Regulation of secretion and clearance of C-type natriuretic peptide in the interaction of vascular endothelial cells and smooth muscle cells

NEP inhibitors enhance C-type natriuretic peptide-induced relaxation in porcine isolated coronary artery

C-type natriuretic peptide (CNP), a local regulator of vascular tone and cell proliferation, is eliminated from the circulation via NPR-C receptors and neutral endopeptidase enzyme (NEP, EC. 3.4.24.11). The increased contractility of coronary arteries in different cardiovascular diseases made us study the possible enhancement of vasodilator capacity of exogenously added CNP with concomitant NEP inhibition on porcine coronary arteries in vitro. CNP (0.006-1.4 microM) concentration dependently relaxed the U46619 (0.07-0.4 microM) precontracted preparations in an almost equally effective manner in the presence and absence of functional endothelium with maximum effects of about 40%. The combined NEP/endothelin-converting enzyme inhibitor (NEP/ECE inhibitor), phosphoramidon (10 microM) or the specific inhibitor of the NEP, thiorphan (10 microM) resulted in an enhanced magnitude of CNP-induced relaxation without significant change in the EC50 both on endothelium intact and endothelium deprived preparations. The inhibition of endothelin receptors by PD 142893 (10 microM) enhanced the relaxing effect of CNP in the presence but not in the absence of functional endothelium indicating a functional antagonism between CNP and endothelin. Our results suggest that inhibition of CNP degradation may endue this endogenous peptide with therapeutic potency in cardiovascular diseases.

Amino-terminal proCNP: a putative marker of cartilage activity in postnatal growth

Recent evidence from rodents and humans shows that C-type natriuretic peptide (CNP) plays an essential role in endochondral bone growth. We recently identified a stable product of proCNP, amino-terminal proCNP (NT-proCNP), which unlike CNP is readily measurable in human and ovine plasma. Hypothesizing that plasma NT-proCNP concentrations reflect in part CNP synthesis within growth plates of rapidly growing cartilage, we studied levels of CNP forms in both children and lambs and related these to age, growth velocity, and biochemical markers of bone turnover. Plasma NT-proCNP levels were elevated at birth and fell progressively with age. Significant associations between plasma NT-proCNP and height velocity, alkaline phosphatase, and type 1 collagen C telopeptide were identified in children (aged 5-18 y). In longitudinal animal studies, elevated plasma concentration of NT-proCNP in 1-wk-old lambs fell progressively to mature adult levels at age 27 wk. Plasma NT-proCNP showed a highly significant association with alkaline phosphatase and metacarpal growth velocity. Glucocorticoids, a treatment known to inhibit cartilage proliferation, reduced metacarpal growth elongation in 4-wk-old lambs and markedly lowered circulating NT-proCNP levels during the treatment period. In summary, NT-proCNP levels in blood show a strong association with growth velocity and markers of bone formation and may well serve as a useful marker of growth plate activity in humans and other mammals.

Target organ damage and non-dipping pattern defined by two sessions of ambulatory blood pressure monitoring in recently diagnosed essential hypertensive patients

OBJECTIVE

To evaluate in a selected population of patients with a recent diagnosis of hypertension whether a reduced nocturnal fall in blood pressure, confirmed by two 24 h ambulatory blood pressure monitoring (ABPM) sessions is associated with more prominent target organ damage (TOD).

METHODS

The study was structured in two phases: in the first, 141 consecutive, recently diagnosed, never-treated essential hypertensives underwent 24 h ABPM twice within 3 weeks; in the second phase, 118 of these patients showing reproducible dipping or non-dipping patterns underwent the following procedures: (1) routine blood chemistry, (2) 24 h urinary collection for microalbuminuria, (3) amydriatic photography of ocular fundi, (4) echocardiography and (5) carotid ultrasonography.

