The effects of inorganic phosphate (P(i)) on force, Ca(2+)-force relationship, ATPase activity, maximal shortening velocity (Vmax) and rate of tension development were investigated in chemically skinned preparations of smooth muscle from the guinea-pig taenia coli. 2. In maximally thiophosphorylated fibres, P(i) in the range 1-40 mM inhibited isometric force, with a reduction of 20% at 20 mM P(i). 3. The relative force was similar at all [Ca2+], i.e. the Ca(2+)-force relationship was not affected, when 20 mM P(i) was present. 4. After photolytic release of ATP from caged ATP in maximally thiophosphorylated fibres in the presence of 20 mM P(i), tension rose to a lower level but with a higher rate constant than in the absence of P(i). 5. Inorganic phosphate (20 mM) did not affect the ATP hydrolysis in fibres activated at intermediate [Ca2+] or by maximal thiophosphorylation. 6. Inorganic phosphate (20 mM) decreased force but did not influence Vmax in maximally activated fibres. At lower levels of activation by Ca2+, P(i) increased the Vmax and decreased force slightly without affecting the degree of myosin light chain phosphorylation. 7. We conclude that P(i) influences cross-bridge reactions associated with force generation in smooth muscle. These reactions are not rate limiting for cross-bridge turnover under isotonic or isometric conditions in maximally activated smooth muscle fibres, since P(i) did not influence Vmax or the rate of ATP turnover. 8. Since P(i) increased Vmax in submaximally activated muscles, we propose that, under these conditions, shortening velocity is rate limited by cross-bridge states, reached early after attachment, which impose a mechanical resistance to shortening.
摘要
研究了无机磷酸盐（P(i)）对豚鼠结肠带平滑肌化学去膜制剂中力、Ca(2+) - 力关系、ATP 酶活性、最大缩短速度（Vmax）和张力发展速率的影响。2. 在最大硫代磷酸化纤维中，1 - 40 mM 范围内的 P(i) 抑制等长力，在 20 mM P(i) 时力降低 20%。3. 当存在 20 mM P(i) 时，在所有[Ca2+]下相对力相似，即 Ca(2+) - 力关系未受影响。4. 在 20 mM P(i) 存在的情况下，最大硫代磷酸化纤维中从笼形 ATP 光解释放 ATP 后，张力上升到较低水平，但速率常数高于不存在 P(i) 时。5. 无机磷酸盐（20 mM）不影响在中等[Ca2+]或通过最大硫代磷酸化激活的纤维中的 ATP 水解。6. 无机磷酸盐（20 mM）降低最大激活纤维中的力，但不影响 Vmax。在较低的 Ca2+ 激活水平下，P(i) 增加 Vmax 并轻微降低力，而不影响肌球蛋白轻链磷酸化程度。7. 我们得出结论，P(i) 影响与平滑肌力产生相关的横桥反应。在最大激活的平滑肌纤维中，在等张或等长条件下，这些反应不是横桥周转的限速因素，因为 P(i) 不影响 Vmax 或 ATP 周转速率。8. 由于 P(i) 在次最大激活的肌肉中增加 Vmax，我们提出，在这些条件下，缩短速度受附着后早期达到的横桥状态限速，这些状态对缩短施加机械阻力。
