Role of adipose tissue in free fatty acid metabolism in hemorrhagic hypotension and shock

Hypoxia inhibits semicarbazide-sensitive amine oxidase activity in adipocytes

Semicarbazide-sensitive amine oxidase (SSAO), an enzyme highly expressed on adipocyte plasma membranes, converts primary amines into aldehydes, ammonium and hydrogen peroxide, and is likely involved in endothelial damage during the course of diabetes and obesity. We investigated whether in vitro, adipocyte SSAO was modulated under hypoxic conditions that is present in adipose tissue from obese or intensive care unit. Physical or pharmacological hypoxia decreased SSAO activity in murine adipocytes and human adipose tissue explants, while enzyme expression was preserved. This effect was time-, dose-dependent and reversible. This down-regulation was confirmed in vivo in subcutaneous adipose tissue from a rat model of hypoxia. Hypoxia-induced suppression in SSAO activity was independent of the HIF-1-α pathway or of oxidative stress, but was partially antagonized by medium acidification. Hypoxia-induced down-regulation of SSAO activity could represent an adaptive mechanism to lower toxic molecules production, and may thus protect from tissue injury during these harmful conditions.

Insulin- and glucagon-independent effects of calcitonin gene-related peptide in the conscious dog

Calcitonin gene-related peptide (CGRP) causes vasodilation in many vascular beds, resulting in hypotension and tachycardia. The current studies were conducted in overnight-fasted conscious dogs to determine the effect of different CGRP dosages on carbohydrate metabolism and catecholamine release resulting from hemodynamic changes. During a pancreatic clamp, dogs received intraportal infusions of CGRP at 13, 26, and 52 (n = 3) or 52, 105, and 210 pmol x kg(-1) x min(-1) (n = 4; 60 minutes at each rate). Blood pressure decreased (P < .05) and the heart rate and hepatic blood flow (HBF) increased a maximum of 100% and 30%, respectively (P < .05). For the five CGRP infusion rates, arterial plasma epinephrine increased approximately 1.3-, 2.4-, 7.4-, 12-fold, and eightfold basal, respectively; norepinephrine increased about 2.3-, 3.3-, 4.1-, 4.6-, and 4.8-fold basal, respectively; and cortisol increased about twofold, 3.4-fold, fivefold, sixfold, and 6.2-fold basal, respectively. At CGRP infusion rates of 52 pmol x kg(-1) x min(-1) or higher, increases (P < .05) occurred for plasma glucose, endogenous glucose production (EndoRa), and net hepatic uptake of gluconeogenic substrates (maximum change, 24 mg/dL, 1.3 mg x kg(-1) x min(-1), and 9.9 micromol x kg(-1) x min(-1), respectively). Arterial blood glycerol concentrations increased only a maximum of 30%. At the two highest CGRP infusion rates, glycerol returned to basal concentrations and arterial plasma nonesterified fatty acids (NEFAs) decreased. The increased net hepatic uptake of gluconeogenic substrates during CGRP infusion was sufficient to account for 49% to 58% of the increase in EndoRa. CGRP has no apparent direct effects on hepatic carbohydrate metabolism, but the catecholamines, at levels similar to those observed during CGRP infusion, stimulate hepatic glycogenolysis. Therefore, some factor(s) other than CGRP, probably an increase in circulating catecholamine concentrations, would appear to be responsible for at least 42% to 51% of the increase in EndoRa.

Changes in ATP and cyclic nucleotide levels during sympathetic nerve stimulation in canine subcutaneous adipose tissue in situ

Subcutaneous adipose tissue in fed, female dogs was isolated. Biopsies of the tissue (30-150 mg) were taken and rapidly frozen in liquid nitrogen before, during and after nerve stimulation (3-4 Hz). In unstimulated adipose tissue the levels of ATP1 were 74+/-7 nmol/g, of cyclic AMP 90 +/- 12 pmol/g and of cyclic PGMP 18 +/- 3 pmol/g (mean+/-S.E.). During sympathetic nerve stimulation the levels of ATP and cyclic GMP fell by 30 and 50% respectively (p less than 0.01), while the cyclic AMP content increased by 50% (p less than 0.05). After nerve stimulation there was a marked increase in glycerol release, and the levels of all three nucleotides returned to control. The fall in ATP during nerve stimulation was essentially eliminated by prior adrenergic alpha-receptor blockade. It is concluded that 1) sympathetic nerve stimulaton induces a rapid, reversible fall in tissue ATP content, which may be related to hypoxia secondary to the vasoconstriction, and 2) lipolytic responses to sympathetic nerve stimulation in vivo are preceeded by small increases in the tissue cyclic AMP level, and a 3-fold increase in the cyclic AMP/cyclic GMP ratio.