Hormones and the Endocrine System

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Water-soluble hormones are secreted by exocytosis, travel freely in the bloodstream, and bind to cell-surface receptors.

Lipid-soluble hormones diffuse across cell membranes, travel in the bloodstream bound to transport proteins, and diffuse through the plasma membrane of target cells.

Slide 17

Signaling by any of these hormones involves three key events:

Reception

Signal transduction

Response

Binding of a hormone to its receptor initiates a signal transduction pathway leading to responses in the cytoplasm, enzyme activation, or a change in gene expression.

signal transduction pathway

Slide 18

Receptor location varies with hormone type

NUCLEUS

Signal

receptor

(a)

(b)

TARGET

CELL

Signal receptor

Transport

protein

Water-

soluble

hormone

Fat-soluble

hormone

Slide 19

Receptor location varies with hormone type

Signal

receptor

TARGET

CELL

Signal receptor

Transport

protein

Water-

soluble

hormone

Fat-soluble

hormone

Gene

regulation

Cytoplasmic

response

Gene

regulation

Cytoplasmic

response

OR

(a)

NUCLEUS

(b)

Slide 20

Pathway for Water-Soluble Hormones

The hormone epinephrine has multiple effects in mediating the body’s response to short-term stress.

Epinephrine binds to receptors on the plasma membrane of liver cells.

This triggers the release of messenger molecules that activate enzymes and result in the release of glucose into the bloodstream.

Slide 21

cAMP

Second

messenger

Adenylyl

cyclase

G protein-coupled

receptor

ATP

GTP

G protein

Epinephrine

Inhibition of

glycogen synthesis

Promotion of

glycogen breakdown

Protein

kinase A

Cell-surface hormone receptors

trigger signal transduction

Slide 22

Pathway for Lipid-Soluble Hormones

The response to a lipid-soluble hormone is usually a change in gene expression.

Steroids, thyroid hormones, and the hormonal form of vitamin D enter target cells and bind to protein receptors in the cytoplasm or nucleus.

Protein-receptor complexes then act as transcription factors in the nucleus, regulating transcription of specific genes.

Slide 23

Steroid hormone receptors