It is about equipotent in blocking beta1- and beta2- receptors.The amount of alpha to beta blockade depends on whether labetalol is administered orally or intravenously (IV). Orally, the ratio of alpha to β blockade is 1:3. Intravenously, alpha to β blockade ratio is 1:7. Thus, the labetalol can be thought to be a beta-blocker with some alpha-blocking effects. By comparison, labetalol is a weaker β-blocker than propranolol, and has a weaker affinity for alpha-receptors compared to Phentolamine.Labetalol possesses intrinsic sympathomimetic activity. In particular, it is a partial agonist at beta2- receptors located in the vascular smooth muscle. Labetalol relaxes vascular smooth muscle by a combination of this partial beta2- agonism and through alpha1- blockade. Overall, this vasodilatory effect can decrease blood pressure.Similar to local anesthetics and sodium channel blocking antiarrhythmics, labetalol also has membrane stabilizing activity. By decreasing sodium entry, labetalol decreases action potential firing and thus has local anesthetic activity. Physiological action
The physiological effects of labetalol when administered acutely (intravenously) are not predictable solely by their receptor blocking effect, i.e. blocking beta1- receptors should decrease heart rate, but labetalol does not. When labetalol is given in acute situations, it decreases the peripheral vascular resistance and systemic blood pressure while having little effect on the heart rate, cardiac output and stroke volume, despite its alpha1-, beta1- and beta2- blocking mechanism. These effects are mainly seen when the person is in the upright position.Long term labetalol use also has different effects from other beta-blocking drugs. Other beta-blockers, such as propranolol, persistently reduce cardiac output during exercise.
Depending on the route of administration and the treated condition, the onset of action can be from two hours up to one or two days after application; and the drug can act much longer than its elimination half-life would suggest. Chemistry
Triamcinolone is a synthetic pregnane corticosteroid and derivative of cortisol (hydrocortisone) and is also known as 1-dehydro-9α-fluoro-16α-hydroxyhydrocortisone or 9α-fluoro-16α-hydroxyprednisolone as well as 9α-fluoro-11β,16α,17α,21-tetrahydroxypregna-1,4-diene-3,20-dione.The substance is a light-sensitive, white to off-white, crystalline powder, or has the form of colourless, matted crystals. It has no odour or is nearly odourless. Information on the melting point varies, partly due to the substances polymorphism: 260 to 263 °C (500 to 505 °F), 264 to 268 °C (507 to 514 °F), or 269 to 271 °C (516 to 520 °F) can be found in the literature.Solubility is 1:500 in water and 1:240 in ethanol; it is slightly soluble in methanol, very slightly soluble in chloroform and diethylether, and practically insoluble in dichloromethane.