NA+ K+ ATPASE MEDIATES RHYTHMIC SPONTANEOUS RELAXATIONS IN CANINE AIRWAY SMOOTH-MUSCLE/

In canine airway smooth muscle, cyclopiazonic acid (CPA; selective blo cker of the sarcoplasmic reticulum Ca2+-pump) evokes a contractile res ponse which is initially mediated via release of internally sequestere d Ca2+, but is later supported almost exclusively by electromechanical coupling. As such, this second component is highly sensitive to inhib ition of voltage-dependent Ca2+-influx (e.g. dihydropyridines, removal of external Ca2+) or to membrane hyperpolarization. In the present st udy, we describe relaxations which occur spontaneously during this sec ond component of the CPA-evoked contraction. These relaxations are als o electromechanically mediated, since they are abolished by depolariza tion of the membrane by high-Kf media. TEA has relatively little effec t on the phasic activity, thus ruling out an involvement of Ca2+-depen dent K+ channels. On the other hand, the phasic activity is abolished by ouabain, by removal of external K+ or by cooling and is markedly sl owed by removal of external Na+. These observations indicate that the phasic activity is mediated by the Na+/K+-ATPase. (C) 1997 Elsevier Sc ience B.V.