Intracellular pH and inorganic phosphate content of heart in vivo: a 31P-NMR study
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L A Katz1, J A Swain, M A Portman
1Laboratory of Cardiac Energetics, National Heart, Lung, and Blood Institute, Bethesda, Maryland 20892.
Insights
Red blood cells minimally impact canine heart 31P-NMR spectra, allowing accurate measurement of myocardial intracellular phosphate and pH. This method enables reliable assessment of cardiac metabolic states.
Area of Science:
- Cardiology
- Biophysics
- Biochemistry
Background:
- 31P-nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for assessing myocardial metabolism.
- Quantifying intracellular phosphate and pH in the heart is crucial for understanding cardiac function and disease.
- The contribution of red blood cells to the 31P-NMR signal can potentially interfere with these measurements.
Purpose of the Study:
- To determine the extent of red blood cell interference in canine cardiac 31P-NMR spectra.
- To establish the feasibility of measuring myocardial intracellular inorganic phosphate (Pi) and pH in vivo.
- To investigate the myocardial intracellular proton-buffering capacity.
Main Methods:
- Utilized a perfluorochemical perfusion emulsion blood substitute (Oxypherol) to replace whole blood in vivo.
- Collected 31P-NMR data from the canine left ventricle using a surface coil.
- Analyzed spectral differences to isolate and quantify blood-borne signals and myocardial metabolites.
Main Results:
- Red blood cells contributed minimally (approx. 10%) to the 31P-NMR spectrum, primarily from 2,3-diphosphoglycerate (2,3-DPG) and phosphodiesters.
- Established a method for relative quantification of intracellular inorganic phosphate (Pi) and pH.
- Under resting conditions, intracellular ATP/Pi was 7.0 ± 0.8, and intracellular pH was 7.10 ± 0.01.
- Myocardial intracellular pH showed minimal changes (approx. 0.1 pH unit) during acute respiratory alkalosis and acidosis (arterial pH 7.0-7.7).
Conclusions:
- Red blood cells have a negligible impact on canine cardiac 31P-NMR spectra, validating its use for myocardial metabolic assessment.
- The developed method allows for reliable in vivo measurement of myocardial intracellular Pi and pH.
- The canine heart possesses an effective intracellular proton-buffering mechanism.
Abstract:
Studies were performed to determine the contribution of red blood cells to the 31P-nuclear magnetic resonance (NMR) spectrum of the canine heart in vivo and the feasibility of measuring myocardial intracellular phosphate and pH. This was accomplished by replacing whole blood with a perfluorochemical perfusion emulsion blood substitute, Oxypherol, and noting the difference in the 31P-NMR spectrum of the heart. NMR data were collected with a NMR transmitter-receiver coil on the surface of the distal portion of the left ventricle. These studies demonstrated that a small (approximately 10%) contribution from 2,3-diphosphoglycerate (2,3-DPG) and phosphodiesters in the blood could be detected. The magnitude and shift of these blood-borne signals permitted the relative quantification of intracellular inorganic phosphate (Pi) content as well as intracellular pH. Under resting conditions, the intracellular ATP/Pi was 7.0 +/- 0.8 (n = 19). This corresponds to a free intracellular Pi content of approximately 0.8 mumol/g wet wt. The intracellular pH was 7.10 +/- 0.01 (n = 19). Acute respiratory alkalosis and acidosis, with the arterial pH ranging from approximately 7.0 to 7.7, resulted in only small changes in the intracellular pH (approximately 0.1 pH unit). These latter results demonstrate an effective myocardial intracellular proton-buffering mechanism in vivo.