Isoproterenol increases peripheral [Ca2+]i and decreases inner [Ca2+]i in single airway smooth muscle cells.

The effects of isoproterenol (Iso) or photolysis of caged adenosine 3',5'-cyclic monophosphate (cAMP) on intracellular Ca 2+ concentration ([Ca 2+ ] i ) were studied in single airway smooth muscle cells. Changes in [Ca 2+ ] i were measured ratiometrically. In cells loaded with 10 μM fura 2-acetoxymethyl ester (AM), superfusion of Iso (10 μM) increased [Ca 2+ ] i in 20 of 22 cells from 153.2 ± 21.3 to 252.8 ± 38.3 nM, and this increase depended on extracellular Ca 2+ and was blocked by ryanodine (50 μM). Photolysis of intracellular caged cAMP increased [Ca 2+ ] i by 104.0 ± 17.3 nM in 20 cells and decreased [Ca 2+ ] i by 49.3 ± 9.4 nM in 10 cells. With modified confocal microscopy, peripheral [Ca 2+ ] i was increased and inner cytosolic [Ca 2+ ] i was decreased during stimulation with Iso. Iso-induced decreases in [Ca 2+ ] i were also observed with conventional optics in cells loaded with 0.5 μM fura 2-AM. However, in these same cells, Iso increased [Ca 2+ ] i in the presence of low concentrations of ryanodine (1-20 μM). We concluded that Iso decreased [Ca 2+ ] i in the inner cytosol but increased [Ca 2+ ] i in the peripheral cytosol by mechanisms that depended on both extracellular and sarcoplasmic reticulum Ca 2+ release channels. Our study suggests that fluorescence from a peripheral cytosol with high [Ca 2+ ] i can sometimes confound the measurement of «cytosolic» [Ca 2+ ] i with conventional optics