Renal parenchymal disease and hypertension

Renal parenchymal disease is the most common cause of secondary hypertension, accounting for 2.5% to 5.0% of all cases. Hypertension associated with renal parenchymal disease occurs as a complication of a wide variety of glomerular and interstitial renal diseases and may accelerate the decline in renal function if inadequately controlled. Renal parenchymal hypertension most probably represents the combined interactions of multiple independent mechanisms: potential factors include impaired sodium handling leading to volume expansion, perturbations of the renin-angiotensin system, alterations in endogenous vasodepressor compounds, and possibly increased activity of vasoactive substances. The past several years have witnessed newer insights into both the pathophysiology and the therapeutics of this disorder. The characterization of endothelin and the nitric oxide (NO)-arginine pathway and their roles in biology and medicine has provided additional new insights with regard to the pathogenesis of hypertension in renal parenchymal disease. For example, methylated L-arginine derivatives that possess NO synthase inhibitor capabilities including NG-N-dimethylarginine and N-monomethyl-L-arginine are found in human plasma and in urine. Patients with chronic uremia have impaired elimination of these compounds, and circulating concentrations of these compounds may increase sufficiently to result in inhibition of NO production. Thus, accumulation of endogenous NO synthase inhibitors might contribute to the hypertension of advanced renal failure. Similarly, it has been proposed that increased endothelium-derived endothelin that results from hypertensive injury to vascular endothelium could lead to further vasoconstriction and worsening of hypertension. Additional insight into this fascinating problem must await further biochemical characterization of some of the mediators and a more precise delineation of their pathophysiological role.

Multiple factors contribute to acetylcholine-induced renal afferent arteriolar vasodilation during myogenic and norepinephrine- and KCl-induced vasoconstriction. Studies in the isolated perfused hydronephrotic kidney

Acetylcholine (ACh) elicits vasodilation by releasing a number of endothelium-derived relaxing factors (EDRFs). We used the isolated perfused hydronephrotic rat kidney to examine the characteristics of ACh-induced vasodilation of renal afferent arterioles during different types of underlying vasoconstriction. Basal arteriolar tone was increased by either elevating perfusion pressure to 180 mm Hg (myogenic), administering 0.3 mumol/L norepinephrine (NE), or elevating medium potassium concentration to 30 mmol/L (KCl). ACh (10 mumol/L) completely reversed myogenic and NE-induced vasoconstriction and reversed KCl-induced vasoconstriction by 80 +/- 5%. However, whereas ACh produced a sustained vasodilation during KCl- and NE-induced vasoconstriction, only a transient reversal of myogenic vasoconstriction was observed, and myogenic tone recovered within 5 to 10 minutes. ACh-induced vasodilation of arterioles preconstricted with KCl was markedly inhibited by either indomethacin (100 mumol/L) or nitro-L-arginine (100 mumol/L) and was completely abolished by pretreatment with both inhibitors. In contrast, indomethacin and nitro-L-arginine had no effect on the transient response to ACh observed during pressure-induced vasoconstriction. In vessels preconstricted with NE, nitro-L-arginine converted the normally sustained response to ACh to a transient vasodilation, which was refractory to both nitric oxide synthase and cyclooxygenase inhibition. Since this component was not observed during KCl-induced vasoconstriction, it may reflect the actions of an, as yet unidentified, endothelium-derived hyperpolarizing factor (EDHF). Our findings thus suggest that prostanoids, nitric oxide, and EDHF all contribute to ACh-induced renal afferent arteriolar vasodilation and that the relative contributions of these individual EDRFs depends on the nature of the underlying renal vascular tone.

Impairment of afferent arteriolar myogenic responsiveness in the galactose-fed rat

