Cross-bridge regulation by Ca2+-dependent phosphorylation in amphibian smooth muscle

A covalent regulatory mechanism involving Ca2+-dependent cross-bridge phosp horylation determines both the number of cycling cross bridges and cycling kinetics in mammalian smooth muscle. Our objective was to determine whether a similar regulatory mechanism governed smooth muscle contraction from a p oikilothermic amphibian in a test of the hypothesis that myosin regulatory light chain (MRLC) phosphorylation could modulate shortening velocity. We m easured MRLC phosphorylation of Rana catesbiana urinary bladder strips at 2 5 degreesC in tonic contractions in response to K+ depolarization, field st imulation, or carbachol stimulation. The force-length relationship was char acterized by a steep ascending limb and a shallow descending limb. There wa s a rapid rise in unloaded shortening velocity early in a contraction, whic h then fell and was maintained at low rates while high force was maintained . In support of the hypothesis, we found a positive correlation of the leve l of myosin phosphorylation and an estimate of tissue shortening velocity. These results suggest that MRLC phosphorylation in amphibian smooth muscle modulates both the number of attached cross bridges (force) and the cross-b ridge cycling kinetics (shortening velocity) as in mammalian smooth muscle.