HS-142-1, a potent antagonist of natriuretic peptides in vitro and in vivo

Pulmonary vasodilation by phosphodiesterase 5 inhibition is enhanced and nitric oxide independent in early pulmonary hypertension after myocardial infarction

Myocardial infarction (MI) may result in pulmonary hypertension (PH). Inhibition of phosphodiesterase 5 (PDE5), the enzyme responsible for the breakdown of cGMP in vascular smooth muscle, has become part of the contemporary therapeutic armamentarium for pulmonary arterial hypertension and may also be beneficial for PH secondary to MI. Nitric oxide (NO) is an important activator of cGMP synthesis and can be enhanced in early PH and decreased in severe PH. In the present study, we investigated if PDE5 inhibition ameliorates pulmonary hemodynamics in swine with PH secondary to MI and whether NO is essential. The PDE5 inhibitor EMD360527 was administered in awake, chronically instrumented swine with or without MI. At rest, PDE5 inhibition produced pulmonary vasodilation as evidenced by a decrease in pulmonary vascular resistance, which was more pronounced in MI ( n = 5) compared with normal swine ( n = 10, P ≤ 0.01) and was accompanied by an increase in stroke volume in MI swine. Both pulmonary vasodilation and increased stroke volume were maintained during exercise, suggesting that this therapy may improve exercise capacity in patients with PH secondary to MI. Interestingly, prior inhibition of NO significantly enhanced ( P ≤ 0.01) pulmonary vasodilation by PDE5 inhibition in both normal ( n = 8) and MI swine ( n = 5, P ≤ 0.05 vs. normal). This suggests that the increased vasodilator responses to PDE5 inhibition after MI were not due to an increase in NO-induced cGMP production. These observations indicate that PDE5 inhibition represents an interesting pharmacotherapeutic approach in early PH after a recent MI to prevent overt PH. NEW & NOTEWORTHY This research article is the first to describe that pulmonary vasodilation to phosphodiesterase 5 inhibition is enhanced and nitric oxide independent in resting and exercising swine with pulmonary hypertension as a result of myocardial infarction. This suggests that phosphodiesterase 5 inhibition can normalize pulmonary hemodynamics in postcapillary pulmonary hypertension after a recent myocardial infarction and may improve exercise capacity.

Chimeric natriuretic peptide ACNP stimulates both natriuretic peptide receptors, the NPRA and NPRB

Here, we investigated the receptor profile of the newly designed natriuretic peptide (NP) ACNP consisting of the N- and C-terminus of human ANP and the ring structure of CNP, its potency/efficacy in stimulating cGMP generation in primary cells, and its stability towards peptidase activity. ACNP stimulated both human natriuretic peptide receptors (NPRs), NPRA and NPRB, as potent as their native ligands in receptor transfected cells. Consequently, ACNP was more efficient in generating cGMP compared to ANP, BNP, and CNP, in primary cells expressing both NPRs. All NPs have been similarly degraded by neprilysin, except the neprilysin-resistant BNP. However, ACNP was fastest degraded in serum, while CNP was most stable. Congruently, CNP but not ACNP reduced blood pressure most significantly after acute peptide infusion in normotensive mice. Our data identify ACNP as the first compound being able to stimulate both natriuretic receptors with similar potency and efficacy as their respective ligands.

Intact acute cardiorenal and humoral responsiveness following chronic subcutaneous administration of the cardiac peptide BNP in experimental heart failure

BACKGROUND

BNP is a cardiac peptide with vasodilating, lusitropic and natriuretic properties mediated by the second messenger cGMP. We have previously shown that chronic subcutaneous (SQ) administration of BNP in experimental CHF resulted in improved haemodynamics and unloading of the heart. However, it is unknown if this will lead to the development of tolerance to exogenous BNP.

METHODS

The current study extends our previous study and compares the cardiorenal effects of acute administration of SQ BNP (5 microg/kg) in a group of dogs (n = 5) with rapid ventricular pacing induced CHF (180 bpm for 10 days) to a separate group of CHF dogs (n = 6), who received chronic SQ BNP (5 microg/kg) three times a day for 10 days.

RESULTS

Acute administration of SQ BNP resulted in similar increases in both plasma cGMP (35+/-5 vs. 29+/-2 pmol/ml) and urinary cGMP excretion (UcGMPV) (6000+/-1000 vs. 4000+/-600 pmol/min) in both the Chronic SQ BNP treated and the Untreated CHF groups (P > 0.05). These were associated with decreased cardiac filling pressures and increased urine flow, which were also similar in both groups.

