RATE OF PHOSPHATE RELEASE AFTER PHOTOLIBERATION OF ADENOSINE 5'-TRIPHOSPHATE IN SLOW AND FAST SKELETAL-MUSCLE FIBERS

Inorganic phosphate (P-i) release was determined by means of a fluores cent P-i-probe in single permeabilized rabbit soleus and psoas muscle fibers. Measurements of P-i release followed photoliberation of simila r to 1.5 mM ATP by flash photolysis of NPE-caged ATP in the absence an d presence of Ca2+ at 15 degrees C. In the absence of Ca2+, P-i releas e occurred with a slow rate of 11 +/- 3 mu M . s(-1) (n = 3) in soleus fibers and 23 +/- 1 mu M . s(-1) (n = 10) in psoas fibers. At saturat ing Ca2+ concentrations (pCa 4.5), photoliberation of ATP was followed by rapid force development. The initial rate of P-i release was 0.57 +/- 0.05 mM . s(-1) in soleus (n = 13) and 4.7 +/- 0.2 mM . s(-1) in p soas (n = 23), corresponding to a rate of P-i release per myosin head of 3.8 . s(-1) in soleus and 31.5 s(-1) in psoas. P-i release declined at a rate of 0.48 s(-1) in soleus and of 5.2 s(-1) in psoas. P-i rele ase in soleus was slightly faster in the presence of an ATP regenerati ng system but slower when 0.5 mM ADP was added. The reduction in the r ate of P-i release results from an initial redistribution of cross-bri dges over different states and a subsequent ADP-sensitive slowing of c ross-bridge detachment.