Previous studies from our laboratory have demonstrated impaired afferent arteriolar responsiveness to pressure in rats 4-6 weeks after the induction of diabetes mellitus. Although the responsible mechanisms mediating this renal autoregulatory defect have not been fully defined, increased polyol metabolism has been implicated as a possible factor involved in the pathogenesis of diabetic complications. We therefore investigated the possible role of this metabolic disturbance in renal autoregulation using the galactose-fed rat, a model characterized by increased polyol pathway activity independent of hyperglycemia or insulin deficiency. Hydronephrosis was induced to permit direct visualization of renal microvessels. Pressure-induced vasoconstriction of afferent arterioles was assessed by quantitating vessel diameter following stepwise increments of renal perfusion pressure (RAP; from 80 to 180 mm Hg) in the hydronephrotic kidneys from control rats and rats fed a 50% galactose diet for 2 or 4 weeks. Vessel diameters were measured from video images by computer-assisted image processing. Control rats exhibited progressive afferent arteriolar vasoconstriction when RAP was increased from 80 to 180 mm Hg (-17.3% +/- 1.0%; P < 0.001). In contrast, myogenic responses to increases in pressure were absent in the afferent arterioles of rats fed a 50% galactose diet for either 2 (-4.1% +/- 1.9%; not significant) or 4 weeks (-2.9 +/- 3.4%; not significant). Our demonstration that the impairment of afferent arteriolar responsiveness to increasing RAP in the normoglycemic galactose-fed rat was identical to that observed in the STZ-diabetic rat suggests that increased polyol accumulation may contribute to the impairment of renal autoregulation in the diabetic rat.

A prospective randomized clinical trial comparing a new oral sustained-release ritodrine with conventional tablets

OBJECTIVES

Evaluation of maternal metabolic and cardiovascular responses to treatment with the new sustained-release oral ritodrine as compared with the conventional tablets.

METHOD

Thirty-two pregnant patients who had successful intravenous tocolysis were randomly assigned to treatment with either ritodrine tablets or sustained-release capsules. After 5 days of the randomly determined first oral treatment each patient was shifted to the alternate ritodrine formulation for a further 5-day course. Each patient underwent metabolic and non-invasive hemodynamic evaluation.

RESULTS

Echocardiographic parameters during treatment with ritodrine tablets were not significantly different from during sustained-release capsules. Mean systolic blood pressure increased significantly during peak drug activity in patients treated with ritodrine tablets and not during treatment with the sustained-release form. Fasting plasma glucose levels were higher in patients on conventional tablets therapy than in patients on sustained-release ritodrine therapy (88.9 +/- 9 mg/dl vs. 78.7 +/- 8 mg/dl, P < 0.05) while levels following a 50-g oral glucose challenge test did not differ significantly (135 +/- 32 mg/dl vs. 124.5 +/- 27 mg/dl)

CONCLUSIONS

Because of fewer metabolic and cardiovascular side-effects, the new oral therapy offers some advantages over the presently available tablets.

Effects of calcium antagonists on renal hemodynamics and progression of nondiabetic chronic renal disease

In recent years, substantial investigative attention has focused on therapeutic regimens that could retard the progression of chronic renal insufficiency. Emphasis has been placed on the effects of antihypertensive treatment on renal hemodynamics and preservation of renal function. It has been suggested that some classes of antihypertensive agents may confer a greater renoprotective effect, especially agents that lower glomerular capillary pressure. Conversely, by virtue of their ability to preferentially dilate the afferent arteriole calcium antagonists theoretically could favor an increase in glomerular capillary pressure thereby accelerating the decline of renal function. In this review we survey the literature critically and conclude that in patients with essential hypertension and in patients with chronic renal insufficiency, calcium antagonists effectively reduce systemic blood pressure while maintaining glomerular filtration rate and effective renal plasma flow. Preliminary results from a few long-term studies suggest that calcium antagonists may even attenuate the decline in renal function of patients with chronic renal failure. The majority of studies in humans, however, have been nonrandomized, of too short duration, or confounded by investigative difficulties precluding definite conclusions whether calcium antagonists have renoprotective effects. Although the possibility that calcium antagonists may retard progression of renal disease remains to be ascertained, the available evidence indicates that calcium antagonists may be used in patients with renal functional impairment without further exacerbating renal function.

