Gen. Pharmac. Vol. 15, No. 2, pp. 75-77, 1984 0306-3623/84 $3.00 + 0.00 Printed in Gr=at Britain. All rights reserved Copyright © 1984 Pergamon Press Ltd MINIREVIEW THE EFFECTS OF STRETCH ON SMOOTH MUSCLE OLLI ARJAMAA Laboratory of Animal Physiology, Department of Biology, University of Turku, SF 20500 Turku 50, Finland (Received 14 June 1983) INTRODUCTION Strips of smooth muscle are widely used in pharma- cological experiments in vitro as a model for various organs; experiments conducted with strips from ves- sels and arteries consisting of smooth muscle are particularly numerous. A prerequisite for evaluating drug effects is to record either the isometric or the isotonic tension of strips. Both types of method provide, however, a stretch being performed on strips to get a reproducibly responsing tissue. Stretch is an important factor in regulating the function of smooth muscles. In the quail's oviduct, which is extremely sensitive to changes in tissue length, the stretch controls the ovum transport (Arjamaa, 1983). Al- though most of the variables (salt solution, tem- perature, pH and oxygenation) involved in construc- tion of a pharmacological experiment are carefully controlled, the length of tissue is either totally omit- ted or only vaguely taken into account. Usually the tissue strip is subjected to a load uncorrelated to the size of the strip, the result of which is that strips are of different lengths. One randomly chosen example: "During equilibration for 90 min the vessel wall was subjected to a passive force of approximately 5 mN" (Brandt et al., 1983). Moulds (1983), reviewing tech- niques for testing isolated blood vessels, mentions nothing about the possible role of tissue length in pharmacological experiments. However, Finkbeiner and Bissada (1980), Price et al. (1981), and Holmberg et al. (1983) do point out the importance of stretching the tissue as a source of error in drug analysis. The purpose of this paper is briefly to review responses of smooth muscle to stretching and to show that length regulates the function of smooth muscle preparations. MECHANICAL ACTIVITY OF SMOOTH MUSCLE When a smooth muscle strip is stretched, different smooth muscles tend to respond similarly. Smooth muscles, either spontaneously active or stimulated by a current, produce a force in which two components can be distinguished (Fig. 1). When smooth muscles lengthen, their passive tension (B) increases ex- ponentially, the steepness of the curve being de- pendent on the type of smooth muscle. In a strip of the guinea-pig's taenia coli, a common model for smooth muscle, a stretch from 100% (=resting 75 length) to 175% increased the passive tension from about 5 mN to 70 mN (Mashima and Yoshida, 1965) whereas in the quail's oviductal strip during the same stretch, the passive tension remained under 5 mN (Arjamaa and Talo, 1981). The active tension (A), usually the response of a tissue quantitatively ana- lyzed in pharmacological experiments, is also de- pendent on tissue length (Fig. 1). The active tension begins to increase on lengthening reaching its max- imum at a certain length; a further stretch results in a fall of the curve of active tension. The active length-tension relation is not symmetrical in vascular smooth muscle and the tension falls more abruptly at greater lengths (Jones, 1981). When a quail's ovi- ductal strip is released back to resting length after a stretch, the amplitude of active tension responds as it did at comparable lengths during stretching, ad- ducing evidence that the decrease of the amplitude at greater lengths is not a result of any tissue damage (Arjamaa and Talo, 1981). Passive tension tends, on the other hand, to follow a so-called hysteresis loop during a stretch-release cycle. During the release, passive tension was always smaller at comparable lengths in the quail's oviduct (Arjamaa and Talo, 1981). The uterus (Csapo, 1960), cat ureter (Weiss et al., 1972), dog ureter (Vereecken et al., 1973), rabbit urinary bladder (Uvelius, 1976), and vascular smooth muscle (Herlihy and Murphy, 1974; Murphy, 1976; Johansson, 1978; Mulvany and Warshaw, 1979) share the same characteristics described in Fig. 1. Contractions can be either of tonic (long term) or of phasic type (short term contractions superimposed on tonic contractions). I. Fig. 1. Length-tension diagram of a smooth muscle. (A) active tension; (= C - B); (B) passive tension; (C) maximum tension; L, length; F, tension.