BBB seminar: Holm Holmsen

Holm Holmsen
Department of Biomedicine, University of Bergen

In the early 1960s one knew that platelets were fragments of the cytoplasm of megakaryocytes in the bone marrow, that they have active energy metabolism, contained large amounts of actin and that they were aggregated by minute amounts of ADP. My first scientific project was to identify the platelet-aggregating principle(s) found extracellularly after treating platelets with collagen. This was ADP. But, did platelets release other low molecular weight substances? One decided to label the releasable, potential substances with [³²P]Pi in order to have a sensitive method. To our great surprise the ADP released was not radioactive, despite the fact that the platelets contained huge amounts of [³²P]-labelled substances. Labelling platelet adenine nucleotides (AN) with radioactive adenine or adenosine gave the same result. Thus, platelets have two compartments of AN: one that participates in metabolism and is not released and another that is non-metabolic and is released. Subcellular fractionation and, particularly, [³¹P]- and [¹H]-NMR studies clearly established that the non-metabolic and releasable pool was contained in the dense granules whereas the metabolic pool was in the cytosol. Further confirmation came with the discovery that (human) patients with severe bleeding (the Holmsen-Weiss Syndrome, unfortunately named "Storage Pool Deficiency") and many species with the Chediak-Higashi Syndrome - actually lacked dense granules and releasable ADP. We also demonstrated a platelet aggregation defect in ADHD patients, effects of hyperbaric conditions and solvents on platelet responses and showed that electromagnetic fields did not affect responses in platelets but they did affect responses in monolayers of fibroblasts. During stimulation of platelets with collagen or thrombin, large amounts of metabolic ATP disappeared and were irreversibly converted to hypoxanthine. Obviously, then, ATP energy was used to drive the platelet functions, aggregation and secretion. Then a search for what ATP was used for in the first few seconds of platelet activation began. Protein phosphorylation (particularly, myosin and protein tyrosine) took place but could not account for the large ATP loss. A much more likely candidate was ATP use in the polyphosphoinositide (PPI) cycle, a major signalling mechanism in platelets. The turnover of the PPI cycle was severely inhibited by elevation of cellular cAMP, which strongly inhibits platelet responses. This turnover was also inhibited by a series of psychotropic drugs, among them chlorpromazine (CPZ) which has been studied in some detail. [¹H]- and [³¹P]-solid-state NMR experiments and measurements of surface pressure/molecular area measurements strongly indicated that CPZ intercalates among negatively charged glycerophospholipids in model membranes. Since human platelets do not contain D₂ receptors, which are the presumed target for most psychotropic drugs, we think that the drug effect on the PPI cycle is receptor-independent, and may be a result of alterations in membrane structure caused by drug intercalation.

Chair: Rolf Kåre Reed <rolf.reed[@]biomed.uib.no>, Dept. of Biomedicine