Use of aequorin to study excitation–contraction coupling in mammalian smooth muscle

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Abstract
Considerable information is available for intact skeletal and cardiac muscle cells on the role of changes in intracellular Ca2+ .**GRAPHIC**. levels during excitation-contraction coupling. .**GRAPHIC**. levels seem to be similarly important in smooth muscle. Only 2 published reports describe the use of indicators to directly measure .**GRAPHIC**. in intact smooth muscle cells. One described the use of murexide in molluscan smooth muscle, the other use of aequorin in single amphibian cells. Very long Ca transients were described for the molluscan muscle, compared with very brief 1 for the amphibian cells. The 1st description of the relationship between Ca transients and contraction of whole, mammalian smooth muscle [dog antral circular smooth muscle] injected with aequorin was presented. The Ca2+ transient observed has a slow rise, a long duration and there was no delay between the peak of the Ca2+ transient and the onset of force. This contrasts with the amphibian findings, but resembles the molluscan data. A delay was observed between the onset of the increase in .**GRAPHIC**. and the onset of the rise in tension which might be explained, by the presence of a threshold .**GRAPHIC**. for concentration.