Postnatal changes in size and actomyosin content of rabbit gastric myocytes.

Maximal tension generated by gastric muscle is three to four times greater in weanlings than in newborn rabbits. To determine if this functional maturation is accompanied by structural changes, we compared length-tension relationships, myocyte number and size, and actomyosin content in muscle from the gastric body of newborn (1 day) and weanling (12 weeks) rabbits. Passive tension at optimal length (Lo) was six times greater in circular smooth muscle strips from weanling rabbits than from newborn rabbits. Active tension at Lo in weanling rabbits was three times greater than in newborn rabbits. For morphometry, muscle cross-sections stretched to Lo in the circular axis were photographed with electron microscopy (5300x). Cell number/unit area was counted in 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 microns2) was less than that in weanlings (13.5 +/- 11.7 microns2). Consequently, the muscle cells occupied similar total areas in newborns and weanlings. We measured actin and myosin heavy chain in full-thickness muscle homogenates using SDS gel electrophoresis and densitometric scanning. Actin and myosin concentrations were lower in newborns (9.6 +/- 1.3 micrograms g-1 wet weight and 5.6 +/- 0.7 micrograms g-1 wet wt, respectively) than in weanlings (17.7 +/- 3.0 micrograms g-1 wet wt and 8.2 +/- 1.6 micrograms g-1 wet wt respectively), each P < 0.01. The proportion of myosin heavy chain isozymes 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.