University of Miami Division of Clinical Pharmacology Therapeutic Rounds: ischemic renal disease

Ischemic renal disease (IRD) is defined as a significant reduction in glomerular filtration rate and/or loss of renal parenchyma caused by hemodynamically significant renal artery stenosis. IRD is a common and often overlooked clinical entity that presents in the setting of extrarenal arteriosclerotic vascular disease in older individuals with azotemia. IRD is an important cause of chronic renal failure and end-stage renal disease (ESRD), and many patients with a presumed diagnosis of hypertensive nephrosclerosis may actually have undiagnosed ischemic nephropathy as the cause of their ESRD. The primary reason for establishing the diagnosis of IRD is the hope that correction of a renal artery stenosis will lead to improvement of renal function or a delay in progression to ESRD. There are six typical clinical settings in which the clinician could suspect IRD: acute renal failure caused by the treatment of hypertension, especially with angiotensin-converting enzyme inhibitors; progressive azotemia in a patient with known renovascular hypertension; acute pulmonary edema superimposed on poorly controlled hypertension and renal failure; progressive azotemia in an elderly patient with refractory or severe hypertension; progressive azotemia in an elderly patient with evidence of atherosclerotic disease; and unexplained progressive azotemia in an elderly patient. It is important for the clinician to identify IRD, because IRD represents a potentially reversible cause of chronic renal failure in a hypertensive patient.

A numerical study of the stiffness of a sarcomere

Relatively recent experimental findings of the significant compliance of the thin (actin) and thick (myosin) filaments have brought into question a number of conclusions based on the assumption of perfect myofilament rigidity. A new model based on a discrete representation of the relevant structures was used to calculate the theoretical stiffness of a sarcomere with compliant myofilaments. Because of the discrete nature of the model, it can be applied to situations in which either the number of links between filaments is low (i.e. partial overlap or partial activation) or the spatial distribution of links is not uniform. The results of this model are discussed and compared to the predictions given by the previously published model by Ford et al. (Ford, L. E., Huxley, A. F. and Simmons, R. M., Journal of Physiology, 1981, 311, 219--249). Although it can be shown that both models give identical results for an infinite number of links, our model consistently predicts a stiffer sarcomere for partial overlap. The differences in the stiffness values as calculated by a model with rigid filaments, the continuous model by Ford et al. and the discrete model presented here would lead to differences in the interpretation of experimental results, especially in the case of partial overlap and partial activation. An explanation for the discrepancies between models is presented together with a way to correct the continuous model to approximate discrete model results.

Surrogate endpoints and renal protection: focus on microalbuminuria

End-stage renal disease (ESRD) is a major public health problem worldwide. The past decade has witnessed increased interest and investigative attention on therapeutic manoeuvres to retard the progression of renal disease. Reduction of albuminuria is regarded as a surrogate endpoint, because this phenomenon seems to be associated with preservation of GFR (principal endpoint) in diabetic nephropathy. Studies from our laboratory suggest that reduction in albuminuria during ACE inhibition constitutes a predictor of an attenuated rate of decline in GFR in early diabetic nephropathy. Because the rate of decline in kidney function is used to assess prognosis and the efficacy of therapy on progression of renal disease, a valid method for the determination of GFR is essential. During the last two decades, several radioactive and non-radioactive filtration markers have been validated and found to be both accurate and precise compared to inulin. Plasma clearance of such filtration markers is frequently applied in clinical trials in diabetic nephropathy because patients suffering from this complication frequently also suffer from diabetic cystopathy, precluding measurements of urine volume. Measurements of serum creatinine, creatinine clearance, calculated creatinine clearance by use of the Cockroft-Gault formula must all be regarded as surrogate measurements for glomerular filtration rate. Consequently, care must be taken to select appropriate filtration markers in order to rigorously evaluate therapeutic trials in progressive renal disease.

