Mechanics of vascular smooth muscle contraction

Modulation of lymphatic muscle contractility by the neuropeptide substance P

Substance P (SP) is a neuropeptide associated with sensory innervation of lymphoid tissue and a suspected modulator of lymphatic function in inflammation. Only a few studies have examined the effects of SP on lymphatic contraction, and it is not clear to what extent SP acts directly on the lymphatic muscle and/or endothelium or indirectly through changes in intraluminal filling pressure secondary to increases in capillary permeability/filtration. We tested the effects of SP on the spontaneous contractions of rat isolated mesenteric lymphatic vessels under isometric and isobaric conditions, hypothesizing that low concentrations would stimulate lymphatic pumping by enhancing lymphatic muscle contraction in a manner complementary to the effect of increased preload. Under isometric conditions, SP (10 nM) dramatically enhanced lymphatic chronotropy and inotropy. Unlike guinea pig lymphatics, SP actions were not blocked by cyclooxygenase or PLA(2) inhibition. In the absence of SP, ramp increases in isometric preload resulted in x approximately 1.6 increases in contraction amplitude (Amp) and x approximately 1.7 increases in frequency (Freq). SP increased Freq by x approximately 2.4, Amp by x approximately 1.9, and the Amp-Freq product (AFP) by x approximately 3.5. Under isobaric conditions, the pressure elevation from 0.5 to 10 cmH(2)O in the absence of SP decreased Amp by x approximately 0.6 and increased Freq by x approximately 1.8. SP caused a modest increase in Amp, a robust increase in Freq at all pressures, and shifted the AFP-pressure relationship upward and leftward. Therefore, SP has substantial positive inotropic and chronotropic effects on rat lymphatic muscle, improving pump efficiency independent of the effects of preload and broadening of the working range of the lymphatic pump.

Effects of heart rate on arterial compliance in men

1. Arterial compliance is a major determinant of left ventricular afterload. In keeping with earlier experimental data obtained in isolated arterial segments, it has recently been shown in the rat that arterial compliance decreases with an increase in heart rate (HR) induced by atrial pacing. 2. To elucidate the potential relevance of this effect in humans, we investigated nine male volunteers (age 20-30 years; mean 26 years). Systemic arterial compliance (SAC) was measured with the diastolic area method and carotid-to-femoral and femoral-to-dorsalis pedis pulse wave velocities (PWV) were measured to determine regional changes in compliance. Heart rate was first lowered with intravenous metoprolol to 56 +/- 2 b.p.m. and then increased by transoesophageal atrial pacing to 80 and 100 b.p.m. 3. Increasing HR from 56 +/- 2 to 80 b.p.m. by pacing increased mean arterial pressure (MAP) from 78 +/- 2 to 98 +/- 1 mmHg (P < 0.001) and then to 102 +/- 2 mmHg (P = NS). Systemic arterial compliance fell from 0.48 +/- 0.06 to 0.33 +/- 0.04 arbitrary compliance units (ACU; P < 0.01), carotid-to-femoral PWV increased from 6.1 +/- 0.3 to 6.8 +/- 0.4 m/s (P < 0.001) and femoral-to-dorsalis pedis PWV increased from 8.9 +/- 0.4 to 10.1 +/- 0.5 m/s (P < 0.001). Pacing at 100 b.p.m did not change MAP, but did lead to a further decrease in SAC (to 0.24 +/- 0.03 ACU; P < 0.05) and further increases in carotid-to-femoral (7.3 +/- 0.4 m/s; P = NS) and femoral-to-dorsalis pedis PWV (11.3 +/- 0.4 m/s; P < 0.001). 4. We conclude that systemic, central and peripheral compliances decrease in vivo with an increase in HR induced by atrial pacing.

On the origin and dynamics of the vasomotion of small arteries

A system of differential equations describing stationary vasomotion is formulated. It incorporates the ionic transports, cell-membrane potential, muscle contraction of the vessel smooth muscle cells, and the mechanics of a thick-walled cylinder. It is shown that the interaction of Ca2+ and K+ fluxes mediated by voltage-gated and voltage-calcium-gated channels, respectively, brings about periodicity of those transports. This results on a time-periodic cytoplasmic calcium concentration, myosin light chains phosphorylation, and crossbridges formation with the attending muscle stress. The vessel's transmural pressure determines a hoop stress. The resultant hoop, elastic, and muscle stresses determine the rate of change of the vessel's diameter: vasomotion. The model results agree with the experimental observations. The sensitivity of the vasomotion's dependence on parameter values and its significance to experimental protocols are examined. Further, it is hypothesized that the dependence of calcium-channel openings on voltage is shifted by changes on transmural pressure. Thus, Harder's experimental results are reproduced, among them the decreasing of vessel diameter with increasing pressure. Those behaviors are associated with a pattern of change of the singularities of the system of equations describing the model. This suggests a functional relationship on the interactions of Ca2+ and K+ fluxes responsible for the myogenic response; it may not result from a single molecular mechanism. The model is constructed so that additional experimental information can be readily incorporated.

