Branched-chain amino acids (BCAAs) are oxidative energy substrates for the heart and may exert
anabolic effects on myocardial
protein. The factors regulating their myocardial uptake in patients with
ischemic heart disease are therefore of interest. To examine whether myocardial BCAA utilization is influenced by the circulating
insulin concentration, in 10 patients with chronic
ischemic heart disease, we measured transmyocardial
amino acid balance during fasting and again during a 90-minute euglycemic
insulin infusion (plasma
insulin, 218+/-25 microU x mL(-1)) with plasma BCAA concentrations held constant by coinfusion. In the fasting state, the myocardial fractional extraction of
leucine (8%),
isoleucine (9%), and
valine (5%) from arterial plasma was slightly greater than that of
glucose (3%), while net myocardial BCAA uptake (
leucine, 409+/-207 nmol x min(-1);
isoleucine, 220+/-144 nmol x min(-1);
valine, 407+/-326 nmol x min(-1); and total BCAA uptake, 1.0+/-0.3 micromol x min(-1)) was about 13% that of
glucose (8+/-2 micromol x min(-1)). During euglycemic
hyperinsulinemia, myocardial
glucose uptake increased 3-fold, but there was no change in the arterial-coronary sinus balance or net myocardial uptake of any BCAA under conditions where their plasma concentrations were held constant. Instead, the myocardial uptake of each BCAA correlated positively with its concentration in arterial plasma. These results demonstrate that in patients with
cardiovascular disease, myocardial utilization of BCAAs is insensitive to the circulating
insulin level and is regulated instead by their availability in arterial plasma.
Hyperinsulinemia reduced the magnitude of both net
glutamate uptake and
alanine release, suggesting a possible salutary effect on myocardial oxidative efficiency.