The pluripotent human erythroleukaemia cell line, HEL, possesses erythrocytic, megakaryocytic and macrophage-like properties. With respect to signal transduction, HEL cells have been used as a model system for platelets, but little attention has been paid to their phagocytic properties. We studied the effects of various receptor agonists on the intracellular free Ca2+ concentration ([Ca2+]i) in HEL cells. Thrombin, platelet-activating factor (PAF), ATP, UTP, prostaglandins E1 and E2 (PGE1 and PGE2), the PGE2 analogue sulprostone and the stable PGI2 analogues iloprost and cicaprost increased [Ca2+]i. ADP was less effective than ATP, and UDP was unable to increase [Ca2+]i. The increases in [Ca2+]i induced by thrombin, PAF, ATP, UTP, iloprost and cicaprost were pertussis toxin-insensitive, whereas the increases induced by PGE2 and sulprostone were completely inhibited by the toxin. The increase in [Ca2+]i induced by PGE1 was partially inhibited by pertussis toxin. PGE2 did not desensitize the increase in [Ca2+]i induced by iloprost, and vice versa. PGE1 desensitized the response to PGE2 and iloprost but not vice versa. Adrenaline potentiated the iloprost- but not the PGE2-induced rise in [Ca2+]i. The phorbol ester phorbol 12-myristate 13-acetate completely blocked the rise in [Ca2+]i induced by ATP and PGE1, whereas the increases induced by thrombin and PAF were only partially inhibited. Agonists increased [Ca2+]i through release from internal stores and sustained Ca2+ influx. Thrombin stimulated Mn2+ influx, which was blocked by Ni2+. Diltiazem, isradipine, gramicidin and 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride (SK&F 96365) did not affect agonist-induced rises in [Ca2+]i. HEL cells contained substantial amounts of beta-glucuronidase which, however, could not be released, and they did not aggregate or generate superoxide. Our data suggest that: (1) HEL cells possess nucleotide receptors with properties similar to those of phagocytes; (2) they possess receptors for PGE2 and PGI2, and PGE1 is an agonist at both receptors; (3) agonist-induced increases in [Ca2+]i are mediated through pertussis toxin-sensitive as well as -insensitive signal transduction pathways; and (4) agonists increase [Ca2+]i by mobilization from internal stores and influx from the extracellular space through cation channels with properties similar to those of phagocytes and platelets.
Signal transduction pathways and gating mechanisms of native TRP-like cation channels in vascular myocytes