METAPROTERENOL SULFATE HAS A PREFERENTIAL EFFECT ON BETA-2 ADRENERGIC RECEPTORS. THE PHARMACOLOGIC EFFECTS OF BETA ADRENERGIC AGONIST DRUGS, INCLUDING METAPROTERENOL, ARE AT LEAST IN PART ATTRIBUTABLE TO STIMULATION THROUGH BETA ADRENERGIC RECEPTORS OF INTRACELLULAR ADENYL CYCLASE, THE ENZYME WHICH CATALYZES THE CONVERSION OF ADENOSINE TRIPHOSPHATE (ATP) TO CYCLIC-3',5'-ADENOSINE MONOPHOSPHATE (C-AMP). INCREASED C-AMP LEVELS ARE ASSOCIATED WITH RELAXATION OF BRONCHIAL SMOOTH MUSCLE AND INHIBITION OF RELEASE OF MEDIATORS OF IMMEDIATE HYPERSENSITIVITY FROM CELLS, ESPECIALLY FROM MAST CELLS.