Calcium antagonists and the progression of chronic renal failure

End-stage renal disease, which signifies irreversible renal failure, constitutes a major and growing public health problem worldwide. The striking increase in end-stage renal disease has catalyzed clinical and investigative focus on pharmacologic interventions to retard progression to this condition. Increasing evidence indicates that some classes of antihypertensive medications may confer a greater effect than others in slowing progression of renal disease despite similar levels of blood pressure reduction. Substantive data indicate that angiotensin converting enzyme inhibition preferentially retards the progression of renal disease, primarily by protecting the injured kidney from hemodynamically mediated glomerular damage. Newer studies suggest that calcium antagonists also have diverse properties, which are independent of their renal microcirculatory effects that might afford renal protection.

Articular joint mechanics with biphasic cartilage layers under dynamic loading

The composition and amount of extracellular matrix produced by chondrocytes are thought to be influenced by the stress and strain states in the vicinity of the chondrocytes. During daily activities, such as walking and running, articular joints are loaded dynamically. In the present study, a solution is proposed to simulate the responses of a joint under dynamic loading. In order to show the characteristics of the proposed solution, numerical simulations were carried out, in which the contact radius, the relative approach displacement between the centers of the contacting bodies, or the contact force were controlled. As a result of the history-dependent material properties of the articular cartilage, the predicted parameters changed nonperiodically, when the controlled parameters varied periodically. For a constant load, the contact radius and the relative displacement between the contacting bodies were predicted to increase at decreasing rates. When the contact force was varied dynamically, the predicted mean values of the contact radius, the relative displacement between the contacting bodies, and the contact pressure at the center of the contact area depended on the amplitude and the duration of the loading. When the relative displacement between the contacting bodies was controlled, the amplitudes and the cycling frequency must be limited to avoid a loss of contact between the articular joint surfaces. The proposed solution is valid for a long but limited time period, the exact extent of which is yet to be determined. It can be used to simulate the effects associated with cartilage degeneration in diseases such as osteoarthritis.

Evaluation of the finite element software ABAQUS for biomechanical modelling of biphasic tissues

The biphasic cartilage model proposed by Mow et al. (1980) has proven successful to capture the essential mechanical features of articular cartilage. In order to analyse the joint contact mechanics in real, anatomical joints, the cartilage model needs to be implemented into a suitable finite element code to approximate the irregular surface geometries of such joints. However, systematic and extensive evaluation of the capacity of commercial software for modelling the contact mechanics with biphasic cartilage layers has not been made. This research was aimed at evaluating the commercial finite element software ABAQUS for analysing biphasic soft tissues. The solutions obtained using ABAQUS were compared with those obtained using other finite element models and analytical solutions for three numerical tests: an unconfined indentation test, a test with the contact of a spherical cartilage surface with a rigid plate, and an axi-symmetric joint contact test. It was concluded that the biphasic cartilage model can be implemented into the commercial finite element software ABAQUS to analyse practical joint contact problems with biphasic articular cartilage layers.

Nested expression and sequential downregulation of the Xenopus caudal genes along the anterior-posterior axis

Expression of the Xenopus Xcad-1 and Xcad-2 genes initiates during early gastrulation exhibiting a dorsoventral asymmetry in their domains of transcription. At mid-gastrulation the ventral preference becomes stronger and the caudal genes take up a posterior localization in their expression, which they will maintain until their downregulation along the dorsal midline. Comparison of the three Xenopus caudal genes revealed a temporal and spatial nested set of expression patterns. The transcription of the caudal genes is sequentially downregulated with the one expressed most caudally (Xcad-2) being shut down first, this sequence is most evident along the dorsal midline. This pattern of expression suggests a role for the caudal genes as posterior determinants along the anteroposterior axis. In chicken, mouse, man and Xenopus three members of the caudal family have been identified in the genome. Even though in Xenopus the Xcad-3 gene has been previously described, in order to obtain a better insight on the role of the caudal genes a comparative study of all three frog genes was performed.

New chart to evaluate weight faltering

A new chart was designed to aid accurate identification of weight faltering and failure to thrive. It provides guidance on the lower limits of expected weight gain for children, whatever their initial centile position. The chart's theoretical basis, the process of its construction, and its evaluation are described in this paper. Evaluation was by a self completion questionnaire, where respondents answered questions about a range of standardised growth patterns, plotted on old and new charts. Forty five health visitors, 28 general practitioner principals and registrars, and nine community paediatricians provided 328 chart ratings. These showed that the new format significantly increased the proportion of correctly rated charts (old: 45 (28%); new: 82 (51%)), with the greatest impact in severe cases. This suggests that the new chart improves the precision of judgments made about weight gain in infancy.

