Interrelationships among platelet responses: studies on the burst in proton liberation, lactate production, and oxygen uptake during platelet aggregation and Ca2+ secretion

The interrelationships between several functional and metabolic platelet responses to physiologic stimuli have been studied in gel-filtered platelets by continuous and simultaneous registration of shape change, aggregation, Ca 24 secretion, oxygen uptake, and proton liberation. At various rates of aerobic and anaerobic energy metabolism. proton liberation quantitatively paralleled the production of lactate. However, immediately following thrombin addition, a lactate-unrelated burst in proton liberation became apparent, which correlated with acid hydrolase secretion but not with dense granule or α-granule secretion, ATP-hydrolysis, or hydrolysis of membrane constituents. Following platelet stimulation with a low dose of thrombin (0.1 U • ml–1), a specific order of cellular responses was recorded: (1) onset of shape change; (2) acceleration of lactate-related proton liberation; (3) onset of aggregation; (4) onset of Ca 24 secretion; and (5) a minor increase in oxygen uptake. Stimulation with a high dose of thrombin (5 U • ml–1) initiated (1) onset of shape change; (2) onset of aggregation, Ca2+ secretion, and lactate-unrelated proton liberation; (3) acceleration of mitochondrial respiration; and (4) start of arachidonate oxygenation. Maximal arachidonate oxygenation was found at 5 U • ml–1 of thrombin and could be inhibited completely without affecting Ca 24 secretion or aggregation. At 0.1 U • ml–1 of thrombin, arachidonate oxygenation was hardly detectable, but when inhibited, Ca 24 secretion (velocity and extent) and aggregation were greatly impaired and became detectable only after a time delay 2–3-fold that of uninhibited suspensions. The results suggest that glyco(geno)lytic but not oxidative ATP production supports aggregation and Ca2+ secretion. They demonstrate that thrombin-induced aggregation precedes Ca 24 secretion and therefore is independent of products secreted from the dense granules.