CYCLIC-AMP STIMULATES CA2+-ATPASE-MEDIATED CA2+ EXTRUSION FROM HUMAN PLATELETS

The effect of cAMP on active Ca2+ extrusion across the plasma membrane of intact human platelets was studied using quin2, a fluorimetric indicator of free Ca2+ in the cytoplasmic compartment ([Ca2+]cyt). Elevations of cAMP were achieved by incubation with dibutyryl-cAMP or by forskolin, which was found to selectively elevate cAMP without affecting cGMP levels. Progress curves of Ca2+ extrusion from quin2-overloaded platelets were measured. The rate vs. [Ca2+]cyt characteristic was calculated as previously described (Johansson, J.S. and Haynes, D.H. (1988) J. Membr. Biol. 104, 147-163). Forskolin, at a maximally effective concentration of 10-mu-M, was shown to stimulate Ca2+ extrusion by increasing by a factor of 1.6 +/- 0.5 the V(m) of a saturable component, previously identified with a Ca2+-Mg2+-ATPase located in the plasma membrane. Neither the K(m) (80 nM) or Hill coefficient (1.7 +/- 0.3) of the Ca2+-ATPase was affected. Forskolin had no effect on the linear, non-saturable component of extrusion (previously identified with a Na+/Ca2+ exchanger) over the [Ca2+]cyt range examined (50-1500 nM). Dibutyryl-cAMP (Bt2-cAMP, 1 mM) stimulated the Ca2+-Mg2+-ATPase component of Ca2+ extrusion by a factor of 2.0 +/- 0.6. Separate experiments showed that 10-mu-M forskolin reduces the resting [Ca2+]cyt from 112 nM to 96 nM. Mathematical analysis showed that this can be accounted for by the above-mentioned increase in V(m) of the pump, countered by a 37-74% increase in the rate constant for passive Ca2+ leakage across the plasma membrane. The results suggest two mechanisms by which prostacyclin-induced elevation of cAMP inhibits platelet aggregation: (a) lowering of resting [Ca2+]cyt and (b) increasing the rate of Ca2+ extrusion after the initial influx or triggered release event.