Nosek T M, Leal-Cardoso J H, McLaughlin M, Godt R E
Department of Physiology and Endocrinology, Medical College of Georgia, Augusta 30912-3000.
Am J Physiol. 1990 Dec;259(6 Pt 1):C933-9. doi: 10.1152/ajpcell.1990.259.6.C933.
It has been widely observed that Pi decreases maximum calcium-activated force (Fmax) and calcium sensitivity of skinned skeletal and cardiac muscle. However, whether a particular ionic species of Pi (i.e., H2PO4-) is responsible for these effects is controversial. To clarify this issue, we examined the influence of Pi and its structural analogue arsenate (Asi) on contraction of skinned rabbit psoas (fast twitch), soleus (slow twitch), and cardiac papillary muscle. Asi decreased Fmax of all three muscles types to a greater extent than Pi. Both Pi and Asi decreased calcium sensitivity of psoas and cardiac muscles, with Asi having the greater effect. The effect of the protonated form of Pi and Asi on Fmax was evaluated by measuring the response to 30 mM total Pi or Asi at pH 7.4, 7.0, 6.6, and 6.2. In psoas fibers we found that both Pi and Asi were more effective in decreasing Fmax as the pH was lowered (i.e., as the concentration of the diprotonated forms increased). On the contrary, soleus and cardiac fibers did not exhibit this behavior. These differences in the effects of Pi and Asi on Fmax in psoas vs. cardiac and soleus muscles may be related to differences in their myosin heavy chains other than the binding site for the gamma-phosphate of ATP which appears to be conserved for all myosins.
人们普遍观察到，无机磷酸盐（Pi）会降低去表皮的骨骼肌和心肌的最大钙激活力（Fmax）以及钙敏感性。然而，Pi的特定离子种类（即H2PO4-）是否导致这些效应仍存在争议。为了阐明这个问题，我们研究了Pi及其结构类似物砷酸盐（Asi）对去表皮的兔腰大肌（快肌纤维）、比目鱼肌（慢肌纤维）和心脏乳头肌收缩的影响。Asi比Pi更能降低所有这三种肌肉类型的Fmax。Pi和Asi均降低了腰大肌和心肌的钙敏感性，且Asi的作用更大。通过测量在pH 7.4、7.0、6.6和6.2时对30 mM总Pi或Asi的反应，评估了Pi和Asi的质子化形式对Fmax的影响。在腰大肌纤维中，我们发现随着pH降低（即随着双质子化形式的浓度增加），Pi和Asi在降低Fmax方面都更有效。相反，比目鱼肌和心脏纤维未表现出这种行为。Pi和Asi对腰大肌与心脏和比目鱼肌Fmax影响的这些差异，可能与它们肌球蛋白重链的差异有关，而不是与ATP的γ-磷酸结合位点有关，该位点似乎在所有肌球蛋白中都是保守的。
