Romero, J. Carlos

Increases in renal venous pressure have been shown to consistently increase renal interstitial pressure; however, not until renal interstitial pressure is increased threefold is a natriuresis noted in normal animals. Since the intrarenal angiotensin II (Ang II) concentration has been postulated to increase with increasing renal venous pressure, the antinatriuretic action of Ang II could override the natriuretic effect of increased renal interstitial pressure. Therefore, the role of Ang II in the natriuretic response to increased renal venous pressure was examined in 10 pentobarbital-anesthetized dogs. Mean arterial pressure, renal blood flow, renal interstitial pressure, glomerular filtration rate, urinary sodium excretion, plasma renin activity, and prostaglandin E2 excretion were measured at renal venous pressures of 3, 15, and 30 mm Hg. The measurements were repeated after the administration of captopril (1 mg/kg i.v. bolus, n = 5) or [Sar1,Ile8]Ang II (50 micrograms/kg i.v. bolus + 50 micrograms/kg/hr infusion, n = 5). Under control conditions, mean arterial pressure, renal blood flow, plasma renin activity, and prostaglandin E2 excretion remained unchanged when renal venous pressure was increased. The elevations in renal venous pressure increased renal interstitial pressure from 7 +/- 2 to 12 +/- 2 and 22 +/- 4 mm Hg, while sodium excretion remained unchanged until renal venous pressure was 30 mm Hg. In the captopril-treated group, increasing renal venous pressure increased renal interstitial pressure as under control conditions; however, sodium excretion (23 +/- 4, 19 +/- 4, and 27 +/- 6 mueq/min) was not significantly increased even at the highest renal venous pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of intrarenal infusions of calcium gluconate (10 and 100 micrograms Ca/kg/min) on renin secretion were studied in anesthetized mongrel dogs. In one group, the two doses of calcium were infused for 30 minutes each (1 ml/min). In a second group, the same doses were administered 30 minutes after the start of infusion of prostaglandin synthesis inhibitors (indomethacin 10 micrograms/kg/min intrarenal or injection of meclofenamate 5 mg/kg i.v. bolus). Mean arterial pressure, renal blood flow, and glomerular filtration rate remained unchanged during the infusion of calcium in both groups. The infusion of 10 micrograms Ca/kg/min increased renin secretion 77% and sodium excretion 123%. During the infusion of 100 micrograms Ca/kg/min, renin secretion was not different from precalcium values, whereas urinary 6-keto-PGF1 alpha, urine flow, sodium, potassium, and calcium excretion rates were increased (p less than 0.05). During the administration of prostaglandin synthesis inhibitors, the urinary 6-keto-PGF1 alpha levels were reduced, and the infusion of 10 micrograms Ca/kg/min failed to increase renin secretion, sodium excretion, or 6-keto-PGF1 alpha excretion rates. The infusion of 100 micrograms Ca/kg/min during prostaglandin synthesis inhibition did not modify urine flow or sodium excretion; however, potassium and calcium excretions increased. It is concluded that 1) the intrarenal infusion of small doses of calcium gluconate is capable of stimulating renin secretion through a prostaglandin-mediated mechanism, and 2) the stimulation of renin secretion as well as the increase in sodium excretion induced by calcium are independent of hemodynamic alterations.

Little is known about the distribution of prostaglandin E2 (PGE2) and prostacyclin (PGI2) production in the canine kidney. To determine the basal and stimulated profiles of PGE2 and PGI2 production along the corticomedullary axis of the dog kidney, a slice (0.5 mm thick, 10-50 mg) was obtained from six equally spaced zones along the axis (zone 1, medullary crest; zones 2 and 3, inner medulla; zone 4, outer medulla; and zones 5 and 6, cortex) and was divided into equal halves. One half of the slice was incubated with Krebs-Ringer buffer containing arachidonic acid (6.6 x 10(-4) M), bradykinin (9.4 x 10(-6) M), or indomethacin (10(-5) M), whereas the remaining half of each slice was similarly incubated in Krebs-Ringer buffer alone. The production of PGE2 and 6-keto-PGF1 alpha (the stable metabolite of PGI2) was determined by radioimmunoassay. Under basal conditions, both PGE2 and 6-keto-PGF1 alpha were highest in the innermost zones of the inner medulla (PGE2, 3,328 +/- 549 pg/mg; 6-keto-PGF 1 alpha, 1,611 +/- 129 pg/mg) and decreased exponentially to low levels in the cortex (PGE2, undetectable; 6-keto-PGF1 alpha, 13 +/- 2 pg/mg); this production was inhibited by indomethacin. Arachidonic acid significantly increased the production of PGE2 in all zones of the kidney and the production of 6-keto-PGF1 alpha only in zones 3-6.(ABSTRACT TRUNCATED AT 250 WORDS)

