New Thoughts On Proteinuria And Management Of Glomerulonephritis
Gregory F. Grauer, DVM, MS, Diplomate, ACVIM (Internal Medicine)
The presence of immune complexes in glomerular capillary walls (glomerulonephritis) is the major cause of proteinuric renal disease. Several studies indicate the incidence of glomerulonephritis in randomly selected dogs is as high as 50% and the resulting glomerular damage is thought to be a common cause of chronic renal insufficiency/renal failure. Amyloidosis, although less common than glomerulonephritis, is the other major cause of proteinuric renal disease. Use of the urine protein/creatinine ratio to identify and quantitate proteinuria has greatly facilitated diagnosis of these glomerular diseases in veterinary medicine, however a kidney biopsy is necessary to differentiate amyloidosis from glomerulonephritis.
Circulating antigen-antibody complexes may be deposited or trapped in the glomerulus or immune complexes may form in situ in the glomerular capillary wall. In situ immune complex formation occurs when circulating antibody reacts with "planted", non-glomerular antigens in the glomerular capillary wall. These antigens may localize in the glomerular capillary wall due to electrical charge interaction or biochemical affinity with the glomerular capillary wall. For example, evidence suggests in situ immune complex formation occurs in dogs with glomerulonephritis associated with heartworm disease.
After formation or deposition of immune complexes in the glomerular capillary wall, several factors including activation of complement, platelet aggregation, infiltration of polymorphonuclear leukocytes, activation of the coagulation system, and fibrin deposition contribute to glomerular damage. Platelet activation and aggregation occur secondary to vascular endothelial damage or antigen-antibody interaction. Platelets, in turn, exacerbate glomerular damage by release of vasoactive and inflammatory substances (principally thromboxanes) and by facilitation of the coagulation cascade. The glomerulus responds to this injury by cellular proliferation, thickening of the glomerular basement membrane, and eventually, hyalinization and sclerosis.
The clinical signs associated with mild to moderate urinary protein loss are usually nonspecific such as weight loss and lethargy, however, if protein loss is severe, edema and or ascites will often occur. If the glomerular disease process is extensive, rendering 3/4 of the nephrons nonfunctional, renal failure and resultant azotemia, PU-PD, anorexia, nausea, and vomiting may occur. Occasionally, signs associated with an underlying infectious, inflammatory, or neoplastic disease may be the reason owners seek veterinary care. Rarely, dogs may be presented with acute dyspnea or severe panting due to pulmonary thromboemboli.
Persistent proteinuria greater than 3.5 g/day will often lead to clinical signs of the nephrotic syndrome. The combination of significant proteinuria, hypoalbuminemia, ascites or edema, and hypercholesterolemia is defined as the nephrotic syndrome. In addition to these clinical signs, hypertension and hypercoagulability are frequent complications in dogs with nephrotic syndrome. Hypertension probably occurs due to a combination of sodium retention, glomerular capillary and arteriolar scarring, decreased renal production of vasodilators, increased responsiveness to normal pressor mechanisms, and activation of the renin-angiotensin system. In one study, 80% of dogs with glomerular disease were found to be hypertensive. Retinal changes, including hemorrhage, detachment, and papilledema, may be the first indication of hypertension and an acute onset of blindness may be the presenting complaint in hypertensive dogs. Blood pressure measurement can help in the evaluation and management of patients with glomerular disease; identification and control of systemic hypertension may attenuate the progression of the glomerular disease.
Hypercoagulability and thromboembolism associated with the nephrotic syndrome occur secondary to several abnormalities in the clotting system. In addition to a mild thrombocytosis, a hypoalbuminemia-related platelet hypersensitivity increases platelet adhesion and aggregation proportional to the magnitude of hypoalbuminemia. Loss of antithrombin III in urine also contributes to hypercoagulability. Antithrombin III works with heparin to inhibit clotting factors and normally plays a role in modulating thrombin and fibrin production. Finally, altered fibrinolysis and increases in the concentration of large molecular weight clotting factors lead to a relative increase in clotting factors vs regulatory proteins. The pulmonary arterial system is the most common location for thromboembolism. Dogs with pulmonary thromboembolism are usually dyspneic and hypoxic with minimal pulmonary parenchymal radiographic abnormalities. Treatment of pulmonary thromboembolism is difficult, often expensive, and frequently unrewarding and therefore, early prophylactic treatment is important.
