Agonist modulation of intracellular calcium oscillations in tracheal smooth muscle cells

!n freshly dissociated porcine trachéal smooth muscle (TSM) cells loaded with fluo-3, the dynamics of intraceliular Ca²⁺ ([Ca²⁺]_i) responses to ACh and endothelin-I (ET-I) were examined using a real-time confocal microscope. ACh (10 nM-IO μM) induced [Ca²⁺]_i oscillations that did not vary in amplitude and persisted in the absence of extracellular Ca2+. Oscillation frequency, but not amplitude, depended on ACh concentration, increasing - 3 fold across the ACh range. Oscillation frequency also slowed during the course of exposure to a given ACh concentration, suggesting muscarinic receptor desensitization. The rise (0.4 to 2 s) and fall (I to 4 s) times of each [Ca], wave did not vary with ACh concentration, but wave velocity increased. ET-1 ( 10 nM-1 u,M) also induced [Ca], oscillations of comparable amplitudes to ACh. However, unlike ACh, oscillation frequency and wave velocity were not dependent on ET-1 concentration. The relatively constant amplitude of [Ca2]i waves in response to varying concentrations of both ACh and ET-1 suggests an all-or-none event at the level of SR Cu4 release in TSM cells. It is likely that control of oscillation frequency and wave velocity are major factors in ACh regulation of [Ca '], but not ET-1. Variations in [Ca], exposure time could play an important role in regulating smooth muscle contractions or other Ca2-dependent processes. Supported by N!H grant HI,51736 and by fellowships from Abbott Laboratories.