Bibliothek

Notizen: Abstract The thiadiazinone derivative [+]-EMD 60263 ((+)-5-(1-(α-ethylimino-3,4-dimethoxybenzyl)-1,2,3,4- tetrahydroquinoline-6-yl)-6-methyl-3,6-dihydro-2H-1,3,4 -thiadiazine-2-on) is a Ca2+-sensitizing agent with only minor phosphodiesterase inhibitory activity. Our aim was to characterize the inotropic and electrophysiological effects of [+]-EMD 60263 and its enantiomer [-]-EMD 60264 in several cardiac muscle preparations. The Ca2+-sensitizing activity resided in the [+]-enantiomer only. [+]-EMD 60263 (3 µM) shifted the EC50 of Ca2+ for contractile activation of skinned fibers of pig heart from 2.41 µM to 0.73 µM, whereas [-]-EMD 60264 (30 µM) was ineffective. In Langendorff-perfused guinea pig hearts, [+]-EMD 60263 and [-]-EMD 60264 induced concentration-dependent positive and negative inotropic effects, respectively; both enantiomers reduced spontaneous heart rate but did not influence perfusion pressure. The maximum increase in force of human atrial trabeculae was 35 % of pre-drug control with [+]-EMD 60263 in comparison to 113 % with forskolin. In guinea-pig papillary muscles, [+]-EMD 60263 and [-]-EMD 60264 had opposite inotropic responses, however, both agents similarly prolonged action potential duration. Both enantiomers concentration-dependently blocked the rapidly activating component IKr of the delayed rectifier in guinea-pig myocytes. The block saturated at potentials positive to +30 mV, closely resembling the effects of the antiarrhythmic agent E-4031 which had been originally used to define IKr. It is concluded, that the positive inotropic action of [+]-EMD 60263 can be explained by prevalence of the Ca2+-sensitizing effect. The accompanying prolongation in action potential duration is caused by block of the IKr component of the delayed rectifier. While the inotropic effects are stereoselective, most of the electrophysiological actions are clearly independent of sterical configuration. The combination of Ca2+-sensitizing with class-III antiarrhythmic action may provide an interesting pharmacological profile of potential therapeutic use.

Notizen: Summary The interaction between rauwolscine and angiotensin II was investigated in the isolated mesenteric artery of the rabbit. Rauwolscine, known as an antagonist at α2-adrenoceptors, did not induce contraction itself but interacted with angiotensin to produce a facilitated response of the vascular tissue. In the presence of rauwolscine, the contractile response of the tissue to angiotensin was markedly enhanced. The degree of facilitation appeared to be dependent on the rauwolscine concentration used rather than that of angiotensin. Moreover, rauwolscine induced a concentration-dependent increase in tension (pD2=6.8) in the presence of even subcontractile concentrations of angiotensin (10−10 mol/l). This effect was not attributable to an indirect action involving presynaptic catecholamines, as revealed by the use of tissue strips from animals pretreated with reserpine or after chemical sympathectomy. Furthermore, an interaction via the prostaglandin system was excluded by negative results obtained with indomethacin. The ‘agonistic effect’ of rauwolscine was significantly attenuated by phentolamine (α1/α2) but not by prazosin (α1) or phenoxybenzamine when applied for only a short time. The α2-antagonist BDF 6143 behaved like rauwolscine whereas the α1-antagonist corynanthine, a stereoisomer of rauwolscine, did not. The results indicate that the ‘rauwolscine effect’ is mediated by a receptor with α2-characteristics. In general, angiotensin appears to interfere with some process which determines the expression of a drug's intrinsic effect.

