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 La³⁺-sensitive and verapamil-insensitive Ca²⁺ influx together with Ca²⁺ release from intracellular Ca²⁺ stores. Incubation of hepatocytes with 2 μM thapsigargin produced a large prolonged increase in [Ca²⁺](i), which was insensitive to both 100 μM La³⁺ and 40 μM 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, La³⁺-sensitive and verapamil-insensitive pathway; 2) the thapsigargin-induced, La³⁺-insensitive and verapamil-insensitive pathway; and 3) the cAMP-induced, La³⁺-sensitive and verapamil-sensitive pathway.