RESULTS

The 92 patients with (>10%) night-time fall in systolic blood pressure (SBP) and diastolic blood pressure (DBP) (dippers) in both monitoring sessions were similar for age, gender, body surface area, smoking habit, clinic BP, 24 h and 48 h BP to the 26 patients with a < or = 10% nocturnal fall (non-dippers) in both sessions. The prevalence of left ventricular hypertrophy (LVH) (defined by two criteria: (1) LV mass index > or = 125 g/m2 in both genders; (2) LV mass index > or = 120 and 100 g/m2 in men and women, respectively) and that of carotid intima-media (IM) thickening (IM thickness > or = 0.8 mm) were significantly higher in non-dippers than in dippers (23 versus 5%, P < 0.01; 50 versus 22%, P < 0.05; and 38 versus 18%, P < 0.05, respectively). There were no differences among the two groups in the prevalence of retinal changes and microalbuminuria. The strength of the association of LV mass index with night-time BP was slightly but significantly greater than that with daytime BP.

CONCLUSIONS

This study suggests that a blunted reduction in nocturnal BP, persisting over time, may play a pivotal role in the development of some expressions of TOD, such as LVH and IM thickening, during the early phase of essential hypertension, despite similar clinic BP, 24 h and 48 h BP levels observed in non-dippers and dippers.

Molecular mechanism of cGMP-mediated smooth muscle relaxation

Contraction and relaxation of smooth muscle is a tightly regulated process involving numerous endogenous substances and their intracellular second messengers. We examine the key role of cyclic guanosine monophosphate (cGMP) in mediating smooth muscle relaxation. We briefly review the current art regarding cGMP generation and degradation, while focusing on the recent identification of the molecular mechanisms underlying cGMP-mediated smooth muscle relaxation. cGMP-induced SM relaxation is mediated mainly by cGMP-dependent protein kinase activation. It involves several molecular events culminating in a reduction in intracellular Ca(2+) concentration and a decrease in the sensitivity of the contractile system to Ca(2+). We propose that the cGMP-induced decrease in Ca(2+) sensitivity is a strategic way to achieve "active relaxation" of the smooth muscle. In summary, we present compelling evidence supporting a key role for cGMP as a mediator of smooth muscle relaxation in physiological and pharmacological settings.

Role of natriuretic peptides in ion transport mechanisms

Natriuretic peptides (NP) act as ligands on the guanylyl cyclase family of receptors. The NP binding site on these receptors is extracellular and the guanylyl cyclase and protein kinase domains are intracellular. The guanylyl cyclase receptor catalyzes the synthesis of the second messenger molecule, cGMP, which activates protein kinase. This in turn is involved in the phosphorylation of various ion transport proteins. Ion transport proteins, which are modulated by NP and are thought to underlie the natriuretic and diuretic actions of NP, include: (a) calcium-activated K+ channels; (b) ATP-sensitive K+ channels; (c) inwardly-rectifying K+ channels; (d) outwardly-rectifying K+ channels; (e) L-type Ca2+ channels; (f) Cl- channels including cystic fibrosis transmembrane conductance regulator Cl- channels; (g) Na+- K+ 2Cl- co-transporter; (h) Na+- K+ ATPase; (i) Na+ channels; (j) stretch-activated channels; and (k) water channels. It appears that NP modulate the kinetics, rather than the conductance, of ion channels. Some of these channels, like the Ca2+, ATP-sensitive K+ and stretch-activated channels, are also involved in NP secretion. In addition, the structural properties of the NP, e.g., ovCNP-22 and ovCNP-39, appear to confer on them the ability to form ion channels. These CNP-formed ion channels can modify the trans-membrane signal transduction and second messenger systems underlying NP-induced pathological effects.

Neutral endopeptidase 3.4.24.11 inhibition potentiates the inhibitory effects of type-C natriuretic peptide on leukotriene D4-induced airway changes

