Evidence for a sodium electrogenic pump in airway smooth muscle

We tested the hypothesis that the resting membrane potential (Em) of guinea pig and bovine airway smooth muscle (ASM) (m. trachealis transversus) might be influenced by a Na+ electrogenic pump. To determine the Em of ASM, glass microelectrodes filled with 3 M KCI and offering an impedance between 40 and 80 M omega were used. After incubation a normal oxygenated physiological salt solution (PSS) [pH, 7.38 +/- 0.02 (SD); temp, 37 +/- 0.5 degrees C (SD)] for 60 min, the Em of guinea pig ASM was -62.5 +/- 1.25 (SE) mV (successful impalement of 437 cells of 37 tracheal preparations from 37 different animals) and that of bovine ASM was -60.0 +/- 1.5 mV (successful impalement of 306 cells of 26 tracheal preparations from 26 different animals). Tracheal preparation consisted of 10-mm-long segments of cervical or intrathoracic portions of the trachea. Approximately 30% of the cells of both species exhibited spontaneous electrical activity (slow waves). Studies to determine whether an electrogenic pump was present revealed that a) ouabain (10(-5) M), a specific inhibitor of the Na+ pump, causes depolarization of ASM in both species; b) exposure of ASM to a K+-free solution resulted in a similar depolarization followed by a hyperpolarization; c) exposure to normal PSS after "Na+ loading" resulted in significant hyperpolarization of ASM when these preparations were returned to normal PSS; and d) Em of ASM decreased on exposure to a low temperature (21, 25, and 29 degrees C) and increased at a high temperature (40 degrees C). It is concluded that an active electrogenic pump is present in guinea pig and bovine airway smooth muscle.