Intrarenal regulation of glomerular filtration rate

Reno- and cardioprotective molecular mechanisms of SGLT2 inhibitors beyond glycemic control: from bedside to bench

Large placebo-controlled clinical trials have shown that sodium-glucose cotransporter-2 inhibitors (SGLT2i) delay the deterioration of renal function and reduce cardiovascular events in a glucose-independent manner, thereby ultimately reducing mortality in patients with chronic kidney disease (CKD) and/or heart failure. These existing clinical data stimulated preclinical studies aiming to understand the observed clinical effects. In animal models, it was shown that the beneficial effect of SGLT2i on the tubuloglomerular feedback (TGF) improves glomerular pressure and reduces tubular workload by improving renal hemodynamics, which appears to be dependent on salt intake. High salt intake might blunt the SGLT2i effects on the TGF. Beyond the salt-dependent effects of SGLT2i on renal hemodynamics, SGLT2i inhibited several key aspects of macrophage-mediated renal inflammation and fibrosis, including inhibiting the differentiation of monocytes to macrophages, promoting the polarization of macrophages from a proinflammatory M1 phenotype to an anti-inflammatory M2 phenotype, and suppressing the activation of inflammasomes and major proinflammatory factors. As macrophages are also important cells mediating atherosclerosis and myocardial remodeling after injury, the inhibitory effects of SGLT2i on macrophage differentiation and inflammatory responses may also play a role in stabilizing atherosclerotic plaques and ameliorating myocardial inflammation and fibrosis. Recent studies suggest that SGLT2i may also act directly on the Na+/H+ exchanger and Late-INa in cardiomyocytes thus reducing Na+ and Ca2+ overload-mediated myocardial damage. In addition, the renal-cardioprotective mechanisms of SGLT2i include systemic effects on the sympathetic nervous system, blood volume, salt excretion, and energy metabolism.

Tissue electrical admittance (electrolyte concentration) in rat renal medulla: effects of furosemide and acetazolamide

Fluctuations of total electrolyte concentration in the renal medulla were estimated from continuous measurement of tissue electrical admittance (reciprocal impedance) by means of needle electrodes placed in the kidney of anaesthetized rats. To compare effects of two diuretic agents with different sites of action, rats received either furosemide, 0.3 mg/kg i.v. followed by an infusion at 0.3 mg/kg.h, or acetazolamide, a single injection of 10 mg/kg. At this dosage similar increases in renal excretion were obtained with either drug. After furosemide (a loop diuretic) admittance fell sharp within first 10 min, then partly recovered and reached a plateau 35 min after injection. Acetazolamide (inhibitor of proximal reabsorption) caused no changes in admittance compared to the pattern observed in untreated control animals. We conclude that dissipation of tissue electrolytes from the renal medulla is not simply a consequence of diuresis and natriuresis but depends critically on the site of transport inhibition in the nephron.

Diuretics. Clinical pharmacology and therapeutic use (Part I)

25 years have elapsed since the introduction of the first effective oral diuretic, chlorothiazide. Diuretics are now amongst the most widely prescribed drugs in clinical practice worldwide. Availability of these drugs has not only brought therapeutic benefit to countless numbers of patients but it has at the same time provided valuable research tools with which to investigate the functional behaviour of the kidney and other electrolyte-transporting tissues. Despite many remaining gaps in our knowledge of the biochemical processes involved in diuretic drug action, available compounds can be divided into 5 groups on the basis of their preferential effects on different segments of the nephron involved in tubular reabsorption of sodium chloride and water. Firstly, there is heterogeneous group of chemicals that share the common property of powerful, short-lived diuretic effects that are complete within 4 to 6 hours. These agents act on the thick ascending limb of Henle's loop and are known as 'high ceiling' or 'loop' diuretics. The second group are the benzothiadiazines and their many related heterocyclic variants, all of which localise their effects to the early portion of the distal tubule. The third group comprises the potassium-sparing diuretics which act exclusively on the Na+-K+/H+ exchange mechanisms in the late distal tubule and cortical collecting duct. The action of drugs in groups 2 and 3 is prolonged to between 12 and 24 hours. The fourth group consists of diuretics that are chemically related to ethacrynic acid but have the unusual property of combining within the same molecule the property of saluresis and uricosuria. These compounds have actions, to different individual extents, in the proximal tubule, thick ascending limb, and early distal tubule and are known as 'polyvalent' diuretics. Finally, there is a mixed group of weak or adjunctive diuretics which includes the vasodilator xanthines such as aminophylline, and the osmotically active compounds such as mannitol. Available evidence on the molecular mechanisms of action of diuretics in each group is reviewed. The haemodynamic, humoral and physical factors involved in control of electrolyte and fluid handling by the kidney in normal conditions and pathological states are discussed in relation to rational choices of different diuretics in the treatment of various oedematous and non-oedematous conditions.

