Leszek Dobrowolski

Differential effect of angiotensin II on blood circulation in the renal medulla and cortex of anaesthetised rats

The renal medulla is sensitive to hypoxia, and a depression of medullary circulation, e.g. in response to angiotensin II (Ang II), could endanger the function of this zone. Earlier data on Ang II effects on medullary vasculature were contradictory. The effects of Ang II on total renal blood flow (RBF), and cortical and medullary blood flow (CBF and MBF: by laser‐Doppler flux) were studied in anaesthetised rats. Ang II infusion (30 ng kg−1 min−1i.v.) decreased RBF 27 ± 2 % (mean ±s.e.m.), whereas MBF increased 12 ± 2 % (both P < 0.001). Non‐selective blockade of Ang II receptors with saralasin (3 μg kg−1 min−1i.v.) increased RBF 12 ± 2 % and decreased MBF 8 ± 2 % (P < 0.001). Blockade of AT1 receptors with losartan (10 mg kg−1) increased CBF 10 ± 2 % (P < 0.002) and did not change MBF. Losartan given during Ang II infusion significantly increased RBF (53 ± 7 %) and decreased MBF (27 ± 7 %). Blockade of AT2 receptors with PD 123319 (50 μg kg−1 min−1i.v.) did not change CBF or MBF. Intramedullary infusion of PD 123319 (10 μg min−1) superimposed on intravenous Ang II infusion did not change RBF, but slightly decreased MBF (4 ± 2 %, P < 0.05). We conclude that in anaesthetised surgically prepared rats, exogenous or endogenous Ang II may not depress medullary circulation. In contrast to the usual vasoconstriction in the cortex, vasodilatation was observed, possibly related to secondary activation of vasodilator paracrine agents rather than to a direct action via AT2 receptors.

The renal medullary interstitium: focus on osmotic hypertonicity

1. There has been continued interest in the functional role of the renal medullary interstitium and intense research in this area has furnished new information regarding the extent, dynamics and mechanisms determining fluctuations in medullary osmotic hypertonicity.

Differential Effect of Frusemide on Renal Medullary and Cortical Blood Flow in the Anaesthetised Rat

In addition to its known effect on renal tubular transport, frusemide (furosemide) has been shown to affect renal circulation. This study in the anaesthetised rat examined the influence of frusemide (bolus 0.25 or 0.5 mg kg‐1 I.V., then infusion delivering the same dose over 1 h) on renal cortical and medullary circulation measured as laser‐Doppler blood (cell) flux. The responses were compared with simultaneously measured changes in renal excretion and in the tissue admittance, an index of medullary ionic hypertonicity of the interstitium. Renal vascular responses to frusemide were significant but not dose dependent. During low‐dose frusemide infusion cortical flux decreased 11.5 ± 0.9% and medullary flux decreased 32.3 ± 3.5% (difference significant at P < 0.001). During high‐dose infusion the decreases were by 13.5 ± 1.4 and 29.3 ± 3.8%, respectively (difference significant at P < 0.001). Sodium excretion increased 15‐fold (by 3.7 ± 0.4 μmol min‐1) and 30‐fold (by 5.9 ± 1.1 μmol min‐1) during low‐ and high‐rate infusion of frusemide, respectively. By contrast, medullary tissue admittance decreased similarly with the two doses: maximally by 13.4 ± 1.4 and 10.9 ± 0.9%, respectively. The observations that an exaggerated post‐frusemide decrease in blood flow within the medulla coincided with decreasing tissue admittance in this zone and that neither medullary blood flow nor admittance changes were related to the dose suggest a causal relationship between interstitial ionic hypertonicity and vascular resistance. We propose that the post‐frusemide decrease in medullary tissue NaCl depressed medullary circulation by inhibiting local generation of vasodilator prostaglandins.

Effects of ATP on rat renal haemodynamics and excretion: role of sodium intake, nitric oxide and cytochrome P450

Aim:  Adenosine‐5′‐triphosphate (ATP) affects intrarenal vascular tone and tubular transport via P2 receptors; however, the actual role of the system in regulation of renal perfusion and excretion remains unclear and is the subject of this whole‐kidney study.

