GI Physiology and Ischemia-Reperfusion Injury MDRs

What makes up the electrical activity of intestinal smooth muscle?

Slow waves and action (spike) potentials

What controls slow waves of intestinal smooth muscle?

Interstitial cells of Cajal

• Serve as a pacemaker

What plexus are the interstitial cells of Cajal associated with?

Myenteric plexus

Slow Waves

Rhythmic, phasic oscillations in smooth muscle resting potentials

By themselves, they do not result in a large enough change in membrane potential to lead to a muscle contraction, but rather lower the threshold for other factors (i.e. stretch, hormones, medications) to result in action potentials and contraction

What determines the maximum rate at which intestinal contractions can occur?

Spike potentials

When do action potentials in the intestines occur?

When in coordination with slow waves, something (i.e. stretch, hormones, medications) lowers the threshold of smooth muscle cells allowing the slow wave to reach threshold causing a rapid depolarization

Migrating Motor Complexes

Small intestinal activity that occurs during periods of fasting or between meals

• Horses are grazers so they have MMCs all the times

Four phases of MMCs

Phase I: period of quiescence

Phase II: Increased action potentials and smooth muscle activity

Phase III: Peak electrical activity and mechanical activity (lasts 5-10 minutes)

Phase IV: Declining activity that leads to phase I

Functions of MMCs

Facilitate transportation of indigestible objects from the stomach to the colon

Prevent overgrowth of bacteria in the small intestine during periods of fasting or between meals

What interrupts MMCs?

Meals

Narcotis, atropine, a2 agonists, stress, and some bacterial toxins

What can induce phase III of MMCs?

Serotonin and serotonin receptor agonists and motilin

What makes up the enteric nervous system?

Ganglia in the myenteric (Auerbach) plexus and submucosal (Meissner) plexus

What are the main source of innervation of longitudinal muscle and outer lamella of circular muscle?

Myenteric neurons

What do the inhibitor neurons of the myenteric plexus do?

Inhibit intestinal sphincter muscles which impedes movement of digesta resulting in increased mixing

What do submucosal neurons innervate?

Inner lamella of circular muscle

What is the main role of the submucosal plexus?

To help control local segmental intestinal secretion, local absorption, and local contraction of the submucosal muscle to help with mixing

Three Primary Types of GI Reflexes

Reflexes that occur entirely within the GI wall enteric nervous system

• These reflexes are responsible for controlling local GI secretion, peristalsis, mixing, contractions, and local inhibitory effects

Reflexes from the GI tract to prevertebral sympathetic ganglia and back to the GI tract

• e.g. gastrocolic reflex - occurs when the stomach fills or stretches which results in colonic contraction (and often subsequent defecation)

Reflexes from the GI tract to the spinal cord or brainstem and back to the GI tract

• e.g. communication between the duodenum to the brainstem and back to the stomach via the vagus nerve to affect gastric motility, pain resulting in decreased GI motility, and stretch of the colon or rectum which is signaled to the spinal cord and leads to colonic, rectal, and abdominal contraction to promote defecation

Stimuli for Secretion of Gastrin

Protein

Gastric distension

(Acid inhibits release of gastrin)

Site of Secretion of Gastrin

G cells located in the stomach, duodenum, and jejunum

What does gastrin stimulate?

Gastric acid secretion

Mucosal growth/repair

Stimuli for Secretion of Cholecystokinin (CCK)

Protein

Fat and fat byproducts such as fatty acids, monoglycerides

Acid

Site of Secretion of Cholecystokinin (CCK)

I cells of the duodenum, jejunum, and ileum

Cholecystokinin Stimulates

Pancreatic enzyme and bicarbonate secretion

Gall bladder contraction to digest fat

Growth of exocrine pancreas

Cholecystokinin Inhibits

Appetite

Gastric acid secretion

Gastric emptying to give time for digestion

Stimuli for Secretion of Gastric Inhibitory Peptide (GIP)

