ATP, THAPSIGARGIN AND CAMP INCREASE CA2+ IN RAT HEPATOCYTES BY ACTIVATING 3 DIFFERENT CA2+ INFLUX PATHWAYS

The intracellular Ca2+ concentration ([Ca2+](i)) in single isolated ra t hepatocytes was measured using fura-2. Extracellular ATP induced La3 +-sensitive and verapamil-insensitive Ca2+ influx together with Ca2+ r elease from intracellular Ca2+ stores. Incubation of hepatocytes with 2 mu M thapsigargin produced a large prolonged increase in [Ca2+](i), which was insensitive to both 100 mu M La3+ and 40 mu M verapamil. Inc ubation with 1 mM dibutyryl cAMP increased [Ca2+](i) in the presence o f 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 path way; and 3) the cAMP-induced, La3+-sensitive and verapamil-sensitive p athway.