Question

What are the three basic actions which the hormones act on target cells?

Answer 1

The three basic actions which the hormones act on target cells based on their distances are:

1. ENDOCRINE ACTION: This is the action of hormones where the hormone is distributed in blood and act by binding on the distant target cells. Example: Amino acid derived hormones such asThyroxine, epinephrine, norepinephrine, etc.,
2. PARACRINE ACTION: This is the type of action where hormone acts locally by diffusing from its source to target cells in the neighbourhood. These hormones are mostly with nervous activity which operate across the synapses.  Example : beta endorphin, enkephalin, estrogens, testosterone
3. AUTOCRINE ACTION: The hormone acts on same cell that produced it. Example: Prostaglandins, thromboxanes, leucotrienes, prostacyclin, Cytokines

For many years the classic categories of hormones based on the chemical structures are used to be steroids, polypeptides, and amino acid and fatty acid derived compounds being classified as exclusive products of endocrine glands. These hormones are released after appropriate signal for their release into the blood. The endocrine hormones travel distant target which it recognises through high affinity receptors on the target. The receptors for each type of hormone is different such as for steroid hormones it is within the nucleus. Similarly it used to be different for glucocorticoid, neurotransmitters,etc., But many of the hormones were observed to act and affect neneighbouring cells or even the same cells that secrete the hormone. Hence based on the distance of action the hormones are classified as Endocrine, Paracrine and Autocrine.

CLASS	ENDOCRINE	PARACRINE	AUTOCRINE
Polypeptide	++++	+
Steroid	++++
Amino acid derived	++++	++++
Fatty acid derived	+		++++

+ (PLUS) sign indicates the extent of secretory type / distance of action.

"Endocrine" means the class of hormones secreted and travelled through the blodd stream to distant target cells.

"Paracrine" indicates that the cell secretes this class of hormones and they travel only to a short distance to neighbouring cellular targets

"Autocrine" means that class of hormones secreted by a cell which act on its own cell.