Chemical messengers

Outline

• Mechanisms of intercellular communication

• Chemical messengers

• Signal transduction mechanisms

Mechanisms of intercellular communication

• Direct through gap junctions

Cells exchange chemicals through connections between cytoplasm

• Indirect through chemical messengers

The messenger must bind to a matching receptor and stimulate a response

Figure 5.1a

Indirect messengers• Autocrine: the cell secretes a

chemical which can bind on its own membrane and stimulates a response

• Paracrine: a cell secretes a chemical which can bind to a receptor on a neighboring cell trigger a response

• Hormone: a chemical travels long distance through the blood before binding to its own target organ and stimulating a response

• Neurotransmitter: influences a distant organ by secreting chemicals which travel through axons

Chemical classification of messengers

• Lipophilic molecules = lipid soluble can cross the phospholipid bilayer cannot be stored by the cell synthesized and released immediately.

ex: steroids (from cholesterol), eicosanoids (from fatty-acids)

• Lipophobic molecules = not soluble in lipid cannot cross the phospholipid bilayer can be stored in vesicles by the cell.

ex: amino acids, peptides, proteins

Transport and elimination

• For autocrines, paracrines and neurotransmitters: diffusion

• For non-water soluble hormones: transport into in the blood bound to carrier proteins for transport (steroids and thyroid hormones)

• Hormones will eventually be degraded by the liver and eliminated by the kidneys each has a characteristic ½ life.

Hormone binding• The hormones must bind

to a matching receptor

• The binding is specific for this hormone receptor affinity (a receptor might have differing affinity for various hormones (ex: epinephrine and norepinephrine)

Signal transduction: the binding of the hormone to its receptor triggers a cell response

- steroid hormone: cross the phospholipid bilayer receptors are inside the cytoplasm or nucleus activates transcription and synthesis of new proteins response slow but sustained

- Lipophobic hormones bind to receptors located on the surface of the cell membrane activates a cascade of enzymes response amplified rapid response but not sustained

Mechanisms of transduction (do not memorize these mechanisms

• The binding of the hormone to its receptor can trigger the opening of a channel, induce enzymatic activation

Application

• Cholera

Readings

• Chp. 5, p. 126-145.

• Discovery: p.128,

• Clinical connections: p. 140.

• Not expected to be known:

• Clinical connections: p. 134