Citations

Adrenomedullin (ADM) is a potent renal vasodilating and natriuretic peptide possessing a six amino acid disulfide ring. Neutral endopeptidase 24.11 (NEP) is localized in greatest abundance in the kidney and cleaves endogenous peptides like atrial natriuretic peptide, which also possesses a disulfide ring. We hypothesized that NEP inhibition potentiates the natriuretic actions of exogenous ADM in anesthetized dogs ( n = 6). We therefore investigated renal function in which one kidney received intrarenal infusion of ADM (1 ng ⋅ kg−1⋅ min−1) while the contralateral kidney served as control before and during the systemic infusion of a NEP inhibitor (Candoxatrilat, 8 μg ⋅ kg−1⋅ min−1; Pfizer). In response to ADM, glomerular filtration rate (GFR) in the ADM kidney did not change, whereas renal blood flow, urine flow (UV), and urinary sodium excretion (UNaV) increased from baseline. Proximal and distal fractional reabsorption of sodium decreased in the ADM-infused kidney. In response to systemic NEP inhibition, UNaV and UV increased further in the ADM kidney. Indeed, ΔUNaV and ΔUV were markedly greater in the ADM kidney compared with the control kidney. Plasma ADM was unchanged during ADM infusion but increased during NEP inhibition. In conclusion, the present investigation is the first to demonstrate that NEP inhibition potentiates the natriuretic and diuretic responses to intrarenal ADM. This potentiation occurs secondary to a decrease in tubular sodium reabsorption. Lastly, the increase in plasma ADM during systemic NEP inhibition supports the conclusion that ADM is a substrate for NEP.

Adrenomedullin (ADM), originally discovered in human pheochromocytoma, is also of renal cell origin and has natriuretic and diuretic actions. The present study was designed to investigate the role of prostaglandins and renal nerves in the renal hemodynamic and natriuretic actions ofADM. ADM was administered intrarenally (1, 5 and 25 ng x kg(-1) x min(-1)) with and without prostaglandin inhibition (meclofenamate, 5 mg/kg intravenous bolus) in anesthetized normal mongrel dogs (n = 5, each). To elucidate the role of renal nerves, ADM was administered intrarenally to the denervated kidney in five dogs. ADM mediated a natriuretic action via increases in glomerular filtration rate and decreases in distal tubular sodium reabsorption, which was attenuated by renal denervation and completely abolished by prostaglandin inhibition. The renal vasodilatation induced by ADM was attenuated by meclofenamate, as well as by renal denervation, although not significantly. Additionally, renal nerves mediated hemodynamic effects of hypertension that were produced by intrarenal infusion of ADM. This study establishes an important mechanistic role for renal prostaglandins as a mediator of ADM-mediated natriuresis at the level of the glomerulus and terminal nephron.

Adrenomedullin (ADM) is a newly described 52-amino acid peptide originally isolated from extracts of human pheochromocytoma and, more recently, detected in human plasma. Based on the report that ADM mRNA and immunoreactivity are present in the kidney, the current study was designed to determine the renal distribution of ADM by immunohistochemistry and the renal biological actions of ADM. In the immunohistochemical studies, the present investigation demonstrated the localization of ADM in glomeruli, cortical distal tubules, and medullary collecting duct cells of the normal canine kidney. In the in vivo studies, ADM was administered (0.25 ng.kg-1.min-1 in group I and 1, 5, and 25 ng.kg-1.min-1 in group II) intrarenally in normal mongrel dogs with the contralateral kidney receiving only saline vehicle. Intrarenal infusion of ADM resulted in a marked diuretic and natriuretic response, whereas the contralateral kidney showed no renal effects. These significant natriuresis and diuresis in the ADM kidney were associated with increases in glomerular filtration rate and fractional sodium excretion and with a decrease in distal tubular sodium reabsorption. Intrarenal infusion of ADM also caused an increase in mean arterial blood pressure and a decrease in heart rate. Plasma concentrations of atrial natriuretic peptide, renin activity, aldosterone, and guanosine 3',5'-cyclic monophosphate were not changed during the infusion of ADM. The current study demonstrates that ADM is present in renal glomerular and tubular cells and is a potent natriuretic peptide that may play an important role in the regulation of sodium excretion.

Endothelin (ET) is a potent vasoconstrictor peptide of endothelial origin, which at low doses results in renal vasoconstriction and diuresis with variable actions on sodium excretion. The current study conducted in four groups of anesthetized dogs was designed to define the role of the ETA and ETB receptor subtypes in the renal actions of low-dose exogenous ET. Group 1 (n = 4) animals served as time controls. In group 2 (n = 6) a systemic ET-1 (5 ng.kg-1.min-1) infusion mediated renal vasoconstriction, antinatriuresis with increases in proximal fractional reabsorption of sodium, and diuresis with a decrease in urine osmolality. In group 3 (n = 6) intrarenal BQ-123 (4 micrograms.kg-1.min-1), a selective ETA antagonist, abolished the systemic ET-1-mediated changes in renal hemodynamics and unmasked a natriuretic action at the level of the proximal tubule. In contrast, the diuretic response of ET was not altered by BQ-123. In group 4 (n = 6) intrarenal sarafotoxin 6-c, a selective ETB receptor agonist, resulted in a diuretic response without a change in sodium excretion. These studies suggest that the ETA receptor contributes to the renal vasoconstriction, whereas the ETB receptor is largely responsible for the diuretic response during exogenous ET. This study also suggests that at low doses ET is natriuretic in vivo by decreasing proximal tubular reabsorption of sodium independent of ETA or ETB receptor activation.

