Pseudotransport of cationic drugs into human smooth muscle cells: comparison of vacuolar (V)‐ATPase‐driven ion trapping with other forms of cellular uptake

Abstract

Cationic drugs frequently exhibit large apparent volumes of distribution, consistent with various forms of sequestration by cells. The contribution of various organelles and intracellular metabolic processes that may mimic cationic drug transport was operationally defined in human vascular smooth muscle cells, large and highly adherent cells that are not specialized in the transport and handling of xenobiotics. The model drugs were procainamide, methoxamine, rhodamine 6G and Hoechst 33258 (millimolar concentration range for the first two agents, micromolar for the two latter; fluorometric determination of cell-associated drugs). Comparatively large amounts of procainamide and methoxamine were taken up by intact cells (>30 nmol/106 cells, maximal in 2 h, reversible upon washout, apparent KM 4.7 and 5.5 mM, respectively). Their uptake was extensively prevented or reversed by pharmacological inhibition of the V-ATPase (bafilomycin A1 or FR 167356), positively correlated with extracellular pH and parallel to a vacuolized cell morphology that involved at least the trans-Golgi. Rhodamine 6G uptake was maximal in 30 min, slowly reversible on washing, morphologically associated with mitochondria and selectively inhibited by treatments that abolish the mitochondrial inner membrane potential (such as oligomycin + antimycin). The uptake of Hoechst 33258, sufficient for morphological nuclear labeling, was quantitatively modest and unaffected by bafilomycin A1 or mitochondrial poisons. V-ATPase-driven ion trapping is a form of low affinity cation pseudo-transport that concerns the uncharged form of the drugs. It is quantitatively important, associated with a vacuolar cytopathology and resistant to mitochondrial poisons in vascular smooth muscle cells.