Agonist‐evoked Ca 2+ wave progression requires Ca 2+ and IP 3

Smooth muscle responds to IP 3 ‐generating agonists by producing Ca 2+ waves. Here, the mechanism of wave progression has been investigated in voltage‐clamped single smooth muscle cells using localized photolysis of caged IP 3 and the caged Ca 2+ buffer diazo‐2. Waves, evoked by the IP 3 ‐generating agonist carbachol (CCh), initiated as a uniform rise in cytoplasmic Ca 2+ concentration ([Ca 2+ ] c ) over a single though substantial length (∼30 µm) of the cell. During regenerative propagation, the wave‐front was about 1/3 the length (∼9 µm) of the initiation site. The wave‐front progressed at a relatively constant velocity although amplitude varied through the cell; differences in sensitivity to IP 3 may explain the amplitude changes. Ca 2+ was required for IP 3 ‐mediated wave progression to occur. Increasing the Ca 2+ buffer capacity in a small (2 µm) region immediately in front of a CCh‐evoked Ca 2+ wave halted progression at the site. However, the wave front does not progress by Ca 2+ ‐dependent positive feedback alone. In support, colliding [Ca 2+ ] c increases from locally released IP 3 did not annihilate but approximately doubled in amplitude. This result suggests that local IP 3 ‐evoked [Ca 2+ ] c increases diffused passively. Failure of local increases in IP 3 to evoke waves appears to arise from the restricted nature of the IP 3 increase. When IP 3 was elevated throughout the cell, a localized increase in Ca 2+ now propagated as a wave. Together, these results suggest that waves initiate over a surprisingly large length of the cell and that both IP 3 and Ca 2+ are required for active propagation of the wave front to occur. J. Cell. Physiol. 224: 334–344, 2010. © 2010 Wiley‐Liss, Inc.