Activation of different pathways for calcium elevation by bradykinin and ATP in rat pheochromocytoma (PC 12) cells

Abstract

We have studied the pathways by which extra-cellular bradykinin and adenosine 5′-triphosphate (ATP) elicit changes in intracellular free calcium ([Ca²⁺]_i) in nerve-growth-factor(NGF)- treated rat pheochromocytoma (PC 12) cells. Both substances caused a significant rise in [Ca²⁺]_i as assessed by fura-2 based micro-fluorimetry. The bradykinin-induced response consisted of an initial Ca²⁺ mobilization from an internal pool followed by a sustained increase in [Ca²⁺]_i, which was due to activation of a small inward current. The initial response always started at a localized site opposite to the cell nucleus. The inward current was partially carried by Ca²⁺ and began with a time lag of about 4 s after the start of the initial transient signal. Stepwise hyperpolarization of the plasma membrane, after activation of the inward current by bradykinin, caused a simultaneous increase in current amplitude and in [Ca²⁺]_i, due to an increase in the driving force for Ca²⁺ influx. With ATP as an agonist the onset of inward current coincided with an increase in [Ca²⁺]_i. Inward current and [Ca²⁺]_i were enhanced during hyperpolarizing steps indicating a substantial Ca²⁺ influx through ATP-activated channels. No release of Ca²⁺ from internal stores, but a large Na⁺ inward current, was observed in Ca²⁺-free external solution after addition of ATP. While the bradykinin-induced responses were much more pronounced in cell bodies than in growth cones, the ATP effects were somewhat variable in cell bodies and more homogeneous in growth cones.