Multiple pathways responsible for the stretch-induced increase in Ca2+ concentration in toad stomach smooth muscle cells

1. A digital imaging microscope with fura-2 as the Ca2+ indicator was used to determine the sources for the rise in intracellular calcium concentratio n ([Ca2+](i)) that occurs when the membrane in a cell-attached patch is str etched. Unitary ionic currents from stretch-activated channels and [Ca2+](i ) images were recorded simultaneously. 2. When suction was applied to the patch pipette to stretch a patch of memb rane, Ca2+- permeable cation channels (stretch-activated channels) opened a nd a global increase in [Ca2+](i) occurred, as well as a greater focal incr ease in the vicinity of the patch pipette. The global changes in [Ca2+](i) occurred only when stretch-activated currents were sufficient to cause memb rane depolarization, as indicated by the reduction in amplitude of the unit ary currents. 3. When Ca2+ was present only in the pipette solution, just the fecal chang e in [Ca2+](i) was obtained. This focal change was not seen when the contri bution from Ca2+ stores was eliminated using caffeine and ryanodine. 4. These results suggest that the opening of stretch-activated channels all ows ions, including Ca2+, to enter the cell. The entry of positive charge t riggers the influx of Ca2+ into the cell by causing membrane depolarization , which presumably activates voltage-gated Ca2+ channels. The entry of Ca2 through stretch-activated channels is also amplified by Ca2+ release from internal stores. This amplification appears to be greater than that obtaine d by activation of whole-cell Ca2+ currents. These multiple pathways whereb y membrane stretch causes a rise in [Ca2+](i) may play a role in stretch-in duced contraction, which is a characteristic of many smooth muscle tissues.