Patterns of urinary beta 2-microglobulin excretion by patients treated with aminoglycosides

Aminoglycoside antibiotics are relatively mild nephrotoxins, but their action is site-specific to the proximal tubule. Therefore, use of these drugs presents a unique opportunity to study the temporal relation between the damage to the cells lining the renal proximal tubule and the subsequent rise in the serum creatinine concentration. Our study of 52 aminoglycoside-treated patients included measurements of daily serum creatinine, daily 24-hour urinary beta 2-microglobulin (beta 2M) excretion, and determination of aminoglycoside tissue accumulation. An elevation in beta 2M excretion above the baseline value occurred in 37 of 52 (71%), whereas the serum creatinine concentration rose in only 17 of 52 (33%) of patients. Even fewer patients (10 of 52) demonstrated all three criteria for aminoglycoside nephrotoxicity. These 10 patients had elevated tissue accumulation, evidence of renal tubular damage, and a rise in serum creatinine concentration. The increased beta 2M excretion greater than 50 mg/day preceded the serum creatinine rise by 2 to 7 days. An abnormal baseline beta 2M was not a risk factor for a subsequent rise in creatinine concentration or vice versa. Although each test is primarily site specific, widespread and severe renal proximal tubular damage, regardless of cause, will eventually lead to an elevation of serum creatinine. Thus, serial monitoring of proximal tubular function with urinary beta 2M excretion has potential value in the assessment of insults to this site, but cannot be expected to explain all changes in serum creatinine.

Glomerular hemodynamics before and after release of 24-hour bilateral ureteral obstruction

Glomerular hemodynamics were studied, by micropuncture, in Munich-Wistar rats submitted to 24-hour bilateral ureteral ligation (BUL). Glomerular capillary pressure (PG), intratubular pressure (PT), and pressure in the first order peritubular capillaries (EAP) were measured with a servonulling device. Single nephron filtration fraction (SNFF) was calculated from arterial and peritubular blood protein concentrations. Single nephron glomerular filtration rate (SNGFR) was both measured by conventional micropuncture techniques and calculated from efferent arteriole blood flow and SNFF. Afferent arteriole blood flow (AABF) and resistance of afferent (Ra) and efferent (Re) arteriole were calculated. Measurements were repeated in the left kidney after releasing the ureter. Sham operated rats were used as control. BUL caused a fall in SNGFR (from 101.8 +/- 9.7 to 40.7 +/- [SEM] 6.0 nl/min/kg body wt), accounted for by a rise in PT (from 14.1 +/- 0.7 to 28.9 +/- 3.1 mm Hg), glomerular hemodynamics (particularly PG and AABF) being unchanged. A marked increase in Ra (from 6.6 +/- 0.7 to 10.8 +/- 1.5 dynes. sec. cm-5) occurred after releasing the ureter, lessening both PG and AABF. Therefore, a low SNGFR was maintained despite the concomitant normalization of PT.

