| {dollar}sp{lcub}31{rcub}{dollar}P NMR methods were used to examine the relationship among creatine kinase (CK) forward (Flux), CK reactant concentrations, and myocardial oxygen consumption rate (MVO{dollar}sb2{dollar}) over a wide range of MVO{dollar}sb2{dollar} in rat hearts perfused with either glucose alone (G-perfused) or pyruvate plus glucose (PG-perfused). The potential regulatory role of the CK reactants was evaluated by CK reaction modeling, Hanes-Woolf plots, and sensitivity calculations based on the CK rate equation. At all MVO{dollar}sb2{dollar} values examined, Flux in the G-perfused hearts was consistently higher and varied to a greater extent than in the PG-perfused hearts. These results contradict the idea that there exists a unique coupling between Flux and MVO{dollar}sb2{dollar} in perfused rat hearts. Results of the reaction modeling suggest that the published values for the CK kinetic constants are probably not applicable to CK in intact rat hearts and that the mechanism of Flux regulation may vary depending on the type of exogenous carbon source. Both the Hanes-Woolf analysis and sensitivity calculations indicate that Flux is primarily determined by free cytosolic (ADP) and somewhat less by (CP).; Using computer simulations based on the Bloch equations, two frequency-selective adiabatic pulses have been developed. The first, a slice selective adiabatic excitation (SSAX) pulse, is capable of generating {dollar}Bsb1{dollar}-independent slice selection in a single shot, a feature presently shared by no other pulse. SSAX should therefore be particularly useful in surface coil applications where multi-slice capability is desired, sample motion is present, and/or subtraction errors are significant. The second pulse achieves its slice selectivity by executing amplitude and frequency modulation in the presence of a time varying {dollar}Bsb0{dollar} field gradient. This gradient modulated adiabatic excitation (GMAX) pulse is extremely insensitive to large variations in {dollar}Bsb1{dollar} magnitude and is therefore quite suitable for surface coil imaging and/or spectroscopy applications which may be hampered by transmission of inhomogeneous {dollar}Bsb1{dollar} fields.; A modified version of the numerical optimization protocol of Ugurbil et al. has been developed. The protocol now addresses factors which dictate the off-resonance performance as well as {dollar}Bsb1{dollar} insensitivity of adiabatic pulses. |
