RELEASE OF CHEMICAL MEDIATORS

Introduction Much of pharmacology is based on interference with the body's own chemical mediators, particularly neurotransmitters, hormones and inflammatory mediators. Common mechanisms involved in the release of such mediators will be discussed Drugs and other agents that affect the various control mechanisms that regulate [Ca2+]i will therefore also affect mediator release, and this accounts for many of the physiological effects that they produce.

Chemical mediators Chemical mediators fall into two main groups: Mediators that are preformed and packaged in storage vesicles-sometimes called storage granules-from which they are released by exocytosis. This large group comprises all the conventional neurotransmitters and neuromodulators and many hormones. It also includes secreted proteins such as cytokines and various growth factors

Chemical mediators (2) Mediators that are produced on demand and are released by diffusion or by membrane carriers. This group includes nitric oxide and many lipid mediators (e.g. prostanoids and endocannabinoids).

Role of [Ca2+]i Calcium ions play a key role in both cases, because a rise in [Ca2+]i initiates exocytosis [Ca2+] is also the main activator of the enzymes responsible for the synthesis of diffusible mediators.

Chemical mediators (3) In addition to mediators that are released from cells, some are formed from precursors in the plasma Two important examples being; kinins Angiotensin These are peptides produced by protease- mediated cleavage of circulating proteins.

EXOCYTOSIS Exocytosis, occurring in response to an increase of [Ca2+]i, is the principal mechanism of transmitter release in the peripheral and central nervous systems, as well as in endocrine cells and mast cells. The secretion of enzymes and other proteins by gastrointestinal and exocrine glands and by vascular endothelial cells is also basically similar.

EXOCYTOSIS (2) Exocytosis involves fusion between the membrane of synaptic vesicles and the inner surface of the plasma membrane. The vesicles are preloaded with stored transmitter, and release occurs in discrete packets, or quanta, each representing the contents of a single vesicle.

EXOCYTOSIS (3) In nerve terminals specialised for fast synaptic transmission (such as muscle, glial cells, neurons), Ca2+ enters through voltage- gated calcium channels, mainly of the N and P type ( Purkinje neurons in cerebellum), the synaptic vesicles are 'docked' at active zones- specialised regions of the presynaptic membrane from which exocytosis occurs, situated close to the relevant calcium channels and opposite receptor- rich zones of the postsynaptic membrane

The SNARE superfamily… 1. Synaptotagmin 2. Synaptobrevin 3. Synaptotaxin

EXOCYTOSIS (4) Calcium causes exocytosis by binding to the vesicle-bound protein synaptotagmin, and this favours association between a second vesicle-bound protein, synaptobrevin, and a related protein, synaptotaxin, on the inner surface of the plasma membrane. This association brings the vesicle membrane into close apposition with the plasma membrane, causing membrane fusion. This group of proteins, known collectively as SNAREs, plays a key role in exocytosis.

EXOCYTOSIS (5) Having undergone exocytosis, the empty vesicle is recaptured by endocytosis and returns to the interior of the terminal, where it fuses with the larger endosomal membrane. The endosome buds off new vesicles, which take up transmitter from the cytosol by means of specific transport proteins and are again docked on the presynaptic membrane.

NON-VESICULAR RELEASE MECHANISMS Acetylcholine, noradrenaline (norepinephrine) and other mediators can leak out of nerve endings from the cytosolic compartment, independently of vesicle fusion, by utilising carriers in the plasma membrane. Drugs such as amphetamines, which release amines from central and peripheral nerve terminals, do so by displacing the endogenous amine from storage vesicles into the cytosol, it escapes via the monoamine transporter in the plasma membrane, a mechanism that does not depend on Ca2+.

NON-VESICULAR RELEASE MECHANISMS (2) Nitric oxide and arachidonic acid metabolites (e.g. prostaglandins) are two important examples of mediators that are released by diffusion across the membrane or by carrier-mediated extrusion, rather than by exocytosis. The mediators are not stored but escape from the cell as soon as they are synthesised. In both cases, the synthetic enzyme is activated by Ca2+, and the moment-to-moment control of the rate of synthesis depends on [Ca2+]i. This kind of release is necessarily slower than the classic exocytotic mechanism, but in the case of nitric oxide is fast enough for it to function as a true transmitter .

EXOCYTOSIS

Mediator Release-Summary Most chemical mediators are packaged into storage vesicles and released by exocytosis. Some are synthesised on demand and released by diffusion or the operation of membrane carriers. Exocytosis occurs in response to increased [Ca2+]i as a result of a Ca2+-mediated interaction between proteins of the synaptic vesicle and the plasma membrane, causing the membranes to fuse. After releasing their contents, vesicles are recycled and reloaded with transmitter.

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