DETERMINANTS OF THE S-ADENOSYLHOMOCYSTEINE (SAH) TECHNIQUE FOR THE LOCAL ASSESSMENT OF CARDIAC FREE CYTOSOLIC ADENOSINE

The S-adenosylhomocysteine (SAH) technique allows the estimation of th e free cytosolic adenosine concentration using the kinetic properties of the enzyme SAH-hydrolase (adenosine + homocysteine reversible arrow SAH + H2O). Besides the cytosolic adenosine concentration, the local SAH signal may also depend on the local homocysteine availability, the continuous production of SAH from S-adenosylmethionine (SAM-->SAH+CH3 ) and the activity of the enzyme SAH-hydrolase. These variables were s tudied with high spatial resolution (sample dry mass 25 mg) in left ve ntricular myocardium from 26 anesthetized open-chest dogs in which hea rt rate averaged 86+/-14 beats/min and mean aortic pressure 96 +/- 17 mmHg, Homocysteine infusion (48 mg/kg i.v.) increased the normal plasm a homocysteine concentration from 5.0 + 0.8 to 586+/-40 mu M after 30m in when the average tissue concentration was 94% of the plasma concent ration and similar in low and high now areas (flow range 0.04 to 1.91 ml/min/g). Local SAH content was 1.18 +/- 0.48 nmol/g under control co nditions and increased to 4.33 +/- 0.59 nmol/g within 60 min following competitive blockage of the SAH-hydrolase by adenosine dialdehyde (10 mu mol/kg i.v,). This increase of the SAH content was slightly more i n high than in low-now areas (P<0.01), Regional SAH-hydrolase activity (9.0+/-0.5 nmol/min/g) was comparable in high and low flow areas. All three variables exhibited an observed variability which was larger th an the methodical variability suggesting significant spatial heterogen eity in the myocardium. A regrouping analysis indicated that between f our and five samples taken from distant sites should be averaged to ob tain a robust estimate of the above metabolic parameters. Reconciling the measurements with a mathematical model of cardiac adenosine metabo lism and fitting of the measured SAH tissue levels gave an estimate of 72 pmol/min/g for the mean transmethylation rate. Estimates of the cy tosolic adenosine concentration of cardiomyocytes and endothelial cell s under control physiological conditions were 24 and 7 mu M, respectiv ely Thus, the present measurements provide a basis for the quantitativ e assessment of the local cytosolic adenosine concentration in relatio n to blood flow. (C) 1997 Academic Press Limited.