Trans-stimulation of 13 NH 4 + efflux provides evidence for the cytosolic origin of tracer in the compartmental analysis of barley roots

The analysis of tracer efflux kinetics is fundamental to membrane transport studies, but requires the rigorous identification of subcellular tracer sources. We present a solution to this problem through the analysis of sharp increases in 13NH4+ efflux from roots of radiolabelled barley ( Hordeum vulgare L.) seedlings, in response to a 100-fold increase in external [NH4+]. By comparing these trans-stimulation data with a mathematical model incorporating changes in subcellular NH4+ fluxes and pool sizes, we show that the cytosol of root cells is the origin of the tracer efflux. Our analysis provides new insight into the rapidly occurring events underlying compensatory flux regulation during transitions from one nutritional steady state to another, and confirms the validity of compartmental analysis by tracer efflux (CATE) in this important model system.