Spare Receptor
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Transcript of Spare Receptor
SPARE RECEPTORS ( RESERVE RECEPTORS)
Maximal efficacy means a state at which receptor mediated signaling is maximal and that, further increase in the drg dose does not produce any additional response.theoretically, it should happen when all the receptors get occupied by the drug. But normally when drugs act on receptors to produce the response all the receptors are not occupied to produce the maximal response by a full agonist i.e each cell or tissue has some receptors unoccupied which may be called spare receptors.
Spare receptor History: Furchgott (1964) showed in an experiment that adrenaline induced contraction of the rabbit aortic strips did not require complete occupation of the α - adrenoceptors in the aortic strips for obtaining maximal response. Nickerson ( 1957) using histamine on a guinea pig ileum preparation. In both cases ,only small parentage of receptors had to be occupied by agonist to produce maximum contraction. The rest of the receptor were ‘spare’ or ‘Reserve’ receptor. The term spare receptors was proposed by Stephenson .
Concept of spare receptors: Maximum response is elicited by agonist at a
concentration that does not require full occupancy of the receptor.
They are not hidden or unavailable. When they are occupied, they can be coupled to
respond. They are not differ from non-spare receptor.
Demonstration of spare receptors:
Spare receptor may be demonstrated by using irreversible antagonist, to prevent binding of an agonist to available receptors. Experimentally, the spare receptor concept can be shown when the agonist can still produce an undiminished maximal response in presence of an irreversible antagonist.
Examples:
1. The amplitude of muscle twitch in response to Ach. Acetylcholine blocked by addition of toxin(curare) .This toxin occupies atleast 50% of receptors. But still max response can be demonstrated. That means, at least 50% of receptor were spare in sense that they were not required for completely normal twitch of Ach.
2. Myocardium also contain large population of spare receptor because the maximal ionotropic response to catecholamine can be elicited even when only less than 10% of receptor occupied.
3. Maximal stimulation of steroidogenesis by leydig cells occurs when only 1% of LH receptor are occupied. Full steroid
induced transcriptional response may only require 10% receptor.
4. About 90% of insulin receptors are just serving as spare receptors.Thus there is an immense functional reserve to make sure that adequate amount of glucose enters into cell.
Clinical significance:
The spare receptor concept could explain why the sensitivity of tissue depend on both affinity of drug & total number of receptor.
It is possible to change the sensitivity of tissue with spare receptor by altering receptor concentration .
An important biological consequence of spare receptor is that it allow low affinity agonists to produce full response at low concentration. This is important because low affinity agonist dissociate rapidly from receptor leading to short biologic response. High affinity agent show slow dissociate rates & slower reversal/termination of response
In analyzing pharmacological properties of ligands or interpreting results with receptor mutants in heterologous expression systems, which often have very high levels of receptor expression, it is essential to understand and account for the spare receptor phenomenon. Many compound that are partial agonist in normal tissues, are full agonist in expression system due to high receptor number
The spare receptor widely misunderstood thinking that the spare receptor are nonfunctional. Although all receptor may not be needed for a maximal response, all contribute to the measured response, thus the potency of full agonist is enhanced by the presence of spare receptor
If tissue with 100(100%) total receptor has 90(90%) spare receptor, this tissue would require only 10(10%) of receptor occupancy to produce maximal (Emax) response. In this case half maximal response will be produced by agonist concentration(EC50) that result in occupancy of only 5(5%) of total receptor per cell. If receptor concentration is doubled to 200 per cell due to activation of protein synthesis, it will still require occupancy of only 10 receptor per cell to produce maximal response & 5 receptor per cell for half maximal response.
SILENT RECEPTORS
Definition:
An agonist may bind to receptor and not produce a pharmacological
response. such receptor is termed as ‘silent receptor’.
Concept of spare receptors:
When drug is withdrawn, the silent receptor still remain
active together with the normally active receptor resulting
in an increased number of functional receptor.
This results in ‘ Rebound hyperactivity ’ or ‘ Withdrawal
phenomenon ’ in neurotransmitter effective system.
This phenomenon was shown by Thesleff (1960) who
showed that upon denervation of striated muscle some
silent receptor(nicotinic) are activated at the
neuromuscular junction.
Examples:
1. Plasma proteins, which are important ingredient of drug
distribution system, act as silent receptor and bind drugs.
2. Some tissue storage sites for drugs have also been termed
as silent receptors.
Eg: Nociceptors :
Each Nociceptors can have a variety of possible threshold
levels.Some do not responds at all to chemical, thermal or mechanical
stimuli, unless injury actually has occurred.