Current microscopic methods to view renal microvasculature reveal only a very limited portion of the total renal volume. Identification of connectivity for postglomerular vessels in the cortex and the medulla during functional states related to changes in sodium excretion will help better to understand the coupling of renal vasculature to tubular function. The purpose of this study was to investigate the possibility of visualizing the continuity of pre- and postglomerular vasculature using three-dimensional micro-computed tomography (micro-CT). Kidneys from normal rats were perfusion fixed in situ at physiological pressure, filled with latex microfil containing lead chromate, and embedded in plastic. The micro-CT scans of the intact kidneys were carried out on a rotating stage illuminated either by a synchrotron x-ray source or a conventional x-ray spectroscopy tube. Images were reconstructed by a filtered backprojection algorithm and volume-rendering techniques were utilized to display the vasculature. The reconstructed images clearly showed the large distribution vessels and the venous drainage of the kidneys, while pre- and postglomerular vessels and their vascular connections throughout the kidney were displayed in great detail. Efferent arterioles showed the characteristics of their peritubular capillary beds in the cortical and medullary regions. The vascular volume of the cortex was 27%, the outer stripe of the outer medulla 18%, the inner stripe of the outer medulla 30%, and the inner medulla 18%. In conclusion, micro-CT is a promising method to evaluate renal vascular architecture relative to physiological and pathological alterations.

Abstract Rapid degradation of prostacyclin (PGI 2 ) inherent to its molecular structure has long been a major limitation in assessing the natriuretic effect of this prostaglandin. The recent availability of the stable PGI 2 analogue iloprost now allows for a comparative study with prostaglandin E 2 (PGE 2 ). In the present study conducted in six anesthetized dogs, the intrarenal effects of two consecutive doses (1 and 4 ng · kg −1 · min −1 ) of PGE 2 on renal blood flow, glomerular filtration rate, and urinary sodium excretion were compared with the effects of two identical doses of iloprost. The selected doses of PGE 2 were those producing a maximal natriuretic and vasodilator response without affecting mean arterial pressure. A washout period was allowed between administration of PGE 2 and iloprost. PGE 2 infusion significantly increased fractional sodium excretion from 0.69±0.1 to 2.79±1.1% and 4.27±1.2%% ( P <.05), respectively. These changes in fractional sodium excretion induced by PGE 2 were associated with significant increases in renal blood flow from 151.1±62 to 185±64.3 and 185.6±64.3 mL/min ( P <.05), respectively; however, no significant alterations were seen in glomerular filtration rate, from 29.5±9.4 to 35.2±12.2 and 32.7±7.8 mL/min (NS), and mean arterial pressure, from 117.6±26 to 113.9±24.1 and 112.3±24.1 mm Hg (NS) during control and PGE 2 infusion. At identical doses, sequential infusion of PGI 2 had no effect on renal blood floww and glomerular filtration rate, producing natriuresis only at the highest dose, a fractional sodium excretion from 0.69±0.1 to 0.8±0.28 mm Hg (NS) and 1.05±0.34% ( P <.05), respectively. In conclusion, the present study confirms that PGE 2 exerts a natriuretic effect during increases in renal blood flow. In contrast, PGI 2 had no hemodynamic effect, and the natriuresis was markedly blunted.

