Polyamine transport and ornithine decarboxylase activity in hypoxic pulmonary artery smooth muscle cells

Hypoxia causes remodeling of the pulmonary circulation that is dependent on increases in lungs polyamine contents. Mechanisms by which polyamines are regulated in hypoxic lung cells are unknown, but ornithine decarboxylase (ODC) activity, the initial enzyme in de novo biosynthesis, is depressed and polyamine transport is augmented in lungs from hypoxic rats (R.-T. Shiao et al. 1990. Am. J. Physiol. 259:L351-L358). To determine if hypoxia directly influences polyamine regulatory mechanisms in pulmonary vascular cells, we examined [14C]spermidine (SPD) transport and ODC activity in bovine main pulmonary artery smooth muscle cells (PASMCs) cultured under standard (culture medium Po2: greater than 100 mm Hg), "normoxic" (culture medium Po2: 50 to 70 mm Hg), or "hypoxic" (culture medium Po2: 18 to 30 mm Hg) conditions. Uptake of [14C]SPD in cells cultured under standard conditions was temperature- and concentration-dependent, exhibited saturation kinetics, and was abolished by metabolic inhibition. Modeling of transport according to Michaelis-Menten kinetics revealed that [14C]SPD uptake in cells cultured under standard conditions was characterized by Km and Vmax values of 0.78 microM and 4.5 pmol/min/10(6) cells, respectively. In comparison to cells cultured under standard conditions, Km was unaffected by culture under normoxic or hypoxic conditions while Vmax was increased to 18 pmol/min/10(6) cells in normoxic cells and to 33 pmol/min/10(6) cells in preparations cultured under hypoxic conditions. Inhibition of ODC with alpha-difluoromethylornithine (DFMO) also induced SPD transport, as evidenced by an increase in the Vmax to 65 pmol/min/10(6) cells. Both hypoxia- and DFMO-induced increases in [14C]SPD transport were suppressed by cycloheximide and actinomycin D, thus highlighting the importance of protein and RNA synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)