Abstract
Apamin, which blocks Ca 2+-dependent increases in K + permeability, antagonizes ATP-induced relaxation of several smooth muscles. The ATP photoaffinity label arylazido aminopropionyl ATP (ANAPP 3), following its photolysis in the presence of the guinea-pig vas deferens, antagonizes contractile responses to ATP. This study was conducted to determine whether apamin antagonizes ATP-induced responses in the guinea-pig vas deferens, and also to evaluate whether ANAPP 3 antagonizes responses to ATP by interfering with Ca 2+-dependent K + permeability changes. Apamin (10 −6 M) potentiated ATP-induced contractions. This potentiation was nonspecific in that responses to norepinephrine, histamine and acetylcholine also were enhanced; responses to KCl were unaffected. To evaluate the possible interactions between the two agents at the same cellular site, the effect of apamin was examined in ANAPP 3-treated tissues. In such tissues apamin did not potentiate the residual responses to ATP; however, apamin was nevertheless able to potentiate responses of ANAPP 3-treated tissues to norepinephrine, histamine and acetylcholine, and responses to KCl remained unaffected. These studies provide additional support for the view that ANAPP 3 antagonizes ATP-induced responses of the guinea-pig vas deferens by blocking P 2-purinergic receptors. The antagonism by ANAPP 3 is not attributable to a blockade of Ca 2+-dependent K + permeability changes.
Published Version
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