Hormone–Target Cell Specificity Hormones circulate to all tissues but only activate cells referred to as target cells Target cells must have specific receptors to which the hormone binds These receptors may be intracellular or located on the plasma membrane Examples of hormone activity ACTH receptors are only found on certain cells of the adrenal cortex Thyroxin receptors are found on nearly all cells of the body

Target Cell Activation Target cell activation depends upon three factors Blood levels of the hormone Relative number of receptors on the target cell The affinity of those receptors for the hormone Up-regulation – target cells form more receptors in response to the hormone Down-regulation – target cells lose receptors in response to the hormone

Receptor Kinetics H+R =HR , K= HR/[H][R] , [HR]/[H]= K × [R] H= free hormone in solution R=unoccupied receptor HR=bound hormone =occupied receptor R0= initial receptor capacity = [R] + [HR] K = affinity constant

Schatchard plot for HR kinetic A linear plot results when the H reacts with a single R class and no cooperativity is present. The negative of the K equals the slope of the line. The R number,R0, equals the intercept with the X axis.

Schatchard plot for HR kinetic An exponential plot results when the H occupancy of one R molecule alters the local affinity of a second nearby molecule for the H. This phenomenon called negative cooperativity.

A, The general shape of a H dose- response curve A, The general shape of a H dose- response curve. Sensitivity is expressed as the concentration of the H that produces half-max response. B, Alterations in dose-response curve results from changes in max responsiveness, sensitivity, or both.

Hormone Concentrations in the Blood Concentrations of circulating hormone reflect: Rate of release Speed of inactivation and removal from the body Hormones are removed from the blood by: Degrading enzymes The kidneys Liver enzyme systems

Control of Hormonal Secretion Feedback control Positive feedback: Stimulus detected, H released, H reaches target cell, H binds R, Effect produced, Feedback to endocrine structure, More H released Negative feedback: Stimulus detected, H released, reaches target cell, binds R, Effect produced, Feedback to endocrine structure, Hormone release shut down Direct nerve control Autonomic nervous system Inhibiting hormones or Releasing hormones Chronotropic control

Feedback Neural Chronal

Control of Hormone Synthesis and Release Blood levels of hormones: Are controlled by negative feedback systems by positive feedback systems (seldom) Vary only within a narrow desirable range Hormones are synthesized and released in response to: Humoral stimuli (substrate or mineral- hormone) Neural stimuli Hormonal stimuli (hormone- hormone)

Humoral Stimuli Humoral stimuli – secretion of hormones in direct response to changing blood levels of ions and nutrients Example: Declining blood Ca2+ concentration stimulates the parathyroid glands to secrete PTH (parathyroid hormone) PTH causes Ca2+ concentrations to rise and the stimulus is removed Figure 17.3a

Neural Stimuli Neural stimuli – nerve fibers stimulate hormone release Preganglionic sympathetic nervous system (SNS) fibers stimulate the adrenal medulla to secrete catecholamines Figure 15.3b

Hormonal Stimuli Hormonal stimuli – release of hormones in response to hormones produced by other endocrine organs The hypothalamic hormones stimulate the anterior pituitary In turn, pituitary hormones stimulate targets to secrete still more hormones Figure 15.3c

Nervous System Modulation The nervous system modifies the stimulation of endocrine glands and their negative feedback mechanisms The nervous system can override normal endocrine controls For example, control of blood glucose levels Normally the endocrine system maintains blood glucose Under stress, the body needs more glucose The hypothalamus and the sympathetic nervous system are activated to supply ample glucose

Chronal control (the circadian rhythms=CR) The origin of CR in H secretion, behavioral and metabolic activity. A clock with a 24-25h cycle is located in the SCN, this free running clock is entrained by environmental light signals to the external 24h day. It has bidirectional relationship with the sleep-wake cycle,too.

Type of cell to cell signaling Endocrine: hormone enters the blood stream Neurocrine (neuroendocrine): also enters the blood Paracrine:through Int. fluid or GJ to another cell type Autocrine: through Int. fluid or gap junction and act on neighboring identical cells or back to the cell of origin Juxtacrine ?