Simultaneous recording of tissue ion content and blood flow in rat renal medulla : evidence on interdependence

The relationship of renal medullary tissue ion concentration and medullary blood flow (MBF) has never been closely evaluated because of limitations of available measuring methods. In an attempt to overcome this difficulty, an integrated probe was developed for simultaneous recording in rat renal medulla of tissue electrical admittance ( Y), an index of interstitial ion concentration, and tissue perfusion with blood (laser-Doppler method). During spontaneous-selective MBF variations tissue Y showed inverse changes ( r = -0.77, P < 0.001). The inverse correlation of the two variables was also seen after MBF has been reduced (-43%) by indomethacin, 5 mg/kg body wt iv ( r = -0.77, P < 0.01). A modest selective MBF reduction (15%) induced by glibenclamide, an inhibitor of ATP-dependent K channels, did not alter medullary tissue admittance. The data support experimentally the concept that the rate of medullary tissue perfusion with blood is one determinant of interstitial solute concentration; however, changes in the latter were demonstrable only with major alterations of the MBF.

OSMOTIC HYPERTONICITY OF THE RENAL MEDULLA DURING CHANGES IN RENAL PERFUSION PRESSURE IN THE RAT

1 The relationship between renal perfusion pressure (RPP) and ion concentration in renal medulla was studied in anaesthetized rats. RPP was changed in steps within the pressure range 130‐80 mmHg, while tissue electrical admittance (Y, index of interstitial ion concentration) and medullary and cortical blood flow (MBF and CBF; laser Doppler flowmetry) were measured, along with glomerular filtration rate (Cin) and renal excretion. 2 With a RPP reduction from 130 to 120 mmHg, tissue Y remained stable; at 100 and 80 mmHg, Y was 5 and 17 % lower, respectively, than at 120 mmHg. 3 CBF fell less than RPP (partial autoregulation) in the range 130‐100 mmHg only. MBF was autoregulated within 120‐100 mmHg, but not above or below this range. 4 Each step of RPP reduction was followed by a decrease in sodium and water excretion (UNaV and V). The osmolality of excised inner medulla fragments was similar at 120 and 105 mmHg (586 ± 45 and 618 ± 35 mosmol (kg H2O)−1, respectively) but lower at 80 mmHg (434 ± 31 mosmol (kg H2O)−1, P < 0.01); the ion concentration changed in parallel. 5 The data show that medullary hypertonicity was well preserved during RPP fluctuations within 130‐100 mmHg, but not below this range. RPP‐dependent changes of UNaV and V were not clearly associated with changes in solute concentration in medullary tissue.

Furosemide-induced renal medullary hypoperfusion in the rat: role of tissue tonicity, prostaglandins and angiotensin II

Furosemide (frusemide)‐induced renal medullary hypoperfusion provides a model for studies of the dependence of local circulation on tissue tonicity. We examined the role of medullary prostaglandins (PG) and adenosine (Ado) as possible mediators of the response to furosemide. Furosemide was infused i.v. at 0.25 mg kg−1 h−1 in anaesthetized rats, untreated or treated with intramedullary indomethacin (Indo) or Ado. An integrated set‐up was used to measure renal medullary laser‐Doppler flux (MBF) and medullary ionic tonicity (electrical admittance, Y), and to infuse Indo and Ado directly into the medulla. The cortical flux was measured on kidney surface. The excretion of water, sodium and total solute was also determined. Intramedullary Indo (1 mg kg−1 h−1) decreased MBF 18 ± 5% and increased tissue Y 14 ± 3% (both significant); the treatment abolished the post‐furosemide decrease in MBF (−22% in untreated group) and enhanced slightly the increase in renal excretion. Intramedullary Ado (5 mg kg−1 h−1) did not change baseline MBF or Y; the post‐furosemide decreases in MBF (−22%) and Y, and the increase in renal excretion were preserved. We conclude that a decrease in intramedullary PG activity secondary to decreased medullary hypertonicity mediates the fall in medullary perfusion in response to furosemide; the hypoperfusion may help restore the initial tonicity. Together with the earlier evidence on the dependence of post‐furosemide medullary hypoperfusion on angiotensin II, the study exposes its interaction with PG in the control of medullary circulation. Adenosine is not involved in medullary vascular responses to decreased tissue hypertonicity.