CONCLUSION

In experimental CHF, chronic SQ BNP administration did not result in the development of tolerance as demonstrated by increases in both plasma cGMP and UcGMPV following acute administration of SQ BNP. This may have important clinical implications, suggesting that chronic BNP administration does not lead to the development of tolerance to acute BNP administration.

Guanylyl cyclase receptors mediate cardiopulmonary vagal reflex actions of ANP

Atrial natriuretic peptide (ANP) potentiates vagal cardiopulmonary reflexes due to chemosensory (Bezold-Jarisch [B-J] reflex) or mechanosensory (ramp baroreflex) activation. The ANP receptor mediating these actions is unknown. We examined the role of particulate guanylyl-cyclase (pGC) receptors in ANP-induced enhancement of cardiopulmonary vagal reflexes. Cardiopulmonary baroreceptor reflex function was assessed by bradycardic responses to ramp blood pressure rises after rapid intravenous methoxamine (100 micro g/kg bolus dose). The B-J reflex was evoked by 3 intravenous doses of serotonin (1 to 10 micro g/kg). In conscious, chronically instrumented rats (n=9), these tests were performed on each animal during randomized infusions of rat ANP (150 ng/kg per minute IV), saline (270 micro L/h IV), the pGC receptor antagonist HS-142-1 (3 mg/kg IV), or combined HS-142-1+ANP treatment. HS-142-1 alone attenuated normal B-J reflex (by 33+/-8%, P<0.05) but not ramp baroreflex responses. As we showed previously, ANP enhanced baroreflex and B-J reflex bradycardia (by approximately 140% and approximately 30%, respectively, P<0.05), compared with saline infusion. These ANP effects were completely blocked by HS-142-1, demonstrating that the cardiopulmonary vagal reflex actions of ANP occurred through pGC natriuretic peptide receptors. Additionally, we have provided evidence for the first time that pGC natriuretic peptide receptors are essential for the full expression of the B-J reflex but not for that of cardiopulmonary vagal baroreflexes. This tonic interaction between pGC natriuretic peptide receptors and cardiopulmonary chemosensitive receptors may be important during pathophysiological activation of B-J reflex, such as with myocardial infarction.

Opposing effects of endothelin-1 on C-type natriuretic peptide actions in rat cardiomyocytes

C-type natriuretic peptide (CNP) and Endothelin-1 are paracrine peptides with opposing vascular and mitogenic actions. In cardiac myocytes, CNP reduced contractility and induced accumulation of cyclic guanosine monophosphate (cGMP). Endothelin-1 caused an increase in contractile amplitude, abolished the negative inotropic effect of CNP, reduced the negative inotropic effect of a membrane permeable cGMP, and inhibited cGMP accumulation induced by CNP. We conclude that endothelin-1 abolishes the negative inotropic effect of CNP. This effect may be mediated by inhibition of the negative inotropic actions of cGMP as well as by reduction of cGMP levels.

C-type natriuretic peptide has a negative inotropic effect on cardiac myocytes

C-type natriuretic peptide (CNP) has vasodilatory and antimitogenic actions, but its role in the control of cardiac function is unclear. We studied the effect of CNP on cultured, beating neonatal rat cardiac myocytes. CNP caused a significant reduction in the amplitude of contraction and a significant accumulation of intracellular cyclic GMP. The effect of a membrane permeable cyclic GMP on cell contraction was similar to that of CNP. CNP caused no change in Ca2+ transients. Blockade of natriuretic peptide receptors abolished the effects of CNP on contraction and accumulation of intracellular cyclic GMP. Blockade of cyclic GMP-dependent protein kinase abolished the effect of CNP on myocyte contraction. We conclude that CNP has a negative inotropic effect on neonatal rat cardiac myocytes. The effect of CNP is mediated via natriuretic peptide receptor(s) causing elevation of intracellular cyclic GMP which possibly activates protein kinase and causes attenuation of myofilament sensitivity to Ca2+.