Lysophosphatidic acid and platelet-derived growth factor synergistically stimulate growth of cultured rat mesangial cells

Lysophosphatidic acid (LPA) is a structurally simple, platelet-derived phospholipid, capable of eliciting a variety of physiological responses. We have demonstrated previously that LPA elicited a marked contractile response in rat mesangial cells (Inoue CN, Forster HG, Epstein M. Circ Res 77:888-896, 1995). In the present study, we examined the potential of this vasoactive substance to induce mesangial cell proliferation. Serum-starved quiescent rat mesangial cells were incubated with either LPA or in combination with platelet-derived growth factor (PDGF). DNA synthesis was assessed by [3H]thymidine incorporation after 24 hr, and cell numbers were determined at 0, 4, and 7 days. LPA- (1 nM-30 microM) stimulated mesangial cell DNA synthesis in a dose-dependent manner. The DNA synthesis stimulated by PDGF (1-100 ng/ml) was characterized by a bell-shaped response curve with a maximum at 40 ng/ml PDGF. The ability of LPA (30 microM) to synergize PDGF was observed over the entire range of PDGF concentrations (1-100 ng/ml). Under optimal concentrations of LPA/PDGF (30 microM40 ng/ml, respectively), mesangial cells displayed a 67-fold increase in [3H]thymidine incorporation, and a 1.9-fold (Day 4) and 2.5-fold (Day 7) increase in cell number as compared with that of quiescent mesangial cells. With an in vitro assay with myelin basic protein as the substrate, both LPA and PDGF induced stimulation of mitogen-activated protein (MAP) kinase activity. In addition, LPA augmented PDGF-induced increase in MAP kinase activity. In summary, these results demonstrate that LPA is mitogenic alone and also acts synergistically in combination with PDGF to promote mesangial cell proliferation. We postulate that these actions of LPA have the potential to play a crucial role in the mitogenic response of mesangial cells seen in a wide array of inflammatory and thrombotic glomerular disorders.

Ischemic renal disease: an emerging cause of chronic renal failure and end-stage renal disease

Ischemic renal disease (IRD) is defined as a clinically important reduction in glomerular filtration rate or loss of renal parenchyma caused by hemodynamically significant renal artery stenosis. IRD is a common and often overlooked clinical entity that presents itself in the setting of extrarenal arteriosclerotic vascular disease in older individuals with azotemia. Eleven to 14% of end-stage renal disease (ESRD) cases are attributable to chronic IRD. A high percentage of patients entering ESRD programs are hypertensive. Many patients with a presumed diagnosis of hypertensive nephrosclerosis actually have undiagnosed ischemic nephropathy as the etiology of their ESRD. It is important for the clinician to identify IRD, because IRD is a potentially reversible cause of chronic renal failure in a hypertensive patient. Atherosclerotic renal artery disease is common among patients with coronary artery disease and aortic and peripheral vascular disease. Atherosclerotic renal artery disease is a progressive disorder, and its progression is associated with loss of renal mass and functioning. A decrease in glomerular filtration rate sufficient to cause an elevation of the serum creatinine concentration requires injury to both kidneys. Consequently, IRD can arise from one of two main clinical situations: bilateral hemodynamically significant renal artery stenosis leading to bilateral renal ischemia; and hemodynamically significant renal artery stenosis in a solitary functioning kidney, or in a kidney that is providing the majority of a patient's glomerular filtration. The primary reason for establishing the diagnosis of IRD is the hope that correction of a renal artery stenosis will lead to improvement of renal function, or a delay in progression to ESRD. There are six major clinical settings in which the clinician could suspect IRD: acute renal failure caused by the treatment of hypertension, especially with angiotensin converting enzyme inhibitors; progressive azotemia in a patient with known renovascular hypertension; acute pulmonary edema superimposed upon poorly controlled hypertension and renal failure; progressive azotemia in an elderly patient with refractory or severe hypertension; progressive azotemia in an elderly patient with evidence of atherosclerotic disease; and unexplained progressive azotemia in an elderly patient. Noninvasive testing modalities that have been used recently include the angiotensin converting enzyme inhibitor renal scan, duplex Doppler sonography, magnetic resonance angiography, and the spiral computed tomography. Treatment methods include percutaneous transluminal angioplasty, endovascular stenting, and surgical revascularization. The results of treatment for preservation of renal function have been encouraging, with stabilization or improvement in renal function observed in a significant proportion of cases.

