Ka: acid ionization constant, a measure of the degree to which an acid ionizes in solution and a constant which has no units. The negative log of the Ka value is the pKa which is also expressed without units.

K_A: a pharmacokinetic term meaning the association constant for the binding of a drug to proteins given in units of L/mole.

K_A: the equilibrium dissociation constant for an agonist binding to receptors.

k_a: a pharmacokinetic term meaning the absorption rate constant given in units of hr^-1.

K_B: the equilibrium dissociation constant for an antagonist binding to receptors.

K_d: also written as K_D, the dissociation equilibrium constant for an agonist.

kDa: kilodaltons, a unified atomic mass unit for measuring the molecular weight of proteins.

K_D: the dissociation equilibrium constant for an agonist.

Kainic acid: a amino acid neurotoxin (its an excitotoxin) originally obtained from a Japanese seaweed (Digenea simplex) and similar in structure to the cyclic form of glutamic acid which stimulates glutamate-sensitive receptors. (See also NMDA and AMPA).

Kainate receptors: sometimes referred to as non-NMDA receptor, kainate receptors are CNS receptors sensitive to excitatory amino acids and make up a separate group of CNS receptors from the NMDA and AMPA receptors although they are structurally similar. These receptors appear to be essential in synaptic plasticity and synaptic transmission and play an important role in long-term potentiation.

Kaliotoxin (KTX): a single chain 38 amino acid 4kDa polypeptide and a blocker of voltage-dependent potassium channels which contain Kv1.1 and Kv1.3 subunits. Kaliotoxin, which shows sequence homology with charybdotoxin, noxiustoxin and iberiotoxin, is found in the venom of the scorpion (Androctonos mauretanicus) and is used as a pharmacological tool to elucidate the molecular pharmacology of the high conductance Ca²⁺-activated K⁺ channels. It interacts with the channel in a one to one manner on the outside of the channel.

Kallidin I: also known as bradykinin, a physiologically active polypeptide which is formed from a blood plasma globulin (bradykininogen which is also known as kallidin II) and which mediates inflammatory responses, increases vasodilation and causes contraction of smooth muscle.

Kallikrein: also known as kininogenin, an enzyme present in blood plasma (plasma kallikrein EC 3.4.21.34), lymph, urine, saliva, pancreatic fluids and tissues (tissue kallikrein) which catalyzes the proteolysis of bradykininogen (kallidin II) to bradykinin (kallidin I).
Sharma JN. Interrelationship between the kallikrein-kinin system and hypertension: a review. Gen Pharmacol (1988) 19(2); 177-187

Kallistatin: a serpin (serine protease inhibitor) and an acidic glycoproteins which forms a stable complex with tissue kallikrein so inhibiting its activities. Kallistatin is known to inhibit angiogenesis and tumour growth.

Kanamycins: a family of broad-spectrum aminoglycoside antibiotic (kanamycins A, B and C) obtained from the soil bacterium Streptomyces kanamyceticus. They binds to bacterial ribosomes and prevents protein synthesis. They have been used to treat tuberculosis.

Kaolin-induced writhing: a method of determining the effects of analgesics by intraperitoneal injection of a suspension of kaolin which causes the animal to writhe in pain. A decrease in the number of writhings indicated analgesic actions of a test compound.
Fujiyoshi T et al. Kaolin-induced writhing in mice, a new model of possible bradykinin-induced pain for assessment of analgesic agents. Agents Actions (1989) Jun;27(3-4); 332-4

Kasugamycin: an antibiotic fungicide isolated from bacteria (Streptomyces kasugaensis) found in a soil sample taken from Kasuga Shrine in Nara, Japan and named after it. It has been used against Pseudomonas infections but is used as an agrochemical for the eradication of, for example, Pyricularia oryzae, a fungal pathogen causing rice blast disease.

Katz, Bernard (1911-2003): German-born biophysicist and physiologist who shared the Nobel Prize for Physiology or Medicine in 1970 with Axelrod and von Euler "for their discoveries concerning the humoral transmitters in the nerve terminals and the mechanism for their storage, release and inactivation".
http://www.nobel.se/medicine/laureates/1970/katz- bio.html

Kava: a narcotic drug used in ethnopharmacology and commonly consumed in the islands of the south Pacific including Fiji and Vanuatu. It is actually the ground up root of a ginger-like shrub Piper methysticum which is mixed with water and drunk. It contains compounds such as kawain, methysticum and yangonin which have analgesic and ethanol-like actions. See http://kavaroot.com/aboutkava_frames.htm

kdkd mouse: a congenic variant of the CBA/Ca mouse which has normal kidneys at birth but from about week eight postpartum begins to develop progressive, lethal autoimmune nephritis.

