The response to thromboxane A2 analogues in human platelets. Discrimination of two binding sites linked to distinct effector systems

Thromboxane A2 (TXA2) induces platelet shape change, secretion, and aggregation. Using a novel TXA2/prostaglandin endoperoxide receptor antagonist, [1r-[1 alpha(Z),2 beta,3 beta,5 alpha]]-(+)-7-[5-[[(1,1'- biphenyl)-4-yl]methoxy]-3-hydroxy-2-(1-piperidinyl) cyclopentyl]-4-heptenoic acid hydrochloride (GR32191), we demonstrate that these responses are mediated by at least two receptor-effector systems. GR32191 non-competitively inhibited platelet aggregation to the TXA2 mimetics, (15S)-hydroxy-11,9-(epoxymethano) prostadienoic acid (U46619) and [1S-(1 alpha,2 beta(5Z),3 alpha (1E,-3S), 4 alpha)]-7-[3-(3-hydroxy-4-(p-iodophenoxy)-1-butenyl)7- oxabicyclo[2.2.1]hept-2yl]-5-heptenoic acid by binding irreversibly to a TXA2/prostaglandin endoperoxide receptor. Dissociation of [3H]GR32191 from human platelets demonstrated two specific binding sites, one which was rapidly dissociating and a site to which binding was essentially irreversible. Stimulation by U46619 of platelets incubated with GR32191 and subsequently washed to expose the reversible binding site failed to aggregate or to secrete [3H]5-hydroxy-tryptamine; formation of inositol phosphates and activation of protein kinase C were markedly suppressed. In contrast, platelet shape change and calcium stimulation remained at 90% of control. Furthermore, stimulation of the reversible binding site with U46619 induced aggregation in the presence of ADP, demonstrating its functional importance in amplifying the response to other agonists. These data suggest that TXA2 mediates platelet activation through at least two receptor-effector systems; one linked to phospholipase C activation, resulting in platelet aggregation and secretion and a second site mediating an increase in cytosolic calcium and platelet shape change.     

A double blind placebo controlled crossover study of prostacyclin in man.

Prostacyclin sodium (PGI₂) was administered in a double blind crossover trial to 6 normal males at infusion rates of 2, 4 and 8 ng/kg/minute. Substantial (p < 0.001) shifts of the log dose response curve of ADP induced platelet aggregation occured during the highest infusion rate of PGI₂. This was associated with a small but significant fall in diastolic blood pressure (?6.3± 1.6 mm Hg, p < 0.01) and a rise in heart rate (+25.5 ± 6.5 beats/minute, p < 0.001). Plasma renin activity rose in a dose related manner with PGI₂ but plasma aldosterone and plasma norepinephrine did not change. Marked facial flushing occured with PGI₂.     

Dopamine, affordance and active inference

The role of dopamine in behaviour and decision-making is often cast in terms of reinforcement learning and optimal decision theory. Here, we present an alternative view that frames the physiology of dopamine in terms of Bayes-optimal behaviour. In this account, dopamine controls the precision or salience of (external or internal) cues that engender action. In other words, dopamine balances bottom-up sensory information and top-down prior beliefs when making hierarchical inferences (predictions) about cues that have affordance. In this paper, we focus on the consequences of changing tonic levels of dopamine firing using simulations of cued sequential movements. Crucially, the predictions driving movements are based upon a hierarchical generative model that infers the context in which movements are made. This means that we can confuse agents by changing the context (order) in which cues are presented. These simulations provide a (Bayes-optimal) model of contextual uncertainty and set switching that can be quantified in terms of behavioural and electrophysiological responses. Furthermore, one can simulate dopaminergic lesions (by changing the precision of prediction errors) to produce pathological behaviours that are reminiscent of those seen in neurological disorders such as Parkinson's disease. We use these simulations to demonstrate how a single functional role for dopamine at the synaptic level can manifest in different ways at the behavioural level.     

Hydrolysis of alphas1- and beta-casein-derived peptides with a broad specificity aminopeptidase and proline specific aminopeptidases from Lactococcus lactis subsp. cremoris AM2

Aminopeptidase hydrolysis of alpha(s)1 - and beta-casein-derived synthetic peptides containing non-consecutive and consecutive proline residues was characterised. Aminopeptidase P (Pep P) (EC 3.4.11.9) or post-proline dipeptidyl aminopeptidase (PPDA) (EC 3.4.14.5) along with lysine-paranitroanilide hydrolase (KpNA-H) (EC 3.4.11.1) activities are required in the degradation of peptides containing non-consecutive proline residues. However, both Pep P and PPDA along with KpNA-H are required for hydrolysis of peptides containing consecutive proline residues. The results demonstrate the mechanism by which combinations of purified general and proline specific aminopeptidases from Lactococcus lactis subsp. cremoris AM2 hydrolyse peptides containing proline residues.