RENAL MEDULLARY COUNTERCURRENT SYSTEM STUDIED WITH HYDROGEN GAS

Effects of striatal nitric oxide production on regional cerebral blood flow and seizure development in rats exposed to extreme hyperoxia

The endogenous vasodilator and signaling molecule nitric oxide has been implicated in cerebral hyperemia, sympathoexcitation, and seizures induced by hyperbaric oxygen (HBO2) at or above 3 atmospheres absolute (ATA). It is unknown whether these events in the onset of central nervous system oxygen toxicity originate within specific brain structures and whether blood flow is diverted to the brain from peripheral organs with high basal flow, such as the kidney. To explore these questions, total and regional cerebral blood flow (CBF) were measured in brain structures of the central autonomic network in anesthetized rats in HBO2 at 6 ATA. Electroencephalogram (EEG) recordings, cardiovascular hemodynamics, and renal blood flow (RBF) were also monitored. As expected, mean arterial blood pressure and total and regional CBF increased preceding EEG spikes while RBF was unaltered. Of the brain structures examined, the earliest rise in CBF occurred in the striatum, suggesting increased neuronal activation. Continuous unilateral or bilateral striatal infusion of the nitric oxide synthase inhibitor N(ω)-nitro-L-arginine methyl ester attenuated CBF responses in that structure, but global EEG discharges persisted and did not differ from controls. Our novel findings indicate that: 1) cerebral hyperemia in extreme HBO2 in rats does not occur at the expense of renal perfusion, highlighting the remarkable autoregulatory capability of the kidney, and 2) in spite of a sentinel increase in striatal blood flow, additional brain structure(s) likely govern the pathogenesis of HBO2-induced seizures because EEG discharge latency was unchanged by local blockade of striatal nitric oxide production and concomitant hyperemia.

Methods of renal blood flow measurement

Variations in regional renal blood flow have been implicated in a variety of disease states. Many techniques have been developed in an attempt to accurately assess these changes. The microsphere technique is the most widely used method at the present time. This technique allows focal measurements to be performed, but there is a conflict between the resolution of the method and the number of microspheres necessary in each sample. New imaging techniques such as tomography and autoradiography enable visual assessment of renal blood flow. Though there is no ideal method, these techniques have opened up new possibilities in the quantification of regional renal blood flow.

Application of missing pulse steady state free precession to the study of renal microcirculation

Missing pulse steady state free precession (MP-SSFP), an extension of steady state free precession (SSFP), was evaluated for its ability to measure slow fluid flows. In experiments using flow phantoms, the MP-SSFP signal was sensitive to fluid velocities in the millimeters per second range. Isolated perfused rabbit kidneys were then used to determine if MP-SSFP could measure perfusion in a biological tissue. The signal intensities in the different anatomical regions of the kidney were observed to be related to the total flow to the organ. Furthermore, increasing the flow sensitivity of the pulse sequence by increasing the gradient strength resulted in decreases in the image signal intensity. The MP-SSFP signal was more sensitive to flow in the medulla than in the cortex. This can be related to slow flow sensitivity of MP-SSFP and the known differences in velocity profiles between these two regions. These results suggest that MP-SSFP may be a powerful tool for the noninvasive measurement of slow fluid flows in different regions of the kidney.

Countercurrent diffusion in the renal cortex of the rabbit

Evidence was obtained for countercurrent diffusion of 14C-n-butanol in the rabbit renal cortex. After injection into the renal artery of perfused, isolated rabbit kidneys, concentrations of both 14C-butanol and tritiated water in the initial samples collected from the renal venous outflow exceeded those of 125I-albumin when the rate of perfusion was low. When flow was increased, the 125I-albumin curve was shifted to earlier times relative to the other labels. These observations confirm the existence of arteriovenous diffusion of 14C-butanol and tritiated water in the kidney. In a second set of experiments, 14C-butanol and tritiated water were infused for 10 or 60 seconds into the renal arteries of anesthetized rabbits, and the kidneys were then removed and frozen in liquid nitrogen. Cores were cut from the kidney surface and sectioned in a cryostat, and the ratio of 14C-butanol to tritiated water was calculated at increasing depths in the cortex. This ratio rose from 0.54 +/- 0.03 (SEM) at the surface to 0.98 +/- 0.07 at 3 mm beneath the surface in kidneys perfused for 10 seconds. This gradient was less steep after 60 seconds of perfusion. The early appearance of 14C-butanol relative to tritiated water in the renal venous outflow and delayed equilibration of 14C-butanol in the outer renal cortical tissue are consistent with counter-current diffusion. It is suggested that this exchange may occur between adjoining interlobular arteries and veins in the cortex and may contribute to high carbon dioxide tensions found near the renal surface.

