Caldesmon association with smooth muscle thin filaments isolated in the presence and absence of calcium

Thin filaments isolated from chicken gizzard smooth muscle in either the presence or the absence of Ca2+ possess identical caldesmon contents. Hence, a 'flip-flop' mechanism, involving Ca2+-dependent association and dissociation of caldesmon and thin filaments, does not appear to operate in vivo and is an unlikely model for caldesmon function.