Relaxation of chemically skinned guinea pig taenia coli smooth muscle from rigor by photolytic release of adenosine-5′-triphosphate

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Abstract
The mechanical events following release of ATP from P3-1-(2-nitro)phenylethyladenosine-5′-triphosphate (caged-ATP) in skinned guinea pig taenia coli smooth muscle in rigor were investigated. A rigor force of about 25–35% of the maximal active force was obtained by removing ATP at the plateau of a maximal active contraction. In the rigor solution free-Mg2+ was 2mm, ionic strength 90mm and pH 7.0. When caged-ATP (12.5mm) was diffused into the preparation there was no change in the rigor force. Photolytic production of about 2mm ATP was achieved with a xenon flash lamp. Following illumination, force decreased with an approximate initial rate constant of 0.7 s−1. The rate of relaxation was increased in the presence of inorganic phosphate (at 3mm: 1.3 s−1; 10mm: 2.2 s−1). At higher Mg2+ concentrations the rate of relaxation was slower (5mm: 0.2 s−1) and at lower concentrations the rate was faster (0.5mm: 1.2 s−1). An increased rate of relaxation was observed when ionic strength was increased to 150mm (2.2 s−1). Phosphate increased the rate of relaxation at the different levels of Mg2+ (0.5–10mm) and ionic strength (90, 150mm). In preparations shortened (by 1–3%) to give reduced rigor force, a small transient increase in tension was recorded after ATP release. In comparison to the rates of ATP-induced dissociation of actomyosin in solution, reported in the literature, the rate of relaxation from rigor is slower. This may reflect a slow rigor cross-bridge dissociation or mechanical interactions not associated with cross-bridges in the muscle fibre. However, the results may also be interpreted on the basis of a model proposed for striated muscle by Goldmanet al. (1984) where the relaxation from rigor in the absence of Ca2+ involves a phase of reattaching cross-bridges whose lifetime in a tension-producing state is influenced by phosphate.