Length and myofilament spacing-dependent changes in calcium sensitivity of skeletal fibres: effects of pH and ionic strength

Abstract

The calcium sensitivity of force was measured in glycerinated rabbit psoas fibres at sarcomere lengths (SL) from 2.3 to 3.4 micron. Increased SL caused calcium sensitivity to increase and the slope of force-calcium relations to decrease. We have hypothesized that length-dependent changes in myofilament lattice spacing and the presence of fixed charge on the myofilaments are important in determining calcium sensitivity. Lattice spacing changes were monitored by measuring fibre diameter (D). D was decreased by increasing SL, decreasing bathing solution pH and by osmotic compression with 3% PVP. 3% PVP caused D to decrease by about 15% at all SLs and pH values tested. Force-calcium relations were measured at different SLs and pH values, with and without 3% PVP in the bathing solutions. At all pH values D at SL 2.3 micron with 3% PVP was comparable to the value at 3.4 micron, without PVP. At pH 7.5 and 7.0 calcium sensitivity was about the same at both SL, although the slope of the force-calcium relation was less at longer SL. The similarity of the calcium sensitivity at the same D, but much different SL, indicates that lattice spacing is important in determining calcium sensitivity, while SL and the degree of myofilament overlap are important in determining the slope of force-calcium relations. In order to test for the role of myofilament charge in determining calcium sensitivity, pH and ionic strength were varied. Decreasing pH caused decreased maximum force and calcium sensitivity. In addition, the influence of SL on calcium sensitivity decreased as pH was lowered, with minimal SL dependence at pH 5.5; even though lattice spacing still decreased with increasing SL. When D was decreased with PVP, calcium sensitivity increased at all SLs in pH 7.5 and 7.0 while the same lattice spacing changes at pH 6.0 and 5.5 resulted in greatly reduced shifts in calcium sensitivity. These results indicate that the effect of lattice spacing on calcium sensitivity depends on myofilament charge. At pH 6.0, even though osmotic compression of the lattice has no effect, increasing SL causes about half the shift in calcium sensitivity seen at pH 7.0. Lowering ionic strength from 200 to 110 mM caused an increase in both the magnitude and length dependence of calcium sensitivity at pH 7.0, while at pH 5.5 both decreased.(ABSTRACT TRUNCATED AT 400 WORDS)

Historical perspective on heart function: the Frank–Starling Law

“…The groups of Moss et al (1983) and Stienen et al (1985) indicated that lattice spacing, rather than length, was responsible for the changes in Ca 2+ -sensitivity in membranepermeabilized skeletal muscles. These results were confirmed in skeletal (Godt and Maughan 1981;Wang and Fuchs 1995;Martyn and Gordon 1988) and cardiac (Wang and Fuchs 1995;McDonald and Moss 1995) muscle preparations which demonstrated that osmotic compression (as a function of muscle width) enhances Ca 2+ sensitivity. These findings were in agreement with the proposition of Hofmann and Fuchs (1987a, 1987a, 1987b who demonstrated that Ca 2+ -binding to cTnC directly regulates cross-bridges interactions (rather than sarcomere length).…”

Ca 2+ -regulated structural changes in troponin

“…Single permeable rabbit skeletal muscle fibers or strips of permeable rabbit cardiac tissue were mounted between a sensitive force transducer and a fast motor, and isometric tension was measured as a function of Ca concentration, as described (35,36).…”

Kinetic Studies of Calcium Binding to the Regulatory Site of Troponin C from Cardiac Muscle

“…Briefly, endogenous TnC was extracted from the fibers, and the TnC-depleted fibers were reconstituted with an exogenous TnC to be tested. Force was measured on the reconstituted fibers as a function of free Ca 2ϩ concentration as described previously (Martyn and Gordon, 1988).…”