Role of bradykinin in the neonatal renal effects of angiotensin converting enzyme inhibition

Effect of prucalopride on sildenafil-induced inhibition of esophageal peristalsis in healthy adults

BACKGROUND AND AIM

Prucalopride, a high-affinity 5-hydroxytryptamine 4 receptor agonist, promotes esophageal peristalsis, while phosphodiesterase type 5 inhibitor sildenafil inhibits esophageal peristalsis. The present study was aimed to evaluate whether prucalopride would augment esophageal peristalsis subsequent to the application of sildenafil.

METHODS

Seventeen healthy adults underwent high-resolution manometry by a catheter with one injection port located in the mid-esophagus. Secondary peristalsis was assessed by rapid air injections after water swallows. Two sessions were randomly performed including acute administration of sildenafil 50 mg after pretreatment with prucalopride or the placebo.

RESULTS

The frequency of primary peristalsis subsequent to the administration of sildenafil was significantly increased by prucalopride (P = 0.02). Prucalopride also significantly increased distal contractile integral of primary peristalsis subsequent to the administration of sildenafil (P = 0.03). No difference in the frequency of secondary peristalsis subsequent to the administration of sildenafil for air injects of 10 mL (P = 0.14) or 20 mL (P = 0.21) was found between prucalopride and placebo. Prucalopride did not change distal contractile integral of secondary peristalsis subsequent to the administration of sildenafil for air injections of 10 mL (P = 0.09) or 20 mL (P = 0.12).

CONCLUSIONS

Prucalopride modulates sildenafil-induced inhibition of primary peristalsis by increasing its effectiveness and peristaltic wave amplitude. Our findings suggest that activation of 5-hydroxytryptamine 4 receptors plays a role in mediating sildenafil-induced inhibition of esophageal primary peristalsis rather than secondary peristalsis.

Influence of sildenafil on esophageal motor function in humans: Studies using high-resolution manometry

BACKGROUND/AIM

Sildenafil induces smooth muscle relaxation of the esophagus by blocking type 5 phosphodiesterase that degrades cyclic guanine monophosphate. We aimed to characterize the effects of sildenafil on esophageal peristalsis and contraction reserve using high-resolution manometry (HRM).

METHODS

Fifteen healthy adults (12 men, age 21-39, mean 27 years) participated in this study using HRM following either sildenafil 50 mg or placebo. HRM with ten wet swallows and five multiple rapid swallows was performed in all participants. HRM metrics included esophagogastric junction contractile integral (EGJ-CI), basal lower esophageal sphincter (LES) pressure, 4-second integrated relaxation pressure (IRP-4s), distal contractile integral (DCI), distal latency, resting upper esophageal sphincter pressure (UESP), and the response to MRS.

RESULTS

Sildenafil significantly lowered EGJ-CI (P < .001), LES pressure (P = .04), IRP-4s (P = .02), and DCI (P < .001). There was no difference in UESP (P = .87) between sildenafil and placebo. Sildenafil significantly decreased peristaltic vigor, inducing absent peristalsis in 12 subjects and ineffective esophageal motility in 3 subjects. Peristaltic response and augmentation following MRS were significantly inhibited following sildenafil (7% vs 100%, P < .001, and none vs 73%, P < .001).

CONCLUSIONS

Sildenafil attenuates EGJ barrier function, resting LES pressure, and LES relaxation. Both esophageal body contractility and contraction reserve are inhibited by sildenafil in healthy adults.

Erection Capability Is Potentiated by Long-Term Sildenafil Treatment: Role of Blood Flow-Induced Endothelial Nitric-Oxide Synthase Phosphorylation

