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Parathyroid hormone regulation of proline uptake by cultured neonatal mouse osteoblastlike cells

Regulation of proline uptake by the synthetic amino‐terminal fragment of bovine parathyroid hormone [bPTH‐(1–34)] has been studied in confluent primary cultures of osteoblastlike cells isolated from neonatal mouse calvaria. The initial velocity of proline transport was increased by 85% in cultures treated with 24 nM bPTH‐(1–34) for 6 h. Cycloheximide, at a concentration that inhibited protein synthesis by 97%, did not prevent this effect. However, adding the inhibitor during the first 1–2 h of hormone treatment did significantly reduce its magnitude. Exposure of cells to the inhibitor alone caused a time‐dependent decrease in the basal rate of proline uptake. In the absence of protein synthesis, the maximal velocity (Vmax) of transport was 60% greater in cultures treated with 24 nM bPTH‐(1–34) than in controls. The concentration of proline at which half‐maximal transport occurred (Km) was unchanged. In cultures treated with cycloheximide alone, proline transport decreased as a first‐order exponential with a half‐life of 250–280 min. Parathyroid hormone significantly reduced this decline, increasing the half‐life of proline transport activity about fourfold. These effects were duplicated by 1 mM DBcAMP. It is concluded that bPTH‐(1–34) increases proline transport in osteoblastlike cells by decreasing the degradation of amino acid transport system A proteins. The hormone may also affect the synthesis of these molecules. These effects appear to be mediated by cAMP.