Depression by relaxin of neurally induced contractile responses in the mouse gastric fundus

Nitric Oxide: From Gastric Motility to Gastric Dysmotility

It is known that nitric oxide (NO) plays a key physiological role in the control of gastrointestinal (GI) motor phenomena. In this respect, NO is considered as the main non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmitter responsible for smooth muscle relaxation. Moreover, many substances (including hormones) have been reported to modulate NO production leading to changes in motor responses, further underlying the importance of this molecule in the control of GI motility. An impaired NO production/release has indeed been reported to be implicated in some GI dysmotility. In this article we wanted to focus on the influence of NO on gastric motility by summarizing knowledge regarding its role in both physiological and pathological conditions. The main role of NO on regulating gastric smooth muscle motor responses, with particular reference to NO synthases expression and signaling pathways, is discussed. A deeper knowledge of nitrergic mechanisms is important for a better understanding of their involvement in gastric pathophysiological conditions of hypo- or hyper-motility states and for future therapeutic approaches. A possible role of substances which, by interfering with NO production, could prove useful in managing such motor disorders has been advanced.

Relaxin influences ileal muscular activity through a dual signaling pathway in mice

AIM

To investigate the signaling pathways involved in the relaxin (RLX) effects on ileal preparations from mice through mechanical and electrophysiological experiments.

METHODS

For mechanical experiments, ileal preparations from female mice were mounted in organ baths containing Krebs-Henseleit solution. The mechanical activity was recorded via force-displacement transducers, which were coupled to a polygraph for continuous recording of isometric tension. Electrophysiological measurements were performed in current- and voltage-clamp conditions by a microelectrode inserted in a single smooth muscle cell (SMC) of the ileal longitudinal layer. Both the membrane passive properties and inward voltage-dependent L-type Ca²⁺ currents were recorded using suitable solutions and voltage stimulation protocols.

RESULTS

Mechanical experiments showed that RLX induced a decay of the basal tension and a reduction in amplitude of the spontaneous contractions. The effects of RLX were partially reduced by 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one (ODQ) or 9-cyclopentyladenine mesylate (9CPA), inhibitors of guanylate cyclase (GC) and adenylate cyclase (AC), respectively, and were abolished in the concomitant presence of both drugs. Electrophysiological experiments demonstrated that RLX directly influenced the biophysical properties of ileal SMCs, decreasing the membrane conductance, hyperpolarizing the resting membrane potential, reducing the L-type calcium current amplitude and affecting its kinetics. The voltage dependence of the current activation and inactivation time constant was significantly speeded by RLX. Each electrophysiological effect of RLX was reduced by ODQ or 9CPA, and abolished in the concomitant presence of both drugs as observed in mechanical experiments.

CONCLUSION

Our new findings demonstrate that RLX influences ileal muscle through a dual mechanism involving both GC and AC.

Relaxin Affects Smooth Muscle Biophysical Properties and Mechanical Activity of the Female Mouse Colon

The hormone relaxin (RLX) has been reported to influence gastrointestinal motility in mice. However, at present, nothing is known about the effects of RLX on the biophysical properties of the gastrointestinal smooth muscle cells (SMCs). Other than extending previous knowledge of RLX on colonic motility, the purpose of this study was to investigate the ability of the hormone to induce changes in resting membrane potential (RMP) and on sarcolemmal ion channels of colonic SMCs of mice that are related to its mechanical activity. To this aim, we used a combined mechanical and electrophysiological approach. In the mechanical experiments, we observed that RLX caused a decay of the basal tone coupled to an increase of the spontaneous contractions, completely abolished by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one (ODQ). The electrophysiological results indicate for the first time that RLX directly affects the SMC biophysical properties inducing hyperpolarization of RMP and cycles of slow hyperpolarization/depolarization oscillations. The effects of RLX on RMP were abolished by ODQ as well as by a specific inhibitor of the cGMP-dependent protein kinase, KT5823. RLX reduced Ca(2+) entry through the voltage-dependent L-type channels and modulated either voltage- or ATP-dependent K(+) channels. These effects were abolished by ODQ, suggesting the involvement of the nitric oxide/guanylate cyclase pathway in the effects of RLX on RMP and ion channel modulation. These actions of RLX on membrane properties may contribute to the regulation of the proximal colon motility by the nitric oxide/cGMP/cGMP-dependent protein kinase pathway.