Hepatorenal syndrome: emerging perspectives of pathophysiology and therapy

Progressive oliguric renal failure (designated "hepatorenal syndrome") commonly complicates the course of patients with advanced hepatic disease. Despite the severe derangement of renal function and ominous prognosis when renal failure develops, minimal and inconsistent pathologic abnormalities of the kidneys are found at autopsy. Furthermore, the kidneys, if transplanted, are capable of normal function, which supports the concept that the renal failure is functional and potentially reversible. In contrast to patients with classical acute failure (ATN), hepatorenal syndrome patients manifest characteristic alterations of renal function including (1) relatively hyperosmolar urine; (2) high creatinine urine:plasma ratio, and (3) a very low urine sodium concentration (< 10 mEq/L). The past several years have witnessed newer insights into both the pathophysiology and the therapeutics of this syndrome. The application of newer methodology such as tracer kinetics has more rigorously delineated the role of a number of pathogenic mechanisms including activation of the sympathetic nervous system. The characterization of endothelin and the nitric oxide-arginine pathway and their roles in biology and medicine has provided additional new insights with regard to the pathogenesis of hepatorenal syndrome. For example, nitric oxide has been proposed to constitute a mediator of both the hyperdynamic circulation and renal failure. Finally, recently initiated therapeutic approaches lend a note of optimism to the future management of a syndrome that is so often incompatible with recovery. These include the acceptance of orthotopic liver transplantation as definitive treatment for patients with end-stage liver disease and attempts to improve renal function by countervailing the decreases in systemic vascular resistance while minimizing concomitant increments in renal vascular resistance. Hopefully, ongoing and future clinical trials will establish the precise contribution of each of these treatment modalities and their respective roles in the therapeutic armamentarium.

Hypertension as a risk factor for progression of chronic renal disease

The declining mortality due to coronary artery disease and stroke has been attributed in part to improved effectiveness and application of antihypertensive therapy, and successful identification and treatment of the population at risk. In striking contrast, end-stage renal disease (ESRD) attributed to hypertension has increased annually for the last decade and will probably worsen through the year 2000. Taken together, patients with diabetic nephropathy and those with hypertensive renal disease account for the majority of new cases annually. The reasons for the striking dissociation between the success with coronary artery disease and stroke on the one hand and the inability to lessen the incidence of ESRD remain to be clarified. Evidence reveals that all levels of untreated hypertension are associated with potentially declining renal function. Data from the Hypertension Detection and Follow-up Program and other studies suggest that antihypertensive treatment can prevent or retard development of progressive renal failure. No data are readily available on repeated measurement of glomerular filtration rate during prolonged treatment of mild hypertension in patients with normal or near-normal renal function. Although the importance of blood pressure control is implicit, a theoretical framework based on data derived from experimental animals suggests that angiotensin-converting enzyme (ACE) inhibitors and calcium antagonists may exert specific renoprotective effects beyond those achieved by blood pressure reduction per se. The results of two recent long-term prospective studies support such a formulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrinsic and extrinsic characteristics of human tumors relevant to radiosurgery: comparative cellular radiosensitivity and hypoxic percentages

We have collected the in vitro x-ray radiation survival characteristics of 181 lines from 12 different classes of exponentially growing human tumor cells (sarcomas, lung cancers, colo-rectal cancers, medulloblastomas, melanoma, breast cancers, prostate cancers, renal cell cancers, grades III and IV brain tumors, ovarian, and head and neck cancers). This information was used to intercompare survival after single high doses of 20-40 Gy for each tumor line. Radiosensitivities could roughly be divided into two groups. The more radiosensitive group included: sarcoma, small-cell lung cancer, non-small cell lung cancer, colorectal cancer, medulloblastoma and melanoma. The more radioresistant group included breast, prostate, renal cell, primary brain tumors, ovarian tumors, and head and neck cancers. Using a model of a 3 cm diameter brain lesion containing about 1.4 x 10(9) oxic cells, the single doses calculated to reduce survival to 1 cell were: sarcoma and small cell lung cancers-22-23 Gy; melanoma-25 Gy; non-small cell lung and colorectal cancer-26 Gy; medullo-blastoma-28 Gy; breast, prostate, renal cell, primary brain tumors, ovarian tumors, and head and neck cancers-30-36 Gy. If, however, tumors contained on average 20 percent hypoxic cells, the dose needed for equivalent cell killing increased by about a factor of 2.6-2.8. Also, there was no correlation between the ranking of relative radiosensitivities of the various classes of tumor cells at high doses (as in radiosurgery) to the sensitivity at low doses (as in conventional fractionated radiotherapy).

CONCLUSION

available information on the intrinsic radiosensitivity of human tumor cells indicates that meaningful differences exist among different histological classes of neoplasm that are relevant to the single high doses used in radioneurosurgery, and which could constitute a basis for "tailoring" the administered dose to the particular neoplasm. However, if intracerebral lesions contain a large number of hypoxic cells (e.g., 20%), this may constitute a significant problem.

