Alpha 2-adrenergic agonists increase cellular lactate efflux.

Abstract

We reported previously that genetic polymorphisms of the alpha 2-adrenergic receptor are associated with hyperinsulinemia, diabetes mellitus, and hypertension in blacks. The evolutionary driving force for maintaining such deleterious mutations in the black population is unknown. Recognizing that vascular alpha 2-adrenergic receptors mediate cold-induced vasoconstriction and that temperature maintenance is a primary thrust of cellular metabolism, we postulated that vascular alpha 2-adrenergic receptors contribute significantly to metabolic heat generation in homeotherms such as humans. Using aerobic lactate production as an indicator of thermogenesis, we measured metabolic heat production in HT29 cells that expressed the gene encoding human vascular alpha 2-adrenergic receptors. Epinephrine, an alpha 2-adrenergic receptor agonist, increased net lactate efflux from 226 +/- 20 to 280 +/- 20 nmol/min (mean +/- SE) (P = .06). Clonidine, a more specific alpha 2-adrenergic agonist, increased lactate efflux from 110 +/- 6 to 156 +/- 8 nmol/min (P < .01). Similarly, in the presence of physiological concentrations of glucose (5.5 mmol/L), insulin increased lactate production from 123 +/- 6 to 175 +/- 10 nmol/min (P < .01). Because differences in aerobic glycolysis may also explain the heat intolerance and abnormal fuel homeostasis found in genetically hypertensive rats, we also measured lactate production in cultured vascular smooth muscle cells isolated from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive control Wistar-Kyoto rats (WKY). Vascular smooth muscle cells from SHRSP had significantly greater lactate efflux compared with cells from normotensive WKY (296 +/- 4 versus 172 +/- 2 nmol/min, P < .001). These differences were not due to abnormalities in glucose uptake, as lactate efflux was greater in SHRSP cells compared with WKY cells when dextrose was replaced with equimolar concentrations of fructose (230 +/- 6 versus 138 +/- 2 nmol/min, P < .001). alpha 2-Adrenergic agonists increase lactate efflux in HT29 cells, and abnormalities in vascular smooth muscle lactate metabolism in genetically hypertensive rats is independent of altered glucose uptake. These data provide support for our hypothesis that balanced polymorphisms of the alpha 2-adrenergic receptor could offer protection against cold stress by increasing the thermogenic response associated with aerobic lactate production.