Biomedical subjects
Publications and source records attributed to David Colquhoun.
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Pharmacology started to develop into a real quantitative science in 1909, when A.V. Hill derived the Langmuir equation in the course of his studies on nicotine and curare. A history of the developments since then shows both brilliant insights and missed opportunities. It also shows that much remains to be done. There is still no mathematical description that can describe quantitatively the actions of agonists on G-protein-coupled receptors, although progress has been greater with agonist-activated ion channels, which are much simpler.
This paper looks at ion channels as an example of the pharmacologist's stock in trade, the action of an agonist on a receptor to produce a response. Looked at in this way, ion channels have been helpful because they are still the only system which is simple enough for quantitative investigation of transduction mechanisms. A short history is given of attempts to elucidate what happens between the time when agonist first binds, and the time when the channel opens.
Steady-state single channel activity from NMDA receptors was recorded at a range of concentrations of both glutamate and glycine. The results were fitted with several plausible mechanisms that describe both binding and gating. The mechanisms we have tested were based on our present understanding of receptor structure, or based on previously proposed mechanisms for these receptors. The steady-state channel properties appear to have virtually no dependence on the concentration of either ligand, other than the frequency of channel activations. This limited the ability to discriminate detail in the mechanism, and, along with the persistence of open-shut correlations in high agonist concentrations, suggests that NMDA channels, unlike other neurotransmitter receptors, cannot open unless all binding sites are occupied. As usual for analyses of NMDA channels, the applicability of our results to physiological observations is limited by uncertainties in synaptic zinc and hydrogen ion concentrations, both of these being known to affect the receptor. The mechanism that we propose, on the basis of steady-state single channel recordings, predicts with fair accuracy the apparent open and shut-time distributions in different concentrations of agonists, correlations between open and shut times, and both the rising and falling phases of the macroscopic response to concentration jumps, and can therefore account for the main features of synaptic currents.
Statins decrease LDL cholesterol and the risk of atherosclerotic cardiovascular disease (CVD). They also decrease coenzyme Q10 (CoQ10), an effect that may negate some of the statin benefit on CVD. We examined the relationship between plasma CoQ10 concentration and CVD in a prospective case-control study of the effect of pravastatin. Plasma samples from 250 LIPID trial patients who over 6 years suffered a recurrent CVD event (CVD death, nonfatal MI or stroke) and 250 matched controls who remained event-free for the same duration of follow-up were assayed for CoQ10 and lipids (cholesterol and cholesterylesters). Mean plasma CoQ10 concentrations were significantly lower in pravastatin-treated patients than in those assigned placebo (0.51 versus 0.60 micromol/L, P = 0.006), and there was a moderate correlation between CoQ10 and common cholesterylesters (Pearson correlation coefficients in patients randomised to placebo, range r = 0.42-0.63). Univariate conditional logistic regression did not suggest any relationship between plasma CoQ10 and the risk of future CVD events (odds ratio 1.18; 95% CI 0.74-1.87; P = 0.49). Instead, we observed a reduction in the rate of recurrent CVD events with increasing ratio of plasma cholesterylarachidonate to cholesteryllinoleate. This study confirms that pravastatin lowers plasma CoQ10 concentrations, but this does not appear to predict the risk of recurrent CVD events.
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A mutation in the epsilon subunit of the human nicotinic acetylcholine receptor (epsilonL78P) is known to cause a congenital slow channel myasthenic syndrome. We have investigated the changes in receptor function that result in the mutant receptor producing prolonged endplate currents, and consequent muscle damage. The rate constants for channel gating and for the binding and dissociation of acetylcholine were investigated by analysis of single ion channel recordings. A conventional mechanism with two non-equivalent binding sites, and variations upon this mechanism, were fitted to data using a maximum likelihood method that uses the Hawkes-Jalali-Colquhoun (HJC) treatment of missed brief events. The mutant receptor produced prolonged activations, bursts of openings that cause a slow decay of simulated synaptic currents. The main reason for the longer bursts of openings seen with mutant receptor was a decrease in the rate of ACh dissociation from diliganded receptors, though the lifetime of individual openings was somewhat increased too. As well as producing long bursts, the mutant receptor also produced many very short openings, though these carry little current. The burst structure for the mutant receptor at low ACh concentration is unusual in that most long bursts appear to start in a very brief monoliganded open state that then usually binds another ACh molecule to produce a long diliganded activation. The first opening is so short that it will usually be missed (together with the shut time that follows it), so the true burst length is likely to be underestimated.
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The alpha1beta heteromeric receptors are likely to be the predominant synaptic form of glycine receptors in the adult. Their activation mechanism was investigated by fitting putative mechanisms to single-channel recordings obtained at four glycine concentrations (10-1000 microm) from rat alpha1beta receptors, expressed in human embryonic kidney 293 cells. The adequacy of each mechanism, with its fitted rate constants, was assessed by comparing experimental dwell time distributions, open-shut correlations, and the concentration-open probability (P(open)) curve with the predictions of the model. A good description was obtained only if the mechanism had three glycine binding sites, allowed both partially and fully liganded openings, and predicted the presence of open-shut correlations. A strong feature of the data was the appearance of an increase in binding affinity as more glycine molecules bind, before the channel opens. One interpretation of this positive binding cooperativity is that binding sites interact, each site sensing the state of ligation of the others. An alternative, and novel, explanation is that agonist binding stabilizes a higher affinity form of the receptor that is produced by a conformational change ("flip") that is separate from, and precedes, channel opening. Both the "interaction" scheme and the flip scheme describe our data well, but the latter has fewer free parameters and above all it offers a mechanism for the affinity increase. Distinguishing between the two mechanisms will be important for our understanding of the structural dynamics of activation in the nicotinic superfamily and is important for our understanding of mutations in these receptors.
