Myosin light chain phosphorylation in contraction of gastric antral smooth muscle from neonate and adult rabbits.

The decreased contractility of gastric antral smooth muscle in the neonate has been attributed to reduced levels of activator calcium. It is generally accepted that calcium-dependent myosin light chain phosphorylation (MLCP) is the key step in the initiation of force development in smooth muscle. In this study, we investigated the relationship between MLCP and force development in gastric antral smooth muscle from neonatal (4-6 d old) and adult rabbits. We tested the hypothesis that the reduced force development of circular smooth muscle from the neonate would be accompanied by decreased levels of MLCP, as compared with data from adult animals. Full thickness muscle strips oriented parallel to the circular muscle layer were examined for their contractile response to acetylcholine (ACh) (10(-8) M to 10(-3) M) or 10(-4) M ACh only. In the latter study, tissues were rapidly frozen in a dry ice-acetone slurry for subsequent MLCP determination. MLCP was determined at times corresponding to 5, 10, 15, 30, and 60 s of stimulation. For each age group, maximal active force developed at an ACh concentration of 10(-4) M and was significantly greater in tissues from adults (1.86 +/- 0.24 N/m2, adult; 0.95 +/- 0.05 N/m2, neonate; p < 0.05). In contrast, no significant differences were observed with respect to basal or agonist-stimulated levels of MLCP. The data suggest that factors other than levels of MLCP contribute to the reduced force-generating capacity of antral smooth muscle from the neonate.