The effects of sodium nitroprusside (N.P.), a pure smooth muscle inhibitor, on platelet function were studied. Platelet-rich plasmas (PRP) from normal controls and from patients receiving N.P. were studied in vitro for aggregation in response to adenosine diphosphate (ADP), epinephrine, and collagen. Platelet ADP release (release reaction) was also investigated. Normal platelets demonstrated marked inhibition of aggregation when incubated with N.P. for 3 min. Prolonging the incubation was without additional effect. ADP and ATP release from platelets in response to collagen was also inhibited. PRP from patients receiving nitroprusside at concentrations between 25 mug/min an 165 mug/min showed inhibition of aggregation when compared to findings prior to the administration of N.P. N.P. acts by inhibiting contractile proteins and thus platelet ADP release and aggregation may depend on contraction of platelet smooth muscle-like protein, thrombosthenin.

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