Abstract
The blood platelet has three morphologically distinct membrane systems. In addition to the plasma membrane the platelet has an 'open canalicular system' (surface-connected intracytoplasmic membrane system) and a microsome-like 'dense tubular system'. The open canalicular and dense tubular systems have been implicated in Ca2+ transport, cyclic nucleotide (cAMP) synthesis and prostaglandin and thromboxane synthesis. Precise definition of the function of the different membrane systems requires analysis of their unique chemical activities. Broken cell preparations are used to advantage for such studies. However, clean separation and definition of the origin and composition of the membrane fractions has been difficult because well-defined marker enzymes for the various membrane systems have not been conclusively established. Platelets were fixed for 5 min in 1% paraformaldehyde-0.2% glutaraldehyde and assayed for K+-dependent p-nitrophenyl phosphatase, Ca2+-, Mg2+-ATPase and adenylate cyclase K+-dependent p-nitrophenyl phosphatase was localized only at the plasma membrane while Ca2+-, Mg2+-ATPase and adenylate cyclase were found relatively segregated to the open canalicular and dense tubular systems. The segregation of these enzymes to separate membrane compartments may have significant implications with regard to understanding platelet function.
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