Keratolytic: a compound which promote the softening and shedding of the horny layers of outer skin by dissolving or breaking down keratin, a protein found in skin and nails. They are used for the removal of corns and calluses and examples include salicylic acid, formaldehyde, gluteraldehyde and silver nitrate.

Ketamine: an anaesthetic chemically related to phencyclidine and used clinically intravenously or intramuscularly to induce and maintain anaesthesia in surgery in both animals and humans. Ketamine produces dissociative anaesthesia where there is a marked loss of sensation, paralysis, amnesia and analgesia occurs but consciousness is not actually lost. It acts by inhibiting the polysynaptic actions of the excitatory neurotransmitter ACh and L-glutamate in the spinal cord and NMDA in the brain. It is a potent non-competitive antagonist at NMDA receptors. Ketamine is also a drug of abuse because of its calming and euphoric effects.

Ketotifen: a 'classic' serotonin antagonist which blocks 5-HT₂ receptor and is an H₁ histamine receptor antagonist. It has a stabilizing effect on mast cells rather like cromoglicate. It is used clinically to treat asthma but is not the drug of choice in this disorder. It may also be effective in the treatment of topical allergies.

Khat: plant which contains psychostimulants and which is chewed for its effects. It is used in ethnopharmacology.

K_i: the equilibrium dissociation constant for a competitive inhibitor of a receptor. Typically the K_i value of a compound is calculated to determine the affinity it has to a particular receptor and this compound may be an agonist or antagonist at this receptor. When calculating K_i values it is common to used a known radiolabelled agonist or antagonist of the receptor and determine to what extent the test compound inhibits it binding. This type of binding assay is commonly employed to determine id a test compound has any affinity for a particular receptor or receptor subtype and how strong that affinity is. It can be calculated from the Cheng-Prussof equation:



where IC₅₀ is the concentration of test compound (ie the unlabelled drug) which inhibits 50% of the radiolabelled compound binding to the receptor, L is the concentration of radioactive ligand used and K_d is the affinity of the radioactive ligand for the receptor. You can find a PDF from www.graphpad.com explaining this in more detail here.

Kinases: a superfamily over a thousand enzymes including isozymes which account for the activities of about 2-3% of the genes in the human genome. They are involved in a large number of signal transduction processes in cell growth, differentiation, metabolism and apoptosis. The most important step in cellular signal transduction mechanisms is the phosphorylation of amino acid residues usually tyrosine, serine or threonine in proteins involved in signal transduction (that is, they transfer phosphate groups from high-energy donors such as ATP) and this phosphorylation is catalyzed by kinases (and phosphatases). Kinases are critical cellular components regulating how a cell respond to exogenous factors such as transmitters, hormones and growth factors. Diseases such as cancer involve unregulated kinase activity such as overexpression and/or up-regulation of kinase genes. The largest group of kinases is the protein kinases and other groups are named after their substrates and include adenylate kinase, creatinine kinase, pyruvate kinase, hexokinase, thymidine kinase, nucleotide diphosphate kinase and tyrosine kinases.

Kinase-linked receptors: a type of receptor in which the cell signal transduction mechanism is the phosphorylation of a protein by a kinase (as opposed to the events at an ion channel-linked receptor, a G protein-coupled receptor or a enzyme-linked receptor). Examples include receptors for epidermal growth factor.

Kinase inhibitors: compounds inhibiting kinases and so disrupting a broad range of cellular signaling mechanisms involved in growth, differentiation, metabolism and apoptosis. Several kinase inhibitors are used clinically to treat tumours and include imatinib to treat chronic myelogenous leukaemia, gastrointestinal stromal tumours and other malignancies and geftinib, a selective inhibitor of epidermal growth factor tyrosine kinase, used against breast and lung tumours (advance or metastatic non-small cell lung cancer).