Renal Vascular Effects of Frusemide in the Rat: Influence of Salt Loading and the Role of Angiotensin II

We showed recently that post‐frusemide (furosemide) natriuresis was associated with a major depression of medullary circulation. In the present study, prior to administration of frusemide the tubular transport of NaCl was modified by loading the animals with 5% saline to elucidate a possible interrelation between the tubular and vascular effects of the drug. Moreover, a possible involvement of the renin‐angiotensin system was examined by pharmacological blockade using captopril, an inhibitor of angiotensin converting enzyme (1 mg kg−1, I.V.), or losartan, a selective inhibitor of angiotensin AT1 receptor (10 mg kg−1, I.V.). The effects of frusemide (0.25 mg kg−1 I.V., then the same dose given over 1 h) on renal medullary and cortical circulation (using laser‐Doppler flowmetry) and renal excretion of sodium (UNaV), water and total solutes were measured in anaesthetised rats. With no pre‐treatment, frusemide decreased the medullary flow (36.6 ± 6.0%) significantly more than the cortical flow (10.1 ± 1.0%; P < 0.001). The difference between the medulla and cortex was not significant in rats which showed high UNaV after hypertonic saline loading (2.0 ± 0.4 vs. 0.4 ± 0.1 μmol min−1 in non‐loaded rats): 21.1 ± 3.9% and 15.8 ± 1.5%, respectively. At very high UNaV (9.5 ± 1.1 μmol min−1) the post‐frusemide decrease in blood flow tended to be smaller in the medulla (7.6 ± 7.7%) than in the cortex (16.2 ± 2.6%). The fall in medullary blood flow was attenuated by pre‐treatment with captopril (22.0 ± 3.3%) and abolished by pre‐treatment with losartan (2.8 ± 11.8%). The decrease in cortical blood flow was not changed by hypertonic saline or angiotensin II blockers. The abolition of the post‐frusemide depression of medullary blood flow by previous salt loading confirms the proposed link between tubular transport status and vasoconstriction. A similar modification of the response by blockade of the renin‐angiotensin system suggests that the system is involved in the mechanism of medullary vasoconstriction.

Atrial peptide natriuresis in the rat without genuine rise in filtration rate or wash‐out of medullary electrolytes.

1. Effects of synthetic atrial natriuretic peptide (ANP) on renal excretion, total renal blood flow (RBF), glomerular filtration rate (GFR) and tissue electrical admittance (reciprocal impedance, an estimate of tissue electrolytes) were determined in pentobarbitone‐anaesthetized rats. GFR was measured both as inulin clearance (Cin) and as a product of renal plasma flow (RPF) and inulin extraction ratio (Ein). 2. With the lowest dose of ANP (0.35 micrograms/(kg min) I.V.) a 5‐fold increase in sodium excretion occurred without measurable change in Cin, RPF x Ein nor medullary electrolyte concentration estimated from tissue electrical admittance. 3. With medium and high dosage (2 and 6 micrograms/(kg min), respectively), major and rapid increases in sodium excretion and urine flow were associated with an acute increase in Cin but not RPF x Ein. 4. The RBF increase observed in all groups of rats was not dose‐related and did not parallel the natriuresis. Electrolyte concentration in the medullary tissue showed a modest transient decrease in rats given medium and high ANP doses. 5. We conclude that pronounced ANP natriuresis can develop in the absence of a measurable increase of GFR, estimated by a method not subject to urinary dead space error (RPF x Ein). The small transient decrease in medullary tissue electrolytes observed with higher peptide doses does not support solute wash‐out as an important mechanism of increased sodium excretion.

High salt intake increases blood pressure in normal rats: putative role of 20-HETE and no evidence on changes in renal vascular reactivity.

Background/Aims. High salt (HS) intake may elevate blood pressure (BP), also in animals without genetic salt sensitivity. The development of salt-dependent hypertension could be mediated by endogenous vasoactive agents; here we examined the role of vasodilator epoxyeicosatrienoic acids (EETs) and vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE). Methods. In conscious Wistar rats on HS diet systolic BP (SBP) was examined after chronic elevation of EETs using 4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (c-AUCB), a blocker of soluble epoxide hydrolase, or after inhibition of 20-HETE with 1-aminobenzotriazole (ABT). Thereafter, in acute experiments the responses of renal artery blood flow (Transonic probe) and renal regional perfusion (laser-Doppler) to intrarenal acetylcholine (ACh) or norepinephrine were determined. Results. HS diet increased urinary 20-HETE excretion. The SBP increase was not reduced by c-AUCB but prevented by ABT until day 5 of HS exposure. Renal vasomotor responses to ACh or norepinephrine were similar on standard and HS diet. ABT but not c-AUCB abolished the responses to ACh. Conclusions. 20-HETE seems to mediate the early-phase HS diet-induced BP increase while EETs are not engaged in the process. Since HS exposure did not alter renal vasodilator responses to Ach, endothelial dysfunction is not a critical factor in the mechanism of salt-induced blood pressure elevation.