Impaired Activity of the Extraneuronal Monoamine Transporter System Known as Uptake-2 in Orct3/Slc22a3-Deficient Mice

Abstract

Two uptake systems that control the extracellular concentrations of released monoamine neurotransmitters such as noradrenaline and adrenaline have been described. Uptake-1 is present at presynaptic nerve endings, whereas uptake-2 is extraneuronal and has been identified in myocardium and vascular and nonvascular smooth muscle cells. The gene encoding the uptake-2 transporter has recently been identified in humans (EMT), rats (OCT3), and mice (Orct3/Slc22a3). To generate an in vivo model for uptake-2, we have inactivated the mouseOrct3 gene. Homozygous mutant mice are viable and fertile with no obvious physiological defect and also show no significant imbalance of noradrenaline or dopamine. However, Orct3-null mice show an impaired uptake-2 activity as measured by accumulation of intravenously administered [³H]MPP⁺ (1-methyl-4-phenylpyridinium). A 72% reduction in MPP⁺ levels was measured in hearts of both male and female Orct3 mutant mice. No significant differences between wild-type and mutant mice were found in any other adult organ or in plasma. When [³H]MPP⁺ was injected into pregnant females, a threefold-reduced MPP⁺accumulation was observed in homozygous mutant embryos but not in their placentas or amniotic fluid. These data show that Orct3is the principal component for uptake-2 function in the adult heart and identify the placenta as a novel site of action of uptake-2 that acts at the fetoplacental interface.

ACKNOWLEDGMENTS

R. Zwart and S. Verhaagh contributed equally to this work.

We thank C. Brouwer and J. Vink for help with the ES cell work in the gene targeting experiment. K. van Veen is acknowledged for the blastocyst injections, N. Bosnie and T. Maidment are acknowledged for taking good care of the mice, and J. W. Jonker is acknowledged for help with the MPP⁺ transport studies. We thank B. Giros and S. Gautron for sharing their unpublished data; A. H. Schinkel, J. W. Jonker, and all members of the lab for critical discussions and reading of the manuscript; and A. Berns for constant help and support.

This work was supported by a grant from the Dutch Cancer Society.