Notizen: Summary Cardiovascular alterations in hypo- and hyperthyroidism have been ascribed to changes of noradrenergic neurotransmission. In the present study the influence of thyroid hormones on adrenoceptors in the rat heart was further characterized. The effect of artificial hypothyroidism (induced by feeding 6-propyl-2-thiouracil, PTU) and hyperthyroidism (induced by daily injections of triiodothyronine, T3) on myocardial adrenoceptor binding, catecholamines, some physiological responses, and their interdependence was examined. 1. The density of myocardial β-adrenergic binding sites (3H-dihydroalprenolol, 3H-DHA) was reduced after PTU (by 38%) and enhanced after T3 treatment (by up to 82%). The increase was dose- and time-dependent and reversible within 4 days. No changes of the affinity of 3H-DHA to its binding sites were observed. Only L-T3 and L-T4 proved to be active, D-T3 and reverse T3 had no effect. The rise in β-adrenoceptor density caused by T3 was prevented by concomitant administration of cycloheximide, indicating its dependence on protein synthesis. 2. The density of myocardial α 1-adrenergic binding sites (3H-prazosin) was significantly reduced in the PTU group (by up to 28%) and even more distinctly by T3 treatment (by up to 50%). K D values remained unaltered. 3. The noradrenaline content and turnover of rat hearts was significantly reduced by T3-induced hyperthyroidism. PTU treatment had no influence on content and turnover of noradrenaline. Plasma noradrenaline as well as adrenaline levels in freely moving rats were increased by PTU treatment 9- and 5-fold, respectively. In T3-injected animals no significant changes were measured. 4. The density of adrenoceptors is known to be inversely correlated with catecholamine levels in several organs. Neither α- nor β-adrenoceptor changes in the myocardium of dysthyroid rats could be attributed to such a homologous regulation, since they still occurred after chemical sympathectomy with 6-hydroxydopamine and adrenalectomy. 5. Hypertrophy of the heart due to T3 could not be explained by prolonged β-adrenergic stimulation because it was not inhibited by 6-hydroxydopamine or high doses of propranolol. A T3-induced tachycardia was recorded in pithed and in intact rats. It was not reduced to normal levels by the β-adrenoceptor antagonist sotalol and, thus, was independent of sympathetic influence. Hypothyroid pithed rats displayed a marked bradycardia, whereas in intact hypothyroid animals a normal heart rate was measured at rest. Obviously, an enhanced availability of catecholamines which seems to reflect an increased release and/or a central nervous compensatory mechanism was responsible for the maintenance of the normal heart rate. 6. In pithed rats the β-adrenoceptor-mediated increase in heart rate was attenuated by PTU treatment. The isoprenaline dose-response curve was shifted to the right, the maximal response was reduced. After T3 injections, the sensitivity to isoprenaline was not affected, but the maximal heart rate that could be obtained was increased. These results are compatible with the β-adrenoceptor changes described above. It is concluded that cardiovascular signs of hypo- and hyperthyroidism can only be explained by a complex interaction of several factors. Beside the changes of adrenoceptor density and an altered sensitivity to noradrenaline, a central nervous regulation and subsequent changes of catecholamine release as well as effects independent of the sympathetic nervous system have to be considered.

Notizen: Summary The influence of removing the endothelial cells on the α-receptor-mediated contractile response in segments of rat aorta was investigated using agonists with a range of affinity for α1 and α2. The preferential α1 were methoxamine, cirazoline, ST 587, and Sgd 101/75 and the preferential α2 were B-HT 920, clonidine, and guanfacine. When the endothelium was intact, the intrinsic activity (compared to noradrenaline) varied widely (0.0–0.7) for both groups of agonists. After removal of the endothelium the intrinsic activity was increased in each case to that of noradrenaline, or close to it. Furthermore, an increase in potency was obtained for each agonist, although to different degrees. No correlation, however, was found between the selectivity of the agonists and the degree of enhancement caused by the removal of the endothelium, in terms of either the intrinsic activity or the potency. Moreover, the use of the selective α2 antagonist rauwolscine on intact tissues did not mimic the effect of removing the endothelium. Therefore, the α-receptors of the endothelium could not be classified as either of the α1 or α2.