Previous studies have demonstrated that the biological actions of atrial natriuretic factor (ANF) are mediated via increases in its intracellular second messenger guanosine 3',5'-cyclic monophosphate (cGMP). Because cGMP egresses rapidly from target cells after ANF binding to particulate guanylate cyclase-linked receptors, extracellular cGMP may be a useful biological marker for the action of ANF in vivo under pathophysiological conditions. The present studies tested the hypothesis that the avid sodium retention and renal ANF resistance characteristic of chronic congestive heart failure (CHF) are associated with attenuated renal cGMP responses to ANF. We assessed the natriuretic and cGMP responses to endogenous and exogenous ANF during the evolution of CHF produced by 6 days of rapid ventricular pacing in conscious dogs (n = 6). Simultaneous measurement of plasma and urinary cGMP concentrations allowed determination of the net renal generation of cGMP, an indicator of the renal contribution to total urinary cGMP excretion. In early CHF, increased sodium excretion and renal cGMP production were observed in association with increases in plasma ANF. Exogenous ANF administration (10 micrograms/kg iv) before CHF also produced parallel increases in sodium excretion and renal cGMP production. In more advanced CHF produced by 6 days of pacing, we observed avid sodium retention in association with reversal of earlier increases in renal cGMP production despite progressive increases in circulating ANF. Natriuretic and renal cGMP responses to exogenous ANF were similarly attenuated in chronic CHF. These studies suggest that 1) renal cGMP production is a useful biological marker for the renal natriuretic action of ANF; 2) endogenous ANF contributes to the maintenance of sodium excretion in early CHF via increases in renal cGMP production; and 3) the avid sodium retention and renal ANF resistance in advanced CHF are, in part, linked to attenuated renal cGMP responses to endogenous and exogenous ANF.

Studies were performed in anesthetized dogs to determine plasma endothelin (ET) concentrations in the presence and absence of experimental congestive heart failure (CHF) produced by rapid ventricular pacing for 8 days. These studies were also designed to determine the effect of exogenous low-dose ET upon integrated cardiorenal and endocrine function in the presence and absence of CHF. In these studies, plasma ET was significantly elevated in CHF (3.25 +/- 0.39 pg/ml) compared with normal (1.03 +/- 0.21 pg/ml) or sham-operated (1.08 +/- 0.27 pg/ml) groups. Compared with the control group, which was characterized by a significant cardiorenal vasoconstrictor response to low-dose ET, a significant attenuation of the vasoconstrictor and antinatriuretic actions of ET was observed in the CHF group. Despite these differential responses, exogenous ET suppressed plasma renin activity (PRA) and activated aldosterone in both control and CHF groups. Thus these studies demonstrate for the first time that experimental CHF is characterized by elevated plasma ET in association with an attenuated cardiorenal response to exogenous ET. In contrast, low-dose ET inhibited PRC and activated aldosterone in the presence and absence of experimental CHF.

β-Adrenergic hyporesponsiveness in congestive heart failure (CHF) is mediated, in part, by nitric oxide (NO). NO and brain natriuretic peptide (BNP) share cGMP as a second messenger. Left ventricular (LV) function and inotropic response to intravenous dobutamine (Dob) were assessed during sequential intracoronary infusion of saline, HS-142-1 (a BNP receptor antagonist), and HS-142-1 + NG-monomethyl-l-arginine (l-NMMA) in anesthetized dogs with CHF due to rapid pacing and in normal dogs during intracoronary infusion of saline, exogenous BNP, and sodium nitroprusside (SNP). In CHF dogs, intracoronary HS-142-1 did not alter the inotropic response to Dob [percent change in first derivative of LV pressure (%ΔdP/d t) 47 ± 4% saline vs. 54 ± 7% HS-142-1, P = not significant]. Addition of intracoronary l-NMMA to HS-142-1 enhanced the response to Dob (%ΔdP/d t 73 ± 8% l-NMMA + HS-142-1, P < 0.05 vs. H142-1). In normal dogs, intracoronary SNP blunted the inotropic response to Dob (%ΔdP/d t 93 ± 6% saline vs. 71 ± 5% SNP, P < 0.05), whereas intracoronary BNP had no effect. In CHF dogs, the time constant of LV pressure decay during isovolumic relaxation increased with intracoronary HS-142-1 (48 ± 4 ms saline vs. 58 ± 5 ms HS-142-1, P < 0.05) and further increased with intracoronary l-NMMA (56 ± 6 ms HS-142-1 vs. 66 ± 7 ms l-NMMA + HS-142-1, P < 0.05). Endogenous BNP and NO preserve diastolic function in CHF, whereas NO but not BNP inhibits β-adrenergic responsiveness.