These are typically referred to as silent or sleeping nociceptors since
their response comes only on the onset of inflammation to the
surrounding tissue. Such as-
a. Human Serum Albumin as a silent receptor for drugs and
endogenous substances.
b. Murine Duffy Antigen receptor for chemokine and Murine CXC
chemokines receptor-2 in murine melanoma tumor growth
Clinical significance:
silent receptor concept is particularly relevant while
considering long term drug usage.
The phenomenon of tolerance which leads to drug dependence can be
explained on this basis.
Normal receptor of neurotransmitter can be blocked on a target organ on chronic use of drug. It is then that these silent receptor become activated and tolerance to drug effects is manifested.
ORPHAN RECEPTORS
Definition of Orphan receptor:
The receptors for which endogenous ligands were not identified are
called orphan receptors.
orphan receptors--receptors without a known ligand, a known signaling
pathway, or a known function.
Orphan receptors are found in a GPCR and Nuclear receptor families.
Orphan GPCR research was started in 1994.
identification of ligands for orphan GPCRs should yield important clues
as to their physiological functions and will help determine whether they
are suitable as drug targets.
Examples:
1. GPCR orphan receptor given name is ‘GPR’ like ‘GPR1’
2. Nuclear receptor like Farnesoid X receptor (FXR)
3. Liver X receptor (LXR)
4. Peroxisome proliferator activated receptor (PPAR).
5. Another orphan receptor site is PCP (1-(1-phenylcyclohexyl)
piperidine) called Phencyclidine binding site in NMDA
receptor, a type of ligand gated ion channel.
Classification of novel orphan GPCRs:
GPCRs Class A or Rhodopsin like receptor family: It represents
heterogeneous group with regard to the physicochemical properties of
their ligands.
Class B or Secretin like family: It includes mostly receptor for
structurally related peptides. Its length is 30-40 amino acids. Subfamily
of class B includes large amino terminal sequences.
Class C receptor family: It consist of metabotropic glutamate and
GABA B receptor. Eg:Ca 2+ sensing receptor.
Methods for identification of ligands:
Two main strategies were developed to search for orphan
receptor ligands. These were based either on the
search for the ligand per se by focused or random screening
of naturally occurring or synthetic compounds
or alternatively through the resolution
of the structure of the NR LBD by X-ray crystallography.
1. Known molecules Database search for Tissue extract
The most orthodox method of ligand fishing may be
employ a tissue extract as the starting material. The
extract of tissue is subjected to a purification procedure.
After the purification steps, which involve a combination
of chromatographies, the ligand molecule is finally
isolated in homogeneity, and its structure is determined.
This was the main approach taken in the early attempts at
ligand fishing.
e.g a.nociceptin and orphanin FQ6 to be isolated from rat and porcine
brain extracts, respectively,.
b. the discoveries of several novel G Protein-Coupled Receptors.
Many novel compounds, all of them peptidie, were discovered using
this method.
2. screening the library of known molecules
Another approach to discovering ligands for the orphan
receptors involves screening the library of known
molecules that includes possible candidate ligands.
such as the biogenic amines, peptides, chemokines,
bioactive lipids, and metabolic pathway intermediates.
In fact, use of this approach led to the discovery of the
most ligands, including bioactive peptides, such as
melanin-concentrating hormone, urotensin II, motilin, and
neuromedin U, and the low molecular weight ligands,
such as sphingosylphosphorylcholine,
lysophosphatidylcholine, bile acids, and free fatty acids.
3. If the target receptor is expected to possibly pair with a
nonpeptidic low molecular weight ligand, this approach can
identify an agonistic molecule, because only rarely will a
completely novel compound be a specific ligand for such a
receptor.
4. Database search for Bioactive peptides are usually
generated by cleavage at the potential processing sites
(cluster of two or three successive basic amino acids) from
the precursor proteins equipped with a secretory signal
sequence.
First orphan receptors to be identified is the estrogen-receptor
related receptors(ERRs). 7 Orexin- Hypothalamic hormone that affect
sleep and appetite. Bile acid – Identified as a ligand for nuclear orphan
receptor.
Biological functions of orphan receptors:
1. All orphan receptors have a very important function that is
specific to each one of them. Thus, they are not inert
molecules, less important than classic receptors.
2. Orphan receptors often play an important role in modulating
the action of classic liganded receptors.
3. Many orphan receptors are important players in
development and cell differentiation. For example, HNF-4_ is
critical for early mouse development as well as for the
development of the liver in vertebrates and arthropods