Effects of hyperbaric stresses on platelet function in vitro
Platelets of divers undergo changes following exposure to high pressures and decompression. Since the latter is associated with formation of intravascular bubbles, platelet-rich plasma (PRP) was treated to reproducible doses of bubbles of air, with and without enrichment by 5% Co2 (to maintain physiological pH). PRP was also subjected to stirring and a 'rocking' procedure. Bubbles caused significant aggregation, measured turbidimetrically, of human (but not bovine) platelets, particularly without pH control, whereas little occurred with stirring and rocking. Bubbling inhibited subsequent ADP-induced aggregation of human platelets, more markedly without pH control. Bovine cells were unaffected, thus apparently less sensitive to bubbles. Since rocking was shown to be equiconvective to bubbling, aggregation associated with bubbling is not due simply to exposure of the gas/liquid interface, shear stress exerted by bubbles possibly being culpable. The importance of pH control in in vitro work was also demonstrated. PRP was next separately equilibrated (by rocking) with 5 atmospheres absolute (ata) pN2O, with 101 ata pN2 (these being approximately narcotically equipotent) and with 100 ata pHe (extremely weakly narcotic). 5 ata pN2O caused least inhibition of ADP-induced aggregation, and hydrostatic pressure (applied independently, using He and a floating plastic barrier which reduced dissolving of gas) was shown to swamp narcotic effects. It was suggested that inhibition by high pressure was due to blocking of prostaglandin synthesis. Finally, the effect of hydrostatic pressures of up to 51 ata on platelet ATP release was examined using a luciferin-luciferase assay. ADP-induced secretion was either completely blocked or partially reduced by high pressure. These effects could be important to divers suffering haemorrhages at great depths. Furthermore, in vitro hyperbaric studies are of potential value in elucidating activation pathways in platelets and other excitatory cells.