Contraction history modulates isotonic shortening velocity in smooth muscle

The effect of contraction history on the isotonic shortening velocity of canine tracheal smooth muscle was investigated. Muscles were contracted isometrically for 20 s at initial lengths of L(o) (length of maximal active force), 85% L(o), or 70% L(o) using electrical field stimulation. Muscles were then allowed to shorten isotonically under different afterloads either with or without first being subjected to a step decrease in length to 70% L(o). Instantaneous velocities were plotted against instantaneous muscle length during isotonic shortening. Regardless of protocol, the velocity at any muscle length during shortening was lower when the muscle was initially activated at a longer length. The isotonic shortening velocity decreased progressively during shortening at a nearly linear rate with respect to instantaneous muscle length under all conditions. Results suggest that a longer muscle length at the time of activation leads to the development of higher loads on the contractile element during subsequent shortening, resulting in a slower shortening velocity. This plasticity of the force-velocity relationship may result from cytostructural reorganization of the smooth muscle cells in response to contractile activation at different muscle lengths.

A mathematical description of the myogenic response in the microcirculation

A mathematical model for description of static and dynamic myogenic responses to change of vascular transmural pressure in the arterioles of skeletal muscle was developed for the purpose of elucidating some basic characteristics of the myogenic vascular control system which have proved difficult to reveal by physiological observations alone. The model, which is a refined version of a previous one (Borgström & Grände 1979), is based on a force-equilibrium in the arteriolar wall, including passive forces related to vascular transmural pressure, wall elasticity, and wall viscosity, an active force related to resting vascular tone, and the active static and dynamic myogenic forces considered to be related to and triggered by wall tension (force) and its rate of change as indicated by our previous results. The effects of biological inertia, of shifts along the length--tension curve of the smooth muscle, and of pressure induced reactions in the more proximal arterial vessels were taken into account in the present force-equilibrium equation for the arterioles. Arteriolar wall viscosity was assumed to decrease with increasing rate of wall movement, a behaviour predicted by the model and corroborated by in vitro observations on larger vessels. The model was found capable of faithfully simulating microvascular myogenic responses in cat skeletal muscle in vivo in response to ramp as well as impulse transmural pressure stimuli over the entire biological range from maximum constriction to dilatation. With such characteristics, it can serve as a useful complement to physiological approaches in attempts to define more precisely the mode of operation of the myogenic control system and to reveal inherent complexities of biophysical factors and of interaction of other control mechanisms in microvascular regulation in vivo, as exemplified by presented tests and preliminary results.

Length dependence of calcium activated isometric force and immediate stiffness in living and glycerol extracted vascular smooth muscle

Vascular smooth muscle series elasticity was examined in living and glycerinated preparations as a function of the tissue length or calcium elicited force. Isometrically contracted smooth muscle strips were submitted to small quick stretches and releases (rise time 1.5 ms). The resulting immediate tension changes were proportion to the length changes for length steps ranging from -0.5% L0 to +2% L0. Plotting the immediate tension changes versus the length steps resulted in force-extension diagrams of the series elasticity (T1-curves). The linear parts of the T1-curves extrapolated to a common abscissa intercept of about -1% to -2% L0 irrespective of the tissue length or the degree of calcium activated force. The slopes of the T1-curves taken as the stiffness of the series elasticity increased in proportion to the isometric tension and depended on the tissue length or the degree of calcium activation in a similar way as tension. It is concluded that tension changes due to changes in the calcium concentration or the tissue length are caused by a change in the number of attached crossbridges. Results obtained in "skinned" fibres were similar to the one obtained in living fibres indicating that electromechanical coupling was not a major factor in determining the decrease in isometric tension and stiffness at short lengths.

