Lab Three - Rat Ileum

Ileum

final section of small intestine, found between jejeunum and large intestine; important for digestion and absorption of nutrients from food

inner circular fibres

smooth muscle cells responsible for narrowing of the ileum when contracted

outer longitudinal fibres

smooth muscle cells responsible for shortening of ileum when contracted

segmentation

contraction of ileum that lacks directional movement; churns chyme with digestive juices to facilitate digestion and nutrient absorption

Peristalsis

rhythmic wave-like contractions that moves chyme through ileum

smooth muscle structure

elongated, spindle-like cells with a single centrally located nucleus; contraction is involuntary and controlled by ANS; NOT striated; made up of myosin and action filaments and intermediate filaments and dense bodies (similar to Z -line in skeletal muscle); Ca2+ comes from extracellular fluid

contraction of smooth muscle

Intracellular calcium concentration increases, calcium ions bind calmodulin, ca2+-calmodulin activates myosin light chain kinase which phosphorylates the light chains in myosin heads and increases myosin ATPase activity allowing it to bind to actin

Regulation of contraction smooth vs skeletal

smooth has ca ions enter from extracellular fluid and activate MLCK, skeletal has ca2+ mainly released from SR in response to APs

myosin phosphorylation smooth vs skeletal

in smooth, phosphorylation by MLCK is essential for myosin-actin crossbridge formation; skeletal Ca ions bind troponin complex and exposes active sight and allowing myosin to bind (no phosphorylation needed)

cross bridge cycling smooth vs skeletal

in smooth much slower, in skeletal faster because contractions are more rapid and forceful

organization of filaments smooth vs skeletal

actin and myosin are present in both but in smooth they are not organized into sarcomeres like in skeletal

neural control smooth vs skeletal

skeletal is controlled by somatic system while smooth is controlled by autonomic system and hormonal control

organ bath

a controlled environment that permits optimal performance of a tissue or organ; controls temperature, supply of gas, and bathing solution

organ bath settings for lab three

temperature: 37 C , gas supply: 95% O2, 5% CO2; Krebs physiological bathing solution

Krebs physiological bathing solution

similar ionic environment to extracellular fluid with high Na+, Cl- and low K+; has Ca2+ to enable contraction, glucose for energy, and a buffering capacity

Transducer

part of the organ bath that measures tension and converts that to electical signals created by muscle contraction and records on a computer

3 Rs of Animal research

Reduce, Refine (to cause less stress to animal), and Replace (if possible to use other methods)

Receptors

specific proteins that drugs bind to to cause a response

Agonists

drugs that bind to a receptor and initiate a response

Antagonists

drugs that bind to a receptor and do NOT cause a response but block agonists from producing a response at that receptor

Drug Receptor Theory (Law of Mass Action)

once a threshold drug concentration is exceeded, a response is measurable and the magnitude is proportional to the concentration, when all receptors are occupied subsequent increase in agonist concentration has no effect

Muscarinic Receptors

Agonists are acetylcholine and carbachol, antagonist is atropine, a GPCR that is part of the cholinergic system and named after muscarine; has five subgroups of receptors but M3 is dominant in smooth muscle; when activated causes contractions

5-HT2A Receptor

agonist of 5-HT (aka serotonin), antagonist of Ketanserin; activation stimulates contraction of smooth muscle

Carbachol

a muscarininc agonist that is a synthetic analogue of Ach, resistant to AchE, antagonism occur with atropine

Basal cholinergic tone

intrinsic tone of muscle contraction due to endogenous release of Ach

EC50

concentration of a drug that causes 50% of the max response

Receptor desensitization

a cells response to a stimulus decreases over time despite continued exposure; the receptor becomes less responsive which reduces the signal inside the cell leading to drug tolerance; can be due to receptor phosphorylation, depletion of intracellular components, receptor internalization, or receptor downregulation

receptor phosphorylation

when receptor is phosphorylated it reduces the activity of the receptor; uncoupling occurs when the receptor is phosphorylated and can no longer activated the associated G-protein

receptor internalization

temporary removal of receptors from the cell surface

receptor downregulation

long term exposure to ligands leads to the degradation of the receptors