During growth and development, dietary intake changes from being predo
minantly liquid in the newborn period to mixed solid liquid meals. The
se alterations in diet vary the functional demands placed on the stoma
ch. It has been shown that, during development, smooth muscle of the s
tomach undergoes changes in the mechanism responsible for the contract
ile process. In this study, we have investigated the possibility that
there are structural changes in two of the major proteins that are res
ponsible for generation of force during smooth muscle contraction: act
in and myosin. Actin and myosin were identified in newborn kittens (1
wk old) and adult gastric smooth muscle using one-dimensional SDS-PACE
. Although both the antrum and fundus of the kitten have significantly
smaller total amounts of actin and myosin per mg protein than the adu
lt, the ratio of actin to myosin is not significantly different betwee
n the age groups. Two different myosin heavy chain (MHC) isoforms, MHC
1 (205 kD) and MHC2 (200 kD), were identified in all tissues. The rela
tive amount of MHC1 remained constant during maturation of the stomach
. We observed an increase in the amount of MHC2 in the adult, which re
sulted in a decreased ratio of MHC1 to MHC2 in the adult. We postulate
that the decreased quantity of actin and myosin in the kitten stomach
and the observed changes in the ratio of the MHC isoforms are related
to changes in the gastric motor that occur during growth and developm
ent.