Superprecipitation of gizzard actomyosin, and tension in gizzard muscle skinned fibers in the presence of nucleotides other than ATP.

1. Nucleotides which are known to be poor substrates for myosin kinase (ITP, GTP, UTP and CTP) are poor relative to ATP at producing Ca2+-sensitive superprecipitation or tension in smooth-muscle-derived experimental systems. The ability of these nucleotides to support Ca2+-sensitive superprecipitation or tension in striated muscle fibers depends on fiber type, but ranges from poor for GTP to excellent for CTP. 2. Thiophosphate analogs of ITP and GTP (ITP gamma S and GTP gamma S) are poor at irreversibly activating superprecipitation in smooth muscle actomyosin relative to ATP gamma S. 3. [gamma-32P]ITP is poor relative to [gamma-32P]ATP as a substrate of the endogenous myosin light-chain kinase in gizzard actomyosin. 4. The results provide further independent evidence in both gizzard actomyosin and skinned fibers that the smooth muscle Ca2+-control system is based on myosin phosphorylation.