b Receptor antagonists block catecholamine-induced tremor and inhibit mast-cell degranulation by catecholamines.
NON-SUBTYPE-SELECTIVE 0 RECEPTOR ANTAGONISTS Propranolol
Propranolol interacts with b1 and b2 receptors with equal affinity, lacks intrinsic sympathomimetic activity, and does not block a receptors.
ABSORPTION, FATE, AND EXCRETION
Propranolol is highly lipophilic, almost completely absorbed after oral administration, and subject to a prominent first-pass effect (hepatic metabolism during the drug's first passage through the portal circulation), such that only ~25% reaches the systemic circulation. Individual variation in hepatic clearance of propranolol contributes to variability in plasma concentrations (~20x) after oral administration. Hepatic extraction of propranolol is saturable, thus the extracted fraction declines as the dose is increased; one hepatic metabolite, 4-hydroxypropranolol, has some b antagonist activity. The bioavailability of propranolol may be increased by the concomitant ingestion of food and during long-term administration of the drug. Propranolol readily enters the CNS. A sustained-release formulation of propranolol (inderal la) maintains therapeutic plasma concentrations over a 24-hour period.
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