Inhibition of ATP-induced calcium influx in HT4 cells by glucocorticoids: involvement of protein kinase A

Aim: In our previous observations, adenosine triphosphate (ATP) was found to
evoke immediate elevations in intracellular free calcium concentration ([Ca²⁺]_i)
in HT4 neuroblastoma cells of mice. We tried to see if a brief pretreatment of
glucocorticoids could inhibit the Ca²⁺ response and reveal the underlying signaling
mechanism. Methods: Measurement of [Ca²⁺]_iwas carried out using the
dual-wavelength fluorescence method with Fura-2 as the indicator. Results: Preincubation
of HT4 cells for 5 min with corticosterone (B) or bovine serum albumin
conjugated corticosterone (B-BSA) inhibited the peak [Ca²⁺]_i increments in a
concentration-dependent manner. Cortisol and dexamethasone had a similar action,
while deoxycorticosterone and cholesterol were ineffective. Both extracellular
Ca²⁺ influx and internal Ca²⁺ release contributed to ATP-induced [Ca²⁺]_ielevation.
The brief treatment with only B attenuated Ca²⁺ influx. Furthermore, the [Ca²⁺]_i
elevation induced by the P2X receptor agonist adenosine 5’-(β,γ-methylene) triphosphate
(β,γ-meATP) was also suppressed. The rapid inhibitory effect of B can
be reproduced by forskolin 1 mmol/L and blocked by H89 20 mmol/L. Neither
nuclear glucocorticoid receptor antagonist mifepristone nor protein kinase C inhibitors
influenced the rapid action of B. Conclusion: Our results suggest that
glucocorticoids modulate P2X receptor-medicated Ca²⁺ influx through a membrane-
initiated, non-genomic and PKA-dependent pathway in HT4 cells.