Myocardial Amino Acid Transport in the Isolated Rabbit Right Ventricular Papillary Muscle

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Abstract
The amino acid analog, alpha-aminoisobutyric acid (AIB) has been used to study myocardial amino acid transport in the isolated rabbit right ventricular papillary muscle. Intracellular AIB accumulation is linear for 2 hours and reaches a plateau at 3 hours at an intracellular/extracellular concentration gradient of 3.4. Anoxia does not inhibit this process whereas simultaneous inhibition of aerobic and anaerobic metabolism inhibits AIB accumulation. Intracellular AIB accumulation follows Michealis-Menten kinetics with a Km of 6.8 x 10-3M and a Vmax of 6.6 µmoles. ml intracellular fluid-1. hour-1. Sodium ion is obligatory for the transport process whereas no absolute potassium or calcium requirement exists. Ouabain, 10-5M, inhibits AIB uptake. Myograph techniques were utilized to determine the effect on AIB accumulation of subjecting unstimulated papillary muscles to various degrees of passive stretch. Muscles incubated at any tension above zero developed a greater intracellular/extracellular fluid AIB concentration ratio than muscles incubated at zero tension. These data suggest that increased myocardial wall tension may be a mechanical stimulus capable of inducing adaptive changes in myocardial metabolism.