MODULATION OF INTRACELLULAR Ca²⁺ TRANSIENTS BY NITRIC OXIDE IN AIRWAY SMOOTH MUSCLE CELLS. ▴ 2308

Nitric oxide (NO) causes tracheal smooth muscle (TSM) relaxation by inhibiting agonist-stimulated elevation of intracellular Ca²⁺([Ca²⁺]_i). via cGMP-dependent mechanisms. In this study, we examined the effect of s-nitroso-n-acetylpenicillamine (SNAP), a NO donor, on the Ca²⁺ response to acetylcholine (ACh). Single, freshly isolated porcine TSM cells were loaded with fura-2/AM and [Ca²⁺]_i was measured as the ratio of fura-2 emissions when excited at 340 and 380 nm, using a video fluorescence imaging system. SNAP (0.1-1.0 μM) inhibited the normal [Ca²⁺]_i response to ACh (0.1-1.0 μM) at an extracellular Ca²⁺ ([Ca²⁺]_o) of either 0 or 2.5 mM. After SNAP inhibition of [Ca²⁺]_i response to ACh in 0 mM Ca²⁺, a large Ca²⁺ response was seen when [Ca²⁺]_o was readjusted to 2.5 mM, indicating that Ca²⁺ influx was not affected by NO. After depleting the sarcoplasmic reticulum (SR) Ca²⁺ pool with ACh and re-exposing to 2.5 mM Ca²⁺ in the presence of SNAP, the subsequent ACh response in the absence of SNAP was unaffected, indicating that NO does not affect reloading of the SR. These results suggest that NO selectively affects SR Ca²⁺ release, but does not affect Ca²⁺ influx or SR reloading. Inhibition of SR Ca²⁺ release can lead to relaxation of TSM. Supported by NIH grants HL51736 and HL07111, Mayo Foundation, and Univ. of Minnesota. MSK was a recipient of an Abbott Senior Fellowship.