Role of intracellular calcium in fast and slow desensitization of P 2 ‐receptors in PC12 cells

Combined whole‐cell patch clamp recording and confocal laser scanning microscopy of [Ca 2+ ] i transients were performed on single PC12 cells to study any correlation between membrane currents induced by ATP and elevation in [Ca 2+ ] i . ATP was applied by pressure from micropipettes near the recorded PC12 cells continuously superfused at a fast rate. Brief (20 ms) pulses of ATP elicited monophasic inward currents and [Ca 2+ ] i increases. Long applications (2 s) of ATP (5 m m ) evoked peak currents which rapidly faded during the pulse and were followed by a large rebound current, interpreted as due to rapid desensitization and recovery of P 2 ‐ receptors. The associated [Ca 2+ ] i increase grew monotonically to a peak reached only after the occurrence of the current rebound, indicating that it is unlikely this cation has a role in fast desensitization. Both membrane currents and [Ca 2+ ] i transients were linearly dependent on holding membrane potential, suggesting that Ca 2+ influx is the predominant cause of [Ca 2+ ] i elevation. This view was supported by experiments carried out in Ca 2+ ‐free solution. Brief pulses of ATP applied after a desensitizing pulse (2 s) of the same elicited smaller inward currents and [Ca 2+ ] i rises indicating a role for [Ca 2+ ] i in controlling slow desensitization of P 2 ‐receptors. This notion was confirmed in experiments with various [Ca 2+ ] i chelators which differentially affected slow desensitization in relation to their buffering capacity, while sparing fast receptor desensitization. These results suggest a role for [Ca 2+ ] i in slow rather than fast desensitization of P 2 ‐receptors, thus proposing this divalent cation as an intracellular factor able to provide an efficient and reversible control over receptor activity induced by ATP.British Journal of Pharmacology (1997) 120 , 1552–1560; doi: 10.1038/sj.bjp.0701060