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Section of Cardiovascular Medicine, Veterans Affairs Connecticut Medical Center and Yale University School of Medicine, New Haven, Connecticut 06520
Physiological increases in circulating insulin level significantly increase myocardial glucose uptake in vivo. To what extent this represents a direct insulin action on the heart or results indirectly from reduction in circulating concentrations of free fatty acids (FFA) is uncertain. To examine this, we measured myocardial glucose, lactate, and FFA extraction in 10 fasting men (ages 49-76 yr) with stable coronary artery disease during sequential intracoronary (10 mU/min, coronary plasma insulin = 140 ± 20 µU/ml) and intravenous (100 mU/min, systemic plasma insulin = 168 ± 26 µU/ml) insulin infusion. Basally, hearts extracted 2 ± 2% of arterial glucose and extracted 27 ± 6% of FFA. Coronary insulin infusion increased glucose extraction to 5 ± 3% (P < 0.01 vs. basal) without changing plasma FFA or heart FFA extraction. Conversion to intravenous infusion lowered plasma FFA by ~50% and heart FFA extraction by ~75%, increasing heart glucose extraction still further to 8 ± 3% (P < 0.01 vs. intracoronary). This suggests the increase in myocardial glucose extraction observed in response to an increment in systemic insulin concentration is mediated equally by a reduction in circulating FFA and by direct insulin action on the heart itself. Coronary insulin infusion increased myocardial lactate extraction as well (from 20 ± 10% to 29 ± 9%, P < 0.05), suggesting the local action may include stimulation of a metabolic step distal to glucose transport and glycolysis.
insulin; myocardium; metabolism; glucose; lactic acid
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