Receptor and Signal Transduction Receptor Concept Dept. Pharmacology, Tzu Chi Univ. T.H. Chiu I. References for the lecture II. Development of receptor theory III. Contribution from physical and biological chemistry IV. Occupational theory V. Subsequent modifications of occupational theory

I. References for the lecture 1. R.R. Ruffolo, Jr. Important concepts of receptor theory. J. Auton. Pharmacol. 2: , Chapter 15, Cell Communication, pp From: Essential Cell Biology, B. Alberts et al., 1998 It provides a general schemes of signal transduction. A good reading materials for the beginners. 3. Signal Transduction, C-H. Heldin & M. Purton,eds. Chapman & Hall, A concise description of major signal transduction pathways you always want but afraid to ask. 4. See the handouts for the other references relating to the receptor concept.

II. Development of receptor theory A. Receptive substance Langley, 1878, studied the mutual antagonistic effects of pilocarpine and atropine on salivary secretion; 1905, studied the antagonism between nicotine and curare on muscle contraction, led to the idea that nicotine and curare acted on the same receptive substance. B. Receptor Ehrlich, 1913, experiments with tissue stains, snake venom, and bacterial toxins (side chain theory to describe the interaction between antigen and antibody, and the concept of specific cell surface receptors as the basis for targeting bioactive agents to the appropriate responsive cells). Awarded Nobel Prize in Medicine in 1908 (immunochemistry).

Experimental observations supporting specific cell surface receptors: 1. Many drug responses are obtained at very low doses or concentrations. 2. Responses can be blocked by other drugs of specific chemical structures (stereo-specificity). 3. The selectivity of agonists and antagonists is extremely dependent on chemical structures, and very small changes in structures can have profound effects on pharmacological activities.

C. Occupancy theory: Number of receptors occupied determines the response Clark, 1926: quantitative mathematical treatment of drug effect Ariens, 1954: introduced intrinsic activity as the effect caused by unit drug-receptor complex Stephenson, 1956: Introduced efficacy as the capacity of a drug to initiate a response Furchgott, 1966: Introduced intrinsic efficacy as a quantal unit for the capacity of a drug to initiate a stimulus from one receptor Spare receptors: Nickerson, 1956 Stephenson, 1956 Furchgott, 1955 (awarded Nobel Prize in Medicine, 1999, for the research on NO) Ariens, 1960

D. Rate theory: Paton, 1961: proposed that the effect was proportional to the rate of drug-receptor interaction, rather than to the number of receptors occupied by the drug. Challenging conceptually, but of limited applicability based on our current understanding or receptor systems. E. Molecular models: Macromolecular perturbation theory, Belleau, 1964 Mobile receptor hypothesis, Cuatrecasas, 1974, explaining the interaction between βadrenoceptor and adenylyl cyclase Allosteric theory, Monod, Wyman & Changeux, 1965

III. Contributions from physical & biological chemistry: Pharmacological concepts Chemical concepts Receptive substance: Langley 1878, 1905 Lock & key fit of ES: Fisher, 1894 Receptor: Ehrlich, 1913 Enzyme kinetics: Henri, 1902 Michaelis-Menten, 1913 Briggs-Haldane, 1925 Lineweaver-Burke, 1934 Occupancy theory: Clark, 1926 Intrinsic activity; Ariens, 1954 Efficacy; Stephenson, 1956 Spare receptors; Nickerson et al., 1956 Intrinsic efficacy, Furchgott, 1966 Induced-fit theory: Koshland, 1958

Pharmacological concepts Chemical concepts Rate theory: Paton, 1961 Macromolecular perturbation theory: Belleau, 1964 Allosteric transition model: Monod, Wyman & Changeux, 1965 Ligand-induced cooperative model: Koshland, Nemethy & Filmer, 1966 Mobile receptor hypothesis: Cuatrecasas, 1974

IV. Occupancy theory: Quantitative treatment by Clark Interaction between drugs and receptors follows Law of Mass Action.

General assumption: 1. One drug molecule reversibly binds to one receptor molecule. 2. A response results from steady-state occupation of receptors. 3. A graded response is obtained. 4. Response is proportional to the number of receptors occupied. 5. E M is proportional to [R T ] occupied. 6. [D] >> [R T ] 7. The occupation of one receptor does not alter the property of other receptors.

V. Modifications of occupancy theory A. Ariens treatment: Effect is dictated by 2 independent parameters. 1. Affinity: the ability of a drug to bind 2. Intrinsic activity: the ability of drugs to induce an effect after binding agonists: possess both affinity and intrinsic activity (1) antagonists: possess affinity but not intrinsic activity (0) partial agonists (or partial antagonists) with intrinsic activity between 0 and In some cases only a small percentage of receptors needs to be occupied to elicit a maximal response.

Assumptions for full agonists are the same as in the Clark treatment. α: effect per unit drug-receptor complex ED 50 (from dose-response curve) as a measure of affinity Maximal effect as a measure of intrinsic activity

B. Stephenson treatment: Introduced a parameter called “stimulus”, and the response is some unknown function (f) of stimulus. Thus, function f dissociates receptor stimulus and tissue response as directly proportional quantities. 1. Effect doe not need to be linearly proportional to receptor occupancy. 2. E M can be achieved with occupation of small percentage of receptors. 3. Different drugs may induce same effects by occupying different percentage of receptors. 4. It is possible for 2 full agonists with intrinsic activity of 1 to have different efficacies. 5. Consequence of spare receptors on the relation between ED 50 and K D (K D >> ED 50 ). ED 50 = concentration required for a half-maximal response K D = concentration required to occupy 50% of receptors

C. Furchgott treatment: Introduced intrinsic efficacy, “ε”, which is the capacity of a drug to initiate a stimulus from one receptor. Thus, intrinsic efficacy was defined as a strictly drug-related term, whereas Stephenson’s efficacy was a drug- and tissue-related term. 2 tissue factors: [R T ] and f (the nature and efficacy of the functions converting receptor stimulus into tissue response) 2 drug factors: K D and ε