Anesthetic regimens commonly administered during studies that assess cardiac structure and function in mice are xylazine-ketamine (XK) and avertin (AV). While it is known that XK anesthesia produces more bradycardia in the mouse, the effects of XK and AV on cardiac function have not been compared. We anesthetized normal adult male Swiss Webster mice with XK or AV. Transthoracic echocardiography and closed-chest cardiac catheterization were performed to assess heart rate (HR), left ventricular (LV) dimensions at end diastole and end systole (LVDd and LVDs, respectively), fractional shortening (FS), LV end-diastolic pressure (LVEDP), the time constant of isovolumic relaxation (τ), and the first derivatives of LV pressure rise and fall (dP/d t max and dP/d t min, respectively). During echocardiography, HR was lower in XK than AV mice (250 ± 14 beats/min in XK vs. 453 ± 24 beats/min in AV, P < 0.05). Preload was increased in XK mice (LVDd: 4.1 ± 0.08 mm in XK vs. 3.8 ± 0.09 mm in AV, P < 0.05). FS, a load-dependent index of systolic function, was increased in XK mice (45 ± 1.2% in XK vs. 40 ± 0.8% in AV, P < 0.05). At LV catheterization, the difference in HR with AV (453 ± 24 beats/min) and XK (342 ± 30 beats/min, P < 0.05) anesthesia was more variable, and no significant differences in systolic or diastolic function were seen in the group as a whole. However, in XK mice with HR <300 beats/min, LVEDP was increased (28 ± 5 vs. 6.2 ± 2 mmHg in mice with HR >300 beats/min, P < 0.05), whereas systolic (LV dP/d t max: 4,402 ± 798 vs. 8,250 ± 415 mmHg/s in mice with HR >300 beats/min, P < 0.05) and diastolic (τ: 23 ± 2 vs. 14 ± 1 ms in mice with HR >300 beats/min, P < 0.05) function were impaired. Compared with AV, XK produces profound bradycardia with effects on loading conditions and ventricular function. The disparate findings at echocardiography and LV catheterization underscore the importance of comprehensive assessment of LV function in the mouse.

Nitric oxide (NO) is an important endothelium-derived relaxing factor that functions via activation of soluble guanylyl cyclase and cGMP generation in vascular smooth muscle. Recently, studies have described the synthesis and secretion of C-type natriuretic peptide (CNP) from endothelial cells. This peptide also mediates relaxation via cGMP but through activation of particulate guanylyl cyclase. We tested the hypothesis that endothelium-dependent relaxations to acetylcholine or bradykinin in isolated canine coronary arteries involve both releases of NO and CNP. Rings of canine coronary arteries were incubated with either inhibitors of NO production ( NG-monomethyl-l-arginine,l-NMMA) or the natriuretic peptide receptor antagonist HS-142-1. CNP caused concentration-dependent relaxations of rings with and without endothelium. These relaxations were attenuated by HS-142-1. Relaxations to acetylcholine and bradykinin were attenuated byl-NMMA alone but not attenuated by HS-142-1 alone. Coinhibition withl-NMMA and HS-142-1 significantly inhibited acetylcholine- and bradykinin-induced relaxation to a magnitude greater than either inhibitor alone. In summary, a novel interaction between the NO and the natriuretic peptide system is demonstrated by increased attenuation of endothelium-dependent relaxations to acetylcholine and bradykinin when both NO synthase and natriuretic peptide receptors are inhibited. These investigations support the concept of release of multiple endothelium-derived factors in response to acetylcholine- and bradykinin-receptor stimulation in endothelial cells, which may include CNP, as well as NO.

The effects on myocardial function and loading conditions of clinically relevant doses of the natriuretic peptides (NP) have not been established. The actions of single doses (100 ng ⋅ kg−1⋅ min−1iv over 30 min) of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) were studied in conscious normal dogs and in dogs with pacing-induced heart failure. All three NP reduced end-diastolic pressure in normal dogs, and ANP and BNP reduced end-diastolic volume. In heart failure ANP and BNP reduced EDP, and ANP reduced EDV. Arterial elastance was unchanged in normal dogs and in dogs with heart failure. ANP increased end-systolic elastance ( Ees) in normal dogs, whereas BNP tended to increase Ees( P = 0.06). In dogs with heart failure, no inotropic effect was seen. In normal dogs, all NP reduced the time constant of isovolumic relaxation (τ), and ANP and BNP reduced τ in dogs with heart failure. Increases in plasma cGMP in dogs with heart failure were blunted. The NP reduced preload and enhanced systolic and diastolic function in normal dogs. Effects of ANP and BNP on preload and diastolic function were maintained in heart failure. Lack of negative inotropic effects in heart failure supports the validity of the NP as therapeutic agents.