Ghrelin and Motilin Depolarize Pacemaker Potentials in Murine Intestinal Cells via the Ghrelin Receptor
Background
The gastrointestinal (GI) tract's motility is crucial for digestion, relying on rhythmic contractions orchestrated by pacemaker cells known as interstitial cells of Cajal (ICCs). These specialized cells generate spontaneous electrical depolarizations, called pacemaker potentials, which set the rhythm for smooth muscle contraction. Disruptions in ICC function are linked to various GI motility disorders. Understanding the hormonal regulation of ICCs is key to developing new treatments. Ghrelin, known for its role in appetite, and motilin are both peptides that influence GI motility, but their precise mechanism of action directly on ICC pacemaker activity was not fully elucidated. This study investigates how these two peptides modulate the electrical behavior of ICCs to better understand their role in controlling gastrointestinal motility.
Study Design
Researchers isolated interstitial cells of Cajal (ICCs) from the small intestines of mice and cultured them. They used the whole-cell patch-clamp technique to measure the electrical pacemaker potentials of these cells. The study examined the effects of applying ghrelin and motilin in a dose-dependent manner. To identify the specific signaling pathway, they co-administered various pharmacological inhibitors, including the ghrelin receptor antagonist [D-Lys] GHRP-6, G protein inhibitors, calcium regulators (Ca2+free solution, thapsigargin), IP3 receptor inhibitors (2-aminoethoxydiphenyl borate, xestospongin C), a Rho kinase inhibitor (Y-27632), and several PKC inhibitors (staurosporine, Go6976, rottlerin).
Results
Both ghrelin and motilin were found to depolarize the pacemaker potentials of ICCs in a dose-dependent manner. The effects of ghrelin were completely blocked by the ghrelin receptor antagonist [D-Lys] GHRP-6, confirming that the action is mediated through the ghrelin receptor. The ghrelin-induced depolarization was also inhibited by blocking the G protein signaling cascade (using intracellular guanosine 5'-diphosphate-β-S). Furthermore, the effect was dependent on both intracellular and extracellular calcium, as it was prevented by pre-treatment with a Ca2+free solution or thapsigargin (an inhibitor of intracellular Ca2+ release).
A key finding was that motilin's ability to depolarize ICC pacemaker potentials was also completely inhibited by [D-Lys] GHRP-6, suggesting that motilin exerts its effect on these specific cells through the ghrelin receptor, not its own classical motilin receptor. The signaling pathway downstream of receptor activation was further dissected. Inhibitors of the
IP3receptor,Rho kinase, andPKC(except for rottlerin) all successfully blocked the ghrelin-induced depolarization, confirming the involvement of theG protein-IP3-Rho kinase-PKCsignaling axis.
Key Findings
- Ghrelin and motilin depolarized pacemaker potentials of murine ICCs in a dose-dependent manner.
- The ghrelin receptor antagonist [D-Lys] GHRP-6 completely inhibited the depolarization induced by both ghrelin and motilin.
- Ghrelin's action is dependent on G protein signaling and requires both intracellular and extracellular Ca2+.
- The signaling cascade involves inositol triphosphate (IP3), Rho kinase, and protein kinase C (PKC).
- Motilin appears to act through the ghrelin receptor, not its own, to depolarize ICCs in the murine small intestine.
Why It Matters
This research clarifies the direct mechanism by which ghrelin and motilin influence gut rhythm at the cellular level, suggesting they act as key modulators of gastrointestinal motility by directly targeting ICC pacemaker cells. For individuals using peptides to enhance gut function or motility, this provides a mechanistic basis for the prokinetic effects of ghrelin mimetics. The discovery that motilin can act through the ghrelin receptor in these cells is particularly significant, revealing a potential crosstalk or redundancy in gut signaling pathways. This implies that targeting the ghrelin receptor system could be a viable strategy for managing GI motility disorders. While this is an in-vitro study on murine cells, it lays the groundwork for developing more targeted therapies. The findings suggest that peptides like GHRP-6, often used for growth hormone stimulation, may have direct and significant effects on gut pacing and motility, a factor to consider in any usage protocol.
