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Notes: Summary The agonist/α2-adrenoceptor interactions at human platelet membranes have been examined in radioligand binding studies with the full agonist ligand 3H-UK-14,304 [5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline] and the antagonist ligand 3H-yohimbine. From association kinetics of different concentrations of 3H-UK-14,304 (0.75−8.1 nmol/l) a K D-value of 2.37 nmol/l in agreement with the high-affinity KD-value (K DH = 1.60 ± 0.15 nmol/1) obtained from equilibrium binding studies was derived. In the presence of Gpp(NH)p about 6% of specific radioligand binding was observed in the association reaction. Addition of Gpp(NH)p at equilibrium resulted in a rapid loss (t 1/2 〈 1 min) of ≈80% of bound radioligand. Dissociation after addition of an excess of phentolamine (10 μmol/l) showed a biphasic time course independent of the radioligand concentration with the proportions of /15 of rapidly (t /12 〈 2 min) and /45 of slowly dissociating ligand (k−1 = 0.033±0.004 min−1). Application of a sequential binding model resulted in K D-values from this approach also in agreement with K DH from equilibrium binding studies. The rank order of potency for different agonists and antagonists to compete for binding with 3H-UK-14,304 indicated an α2-adrenoceptor interaction: (−)adrenaline 〉 clonidine 〉 (−)noradrenaline 〉 (−)isoprenaline and yohimbine = rauwolscine 〉 phentolamine 〉 prazosin 〉- corynanthine 〉 timolol respectively. The analysis of competition isotherms of UK-14,304 versus 3H-yohimbine (Hill-coefficient = 0.59 ± 0.03) showed that the agonist binds to two affinity states of the α2-adrenoceptor, with high (K DH = 1.77 ± 0.50 nmol/l) and low affinity (K DL = 71.2 ± 11.6 nmol/l) respectively. From these experiments a fraction of 56.9%±2.1% of the total number of α2-adrenoceptors (B max = 198.4 ± 8.0 fmol/mg of protein) in the high-affinity state was calculated. Similar results were obtained from 3H-UK-14,304 saturation isotherms according to a two-state binding model (K DH = 1.60±0.15 nmol/l; K DL = 66.2±10.7 nmol/l; B maxH = 57.6% ± 2.3%). Adrenoceptor agonists competed for specific binding of 3H-UK 14,304 and 3H-yohimbine in a manner that suggests that the 3H-UK-14,304 (∼3.5 nmol/l) labeled sites represent predominantly the agonist induced or stabilized high-affinity state of the α2-adrenoceptor. Adrenoceptor antagonists had equal affinities irrespective of the receptor states labeled by the agonist or antagonist radioligand. A loss of the high-affinity binding capacity (B maxH) of the agonist due to the presence of Gpp(NH)p was delineated from 3H-UK-14,304 saturation isotherms. An IC50-value of 0.181 ± 0.007 μmol/l for this Gpp(NH)p-effect was calculated. Divalent cations such as magnesium and manganese (10 mmol/l) increased specific binding of 3H-UK-14,304 by a factor of 3, without any influence on binding of the antagonist 3H-yohimbine. In contrast, sodium chloride strikingly decreased high-affinity binding of the agonist radioligand (IC50 = 41.9 ± 3.7 mmol/l). Unlike Gpp(NH)p, sodium chloride (〉 30 mmol/l) additionally promoted a marked decrease of the affinity of UK-14,304 at the low-affinity binding component. In contrast to the effects on agonist binding, sodium chloride concentrations of 30 to 300 mmol/l increased the binding affinity of the antagonist 3H-yohimbine about 2-fold. The sodium substitute N-methyl-D-glucamine was without effect on binding of 3H-UK-14,304 indicating that the influence of sodium chloride on binding properties was not due to changes in osmolarity. In conclusion these results suggest that 3H-UK-14,304 labels preferentially the agonist induced or stabilized high-affinity state (α2H) of the platelet α2-adrenoceptor.

