Effect of proteases and phospholipases on [3H]yohimbine binding to human platelet membranes

Dimer-based model for heptaspanning membrane receptors

The existence of intramembrane receptor-receptor interactions for heptaspanning membrane receptors is now fully accepted, but a model considering dimers as the basic unit that binds to two ligand molecules is lacking. Here, we propose a two-state-dimer model in which the ligand-induced conformational changes from one component of the dimer are communicated to the other. Our model predicts cooperativity in binding, which is relevant because the other current models fail to address this phenomenon satisfactorily. Our two-state-dimer model also predicts the variety of responses elicited by full or partial agonists, neutral antagonists and inverse agonists. This model can aid our understanding of the operation of heptaspanning receptors and receptor channels, and, potentially, be important for improving the treatment of cardiovascular, neurological and neuropsychyatric diseases.

Proteolytic susceptibility of platelet low density lipoprotein receptor

In order to further characterize low density lipoprotein (LDL)-platelet interaction, we investigated the effect of protease pretreatment of human platelets on the subsequent binding of iodinated LDL (125I-LDL). Our results showed that the platelet LDL receptor had a proteolytic susceptibility different from that of both classical LDL receptors and the fibrinogen receptor. Platelet pretreatment with chymotrypsin, trypsin, and pronase (at 50 micrograms/mL) had no effect on 125I-LDL binding, whereas fibroblast 125I-LDL binding was markedly reduced. Mild proteolytic digestion, however (up to 1 mg/mL), was helpful in characterizing the platelet LDL receptor. Scatchard analysis showed that chymotrypsin did not modify LDL binding characteristics, whereas trypsin and pronase altered maximal number of binding sites (Bmax) without variation in dissociation constant. Trypsin increased Bmax approximately twofold (2156 +/- 327 binding sites on control platelets vs. 5246 +/- 296 on treated platelets, P < 0.001, mean +/- SEM, n = 5), but pronase decreased Bmax about 50% (2017 +/- 275 control vs. 1153 +/- 195 treated, P < 0.001). A minimum of 30 min preincubation was required to detect significant effects, and apparent equilibrium was reached by 60 min. Maximal increase in platelet LDL binding sites induced by trypsin was observed at a protein concentration of 1 mg/mL at 37 degrees C, whereas at 4 degrees C no effect was found. In contrast, maximal pronase-inhibitory effect also was observed at 37 degrees C but at higher protein concentration (10 mg/mL). Aprotinin, phenylmethylsulfonylfluoride, and soybean trypsin inhibitor were capable of fully blocking both the stimulation and the inhibition of platelet LDL binding induced by trypsin and pronase, respectively. Platelet pretreatment with both chymotrypsin and pronase (0.5 mg/mL) activated fibrinogen binding sites to a similar extent as ADP (100 microM). Furthermore, LDL (at a protein concentration of 0.3 mg/mL) increased by 81 +/- 6% the binding of fibrinogen to both protease- and ADP-stimulated platelets, but was unable to activate fibrinogen binding sites in unstimulated platelets. Overall, the results suggest that platelet LDL receptor presents a different proteolytic susceptibility in comparison with both "classical" LDL receptor and fibrinogen receptor.

Triggers and substrates of hippocampal synaptic plasticity

It is widely assumed that behavioral learning reflects adaptive properties of the neuronal networks underlying behavior. Adaptive properties of networks in turn arise from the existence of biochemical mechanisms that regulate the efficacy of synaptic transmission. Considerable progress has been made in the elucidation of the mechanisms involved in synaptic plasticity at central synapses and especially those responsible for the phenomenon of long-term potentiation (LTP) of synaptic transmission in hippocampus. While the nature and the timing requirements of the triggering steps are reasonably well known, there is still a lot of uncertainty concerning the mechanisms responsible for the long-term changes. Several biochemical processes have been proposed to play critical roles in promoting long-lasting modifications of synaptic efficacy. This review examines first the triggers that are necessary to produce LTP in the hippocampus and then the different biochemical processes that have been considered to participate in the maintenance of LTP. Finally, we examine the relationships between LTP and behavioral learning.

Influence of phospholipase A2 on human blood platelet alpha adrenergic receptor function

Exposure of platelets to phospholipase A2 selectively obliterates the aggregation response of human platelets to the action of epinephrine. This treatment does not impair the affinity of the ligand binding although it reduces the number of sites available on platelets. Reduction in the number of alpha receptors caused by phospholipase treatment in no way affected the epinephrine-induced restoration of sensitivity of refractory platelets to the action of agonists.