Characteristics of L-alanine transport in cardiac sarcolemmal vesicles and into isolated cardiac myocytes.

Abstract

During cardiac insults, heart cells synthesise and accumulate alanine as a part of the anaerobic energy production pathway. The transport of alanine presumably influences this pathway, making it important to characterise the L-alanine transporter in the heart. In this study, we have investigated the transport of L-alanine across the sarcolemma using a novel approach, namely utilisation of two preparations: cardiac sarcolemmal vesicles and cardiac myocytes. Both preparations were isolated from the heart of the same mammalian species. L-Alanine uptake in both preparations was sodium dependent. In the sarcolemmal vesicles, the sodium dependent component was electrogenic and saturated with an estimated Michaelis-Menten constant (Km) and maximal reaction velocity (Vmax) of 0.48+/-0.18 mM and 279.97+/-64.17 pmol/mg per min respectively at room temperature. In the isolated myocytes, L-alanine uptake was linear in sodium-containing media, with an estimated Km and Vmax of 9.65+/-0. 76 mM and 169.81+/-13.22 pmol/ microl per min respectively at 10 degreesC for the sodium-dependent component. Inhibition of cotransport by a variety of substrates indicated that L-alanine uptake in the heart is mediated by an A- or ASC-like system. These characteristics of L-alanine transport suggest that under ischaemic conditions, L-alanine efflux will be activated, thus allowing for the continuous utilisation of other amino acids for energy production.