Effects of BQ-123 on systemic and renal hemodynamic responses to endothelin-1 in the rat split hydronephrotic kidney

Adverse effects of drugs on the kidney

The number of drugs presently marketed is countless, their prescription is relentlessly growing, such that the likelihood of adverse effects is strikingly increasing. As many drugs are cleared by the body through kidney excretion, renal adverse events are likely. In this review we shall concisely describe the pathophysiologic mechanisms of renal damage by drugs, the different clinical presentations outlining renal toxicity in the course of pharmacologic treatment, and the main offending agents.

Bosentan normalizes the GFR response to renal nerve stimulation following reversible unilateral ureteric obstruction in the rat

We investigated the renal response to direct renal nerve stimulation, 2 weeks following reversal of 24-h unilateral (left) ureteric obstruction. Renal nerve stimulation caused a 13-15 % fall in renal blood flow, in 4 groups of anesthetized rats following ureteric obstruction (n=9) or a sham operation (n=7) both with (n=9) and without (n=7) treatment with the mixed ET(A/B) receptor antagonist, bosentan. In the sham-operated rats, renal nerve stimulation did not change glomerular filtration rate but reduced urine flow rate (37+/-3 %, P<0.001), and absolute (38+/-4 %, P<0.001) and fractional (35+/-5 %, P<0.01) sodium excretion. Following unilateral ureteric obstruction, renal nerve stimulation increased glomerular filtration rate by 22+/-3 % (P<0.01), but reduced urine flow rate (14+/-2 %, P<0.001) and fractional sodium excretion (23+/-5 %, P<0.01). Bosentan treatment had no effect on baseline or renal responses to renal nerve stimulation in the sham group but normalized the renal response to renal nerve stimulation in the unilateral ureteric obstruction group. We conclude that 14 days after a 24-h period of unilateral ureteric obstruction there is an increase in GFR in response to direct renal nerve stimulation, which is due, in part, to the actions of endothelin at the time of obstruction.

Physiology of endothelin and the kidney

Since its discovery in 1988 as an endothelial cell-derived peptide that exerts the most potent vasoconstriction of any known endogenous compound, endothelin (ET) has emerged as an important regulator of renal physiology and pathophysiology. This review focuses on how the ET system impacts renal function in health; it is apparent that ET regulates multiple aspects of kidney function. These include modulation of glomerular filtration rate and renal blood flow, control of renin release, and regulation of transport of sodium, water, protons, and bicarbonate. These effects are exerted through ET interactions with almost every cell type in the kidney, including mesangial cells, podocytes, endothelium, vascular smooth muscle, every section of the nephron, and renal nerves. In addition, while not the subject of the current review, ET can also indirectly affect renal function through modulation of extrarenal systems, including the vasculature, nervous system, adrenal gland, circulating hormones, and the heart. As will become apparent, these pleiotropic effects of ET are of fundamental physiologic importance in the control of renal function in health. In addition, to help put these effects into perspective, we will also discuss, albeit to a relatively limited extent, how alterations in the ET system can contribute to hypertension and kidney disease.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

ETA and ETB receptors differentially modulate afferent and efferent arteriolar responses to endothelin

The segment-specific actions of endothelin peptides and agonists have not been thoroughly investigated in the renal microcirculation. The current studies were performed to assess the relative contribution of ET(A) and ET(B) receptors to the renal pre- and postglomerular arteriolar responses to ET-1. Experiments determined the effect of selective ET(A) (A-127722; 30 nM) and ET(B) (A-192621; 30 nM) receptor blockade, on arteriolar responses to ET-1 concentrations of 1 pM to 10 nM in rat kidneys using the isolated juxtamedullary nephron technique. Renal perfusion pressure was set at 110 mmHg. Baseline afferent arteriolar diameter was similar in all groups and averaged 17.8+/-0.6 microm (n=14). In control experiments (n=6), ET-1 produced significant concentration-dependent decreases in arteriolar diameter, with 10 nM ET-1 decreasing diameter by 85+/-1%. Selective blockade of ET(A) receptors (n=6) prevented ET-1-mediated vasoconstriction, except at concentrations of 1 and 10 nM. Similarly, the vasoconstrictor profile was right shifted during selective ET(B) receptor blockade (n=4). Combined ET(A) and ET(B) receptor blockade (n=5) completely abolished afferent arteriolar diameter responses to ET-1. ET(B) selective agonists (S6c and IRL-1620) produced disparate responses. S6c produced a concentration-dependent vasoconstriction of afferent arterioles. In contrast, S6c produced a concentration-dependent dilation of efferent arterioles that could be blocked with an ET(B) receptor antagonist. IRL-1620, another ET(B) agonist, was less effective at altering afferent or efferent diameter and produced a small reduction in pre- and postglomerular arteriolar diameter. These data demonstrate that both ET(A) and ET(B) receptors participate in ET-1-mediated vasoconstriction of afferent arterioles. ET(B) receptor stimulation provides a significant vasodilatory influence on the efferent arteriole. Furthermore, since selective ET(A) and ET(B) receptor antagonists abolished preglomerular vasoconstrictor responses at lower ET-1 concentrations, these data support a possible interaction between ET(A) and ET(B) receptors in the control of afferent arteriolar diameter.

Effect of endothelin 1 on proximal reabsorption and tubuloglomerular feedback

Endothelin 1 (ET-1) is a powerful constrictor of the afferent glomerular artery, implicated in the occurrence of both functional and acute renal failure. Besides being produced by the endothelium, ET-1 is also secreted by proximal tubular cells, suggesting that it may act as an endoluminal messenger. The present study is intended to verify whether ET-1 may play a role in the tubuloglomerular feedback system. The experiments were performed in rat superficial glomeruli. In 25 nephrons we measured by the total collection technique the single-nephron glomerular filtration rate (SNGFR; nl/min) and reabsorption rates before (control) and during microinjection (MIJ) of ET-1 10(-7) M into the first proximal convolution or Bowman's space. The SNGFR rose from 27+/-3 to 61+/-11 nl/min (p<0.01), the percent proximal reabsorption rose from 43 to 74%, and the absolute reabsorption rose from 13+/-2 to 46+/-11 nl/min (p<0.01). In additional 23 nephrons the collections were performed at the earliest distal convolution accessible on the renal surface, while MIJ was performed in the last proximal convolution of the same nephrons. The SNGFR rose during MIJ from 22+/-3 to 40+/-6 nl/min (p<0.01), the percent reabsorption rose from 61 to 66% (p>0. 77), and the absolute reabsorption rose from 12+/-2 to 26+/-4 nl/min, (p<0.003). Exposure of the macula densa to intraluminally injected ET-1 causes an abrupt increase in SNGFR of the experimental nephron, in the absence of changes in systemic and renal hemodynamics. During proximal MIJ, ET-1 may have reached the macula densa during the time preceding the beginning of collection and interruption of the delivery of fluid to the distal nephron. ET-1 directly stimulates fractional and absolute volume reabsorption along the proximal tubule. Proximal secretion and/or filtration of ET-1 could represent a physiological mechanism to activate the tubuloglomerular feedback, eliciting a response opposite to that triggered by systemic and intrarenal infusion.