The effects of monovalent and divalent cations on the alpha-adrenoceptor of intact human platelets

Stochastic description of the ligand-receptor interaction of biologically active substances at extremely low doses

Signalling molecules can be effective at extraordinarily low concentrations (down to attomolar levels). To handle such cases, probabilistic methods have been used to describe the formal kinetics of action of biologically active substances in these low doses, although it has been necessary to review what is meant by such a term. The mean numbers of transformed/degraded molecules and their dispersions were calculated for the possible range of ligand-receptor binding schemes. We used both analytical equations and numerical simulations to calculate the coefficients of variation (ratio of standard deviation to mean) and demonstrated that the distribution of the coefficient is highly dependent on the reaction scheme. It may, therefore, be used as an additional factor for discriminating between cooperative and noncooperative models of ligand-receptor interaction over extreme ranges of ligand dilution. The relevance to signalling behaviour is discussed.

Interaction of opioid drugs with human platelet alpha 2-adrenoceptors

Morphine, naltrexone and naloxone inhibited the binding of [3H]clonidine to the alpha 2-adrenoceptors in human platelet membranes, provided that Mg2+ was present in the medium. In the presence of 5'-guanylyl imidophosphate (Gpp(NH)p) or in the absence of Mg2+ morphine did not modify the binding of [3H]clonidine. Neither [D-Ala2,N-MePhe4,Gly5-ol]enkephalin (DAGO), nor [D-Pen2,D-Pen5]enkephalin (DPDPE), nor dynorphin-(1-17) affected the [3H]clonidine binding. The presence of 1 mM naloxone did not alter the affinity of either [3H]clonidine or [3H]yohimbine, but reduced the number of binding sites of [3H]clonidine, having no effect on [3H]yohimbine. Naloxone inhibited the binding of adrenaline to high- but not low-affinity sites. It is concluded that morphine and semisynthetic antagonist derivatives interact with alpha 2-adrenoceptors only in the high-affinity state.

Platelet 3H-rauwolscine binding in patients with panic attacks

Platelet alpha 2 binding was measured in 14 patients with panic attacks and 21 controls using the antagonist 3H-rauwolscine as radioligand. The mean (+/- SD) Bmax values of patients (116 +/- 45 fmoles/mg protein) and controls (106 +/- 32 fmoles/mg protein) was not significantly different (p greater than 0.05, Mann-Whitney U test). There was a nonsignificant trend for the Kd of patients (1.25 +/- 0.67 nM) to be higher than that of controls (0.96 +/- 0.32 nM). The results do not support an adrenergic abnormality in panic patients.

Selective interaction of DG-5128 with a low agonist affinity state of alpha-2 adrenoceptor

Using rat cerebral cortex membranes, the inhibitory effect of DG-5128 against (3H)-clonidine binding was compared between low (alpha 2L) (in the presence of EDTA) and high (alpha 2H) affinity states (in the presence of excess magnesium) of alpha 2-adrenoceptor for agonists. The Ki value (pKi=6.79) of DG-5128 in the alpha 2L state was 6.4 times higher than the value in the alpha 2H state. Thus, DG-5128 produces alpha 2-adrenoceptor antagonism through the selective interaction with an alpha 2L state of the receptor.

Prophylactic lithium treatment of patients with affective disorders is associated with decreased platelet [3H]dihydroergocryptine binding

Intact platelet [3H]dihydroergocryptine binding characteristics have been determined in female patients with a history of affective disorders who have been treated with prophylactic lithium. These results have been compared with results obtained from concurrent experiments in normal female controls and acutely-depressed, female drug-free patients. The platelet density of alpha 2-adrenoceptors in the lithium-treated patients is similar to that of the depressed patients as there are significantly fewer sites per platelet when compared to normal controls. It is concluded that lithium treatment with remission from affective illness is not associated with a change toward normal of alpha 2-adrenoceptor density and represents a trait in patients prone to affective disorder. The problem of comparison of results of platelet alpha 2-adrenoceptor density obtained from different laboratories is discussed with particular reference to the non-uniform choice of [3H]ligands used.

A comparison of alpha 2-adrenoreceptor binding characteristics of intact human platelets identified by [3H]-yohimbine and [3H]-dihydroergocryptine

The binding characteristics of [3H]-yohimbine on intact human platelets were investigated and compared to those of [3H]-dihydroergocryptine. A significant correlation (r = 0.92, P less than 0.01, n = 7) was observed between the inhibition of [3H]-yohimbine binding and of noradrenaline-induced platelet aggregation, indicating that [3H]-yohimbine binds to the alpha 2-adrenoreceptor responsible for noradrenaline-induced platelet aggregation. The binding capacity of [3H]-yohimbine in 20 young male volunteers was 39 +/- 2.2 fmol/10(8) platelets (mean +/- s.e.m.). This is considerably lower than the binding capacity of [3H]-dihydroergocryptine previously reported. Simultaneous analysis of [3H]-yohimbine and [3H]-dihydroergocryptine binding in 10 subjects showed a significantly lower binding capacity for [3H]-yohimbine (P less than 0.001, paired t-test). This difference was not present in every subject, and was consistent within individuals on repeated testing. Possible causes of this difference in radioligand binding are discussed.