood flow and is known to interact with prostaglandins at sites within the kidney. Therefore the role of angiotensin in control of vasa recta blood flow was studied in antidiuretic, Munich-Wistar rats. We also tested the hypothesis that prostaglandins act to modulate the effect of angiotensin. Total renal blood flow was measured by an electromagnetic flow probe, vasa recta blood flow by a dual-slit method. Captopril was used to confirm that angiotensin blockade increased renal blood flow (by 15 +/- 4%). Captopril and saralasin were used to show that angiotensin blockade increased vasa recta blood flow (by 23 +/- 9 and 14 +/- 7%, respectively). The results demonstrate a tonic constrictor effect of angiotensin in the renal medulla. Exogenous angiotensin II, delivered intravenously, failed to mimic the effect of endogenous angiotensin. Indomethacin did not alter blood pressure or renal blood flow but did reduce vasa recta blood flow by 20 +/- 3%, suggesting that prostaglandins act preferentially on the medullary circulation. Nor did it alter the response of blood pressure, of renal blood flow, or of vasa recta blood flow to captopril. Moreover, prior angiotensin blockade with either captopril or saralasin enhanced the medullary vasoconstrictor effect of indomethacin (P less than 0.05). These results are not consistent with the hypothesis that prostaglandins act primarily as angiotensin modulators. They suggest that the medullary interaction between angiotensin and prostaglandins differs from that in the cortex.

Renal medullary plasma flow rate and reabsorption of salt and water from inner medullary collecting duct

It has been proposed that medullary washout secondary to increased blood flow will limit maximal urine osmolality and reabsorption of salt and water from the inner medullary collecting duct. We have tested this prediction. The function of the inner medullary collecting duct was examined by microcatheterization. Acetylcholine was infused directly into the renal circulation, captopril was infused intravenously, and angiotensin II was infused into the renal circulation in rats which also received captopril. Medullary plasma flow rate, measured by dye-dilution in parallel experiments, was not significantly increased by acetylcholine; it was increased 30% (p less than 0.02) by systemic infusion of captopril, and was returned to control by angiotensin II. Acetylcholine increased both urine flow rate and sodium excretion (p less than 0.01, p less than 0.001, respectively), while captopril increased only sodium excretion (p less than 0.025). Angiotensin II blocked the natriuresis due to captopril. None of the treatments altered urine osmolality (p greater than 0.4 in all cases). Acetylcholine increased the loads of water, sodium, chloride, and total solute delivered to the inner medullary collecting duct. Angiotensin II reduced delivery of water and solutes compared with captopril alone. None of the treatments affected load dependency of reabsorption of water, sodium, chloride, or total solute in the inner medullary collecting duct. We conclude that there is, at most, a weak interaction between medullary blood flow and reabsorption from the inner medullary collecting duct.

Load dependency of sodium chloride reabsorption by medullary collecting duct in rat

During antidiuresis, the medullary collecting duct (MCD) reabsorbs sodium in load-dependent fashion. However, attempts to characterize reabsorption when sodium delivery to the MCD is elevated have not led to clear results, largely due to interfering effects of the strategies employed to raise delivery. In the present study, microcatheterization was performed in rats undergoing water diuresis induced solely by infusion of 2.5% dextrose in water, and in rats where solute delivery to the MCD was markedly elevated by the combination of water diuresis with acute potassium chloride loading. The results show that delivery of sodium was elevated by the experimental maneuvers, averaging 7.01 +/- 0.83 mumol . min-1 . g kidney wt-1 compared with a normal antidiuretic value in the literature of 3.50 +/- 0.40 mumol . min-1 . g-1. Sodium and chloride reabsorptions were increased proportionally, indicating that the MCD has a large capacity to transport sodium chloride. Normalized sodium reabsorption remained high, varying in different series between 80 +/- 10 and 96 +/- 1% of the delivered load. Thus the MCD reabsorbed an average of 6.37 +/- 0.70 mumol . min-1 . g-1 of sodium while sodium excretion was 0.52 +/- 0.11 mumol . min-1 . g-1. The results emphasize the importance of MCD sodium chloride reabsorption for determination of final urinary salt excretion, and thus for regulation of body salt balance.

