A new gating NMR technique triggered by pulses for the cardiac pacing of isolated, perfused rat heart is described. This technique is suitable for obtaining spectra at the same positions over the cardiac cycle with the same RF pulse repetition time and with constant heart rate. Accordingly, each recovered magnetization level at these positions during the cycle can be kept constant. It also allows a qualitative analysis of the myocardial energy level during the cycle under the condition of the same or different cardiac workload (rate pressure product, HR x LVP). This method was used to observe cyclical changes in the concentration of adenosine triphosphate (ATP), phosphocreatine (PCr), P(i), and in pH during the cardiac cycle, and confirmed that the levels of ATP and PCr showed reversible decreases, and P(i) showed an antiphasic increase at the peak systole. On the other hand, no significant changes in pH were noted during the cardiac cycle. This pacing-gated technique is an improvement over other gated NMR techniques triggered by the blood pressure or by ECG traces.

number of studies of myocardial energy metabolism using 31 P nuclear magnetic resonance (NMR) have been done on in vitro 1-4 and in vivo [5][6][7] hearts. Cyclical changes in the concentration of adenosine triphosphate (ATP), phosphocreatine (PCr), and inorganic phosphate (Pi) during the cardiac cycle have also been studied using the subject's blood pressure or electrocardiogram (ECG)-gated 31 P NMR techniques. Several studies have demonstrated that cyclical changes in ATP, PCr, and Pi arise in vitro [8][9][10] and in vivo 11 in rat hearts. However, some investigators have not observed any cyclical changes in energy-related phosphate levels in in vitro 12 and in vivo 5,13-15 hearts. Controversy remains regarding the presence of cyclical changes in energy-related phosphate levels. Hoh et al performed electrophoretic analyses in pyrophosphate gels of intact myosin of adult rat myocardium. 16 Their study revealed the presence of 3 distinct components in ventricular myosin (V1, V2, and V3) and an association between the distribution of these myosin components and its adenosine triphosphatase (ATPase) activity.We have developed a pacing-gated NMR technique that is triggered by pulses for cardiac pacing. 10 This is a suitable method for determining cyclical changes in energy-related phosphate compounds in the cardiac cycle because the heart rate and the RF pulse repetition time maintain a constant relationship with pacing. The purpose of the present study was to determine whether cyclical changes in energy-related phosphate levels arise during the cardiac cycle in isolated rat hearts and are affected by differences in myosin isozyme composition. To perform these studies, we used pacing-gated 31 P NMR spectroscopy and manipulated the cardiac workload. Hypo-and hyperthyroid rats, in which myocardial contractility and myocardial energy use should differ as a result of varying myosin isozyme composition, were used in the studies. Methods AnimalsThe experiments were performed on hearts removed from 8-week-old male Wistar rats of the following groups: (1) normal rats (n=10); (2) hypothyroid rats (n=10), who had received 0.8 mg/ml 6-n-propyl-2-thiouracil (PTU; Sigma, St Louis, MO, USA) in drinking water for 21 days, 17 and (3) hyperthyroid rats (n=5), who had received daily intraperitoneal injections of 500 g/kg body weight 3,5,3'-triiodo-L-thyronine (T3; Sigma) for 5 days. 18 Heart PreparationRats were anesthetized intraperitoneally with pentobarbital (Nembutal 20-25 mg/100 g of body weight). The heart was rapidly excised and immersed in the buffer. The aorta was dissected free and mounted onto a Teflon cannula attached to a perfusion apparatus. The isolated, perfused rat heart was suspended in an NMR tube with an outer diameter measuring 15 mm. The heart was perfused in the Langendorff mode under isovolumic conditions at a constant temperature of 37°C and a constant hydrostatic perfusion pressure of 100 cmH2O. We used a modified KrebsHenseleit buffer solution containing 118.0 mmol/L NaCl, 4.7 Whether cyclical changes in e...