The glycine receptor mediates fast synaptic inhibition in the spinal cord and brainstem. Its activation mechanism is not known, despite the physiological importance of this receptor and the fact that it can serve as a prototype for other homopentameric channels. We analyzed single-channel recordings from rat recombinant alpha1 glycine receptors by fitting different mechanisms simultaneously to sets of sequences of openings at four glycine concentrations (10-1000 microm). The adequacy of the mechanism and the rate constants thus fitted was judged by examining how well these described the observed dwell-time distributions, open-shut correlation, and single-channel P(open) dose-response curve. We found that gating efficacy increased as more glycine molecules bind to the channel, but maximum efficacy was reached when only three (of five) potential binding sites are occupied. Successive binding steps are not identical, implying that binding sites can interact while the channel is shut. These interactions can be interpreted in the light of the topology of the binding sites within a homopentamer.
AIMS: Fibrates or nicotinic acid are usually recommended for secondary prevention of coronary heart disease in patients with low plasma levels of both low-density lipoprotein cholesterol (LDL-C) < or =140 mg/dL (< or =3.6 mmol/L) and high-density lipoprotein cholesterol (HDL-C) < or =40 mg/dL (< or =1.03 mmol/L). The LIPID trial, a randomised, placebo-controlled trial in 9014 patients at 87 centres in Australia and New Zealand, provided an opportunity to investigate the effects of an HMG-CoA reductase inhibitor in patients with low LDL-C and low HDL-C. METHODS AND RESULTS: Participants in this post hoc substudy were 2073 patients aged 31-75 years with baseline LDL-C < or =140 mg/dL (< or =3.6 mmol/L), HDL-C < or =40 mg/dL (< or =1.03 mmol/L), and triglyceride < or =300 mg/dL (< or =3.4 mmol/L). The relative risk reduction with pravastatin treatment was 27% for major coronary events (95% CI 8-42%), 27% for coronary heart disease mortality (95% CI 0-47%), 21% for all-cause mortality (95% CI 0-38%), and 51% for stroke (95% CI 24-69%). The number needed to treat to prevent a major coronary event over 6 years was 22. CONCLUSIONS: Treatment with pravastatin in patients with both low LDL-C and low HDL-C significantly reduced major coronary events, stroke, and all-cause mortality. The level of HDL-C is crucial to the risk of recurrent CHD events and, consequently, the benefit of lowering LDL-C.
In the field of ligand-gated ion channels, recent developments, both in the knowledge of structure and in the measurement of function at the single-channel level, have allowed a sensible start to be made on understanding the relationship between structure and function in these proteins. In this review, the cases of glycine, nicotinic ACh and glutamate receptors are compared and contrasted, and problems such as how binding of agonist causes the channel to open, and why partial agonists are partial, are considered. Some observations, both structural and functional, suggest that more attention needs to be paid to conformational changes that occur before the channel opens. Such changes might account for the interaction found between subunits of the glycine receptor while it is still shut and, perhaps, the agonist-dependent structural changes seen in AMPA receptors. They might also complicate our understanding of the binding-gating problem.
Most, but not all, ion channels appear to obey the law of microscopic reversibility (or detailed balance). During the fitting of reaction mechanisms it is therefore often required that cycles in the mechanism should obey microscopic reversibility at all times. In complex reaction mechanisms, especially those that contain cubic arrangements of states, it may not be obvious how to achieve this. Three general methods for imposing microscopic reversibility are described. The first method works by setting the 'obvious' four-state cycles in the correct order. The second method, based on the idea of a spanning tree, works by finding independent cycles (which will often have more than four states) such that the order in which they are set does not matter. The third method uses linear algebra to solve for constrained rates.
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The subunits that compose eukaryotic glutamate ion channel receptors have three transmembrane domains (TMs) and terminate with intracellular tails that are important for controlling channel expression and localization. Truncation of NMDA receptor subunits before the final TM showed that this TM and intracellular tail region are necessary to form functional channels. However, it is shown here that these truncated subunits may be partially rescued by coexpressing the final TM and tail as a separate protein. The whole-cell currents so produced are somewhat lower than with full-length subunits, and they do not show the sag characteristic of currents from channels containing NR1 and NR2A subunits in the continued presence of an agonist. In addition, these truncated subunits were joined to full-length subunits to generate tandems. The functional expression of these tandems confirmed the tetrameric structure of NMDA receptors and also suggested that the subunits making up NMDA receptors are arranged as a dimer of dimers in the receptors with a 1-1-2-2 orientation of the subunits in the channel, and not in an alternating pattern of subunits around the pore. These results may redirect future studies into the mechanism of binding and gating in these receptors toward schemes including dimers, and may also be relevant to studies of glutamate receptor ion channels in general.
The objective was to define the molecular mechanisms underlying congenital myasthenic syndromes (CMS) by studying mutations within genes encoding the acetylcholine receptor (AChR) and related proteins at the neuromuscular junction. It was found that mutations within muscle AChRs are the most common cause of CMS. The majority are located within the epsilon-subunit gene and result in AChR deficiency.