Cell response kinetics: the phenomenon of supercooperativity in aggregation of human platelets.

Concentration-response relationships of human platelet aggregation rates were analyzed for a variety of agonists and inhibitors. Their approximation by the Hill equation showed that the values of the Hill coefficient (h) were agonist-dependent and increased as follows: hADP = hL-EPINEPHRINE = hPAF = hPGH2 = hU46619 less than hPMA less than hA23187 less than hMERTHIOLATE = hARACHIDONATE. The results were interpreted in terms of a model assuming varying degrees of cooperativity for each step of signal transduction involved in platelet aggregation. Super-high values of h (greater than 30) obtained with arachidonate and merthiolate, as well as in the case of inhibition of an arachidonate-induced response by indomethacin and PTA2, suggested that at least one region of signal transduction pathway leading to aggregation exhibited supercooperative properties.