Veterinary Physiology-III

Target organ and receptor types

TARGET ORGANS AND RECEPTOR TYPES

• Target Cell : The cells that contain specific receptors for a hormone
• Tropic hormone – A hormone that stimulates functions of another organ / tissue. Tropic:a nourishing relationship.
• Although all cells are exposed to all hormones, only few cells or organs can respond to all specific hormone, termed as target cells or target organ. This is because of highly specific receptor sites in the target organ.
• The receptors are large protein molecules. Hormone exerts its physiological actions only after binding to specific receptors.
• Despite the diversity of hormones, all hormone receptors can be categorized into one of two types, based on their location
• Membrane-bound receptors called as cell-surface (membrane) receptors
• Receptors located within the cell called as intracellular receptors

Location of Receptor	Classes of Hormones	Principle Mechanism of Action
Cell surface receptors	Proteins and peptides, catecholamines and eicosanoids	Generation of second messengers which alter the activity of other molecules - usually enzymes - within the cell
Intracellular receptors –proteins, floating in cytoplasm and/or nucleus	Steroids and thyroid hormones	Alter transcriptional activity of responsive genes and thereby alter the cell’s protein (enzyme) content