Notes: Summary In a randomized, cross-over, single-dose study of 19 elderly hypertensive patients (aged 62–84 y, SBP 〉 160 mm Hg, DBP 〉 100 mm Hg, creatinine clearance 11–93 ml·min−1) we have studied the pharmacokinetics of the angiotensin converting enzyme (ACE) inhibitor enalapril after a single oral dose of either 10 mg enalapril or 10 mg enalapril + 25 mg hydrochlorothiazide. The pharmacokinetics of enalapril were unaffected by hydrochlorothiazide, but there was a significant reduction in renal clearance and a significant increase in AUC(0–24 h) of enalaprilat after hydrochlorothiazide, resulting in higher serum concentrations of the active drug. This was independent of the individual degree of renal impairment and might be due either to an initial reduction of GFR by hydrochlorothiazide or to interference with the tubular secretion of enalaprilat. The relationships between serum enalaprilat and serum ACE activity were similar after both treatments, both consistent with a value for Ki of enalaprilat of about 0.1 nmol·l−1. Thus, serum ACE activity was not affected by hydrochlorothiazide but completely reflected the pharmacokinetics of enalaprilat in both treatments.

Notes: Summary Carvedilol is a β-blocker with additional vasodilating activity. This study was performed in order to determine whether the vasodilator action of orally administered carvedilol in man is based upon an α-adrenoceptor antagonism exclusively or if evidence for an additional mechanism could be confirmed. The influence of carvedilol (50 mg p.o.) and prazosin (2 mg p.o.) upon the vasoconstrictor effect of noradrenaline and prostaglandin F2α, infused into superficial hand veins, was established in 8 healthy male volunteers. Increasing dosages of the vasoconstrictors below their threshold of systemic activity were employed in order to obtain dose-response curves of the hand veins congested at a venous occlusion pressure of 40 mmHg. These dose-response curves were repeated 1 and 3.5 h after oral administration of either carvedilol, prazosin, or placebo. The ex vivo, in vitro α1-receptor occupancy in plasma was measured before and after each vasoconstrictor dose-response curve, using an α1-radioreceptor binding assay. Washout periods of 48 h were kept between study days, investigating the influence of one orally administered drug upon one of the local vasoconstrictor dose-response curves at a time. In the α1-radioreceptor assay, plasma concentrations from 0.9- to 1.7-fold the equilibrium dissociation constant (K i) of carvedilol could be evaluated 1 as well as 3.5 h after medication, corresponding with a receptor occupancy of 44%–63%. After prazosin, 9–13 times the K i values were determined, which amounts to an α1-adrenoceptor occupation of about 90%–93%. Consequently, the dose-response curves to noradrenaline of the hand veins were attenuated to a greater extent after oral prazosin compared with carvedilol. In contrast, no statistically significant differences between the effects of carvedilol and prazosin could be found as regards the vasoconstriction induced by prostaglandin F2α. An oral placebo did not affect the reproducibility of either vasoconstrictor dose-response curve. We conclude that the relatively weak occupancy at α1-receptors by carvedilol cannot fully explain the effectivity of carvedilol (50 mg p.o.) in inhibiting prostaglandin F2α-induced vasoconstriction when compared with prazosin (2 mg p.o.). An additional mechanism of vasodilation could be responsible for this phenomenon

Notes: Summary Possible circadian changes in the pharmacokinetics and effect on serum angiotensin-converting enzyme (ACE) activity of the ACE inhibitor enalapril have been studied in 8 healthy subjects after oral ingestion of 10 mg enalapril maleate either at 08.00 h or 20.00 h. The time to peak serum concentration (tmax) of enalapril was increased after administration at 20.00 h compared to 08.00 h (2.4 h versus 1.3 h), where as other kinetic parameters were not significantly altered. The 24 h-kinetics of the active metabolite enalaprilat did not differ significantly between the two treatments, but the area under the curve (AUC (0–24)) and the peak serum concentration (Cmax) were slightly higher after intake at 20.00 h. The relationship between the measured serum enalaprilat level and the degree of inhibition of serum ACE was the same after both treatments. Overall, the evening and morning administration of enalapril did not differ markedly in the pharmacokinetics and the time course of ACE inhibition.