Quantitation of proteinuria:
Calculation of the urine protein/creatinine ratio from canine urine samples has been shown to accurately reflect the quantity of protein excreted in the urine over a 24 hour period. This test has greatly facilitated the diagnosis of glomerulonephritis in veterinary medicine. In addition, the magnitude of proteinuria has been shown to roughly correlate with the severity of glomerular lesions making the urine protein/creatinine ratio a useful parameter to assess response to therapy or progression of disease.
Most studies suggest that normal urine protein excretion in dogs is less than 20 mg/kg/24-hrs which roughly correlates with a urine protein/creatinine ratio of less than 1.0. Several studies also suggest that the ratio is accurate in cats. Complete urinalyses are important since hematuria or pyuria may indicate significant non-glomerular proteinuria rendering the urine protein/creatinine ratio inaccurate.
The magnitude of proteinuria does appear to roughly correlate with the nature of the glomerular lesion. In several studies, although there was overlap, urine protein excretion in dogs with glomerulonephritis was greater than in dogs with glomerular atrophy or interstitial nephritis but less than dogs with amyloidosis. Urine and serum protein electrophoresis may help identify the source of the proteinuria and establish a prognosis. Proteinuria associated with hemorrhage into the urinary tract will have an electrophoretic pattern very similar to that of the serum. Early glomerular damage usually results principally in albuminuria, however with progression of the glomerular disease, an increasing amount of globulin may be lost as well. Marked decreases in serum albumin and increased concentrations of larger molecular weight proteins in the serum are suggestive of severe glomerular proteinuria and the nephrotic syndrome.
Proteinuria:Loss of plasma proteins into the urine is one of the earliest functional defects recognized in GN. Consequences of plasma protein loss may include sodium retention, hypercoagulability, muscle wasting, and weight loss. There does not appear to be a relationship between proteinuria and glomerular filtration rate in dogs with GN. Azotemia and renal insufficiency/failure occur as more and more nephrons are irreversibly damaged and become nonfunctional. Late in the disease process, proteinuria in total tends to diminish associated with the decreased number affected glomeruli. Plasma protein loss on an individual remaining nephron basis however, may remain significant. Indeed, individual nephron hyperfiltration and proteinuria have been documented in dogs with the remnant kidney model of renal failure. There is increasing evidence in laboratory animals and human beings that proteinuria can cause glomerular and tubular damage and result in progressive nephron loss. Plasma proteins that have crossed the glomerular capillary wall can accumulate within the glomerular tuft and stimulate mesangial cell proliferation and increased production of mesangial matrix inhuman beings. In addition, excessive amounts of protein in the glomerular filtrate can be toxic to human tubular epithelial cells and can lead to interstitial inflammation, fibrosis, and cell death. Proximal tubular cells normally reabsorb protein from the glomerular filtrate by endocytosis. Albumin and other proteins accumulate in lysosomes and are then degraded into amino acids and returned to the circulation. In proteinuric conditions, excessive lysosomal processing can result in swelling and rupture of lysosomes causing enzymatic damage to the cytoplasm in rat kidneys. Tubular injury may also occur in rats as a consequence of tubular obstruction with proteinaceous casts. Increased glomerular permeability to plasma proteins allows tubular contact with transferrin, complement, and lipoproteins in addition to albumin. Transferrin increases iron uptake by epithelial cells. Once inside the cell the iron ions catalyze the formation of reactive oxygen species that can cause peroxidative injury in rats. Complement proteins can be activated on the brush border of proximal tubular cells cultured from human beings resulting in insertion of a membrane attack complex followed by cytoskeletal damage and cytolysis. Reabsorbed lipoproteins can release lipid moieties that can accumulate into lipid droplets or be oxidized to toxic radicals. All of these processes can irreversibly damage the proximal tubule and result in nephron loss. Exposure to plasma proteins at the apical surface of cultured human tubular cell results in a basolateral release of growth factors, fibronectin, and monocyte chemoattractant protein-1. This process may also induce excessive tubular expression of tranforming growth factor-beta 1 that can result in the interstitial inflammation and scarring typical of end-stage kidney disease. Osteoponin has been detected in protein congested proximal tubular cells from rats in two different models of renal disease. By virtue of its chemoattractant action, osteopontin is a likely mediator of a proximal tubule-dependent inflammatory pathway in response to an excessive protein load. Another mediator of tubulointerstitial damage related to excessive tubular cell protein reabsorption is the up-regulation of tubular-derived endothelin-1 (ET-1). Tubular ET-1 formed in response to increasing concentrations of albumin presented to the proximal tubular epithelium is secreted toward the basolateral cellular compartment and accumulates in the interstitium causing ischemia. Endothelin –1 also binds to receptors on interstitial fibroblasts and causes interstitial cellular proliferation and extracellular matrix production.
