| Abstract: | Hepatocytes in suspension which have accumulated 3H]methotrexate release the antifolate compound into the medium upon exposure to alpha-adrenergic agents. In the presence of metabolic poisons, such as sodium azide, dinitrophenol, or dicumarol, the release of methotrexate is attenuated, indicating that integrity of the cellular metabolic apparatus is required for response to the hormonal stimulus. In the presence of millimolar concentrations of the organic acid, probenecid, release of cellular methotrexate may be reduced (1 mM probenecid) or eliminated (2 mM probenecid), suggesting the involvement of a "membrane carrier." Microtubule poisons such as vincristine, vinblastine, and griseofulvin do not modify epinephrine + isobutyl methyl xanthine (IBMX)-induced release of methotrexate. The involvement of calcium in release of methotrexate from the hepatocyte is substantiated by a dose-dependent response to the calcium ionophore, A23187, in the presence of calcium, with a lack of response in the absence of calcium. These effects of A23187 are not related to inhibition of methotrexate influx. Other putative "calcium antagonists" such as tetracaine, neomycin sulfate, and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester], do not interfere with epinephrine + IBMX-induced release of 3H]methotrexate, suggesting that these agents may not be effective probes of calcium flux in the liver cell. |
