High-energy phosphate responses to tachycardia and inotropic stimulation in left ventricular hypertrophy

Spatially localized nuclear magnetic resonance (NMR) spectroscopy was used to examine the effect of tachycardia and inotropic stimulation on myocardial ATP, creatine phosphate (CrP), and inorganic phosphate (P(i)) in animals with left ventricular hypertrophy (LVH). Studies were performed in eight normal dogs and seven dogs with moderate LVH produced by banding the ascending aorta. ³¹P-NMR spectra were obtained from five layers across the LV wall, while blood flow (BF) was measured with microspheres during control conditions, pacing at 200 and 240 beats/min, and during dobutamine infusion (Dob). Myocardial ATP and CrP levels were normal in the LVH hearts during control conditions. Pacing did not alter the transmural distribution of perfusion or the levels of CrP, ATP, and P(i) in normal hearts. In contrast, in four of seven LVH hearts, pacing decreased the subendocardial/subepicardial (ENDO/EPI) BF ratio and caused depletion of CrP and appearance of P(i) characteristic of ischemia in the subendocardium. Dob produced greater increases in the heart rate x LV systolic pressure product (RPP) and greater increases of P(i) and decreases of CrP in LVH than in normal hearts; however, at comparable elevations of RPP the alterations of P(i) and CrP were similar in both groups. Although Dob decreased the ENDO/EPI in LVH hearts, Dob-induced alterations in CrP and P(i) were uniform across the LV wall. Increasing myocardial BF with adenosine or carbochromen did not reverse the alterations in P(i) or CrP produced by Dob. We conclude that 1) ENDO perfusion abnormalities during tachycardia in LVH do produce ENDO subendocardial ischemia; 2) when the degree of augmentation of mechanical performance is considered, the metabolic changes induced by Dob were similar in normal and LVH hearts; 3) Dob-induced alterations in P(i) and CrP were not related to inadequate perfusion, since increasing coronary BF did not reverse these changes; and 4) alterations of P(i) and CrP during Dob infusion were not more prominent in the ENDO, indicating that the decreased ENDO/EPI flow did not cause ENDO ischemia but may reflect relatively lower O₂ demands in this region during inotropic stimulation.