Interactions between 4-HPR and diet in NMU-induced mammary tumorigenesis

The present study was designed to determine whether the chemopreventive effect of the synthetic retinoid N(4-hydroxyphenyl)retinamide (4-HPR) on mammary tumorigenesis was influenced by diet. Three diets were used: the closed-formula grain-based Wayne Lab Blox, the open-formula grain-based NIH-07, and the casein-based semipurified AIN-76A. Groups of 25 virgin female F-344 rats were fed the experimental diets beginning one week before a single injection of N-methyl-N-nitrosourea (NMU, 45 mg/kg body wt i.v.) at 50 days of age. The experimental design was as follows: Group 1, unsupplemented AIN-76A; Group 2, AIN-76A supplemented with 4-HPR starting seven days before NMU until termination (-7); Group 3, AIN-76A supplemented with 4-HPR seven days after NMU until termination (+7); Group 4, Wayne (no 4-HPR); Group 5, Wayne (4-HPR, -7); Group 6, Wayne (4-HPR, +7); Group 7, NIH-07; Group 8, NIH-07 (4-HPR, -7). 4-HPR [782 mg/kg diet (2 mM)] was given to all supplemented groups. Termination was 25 weeks post-NMU. Analysis of tumor incidence, multiplicity, and latency indicated that 1) control rats fed the AIN-76A diet exhibited significantly higher mammary tumor yields than rats fed unsupplemented natural-ingredient diets (Wayne and NIH-07) and 2) 4-HPR inhibited mammary tumor development in the two grain-based diets but enhanced tumor development in the AIN-76A diet. Animals fed the AIN-76A diet gained weight to a greater extent than those fed the Wayne or NIH-07 diets and exhibited lower levels of circulating 4-HPR.

A quantitative assessment of renal blood flow autoregulation in experimental diabetes

Several approaches have been utilized to describe renal blood flow (RBF) autoregulation in normal and pathological conditions. When describing the relation between RBF and stepwise decrements in renal perfusion pressure (RPP), these methods have several limitations, including: the necessity for predetermining a pressure 'break-point', and establishing constraints on changes in flow. To circumvent these limitations, we successfully utilized a third order polynomial, the cubical parabola, to characterize the autoregulatory responses in untreated streptozotocin (STZ) diabetic and control rats. The nonlinear relationship occurring between RBF and RPP was estimated from individual observations using the equation RBF = a + b x 10(-6) (RPP-c)3. Variables a and c represent RBF and RPP at the inflection point of the curve, respectively; variable b represents the rate of fall of RBF as RPP decreases (shape factor). Variable c was significantly lower in the diabetic group than in the control group whereas variable b was greater in the diabetic group. RBF (a) did not differ between the two groups. In conclusion, we determined that the range of RBF autoregulation in untreated diabetic rats is reset to a lower RPP. Furthermore, the curve below the inflection point declines more rapidly in diabetic rats than in controls. We propose that the equation described herein constitutes a promising and reproducible method for describing RBF autoregulation in vivo.

Successful delivery following myocardial ischemia during the second trimester of pregnancy

Acute myocardial infarction during pregnancy is considered to be associated with approximately 50% mortality of both mother and fetus. However, there are not enough data regarding the role of acute myocardial ischemia. We present a 36-year-old, pregnant, white female who was admitted twice at 18 and 20 weeks of gestation with acute myocardial ischemia. Cardiac catheterization revealed 70-80% stenosis of the mid left anterior descending artery (LAD) with normal antegrade flow and very good retrograde filling of the LAD from distal collaterals of the right coronary artery. Therefore, due to angiographic suggestion of protected LAD territory, we recommended medical therapy and scheduled a vaginal delivery that was successfully completed without cardiovascular complications. A stress thallium test performed 6 months later was normal, supporting our clinical judgment. In conclusion, every case of a pregnant woman with coronary insufficiency should be treated according to individual coronary anatomy and blood supply to the territory of the diseased artery, and should not be based on the old data in the literature. The decision for revascularization prior to delivery versus medical therapy, or Caesarean section versus natural delivery, should be made by a team of a cardiologist and an obstetrician.

