David B. Barnett

[3H]rauwolscine and [3H]yohimbine binding to rat cerebral and human platelet membranes: Possible heterogeneity of α2-adrenoceptors

The specific binding of the alpha 2-adrenoceptor antagonists [3H]yohimbine and [3H]rauwolscine to membranes prepared from rat cerebral cortex and human platelets was rapid, reversible, saturable and of high affinity. Both ligands labelled an identical population of sites in cerebral or platelet membranes though the affinities of both agents were significantly lower in brain. The specific binding of [3H]yohimbine and [3H]rauwolscine in both tissues was displaced by various adrenergic agents in a manner that suggests that the labelled sites probably represent the alpha 2-adrenoceptor. There were, however, significant differences in the affinities of certain alpha-antagonists between cerebral and platelet preparations, even though these agents generated displacement curves with slopes close to unity in both tissues. Thus, whereas yohimbine, rauwolscine, WB4101 and corynanthine were all weaker in cerebral cortex, prazosin was significantly more potent at cortical than in platelet membranes. A possible heterogeneity of alpha 2-adrenoceptors is discussed.

Intravenous infusion of magnesium sulphate after acute myocardial infarction: effects on arrhythmias and mortality

Two hundred patients with acute myocardial infarction were entered into a randomised double-blind trial where they received either intravenous magnesium sulphate or saline for 24 hours after admission to hospital. The incidence of ventricular arrhythmias necessitating treatment was reduced by more than half in the group receiving magnesium sulphate. There were two deaths in the group receiving magnesium sulphate and seven receiving saline placebo. Total cardiac events were significantly reduced in the magnesium treated group. These reductions cannot be attributed to differences in risk factors or therapy between the two groups, either before or during the period of study. These results suggest that magnesium administration reduces the incidence of serious tachyarrhythmias and death after acute myocardial infarction and that this simple regime warrants further study.

Plasma N-terminal pro BNP and cardiotrophin-1 are elevated in aortic stenosis

Echocardiography with Doppler examination of the aortic valve provides a very accurate assessment of the transvalvular gradient and is used to monitor progression of aortic stenosis (AS). Plasma brain natriuretic peptide (BNP) has been shown to correlate with end‐systolic wall stress in patients with AS.

The Use Of A Calcium‐Channel Blocker, Nicardipine, For Severely Asphyxiated Newborn Infants

A continuous infusion of nicardipine was given to four severely asphyxiated fullterm infants who were at high risk for adverse outcome and had abnormal cerebral Doppler haemodynamic studies. The heart rate increased in all four infants and mean arterial blood pressure (MAP) fell in three. Two infants had a sudden and marked fall in MAP, together with severe impairment of skin blood‐flow and a concurrent fall in cerebral blood‐flow velocity. The serum level of nicardipine was <40ng/mL in all cases. The use of nicardipine, and possibly other calcium‐channel blockers, may be associated with marked hypotension, and if there is no cerebral autoregulation, may cause further cerebral hypoperfusion, so use of these drugs in asphyxiated newborn infants should only be attempted if blood pressure is carefully monitored.

Evaluation of a radioreceptor assay for beta-adrenoceptor antagonists in plasma

SummaryA radioreceptor assay (RRA) recently developed in this laboratory for beta-adrenoceptor antagonists in plasma was evaluated in normal volunteers and compared with a radioimmunoassay (RIA) for propranolol. The RRA depends upon the ability of beta-adrenoceptor antagonists to compete with a radiolabelled ligand for beta-adrenoceptor binding sites on lung membranes. Unlike other assays, it measures biologically active drugs including active metabolites of the parent compound. In volunteers given a single oral dose of (±)-propranolol, considerable differences between the two assay methods were demonstrated. In other experiments this difference was shown to relate to the RIAs sensitivity to the inactive (+)-isomer of propranolol and possibly to inactive metabolites. The facility of the RRA in measuring plasma levels of several other non-selective beta-adrenoceptor antagonists was also demonstrated. By employing (−)-propranolol as the standard in the RRA, all of these drugs can be directly compared with a single and relatively simple assay technique.