Pretreatment with Relaxin Does Not Restore NO-Mediated Modulation of Calcium Signal in Coronary Endothelial Cells Isolated from Spontaneously Hypertensive Rats

We demonstrated that in coronary endothelial cells (RCEs) from normotensive Wistar Kyoto rats (WKY), the hormone relaxin (RLX) increases NO production and reduces calcium transients by a NO-related mechanism. Since an impairment of the NO pathway has been described in spontaneously hypertensive rats (SHR), the present study was aimed at exploring RLX effects on RCEs from SHR, hypothesizing that RLX could restore calcium responsiveness to NO. RCEs were isolated from WKY and SHR. Calcium transients were evaluated by image analysis after the administration of angiotensin II or α-thrombin. Angiotensin II (1 µM) caused a prompt rise of [Ca²⁺]i in WKY and SHR RCEs and a rapid decrease, being the decay time higher in SHR than in WKY. NOS inhibition increased calcium transient in WKY, but not in SHR RCEs. Whereas RLX pretreatment (24 h, 60 ng/mL) was ineffective in SHR, it strongly reduced calcium transient in WKY in a NO-dependent way. A similar behavior was measured using 30 U/mL α-thrombin. The current study offers evidence that RLX cannot restore NO responsiveness in SHR, suggesting an accurate selection of patients eligible for RLX treatment of cardiovascular diseases.

Relaxin exerts two opposite effects on mechanical activity and nitric oxide synthase expression in the mouse colon

The hormone relaxin exerts a variety of functions on the smooth muscle of reproductive and nonreproductive organs, most of which occur through a nitric oxide (NO)-mediated mechanism. In the stomach and ileum, relaxin causes muscle relaxation by modulating the activity and expression of different nitric oxide synthase (NOS) isoforms region-dependently. Nothing is known on the effects of relaxin in the colon, the gut region expressing the highest number of neuronal (n) NOSβ-immunoreactive neurons and mainly involved in motor symptoms of pregnancy and menstrual cycle. Therefore, we studied the effects of relaxin exposure in the mouse proximal colon in vitro evaluating muscle mechanical activity and NOS isoform expression. The functional experiments showed that relaxin decreases muscle tone and increases amplitude of spontaneous contractions; the immunohistochemical results showed that relaxin increases nNOSβ and endothelial (e) NOS expression in the neurons and decreases nNOSα and eNOS expression in the smooth muscle cells (SMC). We hypothesized that, in the colon, relaxin primarily increases the activity and expression of nNOSβ and eNOS in the neurons, causing a reduction of the muscle tone. The downregulation of nNOSα and eNOS expression in the SMC associated with increased muscle contractility could be the consequence of continuous exposue of these cells to the NO of neuronal origin. These findings may help to better understand the physiology of NO in the gastrointestinal tract and the role that the "relaxin-NO" system plays in motor disorders such as functional bowel disease.

Relaxin counteracts the altered gastric motility of dystrophic (mdx) mice: functional and immunohistochemical evidence for the involvement of nitric oxide

Impaired gastric motility ascribable to a defective nitric oxide (NO) production has been reported in dystrophic (mdx) mice. Since relaxin upregulates NO biosynthesis, its effects on the motor responses and NO synthase (NOS) expression in the gastric fundus of mdx mice were investigated. Mechanical responses of gastric strips were recorded via force displacement transducers. Evaluation of the three NOS isoforms was performed by immunohistochemistry and Western blot. Wild-type (WT) and mdx mice were distributed into three groups: untreated, relaxin pretreated, and vehicle pretreated. In strips from both untreated and vehicle-pretreated animals, electrical field stimulation (EFS) elicited contractile responses that were greater in mdx than in WT mice. In carbachol-precontracted strips, EFS induced fast relaxant responses that had a lower amplitude in mdx than in WT mice. Only in the mdx mice did relaxin depress the amplitude of the neurally induced excitatory responses and increase that of the inhibitory ones. In the presence of L-NNA, relaxin was ineffective. In relaxin-pretreated mdx mice, the amplitude of the EFS-induced contractile responses was decreased and that of the fast relaxant ones was increased compared with untreated mdx animals. Responses to methacholine or papaverine did not differ among preparations and were not influenced by relaxin. Immunohistochemistry and Western blotting showed a significant decrease in neuronal NOS expression and content in mdx compared with WT mice, which was recovered in the relaxin-pretreated mdx mice. The results suggest that relaxin is able to counteract the altered contractile and relaxant responses in the gastric fundus of mdx mice by upregulating nNOS expression.