Hepatorenal syndrome: emerging perspectives

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 renal failure (ATN), hepatorenal syndrome patients manifest characteristic alterations of renal function including (1) relatively hyperosmolar urine; (2) high creatinine urine to plasma (U:P) ratio, and (3) a very low urine sodium concentration (< 10 mEq/L). During the past several years we have witnessed new insights into both the pathophysiology and the therapeutics of this syndrome. The application of new 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 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 use of transjugular intrahepatic porto-systemic shunt. 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.

Extracorporeal blood purification in the management of patients with hepatic failure

With the increasing success of orthotopic liver transplantation, the time has come for a reassessment of the role of extracorporeal blood purification in the management of patients with liver failure. In those patients with combined liver and kidney failure, both standard hemodialysis and the newer continuous renal replacement therapies have been found to be helpful in maintaining fluid, electrolyte, and acid base balance. In those patients with liver failure but without kidney dysfunction, extracorporeal purification techniques have been found useful for the removal of hepatic toxins and in facilitating the replacement of clotting factors. This review will outline the rationale and documented utility of a wide variety of blood purification modalities used in the management of patients with liver failure.

Patterning of the embryo along the anterior-posterior axis: the role of the caudal genes

Patterning along the anterior-posterior axis takes place during gastrulation and early neurulation. Homeobox genes like Otx-2 and members of the Hox family have been implicated in this process. The caudal genes in Drosophila and C. elegans have been shown to determine posterior fates. In vertebrates, the caudal genes begin their expression during gastrulation and they take up a posterior position. By injecting sense and antisense RNA of the Xenopus caudal gene Xcad-2, we have studied a number of regulatory interactions among homeobox genes along the anterior-posterior axis. Initially, the Xcad-2 and Otx-2 genes are mutually repressed and, by late gastrulation, they mark the posterior- or anterior-most domains of the embryo, respectively. During late gastrulation and neurulation, Xcad-2 plays an additional regulatory function in relation to the Hox genes. Hox genes normally expressed anteriorly are repressed by Xcad-2 overexpression while those normally expressed posteriorly exhibit more anterior expression. The results show that the caudal genes are part of a posterior determining network which during early gastrulation functions in the subdivision of the embryo into anterior head and trunk domains. Later in gastrulation and neurulation these genes play a role in the patterning of the trunk region.

Are calcium antagonists proarrhythmic?

Clinical and experimental studies demonstrate that calcium (Ca2+) overload in myocardial cells is an important factor in the genesis of various serious arrhythmias. Calcium antagonists block voltage-dependent channels and thus reduce entry of Ca2+ into heart cells. Because of their specificity for atrioventricular nodal cells, verapamil and diltiazem are used clinically to treat supraventricular arrhythmias involving transmission in the atrioventricular node. These two drugs and the dihydropyridine (DHP) calcium antagonists have been shown to prevent ventricular ischemic and reperfusion arrhythmias in the laboratory. Despite these data indicating that calcium antagonists are antiarrhythmic, a recent controversy has raised the possibility that certain calcium antagonists are unsafe to use, especially for patients with coronary heart disease. Proarrhythmia has been proposed to be a mechanism contributing to potentially adverse outcomes. Although excessive concentrations of verapamil and diltiazem may cause sino-atrial nodal asystole and varying degrees of atrioventricular block, there is little direct evidence that this contributes to significant proarrhythmia, for example, ventricular tachyarrhythmias. Nonetheless, although it appears paradoxical that agents which block the entry of Ca2+ into heart cells may be considered arrhythmogenic, there are circumstances under which dosage with certain calcium antagonists potentially leads to myocardial Ca2+ overload. For example, bouts of neurohormonal activation brought about by calcium antagonist-induced abrupt reductions in blood pressure may be accompanied each time by significant beta-adrenergic-enhanced influx of Ca2+ through the L-type cardiac calcium channels. This elevates the intracellular Ca2+ concentration and disturbs Ca2+ regulation, especially in diseased hearts whose intracellular Ca2+ regulation has already been compromised, and might induce alterations in cardiac electrical activity. In the present article, interactions among cardiac calcium channels, classes of calcium antagonists, and specific formulations of certain antagonists are considered with respect to directly induced ventricular arrhythmogenesis. Indirect potentially proarrhythmic actions of the calcium antagonists are also discussed. We outline some of the many questions that remain to be answered with respect to the actions of DHP on the heart including that of whether beta-adrenergic stimulation modifies the degree of cardiac Ca2+ channel inhibition by DHP-type calcium antagonists.

