Agonist-Evoked Ca2+ Wave Progression Requires Ca2+ and IP3

Smooth muscle responds to IP3-generating agonists by producing Ca2+ waves. Here, the mechanism of wave progression has been investigated in voltage-clamped single smooth muscle cells using localized photolysis of caged IP3 and the caged Ca2+ buffer diazo-2. Waves, evoked by the IP3-generating agonist carbachol (CCh), initiated as a uniform rise in cytoplasmic Ca2+ concentration ([Ca2+](c)) over a single though substantial length (similar to 30 mu m) of the cell. During regenerative propagation, the wave-front was about 1/3 the-length (similar to 9 mu m) of the initiation site. The wave-front progressed at a relatively constant velocity although amplitude varied through the cell; differences in sensitivity to IP3 may explain the amplitude changes. Ca2+ was required for IP3-mediated wave progression to occur. Increasing the Ca2+ buffer capacity in a small (2 mu m) region immediately in front of a CCh-evoked Ca2+ wave halted progression at the site. However, the wave front does not progress by Ca2+-dependent positive feedback alone. In support, colliding [Ca2+](c) increases from locally released IP3 did not annihilate but approximately doubled in amplitude. This result suggests that local IP3-evoked [Ca2+](c) increases diffused passively. Failure of local increases in IP3 to evoke waves appears to arise from the restricted nature of the IP3 increase. When IP3 was elevated throughout the cell, a localized increase in Ca2+ now propagated as a wave. Together, these results suggest that waves initiate over a surprisingly large length of the cell and that both IP3 and Ca2+ are required for active propagation of the wave front to occur. J. Cell. Physiol. 224: 334-344, 2010. (C) 2010 Wiley-Liss, Inc.