Xamoterol: a partial β1-adrenoceptor agonist which has been used to improve mild heart failure but is deleterious in more severe cases restricting it use.

Xanthines: a class of alkaloids present in beverages such as coffee and tea. Examples include xanthine, theophylline and theobromine. They have varying stimulating effects on the heart and central nervous system and have diuretic effects. The methyl derivatives of xanthine (methylxanthines including theophylline) inhibit phosphodiesterase and some xanthines are purine receptor antagonists with relaxant effects on smooth muscle. Some xanthines are used clinically to treat acute severe asthma and reversible airway obstruction, eg theophylline it is a bronchodilator and pentoxifylline in the treatment of peripheral vascular disease and venous leg ulcers as it is a peripheral vasodilator.



Xanthine oxidase: also known as xanthine oxidoreductases and hypoxanthine oxidase, an enzyme (EC 1.17.3.2) of a molecular weight of about 300kDa which catalyzes the formation of uric acid from hypoxanthine.

Xanthine oxidase inhibitors: compounds which inhibit the actions of xanthine oxidase and which are used clinically in the treatment of gout. Example include allopurinol.

Xenobiotic: a term used to describe a compound which interacts with an organism but which does not occur in the normal metabolic pathways of that organism.

¹³³Xenon clearance method: a method used to measure blood flow and the metabolic rate for oxygen.

Xestoquinone (XQN): a compound isolated from the sea sponge Xestospongia sapra which induces a concentration-dependent Ca²⁺ release from the heavy fraction of fragmented sarcoplasmic reticulum of skeletal muscle by inhibiting Ca²⁺ATPase activity. Xestoquinone is an irreversible inhibitor of pp60v-src, an oncogenic protein tyrosine kinase.

Xipamide: a moderately powerful thiazide-like diuretic which acts on the early distal convoluted tubule to decrease active absorption of sodium and chloride. Xipamide binds to the chloride site of the Na+/Cl- co-transporter and inhibits its actions. It is used clinically to treat oedema and hypertension.

X-linked Hyp mutant mouse: a murine homologue of X-linked hypophosphataemia in humans. Is characterized by renal defects in phosphate reabsorption and vitamin D metabolism and is used in studies of phosphate homeostasis.

XQN: xestoquinone

Xylazine: an animal anaesthetic sometimes used with ketamine as a parenteral anaesthetic. Xylazine is an α2-adrenoceptor agonist used to induce epidural anaesthesia in cats, dogs, cattle, horses and wildlife such as deer and elk. It is also used to induce vomiting in cats after accidental ingestion of poison.

Xylocaine: a local anaesthetic which is used as a dental anaesthetic. It causes reversible block of conduction along nerve fibres.

Xylocholine: also known as TM 10 (as it was the tenth compound to be synthesize in the TM series of choline derivatives), the xylyl ether of choline which has weak nicotinic activity and can antagonize the nicotinic and muscarinic actions of Ach. It also is a weak antagonist of adrenaline and noradrenaline and has weak anaesthetic activity.