Diffusional arteriovenous shunting in the heart

Previous indicator dilution experiments in isolated blood-perfused dog hearts suggested that there was intramyocardial diffusional shunting of water relative to a flow-limited solute, antipyrine. Two sets of studies have been done to assess the importance of this shunting, since it implies the possibility of a diffusional bypass for oxygen and other substances, which may be important in ischemia. Nonconsumed tracers were used to show the phenomenon. In the first set, bolus injections of 133Xe dissolved in saline were made into the coronary inflow and the tracer content of the organ recorded by an external gamma detector. The initial Xe washout was disproportionately rapid at low flows, and the late phase was also relatively retarded. In the second set, boluses of cool saline containing indocyanine green were injected into the coronary arterial inflow while coronary sinus outflow dilution curves were recorded via a thermistor and a dye densitometer over a wide range of flows. The thermal curves showed emergence of heat preceding the dye; the degree of precession was much greater at low flows, and, unlike the dye curves, the thermal dilution curves showed dramatic differences in shape at different flows. A model for diffusional countercurrent exchange shows similar changes in residue curves and outflow dilution curves. The conclusion is that there is diffusional shunting of small lipid-soluble molecules whose diffusion coefficients in tissue are high. While the shunting of heat is great, the shunting of soluble gases will not be large and that of normal substrates will be negligible.

Partition of 125I-iodoantipyrine among erythrocytes, plasma, and renal cortex in the dog

The tissue/blood partition coefficient, lambda tb, defined as the amount of blood having the same tracer content as one unit of tissue at diffusion equilibrium, was determined for 125I-iodoantipyrine (I-Ap) and tritiated water (THO) in the dog kidney cortex. Measurements were made after in vivo equilibration for 75 to 300 s and with liver circulation excluded. In 18 kidneys, lambda tb for I-Ap averaged 1.38 (S.D. 0.13) w/w (weight/weight), without significant correlation to hematocrit (range: 23-43) or to urine pH (range 5.5-8.6). The lambda tb for THO averaged 0.97 (S.D. 0.06) v/w (volume/weight), close to the relative water contents. Erythrocyte/plasma partition for I-Ap was 0.82 w/w, compared to a water partition of 0.72. Thus, at diffusion equilibrium the apparent I-Ap concentration in renal cortical and red cell water exceeds that of plasma water by 14 and 60%, respectively. It follows that I-Ap cannot be used as a general indicator for total tissue water content. When used for measurement of local blood flow and modum Kety, lambda tb must be determined for each tissue and species.

Distribution of blood flow in the dog kidney. I. Saturation rates for inert diffusible tracers, 125I-iodoantipyrine and tritiated water, versus uptake of microspheres under control conditions

Disparate reports on intrarenal blood flow distribution prompted a direct comparison between microspheres (Ms) and inert diffusible tracers (DT). The "tissue sampling technique" for estimating local flow with DT (Kety) was adapted for the dog kidney, using 125I-iodoantipyrine (1-Ap) and tritiated water (THO). Ms (15 micron) were injected 2-3 min prior to 10-15 s DT infusion made during continuous 1 s arterial blood sampling. Tracers were measured in 7 to 20 samples from each of the following zones: Outer, middle and inner cortex (C1, C2, C3), outer and inner halves of outer medulla (OM1, OM2), and inner medulla (IM). I-Ap and THO gave closely similar flow distribution, and average total renal blood flow (RBF) of 3.90 and 3.78 as compared to 3.94 ml/min . g with Ms. Flow in C2 (ml/min . g) was similar with all tracers, and in per cent thereof average local flows were: C1 102, C3 70, OM1 34, OM2 12, and IM 2 with DT versus 117, 53, 12, 3, and 0 with Ms. Zonal flow fractions of total RBF obtained with DT were: C1 0.41, C2 0.33, and C3+medulla 0.26 versus 0.51, 0.33 and 0.16 with Ms. Thus, a Ms surplus in C1 relative to DT flow, representing 10% of total RBF, matched a Ms deficit in C3+medulla. This disparity might result from: (1) Failure of Ms to enter deep afferent arterioles in proportion to blood flow, (2) diffusion of DT from deep portions of the interlobular arteries, and/or (3) postglomerular inward flow of blood and DT.

