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 (approximately 30 microm) of the cell. During regenerative propagation, the wave-front was about 1/3 the length (approximately 9 microm) 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 microm) 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.
McCarron
John G
JG
Strathclyde Institute of Pharmacy & Biomedical Sciences, University of Strathclyde, John Arbuthnott Building, Glasgow, UK. john.mccarron@strath.ac.uk
Chalmers
Susan
S
MacMillan
Debbi
D
Olson
Marnie L
ML
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects*; D002217:Carbachol / Q000494:pharmacology*; D003979:Diazonium Compounds / Q000378:metabolism; D006168:Guinea Pigs; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D010642:Phenoxyacetates / Q000378:metabolism; D010782:Photolysis / Q000187:drug effects
D003979:Diazonium Compounds; D010642:Phenoxyacetates; C063685:diazo-2; D015544:Inositol 1,4,5-Trisphosphate; D002217:Carbachol; D002118:Calcium