Role of nitric oxide produced by constitutive and inducible nitric oxide synthases in the mouse gastric fundus

In the present study, the role of nitric oxide (NO) produced by constitutive and inducible nitric oxide synthases (cNOS and iNOS, resepctively) on the contraction and relaxation of fundus in normal and lipopolysaccharide (LPS)-treated mice was examined. A whole fundic ring isolated from mice pretreated with reserpine was mounted in an organ bath containing Krebs' solution with 0.001 mmol/L atropine. Rings were contracted initially by 5-hydroxytryptamine (5-HT; 0.03 mmol/L) before relaxation was induced using ATP (0.03 mmol/L), ADP (0.03 mmol/L), pentoxifylline (0.002 mmol/L), electrical field stimulation (EFS; 50 V, 1 msec, 50 Hz, 3 min) and L-arginine (0.05 mmol/L). All drugs and EFS induced significant relaxation of isolated rings. The relaxations induced were significantly inhibited by N(G)-nitro-L-arginine methyl ester (L-NAME; 1.0 mmol/L). However, the iNOS inhibitors L-N(6)-(1-iminoethyl) lysine hydrochloride (L-NIL; 1.0 mmol/L) and amino guanidine (AMG; 1.0 mmol/L) had no significant effect on tissue relaxation. Then, the relaxant effects of 0.03 mmol/L ATP were tested on precontracted isolated fundic rings taken from 10 mg/kg LPS-treated animals. The non-selective NOS inhibitor L-NAME (10 mg/kg), the iNOS inhibitors L-NIL (3 mg/kg) and AMG (20 mg/kg) and betamethasone (0.1 mg/kg) were used to examine the role of NO produced by iNOS in the relaxation responses. It was found that the level of contraction induced by 0.03 mmol/L 5-HT in rings isolated from LPS-treated animals was significantly (P < 0.5) less than that in rings from untreated mice. However, precontracted tissues from LPS-treated mice were significantly relaxed by ATP and the relaxation response to ATP was significantly inhibited by L-NIL, ANG and betamethasone, but not by L-NAME. We suggest that, in LPS-treated mice, the production of NO from iNOS produces a reduction in the contractile response, as well as a decrease in NO formation by cNOS, resulting in changes to smooth muscle cell function.

Influence of orexin A on the mechanical activity of mouse gastric strips

The presence of orexins and orexin receptors has been revealed not only in the central nervous system but also in the gastrointestinal tract. The present study was aimed to investigate the influence of orexin A (OXA) on the mechanical activity of fundal and antral strips of the mouse stomach. In the fundus, electrical field stimulation (EFS) elicited tetrodotoxin (TTX)-sensitive, frequency-dependent contractile responses whose amplitude was markedly reduced by OXA and enhanced by the orexin-1 type receptor antagonist SB-334867. In the presence of the NO synthesis inhibitor L-N(G)-nitro arginine (L-NNA), OXA was no longer effective. Methacholine caused a sustained contracture whose amplitude was not influenced by OXA, TTX or L-NNA. In carbachol-precontracted strips, the neurally-induced relaxant responses elicited during EFS were increased in amplitude by OXA. Antral strips showed a spontaneous contractile activity that was unaffected by TTX or L-NNA and transiently depressed by EFS. OXA did not influence either the spontaneous motility or the EFS-induced effects. The results indicate that OXA exerts region-specific effects and that, in the fundus, depresses EFS-induced contractile responses by acting at the nervous level. It is likely that NO is involved in the effects of the peptide.