Generation of immune complexes is dependent on the presence of antigen. The most important treatment for glomerular disease is identification and correction of any underlying disease processes. However, since an antigen source or underlying disease process is often not identified or is impossible to eliminate (e.g. neoplasia), immunosuppressive drugs are often employed in the treatment of glomerulonephritis. Corticosteroids, azathioprine, cyclophosphamide, and cyclosporin have been used clinically or experimentally to prevent immunoglobulin production by B cells or to alter the function of T helper or T suppressor cells. Unfortunately, there are no controlled clinical trials that demonstrate the efficacy of immunosuppressive drugs in the treatment of canine or feline glomerulonephritis. The recent association between hyperadrenocorticism (and long-term exogenous corticosteroid therapy) and glomerulonephritis and thromboembolism in the dog, as well as the lack of consistent therapeutic response to treatment of glomerular disease with corticosteroids, indicates that corticosteroids should be used with caution in dogs with glomerulonephritis. If immunosuppressive drugs are employed, proteinuria should be quantitated frequently to assess the effects of treatment. In some instances, immunosuppressive treatment may exacerbate glomerular lesions and proteinuria.
There is increasing evidence that platelets and their arachidonic acid metabolites (thromboxanes) are integrally involved in the pathogenesis of glomerulonephritis. Beneficial responses to anti-platelet therapy including aspirin, indomethacin, dipyridamole, thromboxane synthetase inhibitors, and platelet-activating factor antagonists have been demonstrated in several studies. Dosage appears to be important when nonspecific cyclooxygenase inhibitors such as aspirin are employed. Theoretically, low dose aspirin therapy (0.5 - 5 mg/kg PO BID) can selectively inhibit platelet cyclooxygenase without preventing beneficial prostacyclin (vasodilator and platelet aggregation antagonist) formation.
Supportive therapy is important in the management of glomerulonephritis and should be aimed at decreasing hypertension, edema, and the tendency for thromboembolism to occur. Sodium restricted diets (< 0.3% dry matter) should be recommended and vasodilators and diuretics may be used as necessary. Angiotensin converting enzyme inhibitors (ACEI) can prevent sodium retention and hypertension in some nephrotic patients. Measurement of antithrombin III and fibrinogen concentrations may be helpful in determining which patients should be treated with anticoagulant therapy. Dogs with antithrombin III concentrations less than 70% of normal and fibrinogen concentrations greater than 300 mg/dl are candidates for therapy. Aspirin and coumadins have been employed for anticoagulant therapy, however low dose aspirin is easily administered on an outpatient basis and does not require extensive monitoring as does coumadin treatment. Since fibrin accumulation within the glomerulus is a frequent consequence of glomerulonephritis, anticoagulant treatment may serve a dual purpose. Finally, protein reduced diets should be recommended in an attempt to decrease glomerular hyperfiltration and the nonimmunologic progression of glomerular disease.
It was recently demonstrated that treatment with enalapril improves renal function and prolongs survival in male Samoyed dogs with hereditary nephritis. This primary glomerular disease results in chronic renal failure in affected dogs prior to one year of age. In addition, in dogs with unilateral nephrectomies and experimentally induced diabetes mellitus, ACEI administration (lisinopril) reduced glomerular transcapillary hydraulic pressure and glomerular cell hypertrophy as well as proteinuria. Finally, in dogs with naturally-occurring glomerulonephritis, treatment with enalapril reduced proteinuria and systolic blood pressure and prevented an increase in serum creatinine concentrations when compared to placebo-treated dogs. Treatment with ACEI probably decreases proteinuria and preserves renal function associated with glomerular disease by several mechanisms in addition to decreasing intraglomerular hypertension and cellular proliferation. Enalapril may prevent the loss of glomerular heparan sulfate that can occur with glomerular disease. Heparan sulfate is a glycosaminoglycan-proteoglycan that contributes to the negative charge of the glomerular capillary wall which in turn hinders the filtration of negatively charged proteins such as albumin. Administration of ACEI is also thought to attenuate proteinuria by decreasing the size of glomerular capillary endothelial cell pores. Attenuation of proteinuria alone may be renoprotective inasmuch as a correlation between proteinuria and renal functional decline has been observed in human beings. In addition, the antiproteinuric and renal protective effects of ACEI may be associated with improved lipoprotein metabolism. Lipid deposition in the glomerular mesangium can contribute to proteinuria and glomerulosclerosis.