Tonic action of endothelin type B and dopamine D3 receptors in spontaneously hypertensive and deoxycorticosterone acetate-salt hypertensive rats: effects of intrarenally applied selective antagonists

Endothelins and renal dopamine contribute to control of renal function and arterial pressure in health and various forms of experimental hypertension, the action is mediated by tonic activity of specific receptors. We determined the action mediated by endothelin type B and by dopamine D3 receptors (ETB-R, D3-R) in anaesthetized spontaneously hypertensive (SHR) and in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. In rats of both hypertension models infused during 60 min into the interstitium of in situ kidney were either ETB-R antagonist, BQ788 (0.67 mg kg-1 BW h-1) or D3-R antagonist, GR103691 (0.2 mg kg-1 BW h-1). Arterial pressure (MAP), renal artery blood flow (RBF, transonic probe) and renal medullary blood flow (MBF, laser-Doppler) were measured along with sodium, water and total solute excretion (UNaV, V, UosmV). Experiments with ETB-R blockade confirmed their tonic vasodilator action in the whole kidney (RBF) and medulla (MBF) in both hypertension models. In SHR only, the first evidence was provided that ETB-R specifically increases transtubular backflux of non-electrolyte solutes. In DOCA-salt rats ETB-R blockade caused an early decrease in water and salt transport whereas an increase was often reported from many previous studies. The most striking effect of D3-R blockade in SHR was a selective increase in MBF, which strongly suggested tonic vasoconstrictor action of these receptors in the renal medulla; this speaks against prevailing opinion that D3 receptors are virtually inactive in SHR. In our model variant of DOCA-salt rats of D3-R blockade clearly caused a rapid major increase in MAP in parallel with depression of renal haemodynamics.

Chymase Dependent Pathway of Angiotensin II Generation and Rapeseed Derived Peptides for Antihypertensive Treatment of Spontaneously Hypertensive Rats

The contribution of chymase, one of the enzymes responsible for angiotensin II generation in non-ACE pathway, remains unclear in the development of hypertension. The aim of the study was to investigate chymase inhibition as potential antihypertensive therapy in spontaneously hypertensive rats (SHR). To block chymase we employed chymostatin, a commercial inhibitor, and new analogues of rapeseed-derived peptides, VWIS and RIY. These simple and easy to obtain peptides not only block chymase, but also possess weak activity to inhibit ACE. This is a first attempt to evaluate the impact of chronic administration of selected inhibitors on blood pressure of SHR in two phases of hypertension. Male SHR (6 or 16 weeks old) were treated daily for two weeks with chymostatin (CH; 2 mg/kg/day), the peptides VWIS (12.5 mg/kg/day) or RIY (7.5 mg/kg/day); control groups received chymostatin solvent (0.15% DMSO in saline) or peptide solvent (saline). The substances were administered intravenously to conscious animals via a chronically cannulated femoral vein. Systolic blood pressure (SBP) was measured by telemetry. Metabolic parameters were measured weekly, and tissue samples were harvested after two weeks of treatment. None of the administered chymase inhibitors affected the development of hypertension in young rats. Only RIY exhibited beneficial properties when administered in the established phase of hypertension: SBP decreased from 165 ± 10 to 157 ± 7 mmHg while the excretion of nitric oxide metabolites increased significantly. The glomerulosclerosis index was lower after RIY treatment in both age groups (significant only in young rats 0.29 ± 0.05 vs 0.48 ± 0.04 in the control group; p < 0.05). Hence, it seems that peptide RIY exhibits some positive effect on renal morphology. The results obtained suggest that the peptide RIY may be a useful tool in the treatment of hypertension, especially in cases when ACE inhibitors are not effective.

Intrarenal vasodilator systems: NO, prostaglandins and bradykinin. An integrative approach

Intrarenal microcirculation is under hormonal, paracrine and neural control. Of particular interest is circulation in the renal medulla: its perfusion seems critical for long term control of arterial pressure. Exposure of the organism to adverse conditions often leads to activation of vasopressor factors, such as renin/angiotensin, renal sympathetic input or vasopressin; this helps maintain arterial pressure but endangers renal circulation. Fortunately, it is protected by intrarenal vasodilators: nitric oxide, prostaglandins, bradykinin and others. The potency of NO to oppose intrarenal vasoconstrictors may differ between individual factors: it is substantial in the case of renal sympathetic input whereas the constrictor influence of angiotensin II in the medulla seems to be offset mostly by intrarenal prostaglandins. Although these are commonly regarded as intrarenal vasodilators, our new data show that this is so only in the renal medulla. In the cortex they exert modest vasoconstriction, probably mediated by EP3 receptors. The role of bradykinin as intrarenal vasodilator is not yet known in sufficient detail, its effect is most pronounced in the inner medulla. The source of vasoactive kinins is uncertain, they could reach intrarenal microvasculature from the sites of synthesis in tubular cells but the synthesis in the vessels themselves cannot be excluded.

Opposed effects of prostaglandin E2 on perfusion of rat renal cortex and medulla: interactions with the renin-angiotensin system

While prostaglandin E(2) (PGE(2)) is an established renal vasodilator, studies of prostaglandin EP receptors suggest that it also has vasoconstrictor potential. Prostaglandin E(2) is much more abundant in the medulla than in the cortex, yet likely differences in effects between zones have not been defined. This study is focused on different vascular effects in the cortex and medulla and interaction with the renin-angiotensin system (RAS). In anaesthetized rats, the effects of cyclo-oxygenase blockade and of PGE(2) infused into the renal artery or renal interstitium were examined. Total renal blood flow was measured by ultrasonic renal artery probe, and local perfusion, separately, of the superficial cortex, outer- and inner medulla, as laser-Doppler fluxes. Indomethacin (5 mg kg(-1) i.v.) increased cortical perfusion (by approximately 10%) and decreased medullary perfusion (by approximately 20%). Renal artery infusion of PGE(2) (15-30 microg kg(-1) h(-1)) increased cortical and medullary perfusion only transiently. Previous inactivation of the RAS using losartan or captopril, and background infusion of exogenous angiotensin II, prevented the transient increase and enhanced the subsequent stable decrease in perfusion. Prostaglandin E(2) infused into the medullary interstitium (7-22 microg kg(-1) h(-1)) increased medullary perfusion by 13%, while cortical perfusion decreased by 6%. Misoprostol, an agonist of constrictor EP(3) receptors, decreased perfusion of the cortex and medulla, with both renal artery and medullary interstitial infusion. In conclusion, in rat renal cortex the dominating stable PGE(2) effect is vasoconstriction, most probably mediated by EP(3) receptors and unrelated to activation of the RAS. Prostaglandin E(2) applied to the cortical or medullary interstitium, a natural route for paracrine agents, induces medullary vasodilatation.