Reversal by relaxin of altered ileal spontaneous contractions in dystrophic (mdx) mice through a nitric oxide-mediated mechanism

Altered nitric oxide (NO) production/release is involved in gastrointestinal motor disorders occurring in dystrophic (mdx) mice. Since the hormone relaxin (RLX) can upregulate NO biosynthesis, its effects on spontaneous motility and NO synthase (NOS) expression in the ileum of dystrophic (mdx) mice were investigated. Mechanical responses of ileal preparations were recorded in vitro via force-displacement transducers. Evaluation of the expression of NOS isoforms was performed by immunohistochemistry and Western blot. Normal and mdx mice were distributed into three groups: untreated, RLX pretreated, and vehicle pretreated. Ileal preparations from the untreated animals showed spontaneous muscular contractions whose amplitude was significantly higher in mdx than in normal mice. Addition of RLX, alone or together with l-arginine, to the bath medium depressed the amplitude of the contractions in the mdx mice, thus reestablishing a motility pattern typical of the normal mice. The NOS inhibitor N(G)-nitro-L-arginine (L-NNA) or the guanylate cyclase inhibitor ODQ reversed the effects of RLX. In RLX-pretreated mdx mice, the amplitude of spontaneous motility was reduced, thus resembling that of the normal mice, and NOS II expression in the muscle coat was increased in respect to the vehicle-pretreated mdx animals. These results indicate that RLX can reverse the altered ileal motility of mdx mice to a normal pattern, likely by upregulating NOS II expression and NO biosynthesis in the ileal smooth muscle.

Relaxin restores altered ileal spontaneous contractions in dystrophic (mdx) mice

We studied the effects of relaxin on ileal contractility in normal and dystrophic (mdx) mice. Ileal preparations from male normal and mdx mice showed spontaneous myogenic contractions whose amplitude was significantly higher in the latter ones. Relaxin added to the bath medium together with l-arginine depressed the amplitude of the spontaneous contractions in the mdx mice to a level similar to that of the normal mice. The nitric oxide synthase (NOS) inhibitor L-N(G)-nitroarginine reverted this effect. In mdx mice pretreated for 18 hours with relaxin, spontaneous motility was greatly reduced in amplitude, resembling that of the normal mice. Concurrently, iNOS expression in the muscle coat was markedly increased. Therefore, in mdx mice, relaxin can restore an ileal motility pattern similar to that of the normal mice by upregulating endogenous NO biosynthesis.

Influence of relaxin on the neurally induced relaxant responses of the mouse gastric fundus

The peptide hormone relaxin has been reported to depress the amplitude of contractile responses in the mouse gastric fundus by upregulating nitric oxide (NO) biosynthesis at the neural level. In the present study, we investigated whether relaxin also influenced nonadrenergic, noncholinergic (NANC) gastric relaxant responses in mice. Female mice in proestrus or estrus were treated for 18 h with relaxin (1 microg s.c.) or vehicle (controls). Mechanical responses of gastric fundal strips were recorded via force-displacement transducers. In carbachol precontracted strips from control mice and in the presence of guanethidine, electrical field stimulation (EFS) elicited fast relaxant responses that may be followed by a sustained relaxation. All relaxant responses were abolished by tetrodotoxin. Relaxin increased the amplitude of the EFS-induced fast relaxation without affecting either the sustained one or the direct smooth muscle response to papaverine. In the presence of the NO synthesis inhibitor L-N(G)-nitro arginine (L-NNA), that abolished the EFS-induced fast relaxation without influencing the sustained one, relaxin was ineffective. In strips from relaxin-pretreated mice, EFS-induced fast relaxations were enhanced in amplitude with respect to the controls, while sustained ones as well as direct smooth muscle responses to papaverine were not changed. Further addition of relaxin to the bath medium did not influence neurally induced fast relaxant responses, whereas L-NNA did. In conclusion, in the mouse gastric fundus, relaxin enhances the neurally induced nitrergic relaxant responses acting at the neural level.