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-y-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 mmphosphocreatine
(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. Vm.x 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 ,umol g-1 min-'. 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.
8. The effects of ionic strength on the behaviour of the skinned smooth muscle fibre may involve an influence on the filament system, but are also compatible with an increased binding of smooth muscle myosin to actin at low ionic strength. The results from activated and rigor muscle suggest that ionic strength does not exclusively affect kinetics of rapid cross-bridge equilibria but may also influence mechanical properties of attached cross-bridge states.