Abstract
The intrinsic phosphocreatine (PCr) T~1~ values measured by time‐dependent magnetization transfer in isolated perfused rat, hamster, and turkey hearts were indistinguishable. The value of 3.5 ± 0.3 s for the rat heart is similar to values measured by other magnetization transfer methods. Irreversibly inhibiting the phosphoryl exchange between PCr and ATP in the rat heart using iodoacetamide changed the apparent T~1~ values of the two exchanging species when measured by inversion recovery: The apparent T~1~ of PCr increased from 1.92 ± 0.06 s to 3.55 ± 0.06 s, in excellent agreement with the intrinsic T~1~, measured by magnetization transfer. The apparent T~1~ of [γ‐P]ATP decreased from 0.92 ± 0.07 s to 0.44 ± 0.03 s. The value for the T~1~ of [γ‐P]ATP in hearts with inhibited phosphoryl exchange was similar to T~1~ values for [α‐P]ATP and [β‐P]ATP, which remained unchanged. This illustrates that apparent T~1~ values for PCr and [γ‐P]ATP measured by inversion recovery in the presence of exchange are average T~1~ values in between the intrinsic values. The large differences between the intrinsic T~1~ measured by magnetization transfer and the T~1~ measured by inversion recovery makes the use of the appropriate value in different applications quantitatively important.