1. Microvascular leakage, a primary feature of inflammation, is well known for worsening the asthmatic condition. Gene expression of and a specific receptor for type-C natriuretic peptide (CNP), initially considered a neuropeptide, have been detected in the human vascular wall and secretion of CNP from vascular endothelial cells has recently been demonstrated. These facts suggest the presence of a vascular natriuretic peptide system and led us to expect that CNP may act beneficially on airway microvascular leakage in asthma. In the present study, we investigated the effects of CNP against leukotriene (LT) D4-induced airway microvascular leakage and bronchoconstriction and how these effects were potentiated by thiorphan, a potent neutral endopeptidase 3.4.24.11 (NEP) inhibitor. 2. Anaesthetized male guinea-pigs, ventilated via a tracheal cannula, were placed into a plethysmograph for 10 min, in order to measure pulmonary mechanics and mean blood pressure, after challenge with 2 micrograms/kg LTD4 and then the extravasation of 20 mg/kg Evans blue dye into airway tissue was investigated to indicate and evaluate microvascular leakage. 3. Intravenous administration of CNP (100, 300 and 1000 micrograms/kg) significantly inhibited the LTD4-induced microvascular leakage and bronchoconstriction in a dose-dependent manner. These inhibitory effects were enhanced by pretreatment with 20 mg/kg thiorphan, suggesting the important role of NEP in the pulmonary metabolism of CNP. 4. We believe that these results are encouraging for the further investigation of the therapeutic applications of exogenous CNP in asthma.

Local expression of C-type natriuretic peptide markedly suppresses neointimal formation in rat injured arteries through an autocrine/paracrine loop

BACKGROUND

In vivo gene transfer into injured arteries may provide a new means to facilitate molecular understanding of and to treat the intractable fibroproliferative arterial diseases. Selection of an optimal molecule to be transferred will be a key to successful gene therapy in the future. We tested the hypothesis that a secreted multifactorial molecule should act more efficiently through an autocrine/paracrine loop to suppress neointimal formation elicited in injured arteries than a simple growth-inhibiting molecule that might be expressed inside cells.

METHODS AND RESULTS

We constructed an adenoviral vector (AdCACNP) expressing C-type natriuretic peptide (CNP), a secreted stimulator of membrane-bound guanyl cyclase. AdCACNP directs cells to secrete large quantities of biologically active CNP. Serum-stimulated DNA synthesis and cell proliferation were only moderately suppressed in arterial smooth muscle cells infected with AdCACNP in vitro. However, when AdCACNP was applied to balloon-injured rat carotid arteries in vivo, neointimal formation was markedly reduced (90% reduction) in an infection-site-specific manner without an increase in plasma CNP level.

CONCLUSIONS

Our results showed that CNP, a secreted multifactorial molecule, was indeed effective in suppressing fibroproliferative response in injured arteries and suggest that the potent antiproliferation effect may not be the most critical factor for the effective suppression of neointimal formation. An adenovirus-mediated expression of CNP could be an effective and site-specific form of molecular intervention in proliferative arterial diseases.

The guanylyl cyclase-deficient mouse defines differential pathways of natriuretic peptide signaling

A genetic model of salt-resistant hypertension has been developed recently through disruption of the guanylyl cyclase-A (GC-A) natriuretic peptide receptor gene (Lopez, M. J., Wong, S. K., Kishimoto, I., Dubois, S., Mach, V., Friesen, J., Garbers, D. L., and Beuve, A. (1995) Nature 378, 65-68). These genetically altered mice were used to determine which of the natural peptides with natriuretic peptide-like structures regulate blood pressure through the GC-A receptor. Atrial natriuretic peptide (ANP) or B-type natriuretic peptide (BNP) half-maximally relaxed precontracted aortic rings in wild-type mice at about 24 nM, but failed to relax such aortas in GC-A null mice, even at micromolar concentrations. C-type natriuretic peptide (CNP), in contrast, caused half-maximal relaxation at concentrations of 335 and 146 nM in aortas from either wild-type or null mice, respectively, suggesting that this peptide acted through a receptor other than GC-A. Since the in vitro results with aortic smooth muscle do not necessarily reflect the physiology of the smaller blood vessels important in blood pressure regulation, the blood pressures of conscious mice infused with the various peptides were determined. ANP caused decreases in blood pressure when infused at rates of 500 ng/kg/min, a rate which resulted in a plasma concentration of 0.8 nM. In the null mice, in contrast, ANP failed to lower blood pressure even at infusion rates of 50 microg/kg/min. Much higher infusion rates for CNP (50 microg/kg/min), which yielded final plasma concentrations of 18.3 nM, were required to lower blood pressure in wild-type mice, but the effects of CNP were not altered in GC-A null mice. Thus, two natriuretic peptides (ANP, BNP) act through GC-A whereas another (CNP) acts through another receptor to regulate blood pressure.