POSTNATAL CHANGES IN SIZE AND ACTOMYOSIN CONTENT OF RABBIT GASTRIC MYOCYTES

Maximal tension generated by gastric muscle is three to four times gre ater in weanlings than in newborn rabbits. To determine if this functi onal maturation is accompanied by structural changes, we compared leng th-tension relationships, myocyte number and size, and actomyosin cont ent in muscle from the gastric body of newborn (1 day) and weanling (1 2 weeks) rabbits. Passive tension at optimal length (L(0)) was six tim es greater in circular smooth muscle strips from weanling rabbits than from newborn rabbits. Active tension at L(0) in weanling rabbits was three times greater than in newborn rabbits. For morphometry, muscle c ross-sections stretched to L(0) in the circular axis were photographed with electron microscopy (5300x). Cell number/unit area was counted i n circular muscle layers from newborns and weanlings. Cross-sectional area of each cell was measured by computerized planimetry. There were 2.5 times more cells per unit area in newborn than in weanling tissue, P<0.001. However, the mean cell area in newborns (5.4+/-4.6 mu m(2)) was less than that in weanlings (13.5+/-11.7 mu m(2)) Consequently, th e muscle cells occupied similar total areas in newborns and weanlings. We measured actin and myosin heavy chain in full-thickness muscle hom ogenates using SDS gel electrophoresis and densitometric scanning. Act in and myosin concentrations were lower in newborns (9.6+/-1.3 mu g g( -1) wet weight and 5.6+/-0.7 mu g g(-1) wet wt, respectively) than in weanlings (17.7+/-3.0 mu g g(-1) wet wt and 8.2+/-1.6 mu g g(-1) wet w t respectively), each P < 0.01. The proportion of myosin heavy chain i sozymes did not change with age. We conclude that there are postnatal increases in cell size and the quantity of actin and myosin in rabbit gastric muscle. The increase in quantity of contractile protein may be in part responsible for age-dependent increases in maximal tension.