ABSTRACTObjective: Amino acid transport and its regulation in vascular endothelial cells remains a largely unexplored area. In this study, we evaluated alanine transport in bovine aortic endothelial cells to assess possible mechanisms of regulation.Methods: Alanine transport into confluent monolayers of endothelial cells was measured using 100 μM [3H]alanine in the presence and absence of external Na+, in cells deprived of serum for 24 hr (SD), and in SD cells exposed to 10% serum (S) for 3 hr (SD + S cells).Results: Our results indicate that although SD did not significantly affect the Na+‐independent transport of alanine when compared to normal cells, serum addition to serum‐deprived cells markedly stimulated the Na+‐dependent uptake of this amino acid through system A. The stimulation of alanine transport pathway(s) by serum was totally abolished by pretreatment of endothelial cells with 10 μM. cycloheximide, suggesting a role of protein synthesis. Serum also induced a marked increase in calcium recycling at the cell membrane, suggesting that calcium is a key element of the serum signaling pathway. Indeed, both BAPTA (20 μM), a cellular calcium chelator, and thapsigargin (1 μM), an agent that depletes intracellular calcium stores, prevented the stimulation of alanine uptake by serum. Finally, pertussis toxin (400 ng/ml), an agent known to inactivate certain G‐protein‐dependent pathways, significantly reduced the serum‐dependent45Ca uptake and [3H]alanine entry. However, the protein kinase C activator PMA (100 nM), significantly reduced the stimulation of alanine uptake by serum but did not affect the stimulation of calcium uptake.Conclusions: Altogether these findings suggest that cell calcium is involved in the regulation of system A by serum in vascular en