Fasting hypoglycaemia secondary to carnitine deficiency: a late consequence of gastric bypass

Twelve years following gastric bypass surgery, a cachectic 69-year-old woman presented with both fasting and postprandial hypoglycaemia. Postprandial symptoms were relieved by dietary modification and acarbose, as is common in such cases. During a supervised fast, symptomatic hypoglycaemia occurred. Concurrent laboratory testing showed suppression of plasma insulin, c-peptide, proinsulin and insulin-like growth factor II. However, beta-hydroxybutyrate was also low, surprising given insulin deficiency. Elevated plasma free fatty acid (FFA) concentrations suggested that lipolysis was not impaired, making cachexia/malnutrition a less likely cause of hypoglycaemia. The apparent diagnosis was failure to counter-regulate—subsequent plasma carnitine measurements showed carnitine deficiency which presumably prevented FFA transport across mitochondrial membranes for ketogenesis. Repletion with high-dose oral carnitine supplements effected resolution of fasting hypoglycaemia.

Abstract 5385: Transport of Free Fatty Acids in Cardiac Myocytes is Mediated by a Plasma Membrane Pump as Revealed by Monitoring Cytoplasmic Unbound Free Fatty Acids

The transport of FFA across the plasma membrane represents one of the earliest points at which FFA metabolism can be controlled by cardiac myocytes. Using novel methods to measure the intracellular unbound concentration of FFA ([FFA i ]), the first direct measurements of FFA transport across cardiac plasma membranes have been performed in freshly isolated cardiac myoctyes. Measurements of the unbound concentrations of FFA (FFA u ) in the aqueous phase were performed using the fluorescent ratio probe ADIFAB. Cardiac myocytes were microinjected with ADIFAB, and the transport of oleate and palmitate was determined by monitoring [FFA i ] using fluorescence ratio microscopy. FFA influx was initiated by rapidly increasing the extracellular concentration of FFA u ([FFA o ]) using FFA-BSA complexes, which clamped [FFA o ] at fixed values. The time course of influx was monitored from the change in [FFA i ], which rose exponentially to a steady state level (k influx ~ 0.01 s −1 ). Once steady state was achieved, efflux was initiated by changing the extracellular media back to zero [FFA o ]. Efflux was monitored by the decrease in [FFA i ] which, like influx, revealed exponential behavior (k efflux ~ 0.02 s −1 ). At steady state [FFA i ] was greater than [FFA o ] by a factor of ~3.5, indicating that during influx FFA are pumped up a concentration gradient. Both the initial rate of transport and the gradient ([FFA i ] > [FFA o ]) revealed saturation with increasing [FFA o ]. The initial rate of influx saturated at [FFA o ] > 200 nM, while the [FFA i ] > [FFA o ] gradient was relatively constant (~ 3.5) but began to decrease and approached 1 at [FFA o ] > 200 nM. The efflux rate constant decreased for [FFA o ] > zero, suggesting that efflux may be regulated by a mechanism that senses the level of circulating FFA u . Our results indicate that the mechanism of FFA transport across cardiac myocytes is regulated by the plasma membrane and allows for the efficient storage and release of FFA from cardiac myocytes. We suggest that this mechanism involves an as yet unknown membrane protein pump which enables the cells to accumulate surprisingly high concentrations of FFA. The ability to measure [FFA i ] and the demonstration of efflux are significant steps in understanding cardiac FFA metabolism. This research has received full or partial funding support from the American Heart Association, AHA Western States Affiliate (California, Nevada & Utah).