Protein

Fat

Carbohydrate

Site of Secretion of Gastric Inhibitory Peptide (GIP)

K cells of the duodenum and jejunum

Gastric Inhibitory Peptide Stimulates

Insulin release

Gastric Inhibitory Peptide (GIP) Inhibits

Gastric acid secretion

Gastric emptying

Stimuli for Secretion of Motilin

Fat

Acid

Nerve

Site of Secretion of Motilin

M cells of duodenum and jejunum

Motilin Stimulates

Gastric motility

Intestinal motility

Stimuli for Secretion of Secretin

Fat

Acid

Site of Secretion of Secretin

S cells of duodenum, jejunum, and ileum

Secretin Stimulates

Pepsin secretion

Pancreatic and biliary bicarbonate secretion

Growth of exocrine pancreas

Secretin Inhibits

Gastric acid secretion

Stimuli for Secretion of Somatostatin

Acid

Site of Secretion of Somatostatin

D cells int he pancreatic islets

Somatostatin Inhibits

Secretion of gastrin, VIP, GIP, secretin, and motilin

Pancreatic exocrine secretion

Gastric acid secretion and motility

Gallbladder contraction

Absorption of amino acids and triglycerides

Gastric Contractions

Most are relatively weak

• Purpose is to promote mixing of gastric contents to aid in digestion and breakdown

20% are strong enough to result in peristaltic action leading to gastric emptying

What alters the rate of gastric emptying?

Largely altered (in an inhibitory fashion) by signals from the duodenum to improve digestion

Inhibitory signals include duodenal distension, irritation of the duodenal mucosa, increased activity of duodenal chyme, increased osmolality of duodenal chyme, and digestive breakdown products in chyme

What increases small intestinal peristalsis?

Gastoenteric reflex

GI hormones (gastrin, CCK, motilin, serotonin, insulin)

Intestinal stretch

MOA of Cisapride, Mosapride, Tegaserod

5-HT4 agonist (serotoniergic drug)

Cisapride, Mosapride, Tegaserod Adverse Effects, Contraindications

Cardiac abnormalities

CP450 metabolism

Cisapride, Mosapride, Tegaserod Comments

Does not cross the blood brain barrier - no anti-emetic or extrapyramidal effects

Pan-GI effects

Metaclopramide MOA

Central dopaminergic and peripheral 5-HT3 antagonist

Metaclopramide Adverse Effects, Contraindications

Central dopaminergic effects can result in extrapyramidal effects including restlessness and at times extreme aggression

Metaclopramide Comments

Stimulates and coordinates esophageal, gastric, and duodenal motility

Improves nausea and vomiting due to central activity

Little to no effect on colonic motility

Bethanechol MOA

Patsympathomimetic choline carbamate - stimulates muscarinic receptors (M1-5)

Bethanechol Adverse Effects, Contraindications

Bradycardia

Vasodilation and hypotension

Bethanechol Comments

Stimulates muscarinic receptors to improve gastrointestinal contraction

Ranitidine MOA

H2 receptor antagonist

Ranitidine Adverse Effects, Contraindications

No significant adverse effects reported in animals

Ranitidine Comments

Prokinetics due to Ach-ase inhibition in the proximal GI tract

Questionable effects when used PO in canine studies

Erythromycin (and Other Macrolide Antimicrobials) MOA

Motilin agonist

Erythromycin (and Other Macrolide Antimicrobials) Adverse Effects, Contraindications

Antimicrobial associated colitis

Erythromycin (and Other Macrolide Antimicrobials) Comments

Affects small intestinal motility more than large intestinal

Lidocaine MOA

Inhibits mesenteric and intestinal neutrophil migration

Modulates inflammation through sodium channel blockade and through decreasing neutrophilic infiltration into the intestinal wall

Lidocaine Adverse Effects, Contraindications

Lidocaine toxicity

Lidocaine Comments

Not a true prokinetic - promotes normal motility through decreasing intestinal inflammation