Despite demonstrated clinical efficacy of sildenafil for the temporary treatment of erectile dysfunction, the possibility that sildenafil used long-term durably augments erectile ability remains unclear. We investigated whether continuous long-term administration of sildenafil at clinically relevant levels to aged rats "primes" the penis for improved erectile ability and involves nitric oxide (NO) or RhoA/Rho-kinase signaling pathways. In aged, but not young rats, sildenafil prolonged erection and increased the protein expressions of phosphorylated endothelial NO synthase (eNOS) at serine-1177 and phosphorylated Akt at serine-473 in penes. Only in the young rat penis, protein expressions of phosphodiesterase-5 and phosphomyosin phosphatase target subunit 1, a marker of Rho-kinase activity, were increased by sildenafil. Sildenafil inhibited phosphodiesterase-5 activity in penes of young and aged rats coincident with assayed free plasma levels of the drug equivalent to clinically therapeutic measurements. We conclude that erectile ability can be enhanced under preconditions of erectile impairment by long-term inhibition of phosphodiesterase-5 and that the effect is mediated by Akt-dependent eNOS phosphorylation. The lack of erectile ability enhancement in young rats by long-term phosphodiesterase-5 inhibition may relate to restrained NO signaling by phosphodiesterase-5 up-regulation, lack of incremental Akt and eNOS phosphorylation, and heightened Rho-kinase signaling in the penis.

Atypical β-Adrenoceptor Subtypes Mediate Relaxations of Rabbit Corpus Cavernosum

This study was performed to characterize the beta-adrenoceptor population in rabbit isolated corpus cavernosum (RbCC) by using nonselective and selective beta-adrenoceptor agonists and antagonists in functional assays. Metaproterenol, ritodrine, fenoterol, and 8-hydroxy-5-[(1R)-1-hydroxy-2-[N-[(1R)-2-(rho-methoxyphenyl)-1-methylethyl]amino]ethyl]carbostyril (TA 2005) (3-100 nmol each) dose dependently relaxed the RbCC preparations. These relaxations were markedly reduced by N(omega)-nitro-L-arginine methyl ester (L-NAME; 10 microM) and 1H-[1,2,4]-oxadiazolo-[4,3,-a]quinoxalin-1-one (ODQ) (10 microM), whereas the adenylyl cyclase inhibitor SQ 22,536 [9-(2-tetrahydrofuryl) adenine] (10 microM) had no effect. In contrast, neither L-NAME nor ODQ affected the isoproterenol-induced RbCC relaxations, but SQ 22,536 abolished this response. Sildenafil (1 microM) significantly potentiated the relaxations induced by beta(2)-agonists without affecting the isoproterenol-evoked relaxations. Rolipram (10 microM) enhanced the relaxations elicited by isoproterenol but had no effect on those induced by the selective beta(2) agonists. Propranolol and (+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride (ICI 118,551) determined a rightward shift in the concentration-response curves to isoproterenol in a noncompetitive manner with a reduction of maximum response at the highest antagonist concentration, with the slope values significantly different from unity. Propranolol and ICI 118,551 had no effect on the relaxations elicited by fenoterol, TA 2005, metaproterenol, and ritodrine. Atenolol and 1-[2-((3-carbamoyl-4-hydroxy)phenoxy) ethylamino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl)-phenoxy]-2-propanol methanesulfonate (CGP 20712A) (0.1-10 microM) failed to affect the relaxations induced by all tested beta-adrenoceptor agonists. Our study revealed the existence of two atypical beta-adrenoceptors in the rabbit erectile tissue. Isoproterenol relaxes the rabbit cavernosal tissue by activating atypical beta-adrenoceptors coupled to adenylyl cyclase pathway, whereas the selective beta(2)-adrenoceptor agonists relax the RbCC tissue through another atypical beta-adrenoceptor subtype coupled to nitric oxide release from the sinusoidal endothelium.