Comparison of the β1 selective affinity of prenalterol and corwin demonstrated by radioligand binding

The ability of the beta 1 partial agonists prenalterol and corwin , to displace the binding of [3H]dihydroalprenolol to the heterogeneous populations of beta-adrenoceptors on rat and rabbit lung membranes was compared. Both drugs exhibited higher affinity for the binding sites in rabbit lung (predominant beta 1) as compared to rat lung (predominant beta 2). Curve fitting analysis of the displacement curves into high affinity (beta 1) and low affinity (beta 2) components indicates that both drugs exhibit beta 1 selective affinity. Further competition experiments performed in the presence of carefully calculated concentrations of highly selective beta 1 (atenolol) or beta 2 (ICI 118,551) antagonists to produce homogeneous receptor subtype populations in rat and rabbit lung membranes respectively, confirmed this beta 1 selective affinity. These results suggest an approximate selective affinity ratio (beta 1 to beta 2) of ten and forty fold for prenalterol and corwin respectively.

Measurement of β-adrenoceptor antagonists in biological fluids using a radioreceptor assay

A new assay for plasma levels of beta-adrenoceptor antagonists is described. The assay depends upon the ability of the drug to compete with a labelled beta-adrenoceptor antagonist (-)-[3H]dihydroalprenolol for beta-adrenoceptor binding sites on lung membranes. The assay is simple to perform, very sensitive and does not require prior extraction of plasma. The assay can also detect bioactive metabolites of these agents and is clearly not limited to a single beta-adrenoceptor antagonist.

Differential rates of down regulation and recovery of rat myocardial β-adrenoceptor subtypes in vivo

We have investigated the differential rate and extent of down regulation and recovery of rat myocardial beta adrenoceptor subtypes during and after short term (up to 72 h) subcutaneous isoprenaline infusions (40 micrograms/kg per h) in vivo using osmotic minipumps. Maximum density (Bmax) of the receptors in ventricular membranes was assessed by radioligand binding, saturation analysis using 125I-pindolol. Groups of animals were sacrificed following various agonist infusion times and then during recovery after removal of minipumps following initial infusion of isoprenaline for 72 h. During agonist infusion, beta 2 adrenoceptors down regulated significantly more rapidly and to a greater extent than the beta 1 subtype (maximum change from control 66% beta 2, 34% beta 1 P less than 0.05). In the recovery phase of the experiments following initial maximum down regulation, beta 2 adrenoceptor density also returned to control values more rapidly (by 24 h) than the beta 1 subtype (greater than 72 h). These results are qualitatively different to those reported in other tissues from this species using selective and non selective catecholamine agonists and in human myocardium in end stage heart failure following chronic sympathetic nervous stimulation. This may be due to differences in tissue cellular composition, the nature/selectivity of the agonist or the length of time of agonist exposure.

Effects of adrenaline and alpha adrenergic antagonists on platelet aggregation in whole blood : Evaluation of electrical impedance aggregometry

The effects of adrenaline and alpha adrenergic antagonists on platelet aggregation in whole blood were assessed by electrical impedance. These effects were then compared to those found in platelet rich plasma assessed by both optical and impedance methods. Our data demonstrate a number of difficulties in attempting to measure platelet responsiveness by electrical impedance and re-emphasise the need for caution in interpretation of in vitro platelet aggregation studies in terms of their clinical relevance.

Heterogenous properties of alpha2 adrenoceptors in particulate and soluble preparations of human platelet and rat and rabbit kidney

Alpha 2 adrenoceptors of the human platelet and rat and rabbit renal cortex were compared in binding studies using the selective antagonist ligand [3H]rauwolscine. Significant differences in the pharmacological characteristics of the alpha 2 adrenoceptor were observed between the tissues with reference to both absolute drug affinities as well as rank order of drug potency. Two subsets of the alpha 2 adrenoceptor sites could be described: one exhibiting equal affinity for the alpha 2 selective diastereoisomers, yohimbine and rauwolscine, and low affinity for the alpha 1 antagonist prazosin (human platelet); the other displaying significantly higher affinity for rauwolscine than yohimbine but also relatively high affinity for prazosin (rat and rabbit kidney). Digitonin solubilised alpha 2 adrenoceptors from these tissues identified by [3H]rauwolscine generally displayed reduced drug affinities. This was most apparent for agonists (10-50-fold lower) indicating separation of the soluble receptors from the guanine nucleotide binding proteins. However, the solubilised alpha 2 adrenoceptors retained the overall pharmacological properties of their respective membrane receptors, including rank order of drug potency, reflecting similar inter-tissue differences. These results suggest that the pharmacological differences in alpha 2 adrenoceptors observed are not species specific and are not related to variation in receptor effector coupling mechanisms or problems of ligand access due to membrane constraints. This may reflect true intrinsic differences in the molecular structure of these receptors.