Glycosylation sites in the atrial natriuretic peptide receptor: oligosaccharide structures are not required for hormone binding

Atrial natriuretic peptide (ANP) is a hormone involved in cardiovascular homeostasis through its natriuretic and vasodilator actions. The ANP receptor that mediates these actions is a glycosylated transmembrane protein coupled to guanylate cyclase. The role of glycosylation in receptor signaling remains unresolved. In this study, we determined, by a combination of HPLC/MS and Edman sequencing, the glycosylation sites in the extracellular domain of ANP receptor (NPR-ECD) from rat expressed in COS-1 cells. HPLC/MS analysis of a tryptic digest of NPR-ECD identified five glycosylated peptide fragments, which were then sequenced by Edman degradation to determine the glycosylation sites. The data revealed Asn-linked glycosylation at five of six potential sites. The type of oligosaccharide structure attached at each site was deduced from the observed masses of the glycosylated peptides as follows: Asn13 (high-mannose), Asn180 (complex), Asn306 (complex), Asn347 (complex), and Asn395 (high-mannose and hybrid types). Glycosylation at Asn180 and Asn347 was partial. The role of glycosyl moieties in ANP binding was examined by enzymatic deglycosylation of NPR-ECD followed by binding assay. NPR-ECD deglycosylated with endoglycosidase F2 and endoglycosidase H retained ANP-binding activity and showed an affinity for ANP similar to that of untreated NPR-ECD. Endoglycosidase treatment of the full-length ANP receptor expressed in COS-1 cells also had no detectable effect on ANP binding. These results suggest that, although glycosylation may be required for folding and transport of the newly synthesized ANP receptor to the cell surface, the oligosaccharide moieties themselves are not involved in hormone binding.

Further attenuation of endothelium-dependent relaxation imparted by natriuretic peptide receptor antagonism

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 (N(G)-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 by L-NMMA alone but not attenuated by HS-142-1 alone. Coinhibition with L-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 guanylyl cyclase family of natriuretic peptide receptors

Guanylyl cyclases are cytoplasmic and membrane-associated enzymes that catalyze the conversion of GTP to cyclic GMP, an intracellular signaling molecule. Molecular cloning has identified a multigene family encoding both soluble and particulate forms of the enzymes. Diffusible agents such as nitric oxide and carbon monoxide activate the soluble guanylyl cyclases. The particulate members of the family share a characteristic domain arrangement, with a single transmembrane span separating a variable extracellular ligand-binding domain from a conserved intracellular regulatory and cyclase catalytic domain. Seven members of the particulate guanylyl cyclase family have been identified, and they include the receptors for natriuretic peptides and Escherichia coli heat-stable enterotoxin. Recently, animal models have been developed to study the role of natriuretic peptides and their guanylyl cyclase-coupled receptors in renal and cardiovascular physiology.

Renal function in high-output heart failure in rats: role of endogenous natriuretic peptides

The physiologic and pathophysiologic importance of natriuretic peptides (NP) has been imperfectly defined. The diminished renal responses to exogenous atrial NP in heart failure have led to the perception that the endogenous NP system might be less effective and thus contribute to renal sodium retention in heart failure. This study tests the hypothesis that in experimental heart failure, the renal responses to an acute volume load are still dependent on the NP system. The specific antagonist HS-142-1 was used to block the effects of NP in a model of high-output heart failure induced by an aortocaval shunt. Plasma cGMP levels and renal cGMP excretion were significantly lower in shunted and sham-operated rats receiving HS-142-1, compared with vehicle-treated controls, indicating effective blockade of guanylate cyclase-coupled receptors. Baseline sodium excretion and urine flow rate were lower in HS-142-1-treated sham-operated rats (15.2+/-1.1 microl/min versus 27.5+/-3.1 microl/min with vehicle, P < 0.001) and in HS-142-1-treated shunted rats (8.1+/-1.3 microl/min versus 19.9+/-2.3 microl/min with vehicle, P < 0.001). After an acute volume load, the diuretic and natriuretic responses were attenuated by HS-142-1 in control and shunted rats. The renal responses were reduced by HS-142-1 to a significantly greater extent in shunted rats than in control rats. HS-142-1 did not induce any significant systemic hemodynamic changes in either group, nor did it alter renal blood flow. However, the GFR in HS-142-1-treated shunted rats was lower than that in vehicle-treated shunted rats, both at baseline (0.6+/-0.3 ml/min versus 2.1+/-0.4 ml/min with vehicle, P < 0.05) and after an acute volume load (1.2+/-0.4 ml/min versus 2.6+/-0.4 ml/min with vehicle, P = 0.01), whereas no such effect was observed in control rats. These data indicate that the maintenance of basal renal function and the responses to acute volume loading are dependent on the NP system. The NP seem to be of particular importance for the maintenance of GFR in this model of experimental heart failure. These observations provide new insights into the importance of the renal NP system in heart failure.

