ENDOTHELIN-3 DECREASES CA2-VESICLES( UPTAKE IN PLATELET MEMBRANE)

To analyze the mechanisms by which endothelin-3 (ET-3) attenuates agon ist-mediated Ca2+ mobilization from an internal pool, we investigated ET-3 effects on Ca-45(2+) uptake in platelet membrane vesicles. They w ere compared to those of thapsigargin (Tg), a specific inhibitor of th e dense tubule Ca2+ pumps. In the absence of ATP, ET-3 up to 1 mu M di d not affect the amount of Ca2+ bound to membrane sites. In the presen ce of ATP, ET-3 dose-dependently reduced the initial rate and the exte nt of Ca2+ uptake (p < 0.001). In comparison, Tg dose-dependently inhi bited both the ATP-independent Ca2+ binding (p < 0.001) and the ATP-de pendent Ca2+ accumulation (p < 0.001), with half-maximal effects at 7 nM. Pretreatment with 1 mu M ET-3 decreased the inhibitory effect of 1 0 nM Tg, but only on the initial rate of ATP-dependent Ca2+ uptake (p = 0.04; n = 6). These results indicate that ET-3 is functionally coupl ed to Ca2+ ATPases of the dense tubules. Its inhibitory effects are pr obably due to inhibition of the catalytic cycle of the Ca2+ pumps. Suc h inhibition could lead to a depletion of Ca2+ stores and therefore to reduced Ca2+ release in response to agonists.