Effects of high fat provision on muscle PDH activation and malonyl-CoA content in moderate exercise

This study examined the effects of elevated free fatty acid (FFA) provision on the regulation of pyruvate dehydrogenase (PDH) activity and malonyl-CoA (M-CoA) content in human skeletal muscle during moderate-intensity exercise. Seven men rested for 30 min and cycled for 10 min at 40% and 10 min at 65% of maximal O2 uptake while being infused with either Intralipid and heparin (Int) or saline (control). Muscle biopsies were taken at 0, 1 (rest-to-exercise transition), 10, and 20 min. Exercise plasma FFA were elevated (0.99 ± 0.11 vs. 0.33 ± 0.03 mM), and the respiratory exchange ratio was reduced during Int (0.87 ± 0.02) vs. control (0.91 ± 0.01). PDH activation was lower during Int at 1 min (1.33 ± 0.19 vs. 2.07 ± 0.14 mmol · min−1 · kg−1 wet muscle) and throughout exercise. Muscle pyruvate was reduced during Int at rest [0.17 ± 0.03 vs. 0.25 ± 0.03 mmol/kg dry muscle (dm)] but increased above control during exercise. NADH was higher during Int vs. control at rest and 1 min of exercise (0.122 ± 0.016 vs. 0.102 ± 0.005 and 0.182 ± 0.016 vs. 0.150 ± 0.016 mmol/kg dm), but not at 10 and 20 min. M-CoA was lower during Int vs. control at rest and 20 min of exercise (1.12 ± 0.22 vs. 1.43 ± 0.17 and 1.33 ± 0.16 vs. 1.84 ± 0.17 μmol/kg dm). The reduced PDH activation with elevated FFA during the rest-to-exercise transition was related to higher mitochondrial NADH at rest and 1 min of exercise and lower muscle pyruvate at rest. The decreased M-CoA may have increased fat oxidation during exercise with elevated FFA by reducing carnitine palmitoyltransferase I inhibition and increasing mitochondrial FFA transport.

Design of exogenous fuel supply systems: adaptive strategies for endurance locomotion

The prolonged performance of submaximal exercise depends on the adequate supply of exogenous fuels (e.g., hepatic glucose) to slow down the use of endogenous substrates (e.g, intramuscular fat or glycogen) and delay their depletion. This paper investigates the adaptive strategies available to vertebrates for increasing the rate of exogenous fuel supply in endurance locomotion. Two steps can be defined for the design of a "good" system: (i) the choice of oxidizable fuels and storage sites that maximize the rate of energy transfer to the working muscle and (ii) the provision of adequate regulatory mechanisms which alter substrate fluxes rapidly in response to work of different intensities. The principal oxidizable fuels used by vertebrates (free fatty acids (FFA), glucose, and lactate) are examined to determine the major constraints on maximal supply rates. The delivery of albumin binding sites to adipose tissue represents a specific constraint on FFA transport to working muscles. Furthermore, because the supply of all exogenous fuels is probably limited by membrane transport, animals requiring the rapid use of oxidizable substrates to sustain locomotion can follow two strategies: (i) switch to endogenous substrates whenever possible to avoid this constraint and (ii) evolve different transmembrane fuel carriers or augment the density of existing ones to increase maximal rates of substrate translocation across cell membranes.

Transport of plasma FFA in cats; effect of hypothalamic stimulation

Two tracer methods, repetitive single injection of [3-H]palmitate and continuous infusion of [1 minus 14-C]palmitate, were applied simultaneously to study tracer kinetics of the free fatty acid (FFA) miscible pool in anesthetized cats before, during, and after electrical stimulation of the hypothalamus. The results indicated that the FFA pool of the cat behaved as though it consisted of two compartments which exchange FFA with each other. The data were analyzed according to a two-compartmental mammillary system model. The space of the central compartment which represents the plasma was found to be 74 ml/kg body wt. No anatomical significance could be unequivocally ascribed to the peripheral compartment. The variables of the model system were determined for each cat, and a highly significant positive linear relationship between the net FFA transport (mumol/min) and the total quantity of FFA (mumol) in both compartments was established by a regression analysis of the control data. No significant changes were observed in this relationship during or following hypothalamic stimulation which had a significant effect on the plasma FFA level. This result suggests that hypothalamic stimulation affects FFA mobilization by modifying the rate of FFA inflow into the circulation, while the mechanisms for clearing FFA from the plasma are not altered.