The mechanism of inhibitory effects of azelastine, an antiallergic and antiasthmatic agent, on depolarization- and alpha1 adrenergic agonist-induced contractions of intact smooth muscle was studied. The effects of azelastine on membrane currents were determined in isolated guinea pig ileum smooth muscle cells with the whole-cell clamp technique; the effects on contraction were evaluated in receptor- and G-protein-coupled, alpha-toxin-permeabilized rabbit femoral artery and portal vein smooth muscle strips. Azelastine (1-20-mu-M), like dihydropyridines, inhibited spontaneous rhythmic and high K+-induced contractions, mainly through inhibition of the voltage-dependent (L-type) Ca++ current. The tonic component of high K+ contractions was inhibited more than the phasic component, correlating to voltage-dependent inhibition of Ca++ current by the drug. Azelastine (IC50 of 0.25-mu-M), a known histamine blocker, also reversibly inhibited alpha-1 agonist-induced contractions in the presence and absence of extracellular Ca++. Both major pathways of pharmacomechanical coupling, agonist-induced Ca++ release from the sarcoplasmic reticulum and Ca++ sensitization of the regulatory/contractile apparatus were blocked by the same concentration of drug in permeabilized as in intact muscle. Inositol 1,4,5-trisphosphate-induced Ca++ release and guanosine 5'O-(tau-thiotriphosphate)-induced Ca++ sensitization, however, were not inhibited. Azelastine at high (> 10-mu-M) concentrations reversibly inhibited Ca++-activated contraction, more potently at lower Ca++ concentration and in phasic smooth muscle, but inhibited neither adenosine 5'-O-(tau-thiotriphosphate)-induced, Ca++-independent nor phorbol ester-induced contractions. These results indicate that azelastine is a genuine Ca++ antagonist that inhibits voltage-gated Ca++ inward current and agonist-induced Ca++ release and Ca++ sensitization.
