Force-velocity relation and rate of ATP hydrolysis in osmotically compressed skinned smooth muscle of the guinea pig

Multiscale and Multiaxial Mechanics of Vascular Smooth Muscle

Mathematical models can facilitate an integrative understanding of the complexity underlying biological structure and function, but they must be informed and validated by empirical data. Uniaxial contraction of an arterial ring is a well-used in vitro approach for studying characteristics of smooth muscle contractility even though this experimental arrangement does not mimic the in vivo vascular geometry or loading. In contrast, biaxial contraction of an inflated and axially extended excised vessel provides broader information, both passive and active, under more realistic conditions. Few investigations have compared these two in vitro approaches directly, namely how their results overlap, how they differ, or if each provides unique complementary information. Toward this end, we present, to our knowledge, a new multiscale mathematical model of arterial contractility accounting for structural and functional constituents at molecular, cellular, and tissue levels. The artery is assumed to be a thick-walled incompressible cylinder described by an anisotropic model of the extracellular matrix and, to our knowledge, novel model of smooth muscle contractility. The latter includes a 3D structural sensitivity to deformation, including microscale muscle filament overlap and filament lattice spacing. The overall model captures uniaxial and biaxial experimental contraction data, which was not possible when accounting for filament overlap alone. The model also enables parameter sensitivity studies, which confirmed that uniaxial contraction tests are not as efficient as biaxial tests for identifying changes in vascular smooth muscle function.

The effect of lattice spacing change on cross-bridge kinetics in chemically skinned rabbit psoas muscle fibers. I. Proportionality between the lattice spacing and the fiber width

Chemically skinned rabbit psoas muscle fibers/bundles were osmotically compressed with a macromolecule dextran T-500 (0-16%, g/100 ml) at 20 degrees C, 200 mM ionic strength, and pH 7.0. The lattice spacing of psoas bundles was measured by equatorial x-ray diffraction studies during relaxation and after rigor induction, and the results were compared with the fiber width measurements by optical microscopy. The purpose of the present study is to determine whether fiber width is a reliable measure of the lattice spacing, and to determine the available spacing for myosin cross-bridges between the thick and thin filaments. We observed that both the lattice spacing and the fiber width decreased with an increase in the dextran concentration during relaxation or after rigor induction, and that the spacing and the fiber width were proportionately related. We further observed that, in the absence of dextran, the lattice spacing (and the fiber width) shrank on a relax-to-rigor transition, whereas in the presence of 16% dextran, the spacing expanded on a relax-to-rigor transition. The cross-over of these plots occurred at the 4-7% dextran concentration. During Ca activation, the fiber width shrank in the absence of dextran, and it slightly expanded in the presence of 14.4% dextran. The degree of expansion was not as large as in the relax-to-rigor transition, and the cross-over occurred at about 11% dextran concentration. We also carried out experiments with dextran T-40 and T-10 to determine the upper limit of the molecular weight that enters the lattice space. We found that the upper limit is about 20 kD.

The effect of the lattice spacing change on cross-bridge kinetics in chemically skinned rabbit psoas muscle fibers. II. Elementary steps affected by the spacing change

The actin-myosin lattice spacing of rabbit psoas fibers was osmotically compressed with a dextran T-500, and its effect on the elementary steps of the cross-bridge cycle was investigated. Experiments were performed at the saturating Ca (pCa 4.5-4.9), 200 mM ionic strength, pH 7.0, and at 20 degrees C, and the results were analyzed by the following cross-bridge scheme: [formula: see text] where A = actin, M = myosin head, S = MgATP, D = MgADP, and P = Pi = phosphate. From MgATP and MgADP studies on exponential process (C) and (D), the association constants of cross-bridges to MgADP (K0), MgATP (K1a), the rate constants of the isomerization of the AM S state (k1b and k-1b), and the rate constants of the cross-bridge detachment step (k2 and k-2) were deduced. From Pi study on process (B), the rate constants of the cross-bridge attachment (power stroke) step (k4- and k-4) and the association constant of Pi ions to cross-bridges (K5) were deduced. From ATP hydrolysis measurement, the rate constant of ADP-isomerization (rate-limiting) step (k6) was deduced. These kinetic constants were studied as functions of dextran concentrations. Our results show that nucleotide binding, the ATP-isomerization, and the cross-bridge detachment steps are minimally affected by the compression. The rate constant of the reverse power stroke step (k-4) decreases with mild compression (0-6.3% dextran), presumably because of the stabilization of the attached cross-bridges in the AM*DP state. The rate constant of the power stroke step (k4) does not change with mild compression, but it decreases with higher compression (> 6.3% dextran), presumably because of an increased difficulty in performing the power stroke. These results are consistent with the observation that isometric tension increases with a low level of compression and decreases with a high level of compression. Our results also show that the association constant K5 of Pi with cross-bridge state AM*D is not changed with compression. Our result further show that the ATP hydrolysis rate decreased with compression, and that the rate constants of the ADP-isomerization step (k6) becomes progressively less with compression. The effect of compression on the power stroke step and rate-limiting step implies that a large-scale molecular rearrangement in the myosin head takes place in these two slow reaction steps.

Limits to shortening in smooth muscle tissues

The extent of shortening in smooth muscle tissues is limited by a number of internal and external factors. In this study, continuous measurements of the stiffness of active muscle were made to characterize the mechanical forces acting to limit shortening. Rabbit ovarian ligament and mesotubarium superius muscles were allowed to shorten as far as possible under light afterloads; under these conditions a stiffness increase was observed that was closely related to the instantaneous muscle length and that was unaffected by other factors influencing the degree of shortening (afterload, time and intensity of activation, temperature, etc.). The results are considered in terms of a hypothesis relating the tissue-based constraints on radial expansion at short lengths to an additional load on the contractile apparatus, an internal force that is externally manifested as an increase in axial stiffness. Changing the cellular volume by varying the tonicity of the bathing medium provided tentative confirmation of the hypothesis.

