Longitudinal force-length relationships of guinea pig ureter were studied in vitro in animals 3 weeks, 3 months, and 3 years of age. Distal segments of 3-week-olds underwent a greater per cent increase in length in response to an applied resting force than those from older age groups. Resting force-length relations of proximal segments were unaffected by age. Active force was higher in distal segments at all ages and at all lengths. Maximum active force increased with age. The increase in force developed between 3 weeks and 3 months of age seemed to be attributable to an increase in contractility since there was an associated increase in active stress; the increase developed between the 3-month- and 3-year-olds was associated with increased muscle cross-sectional area but with no change in active stress. This finding suggests that no change in contractility occurred during this time interval.
Aging Animals