Kindling: a method to induce seizures in an animal model of complex-partial epileptic seizures which has become the most widely studied animal model of limbic epilepsy. It is also used in the study of neural plasticity. Kindling can be induced electrically, chemically or audiogenically. There are many methods of kindling but typically comprise repeated administration of low intensity pulses of electrical stimulation to the amygdaloid body (amygdaloid kindling) or hippocampus, chemically by the intracerebral administration of N-methyl-D-aspartate, pentylenetetrazol or cocaine or audiogenically by exposure of an animal to high-intensity noise.
McIntyre DC, Poulter MO, Gilby K. Kindling: some old and some new. Epilepsy Res (2002) 50(1-2); 79-92

Kinins: a family of structurally related polypeptides such as bradykinin which act locally (they are local hormones or autacoids) to induce vasodilation and contraction of smooth muscle. Examples include bradykinin.

Kininogen: an inactive globulin precursor of a plasma kinins. Examples include bradykininogen.

Klotz plot: a graphical representation of ligand-receptor binding. It is a plot of bound ligand vs log free ligand and is used to determine the appropriate ligand concentration and to determine co-operativity. It is often used as an alternative to a Scatchard plot and assumes equal and independent binding sites on a protein.
Klotz IM, Walker FM, Pivan RB. The binding of organic ions by proteins. J Am Chem Soc (1946) 68:1486-90

KMD3213: a selective antagonist of α_1A- adrenoceptors.

Knock in mouse: a mouse into which a transgene (one from another animal or species) has been inserted into a specific locus in the mouse's genome. This is typically carried out to determine the physiological, biochemical and pharmacological effects of the transgenes or the effects of overexpression of a particular gene as there is a higher number of them in a knock in animal.

Knock out mouse: a mouse which has had both alleles of a particular gene replaced with an inactive allele or disrupted by the insertion of inactive DNA into the allele so that this gene no longer functions normally. Knock out mice are used to determine the role of a particular gene by observing the phenotypes of animals that lack the gene completely.
Conditional knock out mice are mice in which tissue-specific inactivation of a particular gene is achieved usually at a specific time point in the animals life and not a hereditary gene defect. In some cases this can be achieved by the administration of a drug such as tamoxifen if a tamoxifen responsive hormone-binding domain of the estrogen receptor under the control of a tissue-specific promoter is inserted into the transgenic animal.

Konzett-Rossler method: an in vivo method to determine the activity of bronchoactive compounds originally developed by Heribert Konzett and Richard Rossler in 1940. In this method, a tracheal cannula is inserted into the airway of an anaesthetized and artificially ventilated guinea pig and is connected to a spirometer in order to measure changes in the volume of the trachea and bronchioles. The actions of bronchodilators and bronchoconstrictors can be observed as changes in air volume within the trachea and bronchioles.
Konzett H, Roessler R. Versuchsanordnung zu Untersuchungen an der Bronchialmuskulatur. Arch Exp Path Pharmakol (1940) 195; 71-74 (written in German)

Kruskall-Wallis test: a statistical test which is similar to one-way analysis of variance (ANOVA) used to compare the mean values of three or more sets of data when the data are chosen from limited sets of values or if the data is not normally distributed.

Kynurenate: a compound produced by astrocytes in the brain and an endogenous antagonist of both the glycine B receptors and (non-competitively at) the α7 nicotinic receptor. It acts as a broad-spectrum (ie non-specific) antagonist of ionotropic glutamate receptors such as glutamate receptors and is used as a pharmacological tool to investigate these receptors.

Kyneurine: an amino acid intermediate in the pathway from tryptophan to nicotinic acid. It is also metabolised to two other components, the neurotoxic quinolinic acid and the neuroprotective and neuroinhibitory kynurenic acid (which has antagonist activity at ionotropic glutamate receptors and can inhibit presynaptic glutamate release). Kyneurine is excreted in small amounts in the urine as a product of the metabolism of tryptophan.
Ruddick JP et al. Tryptophan metabolism in the central nervous system: medical implications. Expert Rev Mol Med (2006) Aug 31;8(20):1-27

Kynurenine pathway: a major route of L-tryptophan catabolism which results in the formation of nicotinamide adenine dinucleotide and other neuroactive intermediates such as the neurotoxin, quinolinic acid (which may be associated with inflammatory disease of the brain).

Kyotorphin: an opioid analgesic dipeptide (L-tyrosyl-L-arginine) which is synthesized in specific brain regions. It may modulate synaptic transmission and it appears to directly excites cortical neurons and produces analgesia via the release of met-enkephalin.
Prasad C. Bioactive cyclic dipeptides. Peptides (1995);16(1):151-64