Inhibitory influence of phosphate and arsenate on contraction of skinned skeletal and cardiac muscle

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Abstract
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.