Neostigmine MOA

Inhibition of acetylcholinesterase

Neostigmine Adverse Effects, Contraindications

Salivation, lacrimation, diarrhea, bradycardia, bronchoconstriction

Neostigmine Comments

Improves cecal and large intestinal motility more than small intestinal

Domperidone MOA

D2 dopamine receptor antagonist

Domperidone Adverse Effects, Contraindications

Increases lactation in mares

Domperidone Comments

Improves gastric, jejunum, ileum, and colonic motility

Ischemia

Characterized by a reduction or cessation of blood flow to a tissue or organ

Oxygen and nutrient deprivation and toxin/metabolite buildup

Reperfusion

Reintroducing oxygen and nutrients during reperfusion triggers a series of intricate mechanisms including oxidative stress, inflammation, and cellular injury

Compromised endothelial function during reperfusion promotes vascular leakage and edema formation

Restoring blood flow can induce calcium overload, mitochondrial dysfunction, and the activation of apoptotic pathways, leading to cell death

Reactive Oxygen Species in Reperfusion

Central in exacerbating tissue damage

Sudden influx of oxygen can lead to the production of ROS through enzymatic sources such as xanthine oxidase and NADPH oxidase as well as mitochondrial dysfunction

ROS can cause direct oxidative damage to lipids, proteins, and DNA, triggering a cascade of inflammatory responses and cellular injury

Activate immune cells, such as neutrophils and macrophages, which triggers an inflammatory response contributing to tissue injury

Physiological Consequences of Ischemia

During ischemia, the lack of oxygen and nutrient supply impairs cellular metabolism, accumulating waste products and shifting toward anaerobic respiration

• Results in the production of lactic acid, leading to tissue acidosis

• Depletion of ATP reserves compromises energy-dependent cellular processes, impairing ion channel function (Na+/K+ ATPase, disrupting membrane integrity (Ca2+ dysregulation) and compromising cellular viability

Effect of Ischemia on Mitochondria

Disrupts oxygen supply, impairing their ability to carry out oxidative phosphorylation efficiently

ATP production declines and cells switch to anaerobic metabolism, accumulating lactate and metabolic acidosis

The electron transport chain becomes dysunfctional leading to electron leakage and the generation of ROS within the mitochondria

Effect of Reperfusion on Mitochondria

The sudden reintroduction of oxygen exacerbates mitochondrial dysfunction

ROS overwhelm antioxidant defenses, resulting in oxidative stress

ROS directly damage mitochondrial components, further impairing function

Mitochondrial Permeability Transition Pores (mPTPs)

Protein channels in the inner mitochondrial membrane that regulate the exchange of solutes and ions

mPTPs in Ischemia-Reperfusion Injury

During ischemia, the prolonged depletion of ATP and the accumulation of calcium within the mitochondria lead to the opening of mPTPs

• Results in dissipation of the mitochondrial membrane potential, the release of pro-apoptotic factors from the intermembrane space, and the disruption of cellular energy production

Also triggers the mitochondrial permeability transition (MPT), leading to mitochondrial swelling, reupture, and release of mitochondrial contents into the cytosol

Molecular Mechanisms of Ischemia-Reperfusion Injury - Cellular Energy Depletion and ATP Loss

During ischemia, reduced blood flow hampers oxygen and nutrient supply to tissues, impairing ATP production through oxidative phosphorylation

Decline in ATP compromises energy dependent cellular processes, including ion channel function, membrane integrity, and cellular metabolism

ATP depletion triggers activation of energy sensing pathways such as AMP-activated protein kinase (AMPK)

Molecular Mechanisms of Ischemia-Reperfusion Injury - ROS Generation and Oxidative Stress

Reperfusion leads to the sudden influx of oxygen triggering the generation of ROS

Enzymatic sources such as xanthin oxidase, NADPH oxidase, and dysfunctional mitochondria contribute to ROS production

