TY - JOUR

T1 - Functional asymmetry of the urea transporter UT-B in human red blood cells

AU - Leifelt, Jonas

AU - Dziegiel, Morten Hanefeld

AU - Brahm, Jesper

PY - 2024

Y1 - 2024

N2 - We studied urea, thiourea, and methylurea transport and interaction in human red blood cells (RBCs) under conditions of self-exchange (SE), net efflux (NE), and net influx (NI) at pH 7.2. We combined four methods, a four-centrifuge technique, the Millipore-Swinnex filtering technique, the continuous flow tube method, and a continuous pump method to measure the transport of the 14C-labeled compounds. Under SE conditions, both urea and thiourea show perfect Michaelis-Menten kinetics with half-saturation constants, K½,SE (mM), of ≈300 (urea) and ≈20 (thiourea). The solutes show no concentration-dependent saturation under NE conditions. Under NI conditions, transport displays saturation or self-inhibition kinetics with a K½,NI (mM) of ≈210 (urea) and ≈20 (thiourea). Urea, thiourea, and methylurea are competitive inhibitors of the transport of analog solutes. This study supports the hypothesis that the three compounds share the same urea transport system (UT-B). UT-B functions asymmetrically as it saturates from the outside only under SE and NI conditions, whereas it functions as a high-capacity channel-like transporter under NE conditions. When the red blood cell enters the urea-rich kidney tissue, self-inhibition reduces the urea uptake in the cell. When the cell leaves the kidney, the channel-like function of UT-B implies that intracellular urea rapidly equilibrates with external urea. The net result is that the cell during the passage in the kidney capillaries carries urea to the kidney to be excreted while the urea transfer from the kidney via the bloodstream is minimized.NEW & NOTEWORTHY The kinetics of urea transport in red blood cells was determined by means of a combination of four methods that ensures a high time resolution. In the present study, we disclose that the urea transporter UT-B functions highly asymmetric being channel-like with no saturation under conditions of net efflux and saturable under conditions of net influx and self-exchange in the concentration range 1-1,000 mM (pH 7.2 and 25-38 °C).

AB - We studied urea, thiourea, and methylurea transport and interaction in human red blood cells (RBCs) under conditions of self-exchange (SE), net efflux (NE), and net influx (NI) at pH 7.2. We combined four methods, a four-centrifuge technique, the Millipore-Swinnex filtering technique, the continuous flow tube method, and a continuous pump method to measure the transport of the 14C-labeled compounds. Under SE conditions, both urea and thiourea show perfect Michaelis-Menten kinetics with half-saturation constants, K½,SE (mM), of ≈300 (urea) and ≈20 (thiourea). The solutes show no concentration-dependent saturation under NE conditions. Under NI conditions, transport displays saturation or self-inhibition kinetics with a K½,NI (mM) of ≈210 (urea) and ≈20 (thiourea). Urea, thiourea, and methylurea are competitive inhibitors of the transport of analog solutes. This study supports the hypothesis that the three compounds share the same urea transport system (UT-B). UT-B functions asymmetrically as it saturates from the outside only under SE and NI conditions, whereas it functions as a high-capacity channel-like transporter under NE conditions. When the red blood cell enters the urea-rich kidney tissue, self-inhibition reduces the urea uptake in the cell. When the cell leaves the kidney, the channel-like function of UT-B implies that intracellular urea rapidly equilibrates with external urea. The net result is that the cell during the passage in the kidney capillaries carries urea to the kidney to be excreted while the urea transfer from the kidney via the bloodstream is minimized.NEW & NOTEWORTHY The kinetics of urea transport in red blood cells was determined by means of a combination of four methods that ensures a high time resolution. In the present study, we disclose that the urea transporter UT-B functions highly asymmetric being channel-like with no saturation under conditions of net efflux and saturable under conditions of net influx and self-exchange in the concentration range 1-1,000 mM (pH 7.2 and 25-38 °C).

KW - asymmetric function

KW - channel-like efflux

KW - human red blood cells

KW - saturation of influx and self-exchange

KW - urea transporter UT-B

U2 - 10.1152/ajpcell.00435.2023

DO - 10.1152/ajpcell.00435.2023

M3 - Journal article

C2 - 38223930

AN - SCOPUS:85186302163

VL - 326

SP - C905-C916

JO - American Journal of Physiology: Cell Physiology

JF - American Journal of Physiology: Cell Physiology

SN - 0363-6143

IS - 3

ER -