Effect Of HCN1 On The Pace Generation Of The Gastrointestinal Tract

Hypodynamia of gastrointestinal (GI) tract is the most common dynamic disorder among gastrointestinal diseases. The mechanism of hypodynamia lies on reduced peristalsis speed and rhythm disorder. Recent literature reported that the pacemaker of GI tract originated from the interstitial cells of Cajal (ICCs). ICCs are cells that generate rhythmic electric activity of GI-tract smooth muscle, slow wave (SW) which propagates along the network of ICCs and reaches smooth muscle cells via gap junctions between ICC and smooth muscle cell in the end, such as to induce contraction of the smooth muscle periodically. Auxology and gene mutation techniques have documented that abnormal ICC development occurred in c-kit gene ablation or spontaneous mutation w/wv mice, or by inactivation of Kit with anti-Kit antibody, therefore lead to the GI tract disorder of rhythmic peristalsis or rhythm disappearance. So, ICCs are currently recognized as a pacemaker of GI tract dynamia and the regulator of the pace. However, the mechanism of generation of the rhythmic electric activity is not fully known, which seriously hinders the treatment and therapy to multiple GI dynamic diseases in clinic. HCN, abbreviation for hyperpolarization-activated cyclic nucleotide-gated cation non-selective channel, a sort of pace-origin channels, widely distribute in the cells or neurons in charge of rhythmic electric activity in sinoatrial node, atrio-ventricular node, Purkinje fibers, respiration centre in the medulla oblongata and the hippocampus. They are the most clearly understood ion channels associated with generation of spontaneous rhythmic electric activities, and are regulated by classical neurotransmitter such as catecholamine (norepinephrine), Ach, etc, and are modulated by brain-gut peptide which affects intracellular cAMP concentration. Inward current generated by this channel opening, called If, is the main component of the starting inward current by auto rhythm generating cells. As the nature of the GI dynamic pacemaker is similar to that of heart, we wondered whether there were HCN distributing along the GI tract, and their role in dynamic rhythm generation. As we know, there has not been any report on these questions in the world up to now, and the mechanisms of the GI tract dynamic pace generation are unknown. Therefore, we took advantage of the research on mechanisms of cardiac pace generation and slow wave rhythm modulation in respiration center, to see the role of HCN1, a subtype of HCN, in GI tract dynamic pace generation. This research may provide theoretical evidence for the mechanism of GI tract dynamic pace generation, and for the novel drug development.Results1. Distribution of HCN1 in GI tract.(1) HCN1-immunoreactive nanogold labeled ICC usually distributed in cluster around myenteric nerve plexus, the cytoplasmic processes surrounding the nerve bundle formed incomplete"sheath-like structures". Both APUD cells in GI tract and smooth muscle cells were HCN-negative. These data indicate that HCN1 distributes specifically in ICCs.(2) HCN1 widely distributed in gastric antrum, jejunoileum ICC-MY andICC-SM, particularly intensively in ICC-MY. All the HCN1 co-existed with ICC marker, c-Kit. These findings showed HCN1 existed specifically in ICC of GI tract.2. Change of HCN1 during the development of the rat.HCN1 expression in the rat gastric antrum changed during E13-P14. There was just trace expression in gastric antrum of the rat before birth (E13-18). Then the expression increased dramatically after birth (P1-14), especially after P7, significantly increased, and reached adult level at P14. The expression of HCN1 is significantly during the key period of generating rhythmic peristalsis of the rat.3. Effect of HCN1 on electrophysiological activitiesContraction of the GI smooth muscle strip was tested under normal condition in vitro in premise that the muscle strip was able to contract. Muscle strip contraction altered significantly when HCN1 receptor agonist, Alinidine, or HCN1 antagonist ZD7288 was treated. These results suggest that HCN1 might be the starting channel for spontaneous electric rhythem.Conclusion:Specific distribution in ICC and significant increase of expression during key period of generating rhythmic peristalsis of the rat GI tract, show the coincidence of timing that the HCN1 protein expression and generation of the rat GI tract rhythmatic peristalsis; HCN1 distributes specifically in Cajal cells, and the contraction of GI smooth muscle could be affected significantly by HCN1 functional interruption. All together, our findings suggest that HCN1 might be the starting channel for spontaneous electric rhythm. Therefore, HCN1 could be of importance in rhythmic electric activity generation of ICC.