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

1. A digital imaging microscope with fura-2 as the Ca2+ indicator was used to determine the sources for the rise in intracellular calcium concentration ([Ca2+]i) that occurs when the membrane in a cell-attached patch is stretched. 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 membrane, Ca2+-permeable cation channels (stretch-activated channels) opened and a global increase in [Ca2+]i occurred, as well as a greater focal increase in the vicinity of the patch pipette. The global changes in [Ca2+]i occurred only when stretch-activated currents were sufficient to cause membrane depolarization, as indicated by the reduction in amplitude of the unitary currents. 3. When Ca2+ was present only in the pipette solution, just the focal change in [Ca2+]i was obtained. This focal change was not seen when the contribution from Ca2+ stores was eliminated using caffeine and ryanodine. 4. These results suggest that the opening of stretch-activated channels allows ions, including Ca2+, to enter the cell. The entry of positive charge triggers 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 obtained by activation of whole-cell Ca2+ currents. These multiple pathways whereby membrane stretch causes a rise in [Ca2+]i may play a role in stretch-induced contraction, which is a characteristic of many smooth muscle tissues.