Myogenic microvascular responses to change of transmural pressure. A mathematical approach

The recently described static and dynamic myogenic responses in the sympathectomized skeletal muscle microvessels to a given transmural pressure (PT) change applied at different rates (dPT/dt) (Grände & Mellander 1978), were further analysed in this study with a mathematical approach. The hypothesis that myogenic reactions are triggered by and related to wall tension was also tested. The mathematical model was based on a force-equilibrium in the microvessel wall including passive forces related to vascular transmural pressure, elasticity, and wall-viscosity, and active myogenic forces related to wall tension and its rate of change. Great resemblance was demonstrated between microvascular resistance curves obtained with the model and corresponding curves observed in vivo, indicating that the model quite adequately can describe myogenic microvascular resistance responses to transmural pressure stimuli. The results support the myogenic hypothesis in general and, in particular, the concept of an important rate-sensitivity in myogenic microvascular control and are compatible with the view that myogenic reactions are triggered by and related to change of wall tension. The model, in addition, provided data for certain microvascular variables which are difficult to assess by in vivo observations, e.g. Young's modulus of elasticity, wall tension, its rate of change, and internal vessel radius, and it offered a means to define more precisely the role of physical factors like effects of Poiseuille's and Laplace's laws in vascular resistance regulation.

Analysis of the length response to a force step in smooth muscle from rabbit urinary bladder

Responses to isotonic quick release of AC-stimulated smooth muscle strips from rabbit urinary bladder were analysed. Releases were performed at the peak of contraction and at a preset tension level in the contraction and relaxation phase. In other expts. responses at 37 degrees C and 27 degrees C were compared. The length response always consisted of 3 parts: (1) elastic recoil, (2) rapid length change (isotonic transient), (3) steady length change. Qualitatively, phases (1)-(3) could be distinguished also in responses to isotonic quick stretch. The immediate elastic recoils, phase (1), were described by exponential stress-strain relations. Stiffness was found to be somewhat lower during relaxation than during contraction. No effect of temperature on the elastic recoil was seen. The initial velocity in phase (2) was 2-3 times greater than the velocity 100 ms after release. By means of computer analysis of the length records during phases (2) and (3) two decaying exponential processes with widely different time constants could be separated. The time constant of the faster process was of the order of 15-30 ms at 37 degrees C. It decreased with increasing force steps and with increasing temperature. The amount of shortening associated with this process was correlated with the size of the force step, reaching a maximum of about 1.2% of the muscle length. The shortening velocities in phase (3), measured 100 ms after release, were described by Hill's equation. Vmax in the rising part and at the peak of contraction were 0.7 and 0.6 L/s respectively at 37 degrees C. Lower values were found during relaxation and at 27 degrees C. We suggest that part of the elastic recoil in phase (1) occurs in structures associated with the individual cross-bridges, that phase (2) is dominated by a change in the distribution of conformations of bridges in the attached position and that the shortening rate in phase (3) is determined by the entire cycle of events during turnover of bridges after the muscle has adapted to the new load. Observations on the force response to length steps and on shifts from isometric to afterloaded isotonic contraction and vice versa are consistent with this interpretation.

Elasticity of the spinal cord, pia, and denticulate ligament in the dog

The longitudinal elasticities of the dog spinal cord, pia, and denticulate ligaments were obtained by measuring the load-elongation curves. Both pia and denticulate ligaments were moderately elastic, but the spinal cord substance showed an upward curve, which indicated a predominantly viscous character. In situ, the spinal cord and pia were moderately strained. A 40-mm segment of the cord with dorsal and ventral roots severed and denticulate ligaments removed became 1 mm shorter when the cord and pia were transected at both ends. It was estimated that the cord and pia in situ were stressed by a force of 2 to 3 gm, the denticulate ligaments with 3 to 5 gm, and the dura with 50 to 70 gm. The elastic moduli of the spinal cord substance were 1.68 X 10(5) dynes/sq cm for loading between 5 and 10 gm and 1.19 X 10(5) dynes/sq cm between 30 and 35 gm.

Elasticity of the spinal cord dura in the dog

The elasticity of the spinal cord dura in the dog has been investigated histologically, in situ, and by measurement. The dura was composed of collagenous and elastic connective tissue fibers. The collagenous fibers were arranged in longitudinal bundles, straight when stretched and wavy when unstretched, with a delicate network of fine elastic fibers coursing in all directions. Transecting the cord and dura at T-5 caused a separation of 25 to 30 mm of the dura and a 15- to 20-mm gap in the cord. By means of an appropriate sequence of transections of nerve roots and denticulate ligaments within the dura, and transections of the dural sheaths and nerves outside the dura, the strain on the dura was found to be imposed by the attachments of the dural nerve sheaths from T-6 to S-7. The filum terminale was not appreciably strained. By adding weights to a suspended dura, two components of elasticity were found. For loads of 0 to 50 gm, the incremental displacements in the length were large. The elastic modulus was about 4 X 10(6) dynes/sq cm, which was comparable to that of elastic fibers. For loads of 50 to 150 gm the displacements in length were small. The elastic modulus was about 5 X 10(8) dynes/sq cm, which was comparable to that of collagenous fibers.