MECHANICAL-PROPERTIES OF ISOLATED SMOOTH-MUSCLE FROM HUMAN RECTUM ANDINTERNAL ANAL-SPHINCTER

The passive and active length-tension relations of the circular smooth muscle layer of the human distal rectum and the proximal and distal i nternal anal sphincter were investigated. Muscle strips were prepared and mounted in organ baths for recording of isometric tension. Resting lengths (LR) were measured, and the preparations were elongated stepw ise. At each length, the corresponding values for passive tension, spo ntaneous active resting tension, and the submaximal active tension wer e recorded. Elongations of 200-380% of L(R) were possible before a sha rp increase in passive tension occurred. None of the mean tension valu es measured at length for maximal active tension (L(O)) differed signi ficantly among the three muscle types. All strips developed active res ting tension. This tension was myogenic and contributed 10 +/- 3, 23 /- 6, and 27 +/- 6% to the total active performance of rectal and prox imal and distal sphincter preparations, respectively. Collagen constit uted approximately 50% of smooth muscle biopsies, with highest content s in distal internal anal sphincter. This study provides an acceptable method for assessing the optimal experimental length by stretching th e strips in an inactive state to 200% of L(R), followed by individual adjustment of the passive tension to 5 mN/mm2 measured at 200% of LR.