Many anomalies exist in the resting 31P muscle spectra of boys with Duchenne muscular dystrophy (DMD) but few have been reported in Golden Retriever muscular dystrophy (GRMD), the closest existing animal model for DMD. Because GRMD is recommended for preclinical evaluation of therapies and quantitative outcome measures are needed, we investigated anomalies of 31P NMRS in tibial cranial and biceps femoris muscles from 14 GRMD compared to 9 control (CONT) dogs.
Alterations observed in DMD children – low phosphocreatine and high phospho-monoesters and -diesters – were all found in GRMD but increased pH was not. More surprisingly, inorganic phosphate (Pi) appeared to present a prominent splitting with an enhanced Pib resonance at 0.3 ppm downfield of Pia. Assuming that both resonances are Pi, the pH for Pia in GRMD corresponded to a physiological intracellular pHa (6.97 ± 0.05), while pHb approached the extracellular range (7.27 ± 0.10) and correlated with pHa in GRMD (R2 = 0.65). Both Pia and Pib were elevated compared to CONT and Pia increased with age for GRMD (R2 = 0.48, p < 0.001).
Magnetisation transfer experiments between γATP and Pi were conducted to better characterise Pi pools. Equal T1 relaxation times for Pib and Pia did not support a mitochondrial origin of Pib. We suggest that Pib could originate from degenerating hypercontracted cells that have a leaky membrane and inadequate cell homeostasis and pH regulation. Pib showed minimal chemical exchange in all dogs, while the exchange rate of Pia was reduced in GRMD and might extraneously reflect low glycolytic activity in DMD.
Taken together, the ensemble of 31P NMRS alterations identifies muscle dysfunction and could provide useful biomarkers of therapeutic efficacy. Furthermore, among these, two might relate more specifically to dystrophic processes and merit further investigation: one is the existence of the enhanced alkaline Pib pool; the other, mechanisms by which membrane disruption might increase phosphodiesters in dystrophy. Copyright © 2012 John Wiley & Sons, Ltd.