Pre- and postjunctional effects of a thromboxane mimetic in canine bronchi

Thromboxane A2 is believed to participate in causation of hyperreactivity of airway smooth muscle (ASM). We therefore investigated the effects of a thromboxane mimetic (U-46619) on isolated ring segments of canine bronchial ASM (3rd to 5th order). U-46619 (10(-9)-10(-5) M) was found to elicit tonic contraction, membrane depolarization, and oscillations in membrane potential. These effects were sensitive to blockade of thromboxane receptors (using 10(-8) M L 670, 596); the mechanical response was insensitive to 3 x 10(-8) M atropine, 10(-7) M nitrendipine, or exposure to Ca(2+)-free media [(containing 0.5 mM ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA)]. U-46619 also potentiated electrical field stimulation-elicited contractions and excitatory junction potentials without altering the sensitivity to carbachol. This potentiation was also sensitive to L 670,596 but not to blockade of adrenoceptors (using phentolamine plus propranolol). We conclude that canine bronchial ASM possesses both prejunctional and postjunctional thromboxane receptors. The former potentiate cholinergic neurotransmission through a mechanism not involving adrenoceptors. The latter excite the ASM through a mechanism not involving muscarinic receptors and that utilizes intracellular Ca2+ (rather than extracellular Ca2+). Excitation-contraction coupling was found to be pharmacomechanical in nature (rather than electromechanical).