Shock and the kidney: pathophysiology and pharmacological support

This article reviews current knowledge as to the physiological mechanisms that control renal vascular resistence. The contribution of both extrinsic and intrinsic neuro-humoral regulation of both blood flow and glomerular filtration rate are described. The changes that occur both to the renal blood flow and glomerular filtration rate in the pathophysiological situation of "pre-renal uraemia" as well as "acute tubular necrosis" are described. Within this setting pharmacological manoeuvres that may improve both renal blood flow and glomerular filtration rate are discussed. In addition, the indications for and general principles of haemo-and peritoneal dialysis are described.

Countercurrent exchange of heat in the dog kidney

The mechanisms of renal heat clearance were studied in anesthetized dogs by recording renal venous and cortical temperatures during step changes of arterial blood temperature induced by infusion of 0.9% saline of room temperature into the renal artery. The venous thermodilution curves were well described by a three-exponential function with average rate constants: k 1 = 10.4 min -1 , k 2 = 2.2 min -1 , and k 3 = 0.59 min -1 , and intercepts at 34%, 44%, and 22%, respectively. The various components were interpreted as follows:

The slow component mainly represents heat conduction from inner medullary and perirenal tissue as shown by extrarenal temperature recording and by calculating the heat distribution volume. Local cortical recordings showed that cortex is not a homogeneous compartment, and k 2 reflects an average cortical heat clearance, k 1 is much higher than blood flow per gram of tissue in any kidney region and results from heat exchange between intrarenal arteries and veins.

The arteriovenous diffusion of heat explains the higher temperatures observed in the cortex and the renal medulla than in renal venous blood. The arteriovenous thermodilution technique is unsuited for local and total renal blood flow measurements, because of countercurrent diffusion and heat exchange witli perirenal tissue.

The effects of changes in haematocrit on the intrarenal distribution of blood flow in the dog's kidney

1. The effect of changes in the haematocrit of blood perfusing the kidney on its intrarenal distribution was studied in dogs.2. Two types of preparations were employed. (i) In the isolated perfused kidney evidence is presented that flow in the autoregulating preparation represents predominantly cortical flow while flow in the ;low flow non-autoregulating' kidney reflects medullary flow. (ii) In the intact kidney renal blood flow rate and its intrarenal distribution was studied by the injection of (133)Xe into the renal artery and measuring its clearance from the kidney by an external counter.3. In both types of preparation cortical flow was found to be independent of changes in P.C.V. but medullary flow varied inversely with haematocrit.4. A change in the haematocrit of the perfusing blood leads to alteration of its viscosity. It was argued that an increase in viscosity must lead to a reduction in the resistance of the cortical afferent arterioles but that medullary afferent arterioles were not able to respond in this manner.5. These findings demonstrate that changes in total body haematocrit cause a redistribution of blood flow between renal cortex and medulla.

Estimation of coronary blood flow by washout of diffusible indicators

In 13 canine hearts, 158 disappearance curves for ¹³³Xe and antipyrine-¹²⁵I, given by intra-arterial slug injection, were recorded at a wide range of perfusion rates. Flow rates (ml/100 g/min) calculated from these curves by a variety of methods were compared with measured flow rates (F_a) per weight of perfused tissue. Perfusion of isolated, supported hearts and of anterior descending coronary arteries in open-chest dogs provided similar data. The semilogarithmic slope of curves from apex or whole heart decreased with time, particularly at high flow rates. There was a small, consistent difference in shape between antipyrine and xenon curves, suggesting that radioactivity in fat contributed somewhat to this tailing. Estimation of flow rate from the steepest semilog slope yielded an average value of 1.1F_a for all rates; estimation from slope at 30% of peak radioactivity gave 0.9F_a. The curves were closely described by a two-exponential equation which gave flow estimates of 0.95F_a when collimation limited the observations to the heart apex, and lower values when the whole heart was observed. Peak height/area methods gave values of approximately 0.75F_a in spite of various compensations for the impossibility of recording the curve until radioactivity = 0.

