Carrier-mediated uptake of methylglyoxal bis(guanylhydrazone) by rat lungs perfused in situ

Rat lungs perfused in situ were employed to begin investigations of the pathways by which the tissue takes up circulating polyamines (PA). Uptake kinetics were studied using [¹⁴C]methylglyoxal bis(guanylhydrazone) (MGBG), a nonmetabolized substrate analogue thought to enter cells via the PA carrier. Lungs concentrated MGBG from the perfusate at a linear rate for at least 60 min. Uptake was saturable with respect to perfusate MGBG concentration; it exhibited an apparent K(m) of 12.5 μM and V(max) of 0.6 nmol·g lung^-1·min^-1. MGBG (1 μM) uptake was inhibited rapidly and to a similar extent (30-40%) by the naturally occurring PAs spermidine, spermine, or putrescine (50 μM); no additional inhibition of uptake was exerted when all three compounds were present simultaneously (total concentration, 150 μM). No inhibition by 5-hydroxytryptamine was evident. Spermidine produced a half-maximal inhibitory effect at a perfusate concentration of 1.9 μM (vs. 1 μM MGBG). The spermidine-insensitive component of MGBG uptake operated at a V(max) similar to that of the control (total), 1.2 nmol·g^-1·min^-1, but the apparent K(m) was increased 3.5-fold to 44 μM. These observations indicate that MGBG is taken up from the pulmonary circulation by a high-affinity, carrier-mediated, concentrative uptake process that is inhibited, at least in part, by naturally occurring polyamines.