The underreporting of deaths of American Indian children in California, 1979 through 1993

OBJECTIVES

This study linked birth and death certificates to determine misclassification of deaths of American Indian children in California.

METHODS

Birth records for 1979 to 1993 were matched with mortality records through a computerized system.

RESULTS

The number of deaths to American Indians was estimated to be three to four times greater than that reported on death certificates. Children in urban counties and those who died before 1987 were more likely to be misclassified.

CONCLUSIONS

California death certificates identify less than one third of the deaths among American Indian children. Adjusting for racial misclassification provides a more accurate accounting of child mortality among American Indians.

Intrarenal determinants of sodium retention in mild heart failure: effects of angiotensin-converting enzyme inhibition

The onset and the mechanisms leading to Na+ retention in incipient congestive heart failure (CHF) have not been systematically investigated. To investigate renal Na+ handling in the early or mild stages of CHF, Na+ balance and renal clearances were assessed in 10 asymptomatic patients with idiopathic or ischemic dilated cardiomyopathy and mild heart failure (HF) off treatment (left ventricular ejection fraction, 29.7+/-2%) and in 10 matched normal subjects during a diet containing 100 mmol/d of NaCl and after 8 days of high salt intake (250 mmol/d). Six patients were studied again after 6 weeks of treatment with enalapril (5 mg/d P.O.). At the end of the high salt diet, in patients with mild HF the cumulative Na+ balance exceeded by 110 mmol that of normal subjects (F=3.86, P<.001). During high salt intake, renal plasma flow and glomerular filtration rate were similarly increased in both normal subjects and mild HF patients. In spite of comparable increases of filtered Na+ in the two groups, fractional excretion of Na+, fractional clearance of free water, and fractional excretion of K+ (indexes of distal delivery of Na+) increased in normal subjects and were reduced in patients with mild HF. During enalapril treatment, in the mild HF patients the cumulative Na+ balance was restored to normal; furthermore, enalapril significantly attenuated the abnormalities in the distal delivery of Na+. Our results indicate that a defective adaptation of Na+ reabsorption in the proximal nephron is associated with Na+ retention in response to increased salt intake in the early or mild stages of HF. These abnormalities of renal Na+ handling are largely reversed by enalapril.

The calcium antagonist controversy: the emerging importance of drug formulation as a determinant of risk

Calcium antagonists are one of the widely available classes of antihypertensive agents. Their broad appeal is attributable to several features, including their efficacy, their beneficial characteristics such as metabolic neutrality, and the occurrence of relatively few nuisance-type side effects. Despite these attributes, a number of retrospective analyses have suggested that calcium antagonists may be detrimental and may both promote adverse cardiovascular events and increase the risk of cancer by interfering with cellular apoptosis. On the basis of this and other retrospective analyses, Furberg and Psaty (Am J Hypertens 1996; 9: 122-125) have proposed that the use of calcium antagonists as first-line antihypertensive agents should be discontinued. I have previously countered these allegations and have suggested that they are not relevant to the newer calcium antagonist formulations in current use. It is not widely appreciated that different formulations of the same chemical moiety can produce markedly different hemodynamic and neurohormonal effects, due to differences in the rate of drug delivery into the systemic circulation. During chronic treatment with dihydropyridine calcium antagonists, major fluctuations in blood pressure (rapid onset and offset of antihypertensive effects) during the dosing interval may occur for drugs and formulations that are short acting. In contrast, slow-release formulations of otherwise rapidly absorbed dihydropyridines achieve a more gradual and sustained antihypertensive effect. It is probable that newer calcium antagonist formulations that are truly once daily and do not provoke intermittent sympathetic activation or a cardioacceleratory response will not promote adverse cardiovascular events.

