Use of tracer techniques in intensive care research. Part II

Glucose metabolism and catecholamines

Until now, catecholamines were the drugs of choice to treat hypotension during shock states. Catecholamines, however, also have marked metabolic effects, particularly on glucose metabolism, and the degree of this metabolic response is directly related to the beta2-adrenoceptor activity of the individual compound used. Under physiologic conditions, infusing catecholamine is associated with enhanced rates of aerobic glycolysis (resulting in adenosine triphosphate production), glucose release (both from glycogenolysis and gluconeogenesis), and inhibition of insulin-mediated glycogenesis. Consequently, hyperglycemia and hyperlactatemia are the hallmarks of this metabolic response. Under pathophysiologic conditions, the metabolic effects of catecholamines are less predictable because of changes in receptor affinity and density and in drug kinetics and the metabolic capacity of the major gluconeogenic organs, both resulting from the disease per se and the ongoing treatment. It is also well-established that shock states are characterized by a hypermetabolic condition with insulin resistance and increased oxygen demands, which coincide with both compromised tissue microcirculatory perfusion and mitochondrial dysfunction. This, in turn, causes impaired glucose utilization and may lead to inadequate glucose supply and, ultimately, metabolic failure. Based on the landmark studies on intensive insulin use, a crucial role is currently attributed to glucose homeostasis. This article reviews the effects of the various catecholamines on glucose utilization, both under physiologic conditions, as well as during shock states. Because, to date (to our knowledge), no patient data are available, results from relevant animal experiments are discussed. In addition, potential strategies are outlined to influence the catecholamine-induced effects on glucose homeostasis.

Impact of exogenous beta-adrenergic receptor stimulation on hepatosplanchnic oxygen kinetics and metabolic activity in septic shock

OBJECTIVE

To investigate the impact of exogenous beta-adrenergic receptor stimulation on splanchnic blood flow, oxygen kinetics, glucose-precursor flux, and liver metabolism in septic shock.

DESIGN

Prospective trial.

SETTING

University hospital intensive care unit.

PATIENTS

Six patients with hyperdynamic (cardiac index >4.0 L/min/m2) septic shock, all requiring norepinephrine to maintain blood pressure >65 mm Hg.

INTERVENTIONS

We compared norepinephrine and phenylephrine titrated to achieve similar systemic hemodynamics and gas exchange. Splanchnic hemodynamics, oxygen kinetics, and metabolic parameters were measured before, during, and after replacing norepinephrine with phenylephrine.

MEASUREMENTS AND MAIN RESULTS

Splanchnic blood flow and oxygen kinetics were derived from the steady-state indocyanine-green clearance based on hepatic dye extraction and arterial and hepatic venous blood gases. Endogenous glucose production rate was derived from the plasma appearance rate of stable-isotope-labeled glucose using a primed-constant infusion. Splanchnic lactate, alanine (high-performance liquid chromatography) uptake, and hepatic monoethylglycinexylidide (MEGX) (fluorescence polarization immunoassay) formation rates were calculated from splanchnic blood flow and arterial-hepatic venous concentration differences. Replacing norepinephrine with phenylephrine induced no change in systemic hemodynamics or gas exchange. While splanchnic oxygen consumption and alanine uptake rate remained unaffected, splanchnic blood flow, oxygen delivery, and lactate uptake rate were significantly decreased. Glucose production rate also decreased significantly. A return to norepinephrine restored splanchnic blood flow, oxygen delivery, and lactate uptake rate to baseline values, while glucose production rate remained depressed. Hepatic MEGX formation rate was not influenced during the investigation.

CONCLUSIONS

Exogenous beta-adrenergic receptor stimulation determines splanchnic blood flow, oxygen delivery, and glucose precursor flux but not splanchnic oxygen utilization in septic shock. Gluconeogenesis is not directly affiliated to hepatosplanchnic oxygen kinetics. The different response of glucose and MEGX production rates, metabolic pathways of the periportal and perivenous region, may document intrahepatic heterogeneity associated with hepatocellular metabolic compartmentation.