Dual mechanism for cAMP-dependent modulation of Ca²⁺ signalling in articular chondrocytes

The ability of cAMP to modulate the actions of Ca²⁺-mobilizing agonists was studied in single Fura-2-loaded pig articular chondrocytes in primary culture. Forskolin and 8-Br-cAMP increased both the frequency and amplitude of Ca²⁺ oscillations induced by ATP, and, in unstimulated cells, induced single Ca²⁺ transients or even Ca²⁺ oscillations. The cAMP-dependent protein kinase inhibitor H89 totally prevented the effect of cAMP-elevating agents on Ca²⁺ signalling. Forskolin and 8-Br-cAMP promptly increased the rate of Mn²⁺ quenching, when administered in the presence of ATP, suggesting a potentiation of receptor-mediated Ca²⁺ influx. In Ca²⁺-free medium, ATP-induced Ca²⁺ oscillations decreased and stopped after a few cycles: subsequent ATP additions temporarily resumed the activity, an effect that could be mimicked by forskolin. The same agent induced single Ca²⁺ transients in 42% of the cell population maintained in Ca²⁺-free medium. Thapsigargin prevented Ca²⁺ responses to both ATP and forskolin. The results indicate a dual mechanism for cAMP-induced potentiation of Ca²⁺ signalling in articular chondrocytes: an increase of receptor-mediated Ca²⁺ influx and a positive modulation of intracellular Ca²⁺ release.