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

Considerable information is available for intact skeletal and cardiac muscle cells on the role of changes in intracellular Ca2+ (Cai2+) levels during excitation-contraction coupling. Cai2+ levels seem to be similarly important in smooth muscle. However, only two published reports describe the use of indicators to directly measure [Cai2+] in intact smooth muscle cells. One described the use of murexide in molluscan smooth muscle, the other the use of aequorin in single amphibian cells. Their results differed: very long calcium transients were described for the molluscan muscle, compared with very brief ones for the amphibian cells. We present here the first description of the relationship between calcium transients and contraction of whole, mammalian smooth muscle injected with aequorin. The Ca2+ transient we 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. We observed a delay between the onset of the increase in [Cai2+] and the onset of the rise in tension which might be explained, at least in part, by the presence of a threshold [Cai2+] for concentration.