Cardiovascular effects of peptide kinin B2 receptor antagonists in rats

Bradykinin (BK) is a vasoactive peptide reputed to play an important role in cardiovascular homeostasis. In this study, we describe the cardiovascular changes (mean blood pressure (BP) and heart rate (HR)) induced by the i.v. administration (left jugular vein) of two selective kinin B2 receptor antagonist, namely icatibant (0.1-1 micromol/kg as a bolus) and MEN1 1270 (0.1-1 micromol/kg as a bolus or 1 micromol/kg infused in 15 or 60 min), in urethane-anaesthetized or conscious rats with an indwelling catheter implanted in the right carotid artery for BP measurements. In conscious rats, icatibant at 0.1 or 0.3 micromol/kg did not change BP but at 0.1 micromol/kg increased HR at 30 min from administration. MEN1 1270 at 0.1 or 0.3 micromol/kg induced a dose-related increase in BP and a concomitant bradycardia (significant at 0.3 micromol/kg) lasting for 5 or 30 min, respectively. Icatibant at 1 micromol/kg induced a slight (P < 0.05) increase in BP that resolved in 5 min and a biphasic tachycardia (peaks at 30 and 90 min from administration). MEN1 1270 at 1 micromol/kg induced a triphasic change in HR (tachycardia in the first 5 min, bradycardia at 30 min, and tachycardia at 90 and 120 min) and a biphasic change in BP (hypotension at 15 min and hypertension at 30 min). The i.v. infusion of MEN1 1270 (1 micromol/kg in 15 or 60 min) produced hypertension, whereas HR was increased only following the 15-min infusion. In urethane-anaesthetized rats, both icatibant and MEN1 1270 (0.1 micromol/kg as a bolus) increased BP and the onset for this effect was correlated with the time course of the antagonism of BK-induced hypotension, where the effect of MEN1 1270 was more rapid than that of icatibant. These results indicate that kinin B2 receptor antagonists can induce acute cardiovascular effects, and the reason for the different haemodynamic profile between icatibant and MEN1 1270 could be putatively attributed to kinetic characteristics.

[Nitroxidergic (nitrergic) nerve and erectile dysfunction]

In vascular tissues including the corpus cavernosum, the organ function is reciprocally regulated by noradrenergic and non-adrenergic, non-cholinergic (NANC) nerves. NANC nerves innervating the corpus cavernosum is thought to be nitroxidergic (nitrergic) nerves which liberate nitric oxide (NO) produced by neuronal NO synthase, and liberated NO activates soluble guanylate cyclase (sGC) in cavernous smooth muscle cells. Intracellular increase in cyclic (c) GMP by activation of sGC dilates cavernous smooth muscle and then induces penile erection. Nitroxidergic (nitrergic) vasodilator nerves also innervate cavernous arteries and veins which regulate the blood volume in the corpus cavernosum. The order of potency of nitroxidergic nerve functions in these tissues (cavernosum > artery >> vein) may be suitable for producing the erection. Therefore, obstruction of the arteries and impairment of nitroxidergic (nitrergic) nerve function are speculated to be one of the causes for erectile dysfunction (ED). On the other hand, NO derived from the cavernous endothelium may partly contribute to erectile function. Sildenafil (Viagra) is one of the potent therapeutics for ED. The agent is a selective phosphodiesterase type 5 (PDE-V) inhibitor that inhibits degradation of cGMP elevated by NO mainly derived from the nerves. To develop more selective and safer therapeutics for ED, further systematic investigations are required.

Sildenafil inhibits gastroduodenal motility

BACKGROUND

Sildenafil shows an intense and prolonged inhibitory effect on the smooth muscle cells of corpus cavernosum arterioles by blocking phosphodiesterase-type 5, which inactivates the nitric oxide-stimulated cyclic guanosine monophosphate.

AIM

To investigate whether this inhibitory effect is also displayed on the musculature of the gastroduodenal tract.

METHODS

In 16 normal subjects, antroduodenal motility was recorded by means of a low-compliance manometric system. Ten minutes after the appearance of a phase III of the migrating motor complex, a tablet of sildenafil 50 mg, dissolved in 20 mL of water, was infused in the gut of eight patients, or a placebo in the other eight patients, randomly and in a double-blind manner, continuing the recording for 90 min. The frequency and amplitude of antral and duodenal waves, measured during the first 60 min after infusion in the two groups, were compared statistically. In addition, the duration of antral and duodenal phases I, and the number of phases III occurring during the whole 90 min after infusion, were compared in the two groups.

RESULTS

Antral and duodenal wave frequency and amplitude were significantly lower during the first 60 min after sildenafil administration. Both antral and duodenal phases I were significantly longer after sildenafil than after placebo, and the number of phases III which occurred during the 90 min after sildenafil was significantly lower than after placebo.

CONCLUSIONS

Sildenafil inhibits interdigestive motor activity of the antrum and duodenum.

