DIPEPTIDE UPTAKE BY ADENOHYPOPHYSEAL FOLLICULOSTELLATE CELLS

Dipeptide uptake was studied in primary cultures from rat anterior pit uitaries by use of radiolabeled carnosine and the fluorescent dipeptid e derivative beta-Ala-Lys-N-epsilon-AMCA (AMCA is 7-amino-4-methylcoum arin-3-acetic acid). Fluorescence microscopic studies revealed that th e reporter peptide specifically accumulated in the S-100 positive foll iculostellate cells that do not produce any known hormone. The dipepti de derivative was taken up in unmetabolized form by an energy-dependen t saturable process with apparent kinetic constants as follows: Michae lis constant, 19 mu M; maximum velocity, 5.5 nmol . mg protein(-1). h( -1). This high-affinity transporter was strongly affected by inhibitor s of sodium/proton exchangers and thus appeared to be driven by a prot on gradient. Competition studies revealed that the peptide transporter exhibits broad substrate specificity with a preference for hydrophobi c dipeptides. In contrast to free amino acids and the pseudotetrapepti de amastatin, tripeptides were also accepted. Compounds without an alp ha- or beta-amino group, such as captopril, thiorphan, and benzylpenic illin, did not affect uptake of the reporter peptide, although they we re substrates of the well-characterized intestinal and renal dipeptide transporters.