Mecanismos responsables de la relajación neuromuscular en el tracto gastrointestinal / Mechanisms responsible for neuromuscular relaxation in the gastrointestinal tract

El sistema nervioso entérico (SNE) es responsable de la génesis de los patrones motores que aseguran un correcto tránsito intestinal. Las neuronas entéricas se clasifican en aferentes, interneuronas y motoneuronas, que pueden a su vez ser excitatorias, causando contracción, o inhibitorias, provocando la relajación de la musculatura lisa. Los mecanismos de relajación muscular son claves para entender procesos fisiológicos como la relajación de los esfínteres, la acomodación gástrica o la fase descendente del reflejo peristáltico. El óxido nítrico (NO) y el ATP o una purina relacionada son los principales neurotransmisores inhibitorios. Las neuronas nitrérgicas sintetizan NO a partir del enzima nNOS. El NO difunde a través de la membrana celular uniéndose a su receptor, la guanilil ciclasa, y activando posteriormente una serie de mecanismos intracelulares que provocan finalmente una relajación muscular. El ATP actúa como neurotransmisor inhibitorio junto con el NO y el receptor de membrana purinérgico P2Y1 ha sido identificado como elemento clave para entender cómo el ATP relaja la musculatura intestinal. Aunque probablemente ningún clínico duda de la importancia del NO en la fisiopatología motora digestiva, la relevancia de la neurotransmisión purinérgica es aparentemente mucho menor puesto que el ATP no ha sido todavía asociado a una disfunción motora concreta. El objetivo de esta revisión es mostrar el funcionamiento de ambos mecanismos de relajación para poder establecer las bases fisiológicas de posibles disfunciones motoras asociadas a la alteración de la relajación intestinal (AU)

The enteric nervous system (ENS) is responsible for the genesis of motor patterns ensuring an appropriate intestinal transit. Enteric neurons are classified into afferent, interneuron, and motoneuron types, with the latter two being further categorized as excitatory or inhibitory, which cause smooth muscle contraction or inhibition, respectively. Muscle relaxation mechanisms are key for the understanding of physiological processes such as sphincter relaxation, gastric accommodation, or descending peristaltic reflex. Nitric oxide (NO) and ATP or a related purine represent the primary inhibitory neurotransmitters. Nitrergic neurons synthesize NO through nNOS enzyme activity. NO diffuses across the cell membrane to bind its receptor, namely, guanylyl cyclase, and then activates a number of intracellular mechanisms that ultimately result in muscle relaxation. ATP acts as an inhibitory neurotransmitter together with NO, and the purinergic P2Y1 membrane receptor has been identified as a key item in order to understand how ATP may relax intestinal smooth muscle. Although, probably, no clinician doubts the significance of NO in the pathophysiology of digestive motility, the relevance of purinergic neurotransmission is apparently much lower, as ATP has not been associated with any specific motor dysfunction yet. The goal of this review is to discuss the function of both relaxation mechanisms in order to establish the physiological grounds of potential motor dysfunctions arising from impaired intestinal relaxation (AU)

Purinergic and nitrergic neuromuscular transmission mediates spontaneous neuronal activity in the rat colon.

Nitric oxide (NO) and ATP mediate smooth muscle relaxation in the gastrointestinal tract. However, the involvement of these neurotransmitters in spontaneous neuronal activity is unknown. The aim of the present work was to study spontaneous neuromuscular transmission in the rat midcolon. Microelectrode experiments were performed under constant stretch both in circular and longitudinal directions. Spontaneous inhibitory junction potentials (sIJP) were recorded. Tetrodotoxin (1 microM) and apamin (1 microM) depolarized smooth muscle cells and inhibited sIJP. N(omega)-nitro-l-arginine (l-NNA, 1 mM) depolarized smooth muscle cells but did not modify sIJP. In contrast, the P2Y(1) antagonist MRS-2500 (1 microM) did not modify the resting membrane potential (RMP) but reduced sIJP (IC(50) = 3.1 nM). Hexamethonium (200 microM), NF-023 (10 microM), and ondansetron (1 microM) did not modify RMP and sIJP. These results correlate with in vitro (muscle bath) and in vivo (strain gauges) data where l-NNA but not MRS-2500 induced a sustained increase of spontaneous motility. We concluded that, in the rat colon, inhibitory neurons regulate smooth muscle RMP and cause sIJP. In vitro, the release of inhibitory neurotransmitters is independent of nicotinic, P2X, and 5-hydroxytryptamine type 3 receptors. Neuronal NO causes a sustained smooth muscle hyperpolarization that is responsible for a constant inhibition of spontaneous motility. In contrast, ATP acting on P2Y(1) receptors is responsible for sIJP but does not mediate inhibitory neural tone. ATP and NO have complementary physiological functions in the regulation of gastrointestinal motility.

Purinergic and nitrergic junction potential in the human colon.

The aim of the present work is to investigate a putative junction transmission [nitric oxide (NO) and ATP] in the human colon and to characterize the electrophysiological and mechanical responses that might explain different functions from both neurotransmitters. Muscle bath and microelectrode techniques were performed on human colonic circular muscle strips. The NO donor sodium nitroprusside (10 microM), but not the P2Y receptor agonist adenosine 5'-O-2-thiodiphosphate (10 microM), was able to cause a sustained relaxation. NG-nitro-L-arginine (L-NNA) (1 mM), a NO synthase inhibitor, but not 2'-deoxy-N6-methyl adenosine 3',5'-diphosphate tetraammonium salt (MRS 2179) (10 microM), a P2Y antagonist, increased spontaneous motility. Electrical field stimulation (EFS) at 1 Hz caused fast inhibitory junction potentials (fIJPs) and a relaxation sensitive to MRS 2179 (10 microM). EFS at higher frequencies (5 Hz) showed biphasic IJP with fast hyperpolarization sensitive to MRS 2179 followed by sustained hyperpolarization sensitive to L-NNA; both drugs were needed to fully block the EFS relaxation at 2 and 5 Hz. Two consecutive single pulses induced MRS 2179-sensitive fIJPs that showed a rundown. The rundown mechanism was not dependent on the degree of hyperpolarization and was present after incubation with L-NNA (1 mM), hexamethonium (100 microM), MRS 2179 (1 microM), and NF023 (10 microM). We concluded that single pulses elicit ATP release from enteric motor neurons that cause a fIJP and a transient relaxation that is difficult to maintain over time; also, NO is released at higher frequencies causing a sustained hyperpolarization and relaxation. These differences might be responsible for complementary mechanisms of relaxation being phasic (ATP) and tonic (NO).