Catecholamine-induced release of the folic acid analogue, Methotrexate, from rat hepatocytes in suspension. An alpha-adrenergic phenomenon

Studies were undertaken to explore the mechanism(s) for release of the folic acid analogue, methotrexate, from freshly isolated rat hepatocytes in suspension. When cells are at steady state with exchangeable intracellular methotrexate, net efflux of methotrexate is induced by 10 μM epinephrine. This net efflux of methotrexate induced by epinephrine is markedly potentiated by 3-isobutyl-1-methylxanthine at concentrations which do not, alone, result in net loss of methotrexate from the cells. Epinephrine (in the presence of isobutyl methylxanthine) is the most potent of the catecholamines tested in inducing methotrexate efflux; equimolar norepinephrine or phenylephrine are less effective, and isoproterenol is essentially ineffective. This order of potency for the catecholamines suggests an alpha-adrenergic-mediated exit of methotrexate from these cells. This is further supported by the observations that the alpha antagonists phenoxybenzamine and prazosin significantly depress methotrexate efflux induced by epinephrine plus isobutyl methylxanthine, whereas the beta antagonists propranolol and dichloroisoproterenol have no effect on induction of drug exit. Incubation of hepatocytes with the calcium-chelating agent ethylene glycol bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid reduces or eliminates efflux of methotrexate induced by epinephrine or epinephrine plus isobutyl methylxanthine, consistent with calcium involvement in alpha-adrenergic phenomena. Similarly, arginine vasopressin effects methotrexate release by a calcium-dependent mechanism. This calcium-dependent efflux of methotrexate from hepatocytes induced by alpha-adrenergic stimuli in vitro may represent a 'secretory' phenomenon which modulates release of this antifolate into the capillary sinusoid or bile canaliculus when the hepatocyte is in its usual spatial orientation within the liver lobule.