Some quantitative aspects of myoplasmic ATPMg and total internal ATP and ArP levels in resting barnacle muscle fibres.

1. Flash height recorded following the injection of firefly into the external calibration medium depends on the concentration of K-glutamate used, e.g. 120 mM K glutamate reduces flash height by approximately 20 percent. 2. Suspending the fibre in air instead of artificial sea water (ASW) or replacement of NaCl in the bathing medium with Na-glutamate fails to alter flash height. 3. Firefly preparations from DuPont, Packard and SAI give similar myoplasmic ATPMg values viz. 1.1 mM. 4. Analysis of 36 fibres shows the following: myoplasmic ATP = 1.03 +/- 0.06 mM; total ATP (firefly method) = 5.26 +/- 0.12 mmol/kg water; total ATP (enzymic fluorimetry) = 6.27 +/- 0.13 mmol/kg water and ArP = 20.76 +/- 0.59 mmol/kg water. 5. Measurement of ATPMg in samples of myoplasmic aspirate gives a value that is greater than that obtained in situ. 6. Iodoacetate, whether applied externally or internally, reduces resting luminescence in a dose-dependent manner. It also reduces myoplasmic ATP and total ATP. 7. 2-Deoxy-d-glucose fails to reduce myoplasmic ATP but reduces total ATP. 8. Diethylpyrocarbonate, whether applied externally or internally, reduces myoplasmic ATP. It also causes a slow decline in ArP but little change in total ATP. 9. Injection of L-arginine causes a fall in resting luminescence in some fibres while in others it causes a prompt transitory rise. Injection of L-arginine also causes a fall in total ATP. 10. Collectively, these results suggest that the immediate buffering system in the myoplasm is ArP and that ATP supplied by glycolysis lies in a compartment, presumably the interfibrillar space, which is inaccessible to injected firefly.