Adenosine affects the release of Ca2+ from the sarcoplasmic reticulum via A2A receptors in ferret skinned cardiac fibres

In this study, it was shown that adenosine potentiates caffeine-induced Ca2+ release. It was then proposed that the enhancement of the caffeine-induced Ca2+ release might occur by a direct effect on the ryanodine Ca2+ release channel or on other Ca2+ regulation mechanisms. Furthermore, A(2A) receptors may be functional on the ferret cardiac sarcoplasmic reticulum. Using chemically skinned fibres, experiments were conducted on ferret cardiac muscle to find out whether adenosine and the A(1) and A(2A) adenosine receptor agonists (CCPA and CGS 21680) and antagonists (DPCPX and ZM 241385) affected caffeine-induced Ca2+ release and the Ca2+ sensitivity of contractile proteins. Changes in the caffeine-induced contracture brought about by adenosine and by adenosine-receptor agonists and antagonists were recorded in saponin-skinned fibres (50 mu g ml(-1)). Tension-pCa relationships were then obtained by exposing Triton X-100-skinned fibres (1% v/v) sequentially to solutions of decreasing pCa. Adenosine (1-100 nM) and the specific A(2A) receptor agonist CGS 21680(1- 50 nM) produced a concentration-dependant potentiation of the caffeine-induced Ca2+ release from saponin-skinned fibres. The data plotted versus adenosine and CGS 21680 concentrations displayed sigmoid relationships (Hill relationship), with potentiation of Ca2+ release by 22.2 +/- 1.6 (n = 6) and 10.9 +/- 0.4% (n = 6), respectively. In addition, the potentiation of caffeine- induced Ca2+ release by adenosine (50 nM; 15.3 +/- 1.0%; n = 6) and by CGS 21680 (50 nM; 11.2 +/- 0.4%; n = 6) was reduced by the specific A(2A) receptor antagonist ZM 241385 (50 nM) to 8.0 +/- 1.4 (n = 4) and 5.4 +/- 1.2% (n = 4), respectively. The A1 receptor agonist CCPA (1-50 nM) and antagonist DPCPX ( 50 nM) had no significant effects on caffeine responses. In Triton X-100-skinned fibres, the maximal Ca2+-activated tension of the contractile proteins (41.3 +/- 4.1 mN mm(-2); n = 8), the Hill coefficient (n(H) = 2.2 +/- 0.1; n = 8) and the pCa(50) (6.15 +/- 0.05; n = 8) were not significantly modified by adenosine (100 nM) or by CGS 21680 (50 nM).