Summary
The stimulating effect of adrenaline on human platelet phospholipase C (PLC) activation
and responses in vitro (shape change, aggregation and dense granule secretion) was
investigated with respect to its dependence on exogenously added agonists. All experiments
were performed with human gel-filtered platelets pretreated with acetylsalicylic acid
to prevent endogenous stimulation by the arachidonate pathway. (1) Preliminary experiments
demonstrated the presence of trace amounts of extracellular ADP (0.05-0.58 μM) in
non-stimulated platelet suspensions; ADP was effectively converted to ATP by the enzyme
system creatine phosphate (CP)/creatine phosphokinase (CPK). (2) The adrenaline-induced
optical aggregation and single particle (platelet) disappearance in the presence of
trace amounts of ADP were almost abolished by the ADP-scavanger system CP/ CPK. (3)
The response of CP/CPK-treated thrombin- or platelet-activating factor (PAF)-stimulated
platelets was markedly increased by a subsequent addition of adrenaline. When hirudin
or BN 50726 was added just prior to adrenaline to terminate the activation by thrombin
or PAF, respectively, the stimulating effect of adrenaline was also abolished. (4)
CP/CPK-treated, PAF-stimulated platelets rapidly developed decreased responsiveness
to a subsequent addition of PAF. When adrenaline was added instead of a second addition
of PAF, the stimulating effect of adrenaline was gradually decreased and prevented
in parallel with the homologous desensitization of PAF. (5) The weak platelet agonist
serotonin by itself induced only shape change in CP/CPK-treated platelets. Adrenaline
failed to enhance the extent of this serotonin-induced platelet activation. (6) These
results strongly suggest that adrenaline per se is not a platelet agonist in vitro
but acts to enhance the stimulation induced by true agonists.