Renal medullary blood flow: its measurement and physiology

The vasculature of the mammalian renal medulla is complex, having neither discrete input nor output. There is also efficient countercurrent exchange between ascending and descending vasa recta in the vascular bundles. These considerations have hampered measurement of medullary blood flow since they impose pronounced constraints on methods used to assess flow. Three main strategies have been used: (i) indicator extraction; (ii) erythrocyte velocity tracking; and (iii) indicator dilution. These are discussed with respect to their assumptions, requirements, and limitations. There is a consensus that medullary blood flow is autoregulated, albeit over a narrower pressure range than is total renal blood flow. When normalized to gram tissue weight, medullary blood flow in the dog is similar to that in the rat, on the order of 1 to 1.5 mL X min-1 X g-1. This is considerably greater than estimated by the radioiodinated albumin uptake method which has severe conceptual and practical problems. From both theoretical and experimental evidence it seems that urinary concentrating ability is considerably less sensitive to changes in medullary blood flow than is often assumed.

The effect of a natriuretic atrial extract on renal haemodynamics and urinary excretion in anaesthetized rats

1. An extract of cardiac atrial tissue, when injected intravenously into anaesthetized rats, caused a large and rapid increase in renal excretion of sodium chloride. 2. The renal response to atrial extract was associated with increases in total and medullary blood flow in the kidney, as measured by microsphere and albumin uptake methods, respectively. 3. The data suggest that these changes in renal haemodynamics may contribute to, but are unlikely to be the complete cause of, the natriuresis and chloriuresis following injection of atrial tissue extract.

Intrarenal localization of the natriuretic effect of cardiac atrial extract

In anesthetized rats micropuncture and microcatheterization were used to collect tubular fluid from end proximal and distal tubules and from the outer medullary collecting duct. Urine was collected at the papilla tip. Samples were taken from the same sites before and after intravenous injection of atrial tissue extract; rats injected with ventricular extract served as controls. Sodium excretion increased 17-fold after atrial extract, a significantly greater rise than the 3-fold increase after ventricular extract. Clearances of inulin and single nephron filtration rates did not change significantly in either group. Tubular fluid collection results showed a similar reduction (16 to 20%) of proximal fluid and sodium reabsorption in both groups. In the experimental group only, NaCl reabsorption failed to rise in response to increased load in the medullary collecting duct. The resulting fall in fractional reabsorption in the medullary collecting duct accounted for 80% of the natriuresis. We conclude that atrial tissue from rat hearts contains a factor which causes increased renal NaCl excretion by inhibiting transport in the medullary collecting duct.

Lack of effect of barbiturate and ketamine anesthesia on renal blood flow in chronically instrumented rats prepared for micropuncture

Fifteen-micrometer microspheres were used to study renal blood flow in rats which had been cannulated 3 or more days previously. Renal blood flow was assessed while the animals were conscious, after induction of anesthesia with inactin, pentobarbital, or ketamine, and after preparation for renal micropuncture. Blood pressure fell with anesthesia when barbiturates were used and was restored by tracheostomy. In conscious rats, right and left renal blood flows averaged 4.70 +/- 0.24 and 4.64 +/- 0.23 mL/min per gram kidney weight, respectively. These values were essentially unchanged following anesthesia with all agents used and they were also unaffected by surgery. We conclude that, in chronically prepared rats, renal blood flow is not affected by anesthesia or by micropuncture surgery.

Effect of cold water immersion and its combination with alcohol intoxication on urine flow rate of man

Urine flow rate was determined for man before and after immersion in either thermoneutral (33 degrees C) or cold (10 degrees C) water. The effect of alcohol intoxication of a level of approximately 80 mg dL-1 was also evaluated for the cold water immersion. Immersion and cold were additive in their effect, resulting in a mean urine flow rate of 4.25 mL min-1, approximately 3.5 times the preimmersion level. Alcohol intoxication in conjunction with cold water immersion caused a further large increase in urine flow to 8.03 mL min-1. These results permit better evaluation of the importance of volume diuresis as it relates to the reduction of insulative performance of dry-type immersion suits for cold water survival, and to the possible enhancement of "rewarming shock" during therapy for hypothermia victims. The increased urine production observed when alcohol treatment was added to cold immersion provides information for speculation on mechanisms of volume diuresis.