P2Y purinoceptors inhibit exocytosis in adrenal chromaffin cells via modulation of voltage-operated calcium channels

We have used combined membrane capacitance measurements (C-m) and voltage-c lamp recordings to examine the mechanisms underlying modulation of stimulus -secretion coupling by a G(i/o)-coupled purinoceptor (P2Y) in adrenal chrom affin cells. P2Y purinoceptors respond to extracellular ATP and are thought to provide an important inhibitory feedback regulation of catecholamine re lease from central and sympathetic neurons. Inhibition of neurosecretion by other G(i/o) protein-coupled receptors may occur by either inhibition of v oltage-operated Ca2+ channels or modulation of the exocytotic machinery its elf. In this study, we show that the P2Y purinoceptor agonist 2-methylthio ATP (2-MeSATP) significantly inhibits Ca2+ entry and changes in C-m evoked by single 200 msec depolarizations or a train of 20 msec depolarizations (2 .5 Hz). We found that P2Y modulation of secretion declines during a train s uch that only similar to 50% of the modulatory effect remains at the end of a train. The inhibition of both Ca2+ entry and Delta C-m are also attenuat ed by large depolarizing prepulses and treatment with pertussis toxin. Inhi bition of N-type, and to lesser extent P/Q-type, Ca2+ channels contribute t o the modulation of exocytosis by 2-MeSATP. The Ca2+-dependence of exocytos is triggered by either single pulses or trains of depolarizations was unaff ected by 2-MeSATP. When Ca2+ channels were bypassed and exocytosis was evok ed by flash photolysis of caged Ca2+, the inhibitory effect of 2-MeSATP was not observed. Collectively, these data suggest that inhibition of exocytos is by G(i/o)-coupled P2Y purinoceptors results from inhibition of Ca2+ chan nels and the Ca2+ signal controlling exocytosis rather than a direct effect on the secretory machinery.