The role of bradykinin in neonatal hemodynamic effects of ACE inhibition

Angiotensin-converting enzyme (ACE) inhibitors exert their effects by inhibiting angiotensin II (AII) production, but also by inhibiting bradykinin (BK) degradation. In order to clarify whether BK is involved in the systemic effects of ACE inhibition in the newborn period, we investigated the effect of perindoprilat (20 microgram/kg i.v.) in newborn rabbits, with or without the blockade of BK beta(2)-receptors (Hoe 140, 300 microgram/kg s.c.). The bolus infusion of perindoprilat resulted in a marked fall in mean arterial blood pressure (MBP) and a slight decrease in heart rate. BK receptor blockade had no effect on the perindoprilat-induced hypotension but the negative chronotropic effect of ACE inhibition was partly prevented by pretreatment with Hoe 140. We therefore conclude that BK is not involved in neonatal blood pressure regulation but that the ACE inhibition-induced neonatal bradycardia is at least partly BK- mediated.

Inhibition of cyclic GMP-binding cyclic GMP-specific phosphodiesterase (Type 5) by sildenafil and related compounds

The cGMP-binding cGMP-specific phosphodiesterase (PDE5) degrades cGMP and regulates the intracellular level of cGMP in many tissues, including the smooth muscle of the corpus cavernosum of the penis. Sildenafil (Viagra), a specific PDE5 inhibitor, promotes penile erection by blocking the activity of PDE5, which causes cGMP to accumulate in the corpus cavernosum. In the present study, sildenafil, like other PDE5 inhibitors, stimulates cGMP binding to the allosteric sites of PDE5 by interacting at the catalytic site of this enzyme, but the drug does not compete with cGMP for binding at the allosteric sites. Both sildenafil and zaprinast are competitive inhibitors of PDE5, and double-inhibition analysis shows that these two inhibitors added together interact with the catalytic site of PDE5 in a mutually exclusive manner. After site-directed mutagenesis of each of 23 conserved amino acid residues in the catalytic domain of PDE5, the pattern of changes in the IC50 values for sildenafil or UK-122764 is similar to that found for zaprinast. However, among the three inhibitors, sildenafil exhibits the most similar pattern of changes in the IC50 to that found for the affinity of cGMP, implying similar interactions with the catalytic domain. This may explain in part the stronger inhibitory potency of sildenafil for wild-type PDE5 compared with the other inhibitors [sildenafil (Ki = 1 nM) > UK-122764 (Ki = 5 nM) > zaprinast (Ki = 130 nM)]. The affinity of each of these inhibitors for PDE5 is much higher than that of cGMP itself (Km = 2000 nM). It is concluded that residues such as Tyr602, His607, His643, and Asp754 may form important interactions for sildenafil in PDE5, but because these amino acids are conserved in all mammalian PDEs, the selectivity and potency of sildenafil is likely to be provided by a nonconserved residue or residues in the PDE5 catalytic domain.

Protective effect of perindoprilat in the hypoxemia-induced renal dysfunction in the newborn rabbit

The renal effects of acute hypoxemia and the ability of perindoprilat, a potent angiotensin-converting enzyme inhibitor, to prevent these effects were assessed in 31 anesthetized and mechanically ventilated newborn (5 to 8 d of age) rabbits. Renal blood flow (RBF) and GFR were determined by the clearances of para-aminohippuric acid and inulin, respectively. Each animal acted as its own control. In eight normoxemic rabbits (group 1), the i.v. infusion of saline did not change renal and hemodynamic functions. In eight additional rabbits, acute hypoxemia (PaO2= 40 mm Hg) induced a significant decrease in mean blood pressure (-8+/-2%), RBF (-36+/-3%), and GFR (-31+/-3%) and an increase in renal vascular resistance (+50+/-12%). A third group of newborn animals (n=7) was used to determine the renal effects of perindoprilat administration (20 microg/kg) under normoxemic conditions. RBF significantly increased (+15+/-2%) and renal vascular resistance significantly decreased (-15+/-3%), whereas GFR, mean blood pressure, and filtration fraction did not change significantly. In group 4 (n=7), perindoprilat infusion completely prevented the hypoxemia-induced alterations in GFR and renal vascular resistance and partially prevented the fall in RBF. These results demonstrate that angiotensin II modulates the renal immature microcirculation and that inhibition of its formation effectively prevents the hypoxemia-induced decrease in GFR.