Lack of effect on phosphoinositide (PI) hydrolysis and [Ca²⁺]_<i>i</i> by actin polymerization inhibitors.

A: PI hydrolysis in response to ACh was measured in muscle cells labeled with myo [3H] inositol. Cultured smooth muscle cells were pretreated with the PLC-β inhibitor U73122 (10 μM for 10 min) or, the actin polymerization inhibitor latrunculin (1 μM, 1 h) followed by 1 μM ACh for 1 min, and PI hydrolysis was measured as an increase in water-soluble [3H] inositol formation. PI hydrolysis is expressed as [3H] inositol phosphate formation in counts per minute (cpm) per mg protein. B: Isolated smooth muscle cells were loaded with 5 μM fura-2 and treated with 1 μM ACh in the presence or absence of the actin polymerization inhibitor latrunculin (1 μM, 1 h). The cells were alternately excited at 380 nm and 340 nm. Results are expressed as a 340/380 ratio, and an increase in ratio reflects an increase in cytosolic Ca2+. Values are means ± SE of ten cells. **P<0.001.