Effect of hemorrhage and retransfusion on intrarenal distribution of blood flow in dogs

Distribution of intrarenal blood flow was studied in 12 dogs anesthetized with Nembutal. Medullary blood flow was estimated by local clearance of hydrogen gas from the outer medulla measured polarographically with needleshaped platinum electrodes, and by local clearance of (85)Kr and mean transit time of (32)P-labeled erythrocytes measured with a small semiconductor detector placed in the outer medulla. Cortical blood flow was estimated from cortical red cell transit time and from total renal blood flow measured by electromagnetic flowmeter. Bleeding to a mean arterial pressure of 50-65 mm Hg in the course of 8-20 min reduced cortical and medullary blood flow on the average to the same extent. In half of the experiments both cortical and medullary blood flow were reduced proportionately less than mean arterial pressure during the first half hour of bleeding. Maintenance of mean arterial pressure at 50-65 mm Hg in all cases led to progressive reduction of both cortical and medullary blood flow, out of proportion to the reduction of arterial pressure. A two step bleeding procedure used in two experiments also led to uniform reduction of renal blood flow. Reinfusion of blood after 2-3 hr of hypotension increased total renal blood flow to an average of 82% and outer medullary hydrogen clearance to an average of 92% of control values. All dogs survived the experiment without evidence of renal failure. It is concluded that hemorrhagic hypotension in dogs leads to a progressive and fairly uniform rise in renal vascular resistance, without any selective hemodynamic response in the juxtamedullary circulation.

Limitation of the use of inert gases in the measurement of small gut mucosal blood flow

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Diffusion processes invalidating the intra-arterial krypton-85 beta-particle clearance method for measurement of skin blood flow in man

Krypton-85 dissolved in 0.9% NaCl was injected into the femoral artery of man; β particles were measured over the skin of the crus. Cutaneous mean blood flow was calculated using the integral equation of Zierler based on the mean transit time analysis of clearance curves recorded from the cutaneous tissue; these calculations gave estimates which were only about one-fifth those obtained by other methods. The low estimates were caused by an intercom-partmental exchange of 85 krypton between the cutaneous and subcutaneous tissues, an exchange implying that both tissues influence the shape of the clearance curve recorded from the cutaneous tissue alone. This intercompart-mental exchange was effected not only by diffusion directly from the cutaneous to the subcutaneous tissue, but presumably also by transport from the blood streaming between the two tissues. The clearance rate of the tail of the curve was shown to correspond to that of the subcutaneous tissue. The errors distorting the clearance curve of β particles may invalidate the use of intra-arterial injection and measurement of clearance curves of β particles for measurement of blood flow in other tissues.

Renal medullary heat clearance in the dog

Renal medullary heat clearance was investigated in anesthetized dogs to evaluate countercurrent exchange of heat. Sudden changes in the temperature of renal arterial blood were induced by infusion of 3 to 7 ml/min of saline at room temperature. The resulting changes in medullary temperature, measured with fine thermocouples, started with increasing delay from cortex towards the papilla. Average rate constant for heat uptake in the inner medulla was 0.61 min -1 . In the outer medulla, rate constants from 0.6 to 5.0 min -1 were observed, with the higher values in the subcortical zone. Inner medullary clearance rose during diuresis, but was practically uninfluenced by lowering perfusion pressure to 35 mm Hg. Calculated heat conduction (diffusion) from cortex into medulla was close to observed clearances, which are thus mainly determined by heat conduction. Clearance due to flow in vasa recta and loops of Henle was small, presumably due to efficient countercurrent heat exchange, whereas urine flow in collecting ducts is not subject to this effect. Formulas for countercurrent exchange not taking into account length diffusion (parallel to the vasa recta) are not valid for medullary heat exchange. The present heat clearance technique cannot be used for estimating medullary blood flow.

The action of pitressin on solute permeability of the rabbit nephron in vivo

The isotopic equilibration of urea, thiourea, and inulin between urine and plasma was determined in rabbits in the presence or absence of antidiuretic hormone (ADH). Animals were anesthetized with ethanol and permitted to reach steady state after completion of surgery. Tracer was then administered by intraarterial infusion in such a manner that a high constant specific activity in plasma was rapidly attained. Urine flow was kept independent of ADH by addition of mannitol. Urea/creatinine clearance ratios and the accumulation of urea in renal medulla and papilla also remained unaffected by ADH. Under these conditions, thiourea and inulin at all times approached equilibrium, at similar rates. In the absence of ADH, urea also equilibrated at a rate similar to that of inulin. The addition of ADH, however, significantly prolonged the delay before urinary urea reached the high constant specific activity of plasma urea. These observations are interpreted in terms of a specific effect of the hormone on the solute permeability of the nephron.