ROS induce oxidative stress by damaing lipids, proteins and DNA

Molecular Mechanisms of Ischemia-Reperfusion Injury - Inflammatory Response and Immune Activation

Ischemia leads to the release of DAMPs from injured cells which activate PRRs

• Triggers the production of pro-inflammatory cytokines, promoting leukocyte infiltration and amplifying the inflammatory cascade

Molecular Mechanisms of Ischemia-Reperfusion Injury - Endothelial Dysfunction and Vascular Injury

ROS, pro-inflammatory cytokines, and proteases disrupt the endothelial barrier integrity, resulting in increased vascular permeability and edema formation

Endothelial dysfunction also promotes platelet activation, thrombosis, and leukocyte adhesion, further compromising blood flow and exacerbating tissue injury

Molecular Mechanisms of Ischemia-Reperfusion Injury - Calcium Overload and Mitochondrial Dysfunction

Ischemia disrupts calcium homeostasis, leading to intracellular calcium overload upon reperfusion

• Influx of calcium triggers mitochondrial calcium uptake, impairing mitochondrial function

Elevated mitochondrial calcium levels contribute to mPTP opening, which leads to mitochondrial membrane depolarization, cytochrome c release, adn activation of apoptotic pathways

Molecular Mechanisms of Ischemia-Reperfusion Injury - Cell Death Pathways

Activates multiple cell death pathways including apoptosis, necrosis, and programmed necrosis (necroptosis)

Apoptosis primarily mediated through the intrinsic mitochondrial pathway, involving cytochrome c release, apoptosome formation, and caspase activation

Necrosis occurs as a consequence of severe cellular damage and energy depletion

Necroptosis, a regulated form of necrosis, is mediated by receptor-interacting protein kinases (RIPKs) and contributes to tissue injury during ischemia-reperfusion

Molecular Mechanisms of Ischemia-Reperfusion Injury - Inflammatory Mediators and Chemotaxis

Pro-inflammatory cytokines, chemokines, and lipid mediators activate immune cells infiltrating the injured tissue

Neutrophils release proteases, ROS, and other inflammatory molecules, exacerbating tissue injury through collateral damage

Role of ROS During Hypoxia

Involvement in oxygen sensing and the activation of hypoxia-inducible factor (HIF)

• HIF is a transcription factor that plays a central role in cellular adaptation to low oxygen levels

• Under normoxic conditions, HIF is hydroxylated by prolyl hydroxylase enzymes, targeting it for degradation

• Under hypoxia, the production of ROS inhibits prolyl hydroxylase activity, stabilizing HIF and allowing it to translocate to the nucleus

  • HIF induces transcription of genes involved in angiogenesis, glycolysis, erythropoiesis, and other adaptive resonses to hypoxia

ROS in Redox Signaling Pathways

Redox-sensitive pathways mediated by ROS include mitogen-activated protein kinases (MAPKs), NF-kB, and the Keap1-Nrf2 antioxidant response pathway

ROS can also act as secondary messengers in cellular signaling cascades

Pharmacological Interventions for Ischemia-Reperfusion

Antioxidants: N-acetylcysteine (NAC), Vitamin C, Vitamin E

Mitochondrial protective agents: cyclosporine A, MitoQ, and GJA1-20k

Adenosine Receptor Agonists

Nitric oxide (NO) donors: nitroglycerine and sodium nitroprusside

Ischemic Conditioning for Ischemia-Reperfusion Injury

Preconditioning refers to subjecting tissues to brief, repetitive ischemia before the prolonged ischemic insult

Postconditioning involves applying brief reperfusion and ischemia cycles immediately after the initial ischemic insult

Stem Cell Therapy for Ischemia-Reperfusion Injury

Promote tissue repair and regeneration

Therapeutic Hypothermia for Ischemia-Reperfusion Injury

Induce mild hypothermia in patients undergoing reperfusion

Helps reduce metabolic demands, attenuate inflammation, and limit free radical production