Dexfenfluramine increases pulmonary artery smooth muscle intracellular Ca2+, independent of membrane potential

The anorexic agent dexfenfluramine causes the development of primary pulmonary hypertension in susceptible patients by an unknown mechanism that may include changes in K+-channel activity and intracellular Ca2+ concentration ([Ca2+]i). We investigated the dose-dependent effects of dexfenfluramine on [Ca2+]i, K+ current, and membrane potential in freshly dispersed rat pulmonary artery smooth muscle cells. Dexfenfluramine caused a dose-dependent (1-1,000 microM) increase in [Ca2+]i, even at concentrations lower than those necessary to inhibit K+ currents (10 microM) and cause membrane depolarization (100 microM). The [Ca2+]i response to 1 and 10 microM dexfenfluramine was completely abolished by pretreatment of the cells with 0.1 microM thapsigargin, whereas the response to 100 microM dexfenfluramine was reduced. CoCl2 (1 mM), removal of extracellular Ca2+, and pretreatment with caffeine (1 mM) reduced but did not abolish the response to 100 microM dexfenfluramine. We conclude that dexfenfluramine increases [Ca2+]i in rat pulmonary artery smooth muscle cells by both release of Ca2+ from the sarcoplasmic reticulum and influx of extracellular Ca2+.