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 neo nate has been attributed to reduced levels of activator calcium. It is generally accepted that calcium-dependent myosin light chain phosphor ylation (MLCP) is the key step in the initiation of force development in smooth muscle. In this study, we investigated the relationship betw een MLCP and force de 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 neon ate would be accompanied by decreased levels of MLCP, as compared with data from adult animals. Full thickness muscle strips oriented parall el to the circular muscle layer were examined for their contractile re sponse to acetylcholine (ACh) (10(-8) M to 10(-3) M) or 10(-4) M ACh o nly. In the latter study, tissues were rapidly frozen in a dry ice-ace tone slurry for subsequent MLCP determination. MLCP was determined at times corresponding to 5, 10, 15, 30, and 60 s of stimulation. For eac h 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/m(2), adult; 0.95 +/- 0.05 N/m(2), neonate; p < 0.05). In co ntrast, no significant differences were observed with respect to basal or agonist-stimulated levels of MLCP, The data suggest that factors o ther than levels of MLCP contribute to the reduced force-generating ca pacity of antral smooth muscle from the neonate.