Supercontracted state of vertebrate smooth muscle cell fragments reveals myofilament lengths

Isolated cell preparations from chicken gizzard smooth muscle typically contain a mixture of cell fragments and whole cells. Both species are spontaneously permeable and may be preloaded with externally applied phalloidin and antibodies and then induced to contract with Mg ATP. Labeling with antibodies revealed that the cell fragments specifically lacked certain cytoskeletal proteins (vinculin, filamin) and were depleted to various degrees in others (desmin, alpha-actinin). The cell fragments showed a unique mode of supercontraction that involved the protrusion of actin filaments through the cell surface during the terminal phase of shortening. In the presence of dextran, to minimize protein loss, the supercontracted products were star-like in form, comprising long actin bundles radiating in all directions from a central core containing myosin, desmin, and alpha-actinin. It is concluded that supercontraction is facilitated by an effective uncoupling of the contractile apparatus from the cytoskeleton, due to partial degradation of the latter, which allows unhindered sliding of actin over myosin. Homogenization of the cell fragments before or after supercontraction produced linear bipolar dimer structures composed of two oppositely polarized bundles of actin flanking a central bundle of myosin filaments. Actin filaments were shown to extend the whole length of the bundles and their length averaged integral to 4.5 microns. Myosin filaments in the supercontracted dimers averaged 1.6 microns in length. The results, showing for the first time the high actin to myosin filament length ratio in smooth muscle are readily consistent with the slow speed of shortening of this tissue. Other implications of the results are also discussed.

Calcium sensitivity and energetics of contraction in skinned smooth muscle of the guinea pig taenia coli at altered pH

Calcium-sensitivity of contraction, force-velocity relation and ATP hydrolysis rate at different pH (6.2-7.8) were investigated in skinned smooth muscle preparations from the guinea pig taenia coli. Varied free-calcium levels were buffered by 4 mM BAPTA (1,2-bis(2-aminophenoxy)-ethane-N,N,N'N'-tetraacetic acid) which has calcium binding properties little affected by pH. A small increase of calcium-sensitivity of contraction was seen at pH 6.2 compared to 6.9 and 7.8 (ED50 shift of about 0.15 pCa units). The isometric force and Vmax in fibres activated either by calcium or by thiophosphorylation of the myosin light chains were each reduced by about 15% at pH 6.2 compared to 6.9 and 7.8. Following an isotonic quick release the shortening velocity decreases with time. This effect was more pronounced at pH 6.2 than at pH 6.9 or 7.8. The ATP hydrolysis rates in relaxed and thiophosphorylated fibres were essentially unaffected by alteration in pH between 6.2 and 7.8. Due to the lower force, energetic cost of force maintenance was thus somewhat increased at pH 6.2. These results suggest that pH alteration between 6.2 and 7.8 have effects on the properties of the contractile machinery of the smooth muscle in the skinned guinea pig taenia coli. The effects are however small and therefore probably of little functional importance over a pH range which should cover most cases of intracellular pH alteration under physiological or pathophysiological conditions.

Cross-bridge behaviour in skinned smooth muscle of the guinea-pig taenia coli at altered ionic strength

1. The effects of varied levels (25-300 mM) of ionic strength on mechanical properties and ATP hydrolysis rate of chemically skinned guinea-pig taenia coli fibres were investigated. 2. The tension development following activation by calcium (pCa 4.8), and relaxation following removal of calcium (pCa 9), were slower in 25 mM compared to 150 mM ionic strength. In fibres activated by thiophosphorylation of myosin light chains, by exposure to ATP-gamma-S, the tension development was rapid and independent of ionic strength. 3. The maximal shortening velocity (Vmax) was obtained from force-velocity relations determined by the quick-release method. The rate of ATP hydrolysis (JATP) was determined by measurement of pyruvate released from phosphoenolpyruvate (PEP). In order to obtain maximal Vmax and JATP at a Mg-ATP concentration of 1 mM, an ATP regenerating system was required. In thiophosphorylated fibres 2 mM-phosphocreatine (PCr) or 3.2 mM-PEP were adequate for maximal Vmax and JATP respectively. In calcium-activated fibres 5 mM-PCr was required for maximal Vmax. 4. The isometric force of thiophosphorylated fibres showed a biphasic dependence on ionic strength with a maximum at 90 mM. Vmax was essentially unchanged between 50 and 200 mM ionic strength. At 25 mM ionic strength, isometric force and Vmax were decreased by, respectively, about 15 and 25%. At 250 mM ionic strength, isometric force and Vmax were decreased by, respectively, 47 and 33%. 5. Vmax decreased with decreasing [Mg-ATP]. At [Mg-ATP] less than 0.1 mM there was no difference in Vmax between 35 and 150 mM ionic strength. At 250 mM ionic strength Vmax was lower than that at 150 mM at all [Mg-ATP]. 6. JATP during contraction in thiophosphorylated fibres at 35, 150 and 250 mM ionic strength was respectively, 0.62, 0.98 and 0.93 mumol g-1 min-1. The energetic tension cost (JATP/force) increased with ionic strength. 7. The force response to a quick stretch was investigated in the relaxed, contracted and rigor states at 25, 150 and 250 mM ionic strength. Stiffness in the relaxed state increased with speed of stretch and was higher the lower the ionic strength. In the contracted and rigor states, stiffness was also affected by ionic strength, but the relative effect in the contracted state was small.(ABSTRACT TRUNCATED AT 400 WORDS)