ATP, thapsigargin and cAMP increase Ca2+ in rat hepatocytes by activating three different Ca2+ influx pathways.

The intracellular Ca2+ concentration ([Ca2+]i) in single isolated rat hepatocytes was measured using fura-2. Extracellular ATP induced La(3+)-sensitive and verapamil-insensitive Ca2+ influx together with Ca2+ release from intracellular Ca2+ stores. Incubation of hepatocytes with 2 microM thapsigargin produced a large prolonged increase in [Ca2+]i, which was insensitive to both 100 microM La3+ and 40 microM verapamil. Incubation with 1 mM dibutyryl cAMP increased [Ca2+]i in the presence of extracellular Ca2+ but did not in the absence of extracellular Ca2+, indicating that dibutyryl cAMP induces Ca2+ influx which was found to be sensitive to both La3+ and verapamil, without mobilizing Ca2+ from the intracellular pools. This study shows the presence of at least 3 different Ca2+ influx pathways in the plasma membrane: that is, 1) the ATP-induced, La(3+)-sensitive and verapamil-insensitive pathway; 2) the thapsigargin-induced, La(3+)-insensitive and verapamil-insensitive pathway; and 3) the cAMP-induced, La(3+)-sensitive and verapamil-sensitive pathway.