Alteration of cardiac energy state during development of hypertension in rats of the Lyon strain: a 31P-NMR study on the isolated rat heart.

The effect of different chronic blood pressure levels on cardiac energy metabolism was studied by 31P-NMR spectroscopy in perfused hearts from the Lyon strains of hypertensive (LH), normotensive (LN) and hypotensive (LL) rats at the ages of 12 and 21 weeks. The in vivo assessment of haemodynamic parameters measured at 21 weeks in anaesthetized rats with an ultraminiature catheter pressure transducer confirmed that left ventricular systolic pressure and mean aortic pressure were significantly greater (+25%) in LH rats than in LN and LL rats. In the LL animals, left ventricular systolic pressure was slightly reduced (-10%) and cardiac contractility (estimated by LV dP/dtmax) showed a 24% decreased compared to normotensive animals. The energy state of the cardiomyocytes was characterized at different work levels of isolated rat hearts, by determining the concentration of the free phosphorylated compounds at each work level. Changes in workload were induced by varying the calcium concentration in the perfusion fluid. Increasing extracellular calcium concentration resulted in a similar increase in left ventricular developed pressure (LVDP) in all groups studied. Intracellular pH was not influenced by either the age of the animals or the level of cardiac work, in the three groups of animals. ATP content of the LN and LL rats remained constant during the whole perfusion period while the 12 week-old LH rats showed a decreased ATP content with increasing cardiac work. In the older LH rats, ATP content was decreased at the highest work level (corresponding to 2 mM calcium). In response to the increase in work, phosphocreatine (PCr) content diminished and inorganic phosphate (Pi) content increased in both LN, LH and LL animals. PCr degradation and Pi accumulation were higher in the LH rats and less in LL rats compared to the LN. These changes were more important in the younger than in the older hypertensive animals. The relationship between LVDP and [Pi]/[PCr] indicates that oxidative metabolism is maximally activated in the young hypertensive rats and suggests that this maximal activation represents an adaptive phase to the increase in blood pressure. Since the difference between the metabolic pattern of the 21 week-old LH rats and age-matched LN rats was less pronounced, it is likely that a compensatory stage has been reached at that age.