Protein kinase C and calcium channel activation as determinants of renal vasoconstriction by angiotensin II and endothelin

The mechanisms mediating renal microvascular constriction induced by angiotensin II (Ang II) and endothelin (ET) have not been fully established. In the present study, we have determined the effects of isradipine, a dihydropyridine calcium antagonist, on Ang II- or ET-induced constriction of afferent arterioles (AAs) and efferent arterioles (EAs) using the isolated perfused hydronephrotic kidney. Ang II (0.3 nmol/L) and ET (0.3 nmol/L) constricted AAs by 36 +/- 2% and 29 +/- 3%, respectively. Isradipine reversed AA constriction induced by both peptides. However, Ang II-induced AA constriction was more sensitive to isradipine than ET-induced constriction (half-maximal inhibitory concentration [IC50], 1.2 +/- 0.2 nmol/L [n = 12] versus 170 +/- 65 nmol/L [n = 19]; P < .01). The sensitivity of Ang II-induced AA constriction to isradipine was identical to that of KCI-induced AA constriction (IC50, 4.2 +/- 0.9 nmol/L; n = 12). Pretreatment with staurosporine (50 nmol/L), a protein kinase C inhibitor, enhanced the sensitivity of ET-induced AA constriction to isradipine (4.3 +/- 1.7 nmol/L, n = 14), rendering it identical to that of KCl-induced AA constriction. Ang II and ET decreased EA diameter by 26 +/- 2% (n = 12) and 12 +/- 2% (n = 8), respectively. In contrast to AA constriction, EA constriction induced by both peptides was relatively refractory to isradipine.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin-converting enzyme inhibitors and progression of nondiabetic chronic renal disease

In recent years, considerable attention has focused on the possibility that some classes of antihypertensive agents may confer a greater renoprotective effect in retarding the progression of chronic renal insufficiency. Experimental studies in rats have demonstrated that the sustained increase in glomerular capillary pressure evoked in response to loss of renal mass produces a destructive sclerosing reaction. Administration of angiotensin-converting enzyme inhibitors decreases glomerular capillary pressure with a resultant reduction of glomerular sclerosis, suggesting that angiotensin-converting enzyme inhibitor therapy may protect the injured kidney from hemodynamically mediated glomerular damage. On the basis of these experimental observations, many studies have attempted to investigate whether angiotensin-converting enzyme inhibitors can slow the progression of chronic renal disease in humans. We surveyed the literature critically and concluded that, the data from animals notwithstanding, the majority of studies in humans have been nonrandomized, of too short duration, or confounded by investigative difficulties. Therefore, we cannot yet conclude that angiotensin-converting enzyme inhibitors modify the rate at which renal disease progresses in nondiabetic patients.

Calcium antagonists and the kidney. Implications for renal protection

During the past decade, attention has focused on the effects of calcium antagonists on renal function. When administered in vitro to the isolated perfused kidney, calcium antagonists exhibit consistent actions permitting characterization of their renal effects. Calcium antagonists do not affect the vasodilated isolated perfused kidney, but they alter dramatically the response of this preparation to vasoconstrictor agents. Our recent studies using the isolated perfused hydronephrotic rat kidney model, which permits direct visualization of afferent and efferent arterioles, demonstrated that the preferential augmentation of glomerular filtration rate observed in the isolated perfused kidney is attributable to preferential vasodilation of preglomerular vessels. Although the clinical implications of such observations have not been fully delineated, the results of recent studies indicate that calcium antagonists exert salutary effects on renal function in clinical settings characterized by impaired renal hemodynamics. Such disorders include radiocontrast-induced nephrotoxicity and transplant-associated acute renal insufficiency. It is apparent, however, that the renal hemodynamic effects of calcium antagonists commend their use in the management of essential hypertension.

Calcium antagonists and the kidney: future therapeutic perspectives

During the past decade, attention has focused on the effects of calcium antagonists on renal function. Recent studies using diverse videomicroscopic techniques including the isolated perfused hydronephrotic rat kidney model, which permits direct visualization of afferent and efferent arterioles, have demonstrated that calcium antagonists acutely antagonize preglomerular vasoconstriction. In contrast, most studies suggest that the efferent arteriole appears to be refractory to the vasodilatory effects of these agents. Although the clinical implications of such observations have not been fully delineated, the results of recent studies indicate that calcium antagonists exert salutary effects on renal function in clinical settings characterized by impaired renal hemodynamics, including transplant-associated acute renal insufficiency and, possibly, cyclosporine nephrotoxicity. Evidence has accrued to suggest that calcium antagonists also may be protective against acute radiocontrast-induced nephrotoxicity. Finally, the renal hemodynamic and natriuretic effects of calcium antagonists commend their use as antihypertensive agents in the management of essential hypertension and transplant-associated hypertension.