Insulin responsiveness of neonatal pig myocardium

The transition from preferential use of carbohydrates to fatty acids for energy production by neonatal mammalian myocardium might be hormonally influenced. In neonatal pigs (between 1 and 4 days of age), plasma glucose, lactate, and glucagon concentrations remained constant at adult levels, while plasma triglycerides and insulin content remained stable at substantially lower levels. Insulin, whether administered in vivo or during perfusion, increased myocardial glucose utilization [GU] and lactate production [LP]. Stimulation of GU and LP by insulin exposure in vivo persisted for at least 1 h during perfusions after insulin was removed. Exogenous lactate (0.5 mM) diminished insulin-stimulated GU and LP; and was used as a substrate at 1.25 mM. Thus, insulin augmented glycolytic activity of neonatal piglet myocardium; however, exogenous lactate, within the physiological range, altered the nature of the myocardium's response by, as yet, undetermined mechanisms. Therefore, the transition from carbohydrate to fatty acid utilization for energy production by the neonatal myocardium does not appear to result from decreased carbohydrate metabolism or reduced myocardial responsiveness to insulin.