Distal Tubule [Na+] and juxtaglomerular apparatus Renin activity in uranyl nitrate induced acute renal failure in the rat. An evaluation of the role of tubuloglomerular feedback

It has been previously demonstrated that single neophron filtration rate, whole kidney glomerular filtration rate and total renal blood flow decreased by 30-35% 6 h after uranyl nitrate induced acute renal failure in the rat. In order to evaluate a role of the renin-angiotensin system in the initiating phase (0-6 h) of this model of acute renal failure determinations of plasma renin activity, superficial (S) and deep (D) juxtaglomerular apparatus (JGA) renin activity and distal nephron [Na+] were obtained. Plasma renin activity increased from the control value of 1.5 +/- 0.3 (S.E.M.) to 2.9 +/- 0.4 ng/ml/h (P less than 0.005) at 6 h. Mean renin activity in S- and D-JGA's of control rats was 6.99 +/- 0.41 and 2.67 +/- 0.21 ng/JGA/h, respectively. After uranyl nitrate, renin activity in S-JGA's increased to 13.62 +/- 0.80 ng/JGA/h (P less than 0.001) at 2 h and remained elevated, 12.56 +/- 0.90 and 12.75 +/- 0.87 ng/JGA/h at 4 and 6 h. D-JGA renin activity increased (P less than 0.05) to 7.04 +/- 0.53, 6.23 +/- 0.31 and 3.44 +/- 0.33 ng/JGA/h at 2, 4 and 6 h after uranyl nitrate. Distal tubule [Na+], 27 samples in 6 rats, increased from a mean control value of 53.7 +/- 1.2 mEq/l to 116.9 +/- 2.5 mEq/l, 24 samples in 6 rats (P less than 0.001). Prompt increases in JGA renin activity were observed in the initiating phase of acute renal failure, suggesting a role for the renin-angiotensin system in the pathophysiology of this nephrotoxic model. The association of increased JGA renin activity and increased distal [Na+] is consistent with a role for the tubuloglomerular feedback mechanism in the initiating phase of uranyl nitrate induced acute renal failure in the rat.

Hepatorenal syndrome: a disease mediated by the intrarenal action of renin

The functional renal failure accompanying advanced liver disease is characterized by azotemia, a urine of very low sodium concentration and systemic hypotension with decreased renal perfusion and high renal vascular resistance. Patients with this disorder have a markedly reduced ability to excrete free water and develop hyponatremia, ascites and edema. It is postulated that this renal dysfunction is due to hepatic failure to make renin substrate. Renin released from the kidney is thus unable to exert its pressor effect. The resultant hypotension and renal hypoperfusion continue to stimulate excessive synthesis and release of renin. It is postulated that the overdriven renal renin system increases renovascular resistance at the level of the glomerular arterioles. This causes decreased renal blood flow and decreased glomerular filtration rate leading to salt and water retention and azotemia. Since no renin substrate is available for human infusion, this hypothesis could be tested either by infusion of angiotensin II to restore systemic blood pressure and renal perfusion or by beta adrenergic blockade with propranolol to attempt to decrease the intrarenal effects of renin, restore glomerular blood flow and filtration and thus return of renal function.

The role of the kidney in essential hypertension

1. Many forms of human and experimental hypertension begin with compromised renal function. Essential hypertension may be another such case. 2. The kidneys of subjects with essential hypertension excrete normal amounts of salt and water at higher-than-normal renal perfusing pressures. Other overt signs of renal dysfunction are few; renal disease is excluded by definition. However, renal blood flow and glomerular filtration rate are usually less than normal in essential hypertension. 3. Renal afferent resistance can be calculated from arterial pressure, renal blood flow, and an estimate of glomerular capillary pressure. These calculations indicate that afferent resistance is increased to two or more times normal in essential hypertension. 4. It is not clear whether afferent constriction causes hypertension or results from it. The ability of high pressure to produce vascular damage points to the latter. But, most essential hypertensives show low-to-normal plasma renin levels and a marked afferent dilation after saline loading. These observations do not suggest nephrosclerosis: they are consistent with a causal role for afferent constriction. 5. We can speculate that, in essential hypertension, there is a defect in one of the mechanisms that sets afferent resistance. Afferent constriction could result from extrinsic influences (neural or humoral) or something totally within the kidney, such as abnormal handling of information from the macula densa. 6. The effect of afferent constriction on salt-and-water excretion would theoretically be offset by elevated arterial pressure so that the actual salt-and-water excretion would be normal, but only so long as the arterial pressure remained elevated.