Two Distinct ATP Receptors Activate Calcium Entry and Internal Calcium Release in Bovine Chromaffin Cells

Abstract: ATP, an established neurotransmitter, causes elevation of cytosolic Ca²⁺ and catecholamine secretion when applied to chromaffin cells in the intact adrenal gland. The ATP-induced rise in Ca²⁺ is due both to release from internal stores and to entry across the plasma membrane. The latter source of Ca²⁺ causes secretion; the primary role of Ca²⁺ released from internal stores remains undetermined. In this article, we have studied the nucleotide specificity for activating the two types of Ca²⁺ increases. The agonist potency order for the increase in fluorescence from fura-2-loaded chromaffin cells due to release of Ca²⁺ from internal stores is ATP = UTP > ADP > 2-methylthio-ATP, α,β-methylene ATP, identifying the receptor as a P_2U purinoceptor. The potency order for secretion is 2-methylthio-ATP > ATP > α,β-methylene ATP, ADP, UTP, placing the receptor in the P_2Y subtype. Thus, two distinct receptors are responsible for Ca²⁺ release and secretion. Agonists were more effective in the absence of extracellular Mg²⁺, suggesting that ATP uncomplexed with divalent cations binds preferentially to both receptors. The low response of both receptors to ADP distinguishes them from the ATP receptor on these cells that inhibits voltage-dependent Ca²⁺ current and secretion.