Dependence of the length-tension relationship on agonist concentration in vascular smooth muscle

This study examines the dependence of the length-tension (L-T) relationship in vascular smooth muscle on its level of activation. A horizontal shift of the L-T relationship with a change in activation level has been shown in striated muscle when L-T curves could not be superimposed. Active force at each length was normalized to the maximum active force in each curve. Indices of a horizontal shift of a L-T curve include the initial length for an active response (Li) and the length for maximum active force (Lmax). In this study normalized L-T curves were obtained from rings of the dog anterior tibial artery at low (approximately ED50) and high (maximal activation) concentrations of potassium (K+), norepinephrine (NE), and calcium (Ca2+). The normalized curve with a low concentration of K+ or NE was shifted to the right of the curve obtained with a high concentration. Li and Lmax were significantly longer for a low concentration of K+ or NE than a high concentration. With the same concentration of NE (10(-5) M) no difference in the normalized L-T curves, in Li, or in Lmax were found when low (0.085 mM) Ca2+ experiments were compared to normal (1.7 mM) Ca2+ experiments. It may be concluded that the length-tension relationship in vascular smooth muscle is shifted to longer lengths with a decrease in the concentration of a chemical agonist but not by a decrease in external calcium. We suggest that a concentration dependent shift in the length-tension relationship may have a role in the regulation of blood flow.     

The sarcomere length-tension relation in skeletal muscle

Tension development during isometric tetani in single fibers of frog semitendinosus muscle occurs in three phases: (a) in initial fast-rise phase; (b) a slow-rise phase; and (c) a plateau, which lasts greater than 10 s. The slow-rise phase has previously been assumed to rise out of a progressive increase of sarcomere length dispersion along the fiber (Gordon et al. 1966. J. Physiol. [Lond.]. 184:143--169;184:170-- 192). Consequently, the "true" tetanic tension has been considered to be the one existing before the onset of the slow-rise phase; this is obtained by extrapolating the slowly rising tension back to the start of the tetanus. In the study by Gordon et al. (1966. J. Physiol. [Lond.] 184:170--192), as well as in the present study, the relation between this extrapolated tension and sarcomere length gave the familiar linear descending limb of the length-tension relation. We tested the assumption that the slow rise of tension was due to a progressive increase in sarcomere length dispersion. During the fast rise, the slow rise, and the plateau of tension, the sarcomere length dispersion at any area along the muscle was less than 4% of the average sarcomere length. Therefore, a progressive increase of sarcomere length dispersion during contraction appears unable to account for the slow rise of tetanic tension. A sarcomere length-tension relation was constructed from the levels of tension and sarcomere length measured during the plateau. Tension was independent of sarcomere length between 1.9 and 2.6 microgram, and declined to 50% maximal at 3.4 microgram. This result is difficult to reconcile with the cross-bridge model of force generation.     

Action of FMRFamide on longitudinal muscle of the leech,Hirudo medicinalis

1. Nerve terminals associated with longitudinal muscle in the leech show FMRFamide-like immunoreactivity. 2. Structure-activity studies using FMRFamide analogs show that the C-terminal RFamide portion of the molecule is crucial for biological activity on leech longitudinal muscle. 3. The putative protease inhibitor FA (Phe-Ala) increases the peak tension produced by longitudinal muscle in response to superfused FMRFamide and the majority of its analogs, suggesting the presence of peripheral proteases capable of degrading RFamide peptides. 4. FMRFamide decreases the relaxation rate of neurally evoked contractions of longitudinal muscle. FA also decreases the relaxation rate of neurally evoked contractions. 5. Intact and isolated muscle cells respond to superfused FMRFamide with a conductance increase, that leads to depolarization and often with a delayed conductance decrease as the membrane potential is restored to resting levels. 6. The depolarizing response of isolated muscle cells to FMRFamide is dependent on external calcium.     

Selected contribution: effect of chronic passive length change on airway smooth muscle length-tension relationship.

The ability of rabbit trachealis to undergo plastic adaptation to chronic shortening or lengthening was assessed by setting the muscle preparations at three lengths for 24 h in relaxed state: a reference length in which applied force was approximately 1-2% of maximal active force (P(o)) and lengths considerably shorter and longer than the reference. Passive and active length-tension (L-T) curves for the preparations were then obtained by electrical field stimulation at progressively increasing muscle length. Classically shaped L-T curves were obtained with a distinct optimal length (L(o)) at which P(o) developed; however, both the active and passive L-T curves were shifted, whereas P(o) remained unchanged. L(o) was 72% and 148% that of the reference preparations for the passively shortened and lengthened muscles, respectively. The results suggest that chronic narrowing of the airways could induce a shift in the L-T relationship of smooth muscle, resulting in a maintained potential for maximal force production.     

The GABA-ergic regulation of the Lymnaea stagnalis (L.) dorsal longitudinal muscle contractions

Presence of GABA-immunoreactive fibres in the nerve cervicalis inferior was shown as well as their regulatory effects upon contractions of the dorsal longitudinal muscle in Lymnaea stagnalis. The exogenous GABA affected the amplitude of evoked muscle contractions diminishing them at concentrations from 10(-8) M and augmenting them from above 10(-5) M. Picrotoxin produced opposite effects on the muscle contractions in the same concentration range. Bicucullin did not affect the contractions. The findings are discussed from the standpoint of the supposed functional heterogeneity of the GABA receptors in the mollusc nerve-muscle junction.