Effect of endogenous natriuretic peptide system on ventricular and coronary function in failing heart

Ventricular concentrations of atrial, brain (BNP) and C-type natriuretic peptide are enhanced in congestive heart failure (CHF). Natriuretic peptide receptors are present on ventricular myocytes and stimulate guanosine 3',5'-cyclic monophosphate (cGMP) production. cGMP has been demonstrated to affect myocyte function in vitro. Thus we hypothesized that the intracardiac natriuretic peptide system may modulate myocardial and coronary function in CHF. To test this hypothesis, the effects of an intracoronary infusion of the natriuretic peptide receptor antagonist HS-142-1 on ventricular and coronary function were examined in anesthetized dogs with chronic CHF. To determine whether receptor stimulation had contrasting effects to those of receptor blockade, intracoronary BNP was infused in anesthetized normal and CHF dogs. Low-dose HS-142-1 delayed and slowed left ventricular (LV) relaxation and decreased coronary blood flow without changes in LV pressures. Higher doses further impaired LV relaxation without further decreases in coronary blood flow. In normal and CHF dogs, exogenous BNP produced the opposite effect with a quicker onset and faster rate of LV relaxation without effects on LV pressures or coronary blood flow. The endogenous natriuretic peptide system has an autocrine-paracrine role to modulate LV and coronary vascular function in CHF.

Role of guanylyl cyclase receptors for CNP in salt secretion by shark rectal gland

The role of C-type natriuretic peptide (CNP) and its guanylyl cyclase-linked receptors in mediating salt secretion by the rectal gland of the spiny dogfish shark (Squalus acanthias) was investigated using HS-142-1, a competitive inhibitor of the binding of natriuretic peptides to their guanylyl cyclase receptors. CNP binds to receptors and activates guanylyl cyclase in rectal gland membranes in a way that is inhibited by HS-142-1. Guanylyl cyclase activation in rectal gland membranes is far more sensitive to CNP than to atrial natriuretic peptide, whereas the reverse is true for membranes derived from mammalian (rabbit) renal collecting duct cells. HS-142-1 inhibited the stimulatory effect of CNP on ouabain-inhibitable oxygen consumption by rectal gland tubules. In explanted rectal glands continuously perfused with blood from intact donor sharks, HS-142-1 inhibited the increase in salt secretion normally provoked by infusing isotonic saline solutions into the donor animal. These results strongly support the view that CNP released into the systemic circulation in response to volume expansion mediates the secretion of chloride by the rectal gland via receptors linked to guanylyl cyclase.

Renal effects of high-dose natriuretic peptide receptor blockade in rats with congestive heart failure

Previous studies suggest that elevated plasma atrial natriuretic peptide (ANP) levels participate in regulating renal excretory function in rats with congestive heart failure (CHF). To define the role of natriuretic peptides (NPs) in the regulation of renal function in CHF, the renal responses to HS-142-1 (HS), a potent NP receptor antagonist, were studied in anesthetized rats subjected to coronary ligation that developed left ventricular infarction and CHF or in sham-operated (SO) control rats. Plasma ANP levels averaged > 14-fold higher in rats with CHF than in SO rats. In response to HS (20 mg/kg IV bolus), both mean arterial pressure and renal vascular resistance increased in rats with CHF but not in SO rats; glomerular filtration rate (GFR, 1.26 +/- 0.04 versus 0.76 +/- 0.11 mL/min) and renal plasma flow rate (RPF, 3.52 +/- 0.27 versus 2.70 +/- 0.32 mL/min) were significantly reduced in rats with CHF; and in SO rats, GFR (1.26 +/- 0.06 versus 1.20 +/- 0.07 mL/min) and RPF (3.98 +/- 0.21 versus 3.99 +/- 0.18 mL/min) were not significantly affected by HS. The sodium excretion rate (0.18 +/- 0.04 to 0.06 +/- 0.01 muEq/min) and fractional sodium excretion (0.01 +/- 0.02% to 0.04 +/- 0.01%) also fell markedly after HS administration in rats with CHF, but these parameters were unchanged in SO rats. These data indicate that NPs play a critical role in maintaining renal hemodynamic function and inhibiting tubule sodium reabsorption in rats with CHF, thus opposing sodium retention and preserving sodium balance in this model.