AJP - Cell Physiology, Vol 257, Issue 6 C1149-C1157, Copyright © 1989 by American Physiological Society
Linear dependence of muscle phosphocreatine kinetics on total creatine content
R. A. Meyer
Department of Physiology, Michigan State University, East Lansing 48824.
Phosphorus nuclear magnetic resonance (NMR) spectra and twitch tension were
recorded during stimulation of gastrocnemius muscles of pentobarbital
sodium-anesthetized rats which had been fed the creatine analogue
beta-guanidinopropionic acid (beta-GPA, 2% diet) for periods from 0
(control) to 8 wk. Total creatine content of unstimulated muscles decreased
by 42, 67, 82, and 88% compared with controls after 2-, 4-, 6-, and 8-wk
feeding, respectively. The staircase effect observed in control muscles
during 8 min of twitch stimulation at 0.25, 0.5, and 0.75 Hz was reduced
after 2- to 8-wk beta-GPA feeding. However, after 6- to 8-wk feeding, the
twitch force at the end of 8 min of stimulation was not different from
controls. The time constant for phosphocreatine (PCr) changes at the onset
of and during recovery after stimulation was proportional to total creatine
content. The relationship between PCr content and twitch rate times force
at the end of stimulation was linear, with slope proportional to total
creatine content. PCr content in beta-GPA-fed animals was transiently
greater during recovery than before stimulation, suggesting a regulatory
effect of the inorganic phosphate released by hydrolysis of phosphorylated
beta-GPA. The results are consistent with linear models of respiratory
control in which the creatine kinase reaction acts as a simple buffer of
adenine nucleotide levels.
