Urinary sodium excretion and proximal tubule reabsorption in rats with high-output heart failure

Aortocaval fistula in rat: a unique model of volume-overload congestive heart failure and cardiac hypertrophy

Despite continuous progress in our understanding of the pathogenesis of congestive heart failure (CHF) and its management, mortality remains high. Therefore, development of reliable experimental models of CHF and cardiac hypertrophy is essential to better understand disease progression and allow new therapy developement. The aortocaval fistula (ACF) model, first described in dogs almost a century ago, has been adopted in rodents by several groups including ours. Although considered to be a model of high-output heart failure, its long-term renal and cardiac manifestations are similar to those seen in patients with low-output CHF. These include Na⁺-retention, cardiac hypertrophy and increased activity of both vasoconstrictor/antinatriureticneurohormonal systems and compensatory vasodilating/natriuretic systems. Previous data from our group and others suggest that progression of cardiorenal pathophysiology in this model is largely determined by balance between opposing hormonal forces, as reflected in states of CHF decompensation that are characterized by overactivation of vasoconstrictive/Na⁺-retaining systems. Thus, ACF serves as a simple, cheap, and reproducible platform to investigate the pathogenesis of CHF and to examine efficacy of new therapeutic approaches. Hereby, we will focus on the neurohormonal, renal, and cardiac manifestations of the ACF model in rats, with special emphasis on our own experience.

Clinical pharmacologic implications in diuretic selection

Logical and comprehensive diuretic therapy is based on a sound knowledge of renal physiologic principles. Knowledge of the sites within the nephron at which the currently available diuretics work and their modes of action provide the clinician with a basis upon which to arrive at therapeutic decisions. True diuretic resistance is uncommon, but when it occurs, sequential nephron blockade can frequently reverse the refractoriness initially exhibited by the patient.

Renal sodium retention during volume expansion in experimental nephrotic syndrome

We have studied sodium retention during volume expansion in rats with autologous immune complex nephropathy (AICN), a model of nephrotic syndrome (NS) in which GFR after volume expansion was not different from that in adjuvant-injected controls (C). AICN rats developed heavy proteinuria (298 +/- 27 vs. less than 10 mg/day), hypoalbuminemia (2.14 +/- 0.15 vs. 3.08 +/- 0.12 g/100 ml) and hypercholesterolemia (181 +/- 22 vs. 58 +/- 4 mg/100 ml). After saline, there were no significant differences in blood pressure (119 +/- 2 vs. 114 +/- 2 mm Hg), renal plasma flow (4.9 +/- 0.41 vs. 4.1 +/- 0.28 ml/min), inulin clearance (1.37 +/- 0.06 vs. 1.55 +/- 0.10 ml/min), or SNGFR (47 +/- 2 vs. 53 +/- 4 nl/min). Sodium excretion, however, was significantly lower in NS rats (4.7 +/- 1.1 vs. 9.2 +/- 1.2 muEq/min). Proximal sodium reabsorption was decreased in NS rats (35 +/- 2 vs. 41 +/- 2%, 2.5 +/- 0.2 vs. 3.3 +/- 0.2 nEq/min). Sodium delivery into the loop, however, was equal in NS and C, since the slightly lower filtered load in NS rats offset the depression in proximal reabsorption. Sodium reabsorption by the loop and by the distal convoluted tubules were equal in NS and C. Thus, sodium delivered into the cortical collecting ducts was the same in both groups (0.33 +/- 0.17 vs. 0.34 +/- 0.07 nEq/min; 4.5 +/- 0.6% of filtered sodium vs. 4.4 +/- 0.3%). The percent of filtered sodium excreted in the urine, however, was significantly lower in the NS rats, 2.18 +/- 0.48% vs. 4.0 +/- 0.58%. We conclude that antinatriuresis in this model of NS is determined beyond the superficial late distal convoluted tubule. The inability to excrete the sodium load during volume expansion is due to either enhanced reabsorption by the collecting duct or to abnormal function in deep nephrons.

Diuretics: mechanism of action and clinical application

Despite the bewildering number of diuretics available to the physician, these drugs can be divided into 4 main groups, characterised by their site of action on sodium reabsorption in the kidney. Drugs acting on the ascending limb of the loop of Henle have a powerful but short acting diuretic effect; they include frusemide, ethacrynic acid and bumetanide. The benzothiadiazines and related compounds have a moderate diuretic action spread over a longer period, whilst the potassium-sparing diuretics, triamterene, amiloride and spironolactone, have only a weak diuretic effect but a marked ability to diminish urinary potassium excretion. The fourth group is made up of miscellaneous substances which function as vasodilator or osmotic agents. The pathogenesis of oedema formation in heart failure is outlined and a logical approach to treatment suggested. Duiretics are being increasingly used in the treatment of non-oedematous states, in particular hypertension, diabetes insipidus and hypercalciuria; their exact role in pregnancy and acute renal failure remains controversial. Side-effects can be related to their effect on electrolyte excretion and include hypokalaemia, hyponatraemia, hyperkalaemia and hyperuricaemia. The incidence of disturbed carbohydrate tolerance in previously normal individuals is low. Other less common side-effects are also discussed.