Age‐related changes in rat colon mechanics

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Abstract
To determine whether myogenic factors are responsible for slowed colonic transit in senescent rats, maximum shortening velocity (V0), compliance of the series elastic component (SEC), measurements of passive force in calcium‐depleted tissue and peak isometric force (F0) were examined in proximal and distal colonic circular smooth muscle from 6‐ and 30‐monthold Fischer rats (n = 5). After mucosa was removed, measurements were made on strips stimulated with 80 m>m KCl in a 2.5 m>m Ca2+ Kreb's solution. Muscle strips were quick released at peak isometric force (F0) to afterloads of 60% of F0. The changes in muscle length from zero to 40 msec and 1 to 2 sec after release during isotonic contraction were used to calculate the SEC and V0 as a fraction of total muscle length. Passive force (Fp) was measured in 2.5 m>m Ca2+ Kreb's solution and in a zero Ca2+, 0.1 m>m EGTA solution to determine the contribution of contractile and passive elements to passive force. The results of these studies indicate there is no difference in the V0 (L0/sec) of adult (8.4 ± 1.5) and aged (7.5 ± 2.0) animals (P ± 0.05). Peak force (F0) in the distal colon of the aged rats was greater than adult rats (1.23 ± 0.1 vs 0.85 ± 0.01 kg/cm2, P = 0.05). The stiffness of the parallel elastic component and the length‐tension relationship were similar in adult and aged animals. Negligible decreases in Fp were observed in zero calcium medium. However, basal contractile tone was elevated in aged animals (P = 0.05). These studies indicate basic differences in aged colonic circular muscle that may contribute to altered bowel transit and function during ageing.