Endothelin and nitric oxide in hepatorenal syndrome: a balance reset

The newly described endothelium-dependent vasoactive substances, nitric oxide and endothelins, have been the subject of intense investigative interest and have been demonstrated to promote a wide array of autocrine and paracrine functions. Recent data highlight their emerging role as potential important mediators of renal failure in general, thus suggesting that they might also contribute to renal dysfunction in the setting of advanced hepatic disease. Most but not all investigators, have demonstrated that circulating ET-1 levels are elevated in patients with advanced liver disease, and some reports have suggested that the magnitude of the elevation correlates with the degree of renal dysfunction. Concomitantly, several investigators have demonstrated NO overproduction as assessed by elevated levels of NO2- and NO3- in patients with advanced liver disease. It has been suggested that serum nitrite/nitrate levels are highest in patients with functional renal failure (i.e., HRS) and that these levels correlate with the magnitude of endotoxemia. Although large voids in our knowledge remain, the available evidence suggest that a reset balance between vasoconstrictor and vasodilatory stimuli may contribute to the renal hemodynamic abnormalities that characterize the renal functional abnormalities of liver disease.

The dorsalizing and neural inducing gene follistatin is an antagonist of BMP-4

Specific signaling molecules play a pivotal role in the induction and specification of tissues during early vertebrate embryogenesis. BMP-4 specifies ventral mesoderm differentiation and inhibits neural induction in Xenopus, whereas three molecules secreted from the organizer, noggin, follistatin and chordin dorsalize mesoderm and promote neural induction. Here we report that follistatin antagonizes the activities of BMP-4 in frog embryos and mouse teratocarcinoma cells. In Xenopus embryos follistatin blocks the ventralizing effect of BMP-4. In mouse P19 cells follistatin promotes neural differentiation. BMP-4 antagonizes the action of follistatin and prevents neural differentiation. In addition we show that the follistatin and BMP-4 proteins can interact directly in vitro. These data provide evidence that follistatin might play a role in modulating BMP-4 activity in vivo.

Video-microscopic assessment of the role of tissue angiotensin-converting enzyme in the control of the renal microcirculation

In the present study, we assessed the role of tissue angiotensin-converting enzyme as a determinant of intrarenal hemodynamics by using the angiotensin-converting enzyme inhibitor trandolaprilat and the angiotensin II receptor antagonist losartan. Afferent and efferent arteriolar diameters were measured with computer-assisted vessel imaging in isolated perfused hydronephrotic rat kidneys. In response to the addition of 1.0 nM angiotensin I, afferent arterioles constricted by 27.3 +/- 2.4% and efferent arterioles by 20.9 +/- 2.4%. These constrictions were similar to those observed after the administration of 0.3 nM angiotensin 11 (33.7 +/- 2.3% and 20.9 +/- 2.4% in afferent and efferent arterioles, respectively). Pretreatment with the angiotensin-converting enzyme inhibitor trandolaprilat (0.1-10 microM) blunted the angiotensin I-induced constriction of afferent arterioles (12.7 +/- 1.4%) and completely abolished the angiotensin I-induced constriction of efferent arterioles. Subsequent addition of angiotensin II to the perfusate resulted in a marked decrease of afferent (39.9 +/- 1.8%) and efferent (27.8 +/- 3.3%) arteriolar diameters. Pretreatment with the angiotensin II receptor antagonist losartan completely blocked the angiotensin I-induced constriction of both afferent and efferent arterioles. Collectively, these data suggest that angiotensin I affects renal microvessels through its conversion to angiotensin II, mediated by locally available tissue angiotensin-converting enzyme, which subserves the local control of the renal microcirculation.

