Hormone Action and Target Cell Specificity

Target Cell Receptors and Activation

Hormones exert their effects only on specific target cells that possess the appropriate receptors. The degree of target cell activation is determined by several factors:

• Blood levels of hormone: Higher concentrations generally increase the likelihood of activation.

• Relative number of target cell receptors: More receptors increase sensitivity to the hormone.

• Affinity (strength) of binding: Stronger binding between hormone and receptor enhances the response.

Cells can regulate their sensitivity to hormones by adjusting receptor numbers:

• Up-regulation: Target cells increase receptor numbers in response to persistent low hormone levels, making them more sensitive.

• Down-regulation: Target cells decrease receptor numbers in response to persistent high hormone levels, reducing sensitivity and preventing overstimulation.

Hormone Transport, Onset, and Duration

Hormone Circulation and Half-life

Hormones circulate in the blood either freely or bound to carrier proteins. Steroid and thyroid hormones are typically bound to plasma proteins, while other hormones circulate freely. The concentration of a hormone in the blood depends on:

• Rate of release from endocrine glands

• Speed of inactivation and removal from the body (by enzymes, liver, or kidneys)

Half-life is the time required for the concentration of a hormone in the blood to decrease by half. This can range from seconds to days, depending on the hormone.

Onset and Duration of Hormone Effects

Hormones vary in how quickly they trigger responses and how long their effects last:

• Some hormones act almost immediately, while others (especially steroids) may take hours or days.

• Some hormones are secreted in inactive forms and must be activated in target tissues.

• Duration of action ranges from seconds to hours; effects may disappear quickly or persist even at low hormone levels.

• Water-soluble and lipid-soluble hormones differ in their onset and duration of activity.

Comparison of Lipid-Soluble and Water-Soluble Hormones

Key Differences

Lipid-soluble and water-soluble hormones differ in their chemical nature, transport, receptor location, and mechanism of action. The following table summarizes these differences:

Interaction of Hormones at Target Cells

Types of Hormonal Interactions

Multiple hormones can influence the same target cell simultaneously, resulting in different types of interactions:

• Permissiveness: One hormone requires the presence of another to exert its full effect. Example: Reproductive hormones need thyroid hormone to have their effect.

• Synergism: Two or more hormones produce similar effects, and their combined effect is amplified. Example: Glucagon and epinephrine both cause the liver to release glucose.

• Antagonism: One or more hormones oppose the action of another hormone. Example: Insulin and glucagon have opposite effects on blood glucose levels.

Hypothalamic Control of Pituitary Hormones

Structure and Function of the Pituitary Gland

The hypothalamus is connected to the pituitary gland (hypophysis) via the infundibulum. The pituitary gland has two major lobes:

• Posterior pituitary (neurohypophysis): Composed of neural tissue, stores and secretes two neurohormones (oxytocin and ADH).

• Anterior pituitary (adenohypophysis): Composed of glandular tissue, manufactures and secretes six hormones.

Posterior Pituitary and Hypothalamic Relationships

The posterior pituitary contains pituicytes (support cells) and axon terminals of hypothalamic neurons. Axons form the hypothalamic-hypophyseal tract, which:

• Arises from neuron cell bodies in the paraventricular and supraoptic nuclei of the hypothalamus.

• Runs through the infundibulum to the posterior pituitary.

• Stores and secretes oxytocin and antidiuretic hormone (ADH) when hypothalamic neurons fire action potentials.

Additional info: The anterior pituitary is connected to the hypothalamus via the hypophyseal portal system, allowing hypothalamic hormones to regulate anterior pituitary hormone secretion.