Abstract — — Chronically infusing a subpressor dose of angiotensin (Ang) II increases blood pressure via poorly defined mechanisms. We found that this hypertensive response is accompanied by increased oxidant stress and is prevented by blocking endothelin (ET) receptors. Thus, we now tested whether blocking oxidant stress decreases both blood pressure and ET levels. We infused Sprague-Dawley rats (via osmotic pumps) with either vehicle (group 1) or Ang II (5 ng · kg −1 · min −1 ; groups 2 to 4) for 15 days. Groups 3 and 4 also received either tempol in the drinking water (1 mmol/L) or vitamin E (5000 IU/kg diet), respectively, for 15 days. We measured systolic blood pressure (SBP) and urinary nitrite excretion every 3 days, and on day 15 we measured systemic and renal venous plasma levels of ET, isoprostanes, and thiobarbituric acid reactive substances (TBARS). SBP in Group 1 did not change throughout the study, whereas Ang II increased SBP (from 132±5 to 151±7 mm Hg). In addition, Ang II increased the systemic and renal venous levels of isoprostanes, TBARS, and ET and caused a transient decrease in urinary nitrites (that returned to control levels by day 9). Both tempol and vitamin E prevented Ang II-induced hypertension and either prevented or tended to blunt the increase in systemic and renal isoprostanes, TBARS, and ET. Finally, both antioxidants abolished the transient decrease in urinary nitrites. These results together with our previous study suggest that subpressor-dose Ang II increases oxidant stress (and isoprostanes). This in turn increases ET levels, which participate in the hypertensive response to Ang II.

Hypercholesterolemia and hypertension are both risk factors for end-stage renal disease. This study was designed to examine whether their coexistence augmented impairment in renal function and redox status. Regional renal hemodynamics and function in response to vasoactive challenges with acetylcholine or sodium nitroprusside were quantified by using electron-beam computed tomography in pigs after 12 weeks of either a normal (n=10) or hypercholesterolemic (n=10) diet, renovascular hypertension (n=7), or combined hypercholesterolemia+hypertension (n=6). The hypercholesterolemic and hypercholesterolemic+hypertensive groups had significantly increased serum cholesterol levels, whereas in the hypertensive and hypercholesterolemic+hypertensive groups, mean arterial pressure was significantly elevated compared with the group fed a normal diet. Basal regional renal perfusion and glomerular filtration rates were similar among the groups. In response to acetylcholine, cortical perfusion increased in normal animals (15.6±4.7%, P =0.002) but not in hypercholesterolemic or hypertensive animals (8.0±7.4% and 8.2±5.9%, respectively; P >0.05). Moreover, in the hypercholesterolemic+hypertensive group, cortical perfusion response was further attenuated (2.5±4.8%, P =0.02) and significantly different from the group fed a normal diet ( P <0.05). The response to sodium nitroprusside followed a similar pattern, and the impairment was augmented in the hypercholesterolemic+hypertensive group. The functional abnormalities in hypercholesterolemia or hypertension were associated with a decrease in systemic and/or renal tissue levels of oxygen radical scavengers that was again accentuated in hypercholesterolemia+hypertension. These results demonstrate that concurrent hypercholesterolemia and hypertension have a greater detrimental effect on renal perfusion responses compared with hypercholesterolemia or hypertension alone, associated with a marked pro-oxidant shift in redox status. These effects may potentially augment renal functional impairment and play a role in the initiation and progression of renal injury in hypertension and atherosclerosis.

The pathophysiological mechanisms responsible for maintenance of chronic renovascular hypertension remain undefined. Excess angiotensin II generation may lead to release of reactive oxygen species and increased vasoconstrictor activity. To examine the potential involvement of oxidation-sensitive mechanisms in the pathophysiology of renovascular hypertension, blood samples were collected and renal blood flow measured with electron-beam computed tomography in pigs 5 and 10 weeks after induction of unilateral renal artery stenosis (n=7) or sham operation (n=7). Five weeks after procedure, plasma renin activity and mean arterial pressure were elevated in hypertensive pigs. Levels of prostaglandin F2α (PGF 2α )–isoprostanes, vasoconstrictors and markers of oxidative stress, also were significantly increased (157±21 versus 99±16 pg/mL; P <0.05) and correlated with both plasma renin activity ( r =0.83) and arterial pressure ( r =0.82). By 10 weeks, plasma renin activity returned to baseline but arterial pressure remained elevated (144±10 versus 115±5 mm Hg; P <0.05). Isoprostane levels remained high and still correlated directly with the increase in arterial pressure ( r =0.7) but not with plasma renin activity. Stenotic kidney blood flow was decreased at both studies. In shock-frozen cortical tissue, ex vivo endogenous intracellular radical scavengers were significantly decreased in both kidneys. The present study demonstrates, for the first time, that in early renovascular hypertension, an increase in plasma renin activity and arterial pressure is associated with increased systemic oxidative stress. When plasma renin activity later declines, PGF 2α -isoprostanes remain elevated, possibly due to local activation or slow responses to angiotensin II, and may participate in sustenance of arterial pressure. Moreover, oxidation-sensitive mechanisms may influence ischemic and hypertensive parenchymal renal injury.