An improved solution for the contact of two biphasic cartilage layers

The purpose of this study was to present a solution for the contact of two biphasic cartilage layers which can be used for dynamic loading, is not restricted to predictions over small time periods, and predicts biologically meaningful changes in contact areas over time. The proposed solution was based on the work of Ateshian et al. (1994, J. Biomechanics 27, 1347-1360) who retained the first term of an asymptotic expansion and used an approximate integration which is valid for short time periods. The solution proposed here uses an exact integration, is valid over long time periods, and can be used for increasing loading. The new solution corrects a limitation of the work by Ateshian et al., which manifests itself immediately (i.e. at time t = 0+ s): the rate of change in the contact radius (and therefore, the contact area) is increasing in Ateshian et al.'s solution for a constant force, whereas it is decreasing in the new solution. An increasing rate of change in the contact radius suggests that the contact radius (area) is unbounded, and a steady-state solution cannot be reached, which is physically not correct for the contact of two joint surfaces. In the new solution, the contact radius reaches a steady-state value given sufficient time.

Theoretical considerations on myofibril stiffness

A discrete model of the interaction between individual myofilaments was developed to study the stiffness of a sarcomere for the case in which filament compliance is not negligible. Our model retains, in the limit, the characteristics of the previously published model by Ford et al. (Ford, L. E., A. F. Huxley, and R. M. Simmons. 1981. The relation between stiffness and filament overlap in stimulated frog muscle fibres. J. Physiol. 311:219-249). In addition, the model is able to model the interaction in cases in which few cross-bridges are attached, or when the distribution of attached cross-bridges is not uniform. Our results confirm previous indications that it might be impossible to calculate the number of attached cross-bridges by using only stiffness measurements in quick-stretch (or release) experiments.

Randomized placebo-controlled clinical trial of high-dose interleukin-2 in combination with a soluble p75 tumor necrosis factor receptor immunoglobulin G chimera in patients with advanced melanoma and renal cell carcinoma

PURPOSE

A randomized, double-blind, placebo-controlled trial was performed to compare the toxicity and biologic effects of treatment with high-dose intravenous (IV) bolus interleukin-2 (IL-2) plus the recombinant human soluble p75 tumor necrosis factor (TNF) receptor immunoglobulin G (IgG) chimera (rhuTNFR:Fc) with high-dose IL-2 alone in patients with advanced melanoma and renal cell carcinoma.

PATIENTS AND METHODS

Twenty patients with advanced melanoma or renal cell carcinoma were randomized to receive IL-2 (Chiron, Emeryville, CA) 600,000 IU/kg every 8 hours on days 1 to 5 and 15 to 19 (maximum, 28 doses) combined with placebo or the rhuTNFR:Fc fusion protein (Immunex, Seattle, WA) 10 mg/m2 on days 1 and 15 and 5 mg/m2 on days 3, 5, 17, and 19. The impact of rhuTNFR:Fc on IL-2 toxicity and biologic effects was evaluated.

RESULTS

No clinically significant difference in toxicity was observed in the two treatment arms. The adjusted median number of IL-2 doses administered during cycle 1 was 24.5 (range, seven to 28) and 21.5 (range, five to 27) for the placebo and rhuTNFR:Fc arms, respectively (P = .544). IL-2-induced TNF bioactivity, neutrophil chemotactic defect, and serum IL-6, IL-8, and IL-1 receptor antagonist (IL-1RA) induction were suppressed by rhuTNFR:Fc. Two of nine assessable patients (22%) on IL-2/placebo and three of 10 patients (30%) on IL-2/rhuTNFR:Fc responded.

CONCLUSION

Despite evidence of in vitro neutralization of TNF functional activity and partial inhibition of other secondary biologic effects of IL-2, rhuTNFR:Fc does not reduce the clinical toxicity associated with high-dose IL-2 therapy. These results suggest that the toxicity and antitumor effects of IL-2 treatment are independent of circulating TNF.