Gen. Pharmac. Vol. 15, No. 2, pp. 75-77, 1984 Printed in Gr=at Britain. All rights reserved
0306-3623/84 $3.00 + 0.00 Copyright © 1984 Pergamon Press Ltd
MINIREVIEW THE EFFECTS OF STRETCH ON SMOOTH MUSCLE O L L I ARJAMAA
Laboratory of Animal Physiology, Department of Biology, University of Turku, SF 20500 Turku 50, Finland (Received 14 June 1983)
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 analyzed in pharmacological experiments, is also dependent on tissue length (Fig. 1). The active tension begins to increase on lengthening reaching its maximum 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 oviductal strip is released back to resting length after a stretch, the amplitude of active tension responds as it did at comparable lengths during stretching, adducing 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).
Strips of smooth muscle are widely used in pharmacological experiments in vitro as a model for various organs; experiments conducted with strips from vessels 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). Although most of the variables (salt solution, temperature, pH and oxygenation) involved in construction of a pharmacological experiment are carefully controlled, the length of tissue is either totally omitted 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 techniques 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 S M O O T H 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 exponentially, the steepness of the curve being dependent 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
Fig. 1. Length-tension diagram of a smooth muscle. (A) active tension; (= C - B); (B) passive tension; (C) maximum tension; L, length; F, tension. 75
The length-tension relationship of smooth muscle cannot as yet, be completely explained in structural terms (Meiss, 1978). The properties of passive tension during a stretch are probably due to changes in the connective framework of smooth muscle fibres (Conrad et al., 1966; Mulvany and Warshaw, 1979). The mechanism behind the active tension is unclear. Lakatta and Jewell (1977) have suggested that in the heart muscle, an increased influx of Ca 2+ into muscle fibres functions in the Frank-Starling relation describing the active tension. Although not directly shown, a stretch may increase at cellular level the accessibility of Ca 2+ for the contraction relaxation cycle of smooth muscle and in this way increase the contractile force (Johansson, 1978; Arjamaa, 1982). A stretch may also directly affect the contractile apparatus: according to Hibberd and Jewell (1982), the sensitivity of the contractile system to Ca ,`+ increases with length in a strip of the rat ventricle. Either a quick (mm/msec) or a slow (mm/min) stretch can be performed on a strip of smooth muscle. In the former, the aim is usually to analyse the behaviour of the contractile apparatus (e.g. Meiss, 1978) while in the latter, a stretch may mimic some physiological phenomena, e.g. ovum transport in the avian oviduct (Arjamaa and Talo, 1981). Probably mechanisms behind responses to slow and fast stretches are different (Buchthal et al., 1956; Burnstock and Prosser, 1960). Since the slow stretch is the case applied to most pharmacological experiments, the present paper discusses slow stretches only.
mally contract only in response to stimulation of nerves (Creed, 1979); stimulation may summate junction potentials which produce a spike. A stretch changes the membrane potential of smooth muscles and thereby the excitability of fibres. In the taenia coli, a stretch is followed by a depolarization and by an increase in electrical activity (Billbring, 1955). In the quail's oviduct, a small stretch first hyperpolarizes the tissue concomitantly with an increased electrical activity, and a further stretch depolarizes the oviduct concomitantly with a decreased electrical activity (Arjamaa and Talo, 1983). The ureter becomes spontaneously active if depolarized by stretching (Holman and Neild, 1979). Thus, a stretch may either increase or decrease the electrical activity depending on the type of smooth muscle and on the level of stretching. As a consequence, when a stretch affects the electrical activity which is prerequisite to mechanical activity, it has its effects on the active tension as well. Bfilbring and Kuriyama (1963) first described the interrelation between membrane potential, electrical activity, and tension in the taenia coll. They were, however, unable to provide any explanation for their finding. Later, Arjamaa and Yalo (1981), Arjamaa (1982) and Arjamaa and Talo (1983) found the same interdependence of the three factors in the quail's oviduct. Although this interrelation in smooth muscle is not fully understood at the present stage of knowledge, it can be concluded that a stretch affects the excitability of smooth muscle fibres through these factors.
ELECTRICAL ACTIVITY OF S M O O T H MUSCLE
Mechanical activity of smooth muscle is preceded and regulated by membrane potential and electrical activity. A change in membrane potential, either of spontaneous origin, or evoked by a drug or electrical current, leads to action potentials which in turn trigger contractions (=excitation~zontraction coupling) (Casteels, 1981). Tension development can occur, however, without necessarily being accompanied by a change of the membrane potential (=pharmacomechanical coupling) (Somlyo and Somlyo, 1968). The membrane potential is always more negative than - 6 0 m V in smooth muscles which are not spontaneously active (Holman and Neild, 1979). In spontaneously active fibres, the membrane potential is about - 5 0 mV (Creed, 1979). Creed (1979) describes three types of smooth muscles on the basis of their electrical activity. Muscles with inherent myogenic activity are of the first type; the activity originates within the muscle since tetrodotoxin, which normally blocks nerves, does not have any effect on fibres. In the taenia coli (Bfilbring, 1955) and in the quail's oviduct (Arjamaa, 1982), typical smooth muscles of this category, a stretch increases the spontaneous myogenic frequency. According to Creed (1979), electrically inexcitable smooth muscles belong to a second group. In arteries and tracheal smooth muscles, the membrane potential is steady, active electrical responses are small and restricted to a few fibres, and activity in initiated by nerves. Smooth muscles with intermediate properties, like the vas deferens, are not spontaneously active and nor-
From Fig. 1 it is evident that if an arbitrary load is applied to a smooth muscle strip, as is normally the case, the length of the tissue would be unknown. As a result, the tissue may be working either on the ascending or descending limb of the active tension curve A. Therefore, a routine placement of the strip at a certain tension is not ideal. Since each agonist exhibited a maximum response at unique lengths or tension, Finkbeiner and Bissada (1980) suggested that these lengths or tension for each agonist should be determined. Price et al. (1981) too, found that sensitivity of vascular smooth muscle depended on length and that the length sensitivity relationship was similar to the length-active tension relationship. Vessels set at different lengths even produced different EDs0 values; they also showed that sensitivity did not depend on the method of stimulation. In view of the fact that there is a wide variation in responses to drugs between individual experiments made with smooth muscle and that the repetition of tests on smooth muscle is sometimes fraught with great difficulties, the level of stretch should be taken into account as a possible source of error. A rough method for taking up the stretch effect is to measure the length tension curve for the smooth muscle used and then to make all the experiments at the length which produced the maximum force. This suggestion was originally made by Finkbeiner and Bissada (1980). Previously Biilbring and Kuriyama (1963) discussed the same problem and concluded that responses of the taenia coli strips correlated better to weight/length ratio than to length alone
Effects of stretch on smooth muscle since the definition o f the initial muscle length was difficult. However, neither o f these suggestions have been taken into account in pharmacological experiments. In fact, we have not found a single p h a r m a cological paper which refers to Bfilbring and Kuriyama (1963). A l t h o u g h the small size o f s m o o t h muscle strips does not readily allow a length-tension curve to be recorded or permit the accurate weighing o f strips, we feel that the omission o f the stretch effect reveals a serious deficiency in experimental design if tension is to be recorded. A c k n o w l e d g e m e n t ~ h e author wishes to thank Dr Antti Talo for his criticism and valuable suggestions. REFERENCES
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