We tested the hypothesis that angiotensin II (Ang II)–induced stimulations of endothelin (ET) and isoprostanes are implicated in the slow pressor responses to Ang II. We infused either vehicle (group 1) or Ang II (groups 2 to 4) intravenously at 5 ng/kg per minute via osmotic pumps for 15 days into Sprague-Dawley rats. Groups 3 and 4 received 30 mg/kg per day of either losartan (Ang II type 1 receptor blocker) or bosentan (ETAand ETBreceptor blocker) in their drinking water. We measured systolic blood pressure (SBP) every 3 days during the infusion. Plasma levels of Ang II, ET, isoprostanes, and urinary nitrites were determined at 15 days. Vehicle infusion did not change SBP (from 138±13 to 136±2 mm Hg at day 15). Circulating Ang II, ET, and isoprostane levels were 35±9, 39±3, and 111±10 pg/mL, respectively, whereas urinary nitrites were 2.3±0.4 μg/d. Ang II increased SBP (from 133±10 to 158±8 mm Hg), plasma Ang II (179±77 pg/mL), and isoprostanes (156±19 pg/mL) without altering ET levels (38±5 pg/mL) or urinary nitrites (1.8±0.5 μg/d). Losartan prevented Ang II–induced increases in SBP and isoprostanes (SBP went from 137±5 to 120±4 mm Hg; isoprostanes were 115±15 pg/mL) while increasing urinary nitrite levels (5.2±1.1 μg/d). Losartan did not alter Ang II (141±57 pg/mL) or ET (40±4 pg/mL) levels. Bosentan also blocked Ang II–induced hypertension (from 135±4 to 139±3 mm Hg) but did not decrease isoprostanes (146±14 pg/mL). Ang II (63±11 pg/mL), ET levels (46±2 pg/mL), and urinary nitrites (2.8±0.4 μg/d) were not altered. In conclusion, our results suggest that low-dose Ang II increases isoprostanes via its Ang II type 1 receptor and causes an ET-dependent hypertension, without altering circulating ET levels.

Abstract —The present study was performed to determine whether physiologically relevant doses of angiotensin II (Ang II), which do not affect renal hemodynamics but do cause slow response hypertension, result in oxidative stress as measured by production of vasoconstrictor F 2 -isoprostane, a prostaglandin-like non–cyclooxygenase-produced arachidonic acid metabolite that is the end product of lipid peroxidation. Rats were instrumented with abdominal aortic and left femoral venous catheters, and before and throughout Ang II (or saline) infusion, all rats received enalapril (250 mg/L). Four days after the initiation of enalapril, rats were infused with Ang II (10 ng · kg −1 · min −1 , n=6) or saline (n=6) for 14 days. Mean arterial pressure was measured 24 hours per day, and on day 12, glomerular filtration rate and renal plasma flow were measured. Mean arterial pressure in control rats averaged 85±1 mm Hg, and with Ang II infusion, mean arterial pressure increased slowly and reached a plateau on day 3, averaging 117±2 mm Hg ( P <0.0001 compared with enalapril alone). Glomerular filtration rate and renal plasma flow were not affected by Ang II. Free F 2 -isoprostanes in plasma increased by 54% with Ang II ( P <0.01), and the production of F 2 -isoprostanes esterified in plasma lipids tended to be higher with Ang II also but did not reach significance ( P =0.1). These studies suggest that low doses of Ang II are capable of producing oxidative stress in animals. Whether oxidative stress plays a causative role in Ang II–mediated slow-response hypertension or is secondary to the hypertension is not clear from these data and will require further study.