Notizen: Summary Altered thyroid states are known to produce profound changes in sympathetic mechanisms. The influence of thyroid hormones on plasma catecholamines, adrenal catecholamines and on circulatory parameters were studied in pithed rats. Animals were made hypothyroid by feeding 6-propyl-2-thiouracil for 6 weeks. Hyperthyroidism was induced by triiodothyronine injections (0.5 mg/kg body weight) for 7 days. 1. The basal heart rate was decreased in hypothyroidism and accelerated in hyperthyroidism. Basal diastolic blood pressure was reduced in both dysthyroid states. 2. Electrical stimulation of autonomic outflow from the spinal cord induced tachycardia and increased diastolic blood pressure. The increase in heart rate due to electrical stimulation was reduced in hypo- and hyperthyroidism. In hyperthyroid animals this may be due to the already accelerated basal heart rate. The initial rise in diastolic blood pressure was decreased in the hypothyroid and increased in the hyperthyroid state. 3. Basal adrenaline plasma levels were higher than control values in hypothyroidism and unaltered in hyperthyroidism. Basal noradrenaline levels were not significantly influenced by either thyroid state. 4. The increase in plasma noradrenaline during electrical stimulation was enhanced in hyperthyroid animals and remained unaltered by hypothyroidism. 5. The increase in plasma adrenaline during electrical stimulation was significantly changed in both dysthyroid states. Hyperthyroidism reduced the initial peak of adrenaline release. Hypothyroidism raised plasma adrenaline in the steady state after sustained stimulation. 6. Elevated basal and stimulated adrenaline plasma levels corresponded to an increased adrenaline content and enhanced phenylethanolamine-N-methyltransferase activity in adrenal glands of hypothyroid rats. The significant influence of thyroid hormones on plasma catecholamines seems to contribute but cannot explain completely the circulatory changes in hypo- and hyperthyroidism. The role of additional factors such as receptor changes and central nervous mechanisms is discussed.

Notizen: Summary Experiments on the isolated saphenous vein of the rabbit have been performed in order to determine whether a coexistence of postjunctional α1- and α2-adrenoceptor subtypes can be demonstrated also under in vitro conditions. 1. Prazosin, selective for α1-receptors, and rauwolscine, selective for α2-receptors, were used to antagonize the contractile response of the agonists phenylephrine (α1), B-HT 920 (α2) and noradrenaline (α1/α2). Each of the antagonists was equipotent against all three agonists; the effect of neither antagonist fulfilled the criteria for a competitive antagonism. The preferential α1-receptor antagonist phenoxybenzamine, applied for irreversible blockade of α-receptors, reduced the effect of phenylephrine and B-HT 920 to the same degree. Thus, the results obtained with α-receptor antagonists cannot be reconciled with the coexistence of α1- and α2-receptors or the existence of only one of them. 2. Contractile responses induced by α1- and α2-receptor agonists, respectively, could be differentiated by the calcium entry blocker nitrendipine. The effect of B-HT 920 was decreased whereas that of phenylephrine was hardly affected. 3. Furthermore, we investigated the influence of angiotensin II on the effects of the α-adrenoceptor agonists as well as the antagonists. We observed firstly, that angiotensin, acting postsynaptically, potentiates the contractile response of certain α2-receptor agonists and secondly, that in the presence of angiotensin the characteristics of the receptors as revealed by B-HT920 are converted to typical α2-adrenoceptors. It is concluded that the postjunctional α2-receptors of the saphenous vein require the blood borne substance angiotensin for their expression.

Notizen: Summary We have investigated the interaction of the α 2-adrenoceptor agonist B-HT 920 with the α-adrenoceptors in the rabbit aorta using various experimental conditions. In standard Krebs-Henseleit solution B-HT 920 behaved as an antagonist when tested against the α 1-agonist phenylephrine. However, it behaved as a partial agonist at α-receptors when subcontractile concentrations of various spasmogens (angiotensin II, serotonin, prostaglandin F2α ) were applied, although no change in the affinity of the receptor to B-HT 920 was observed. By means of the selective antagonists prazosin and rauwolscine it was established that B-HT 920 activated α 1-renoceptors. The same agonistic effects of B-HT 920 were obtained after pretreatment of the animal with reserpine or in the presence of ouabain. The various treatments used (except reserpine) did not influence the contractile response to phenylephrine. The contractile response to B-HT 920 was found to be highly susceptible to the calcium entry blocker nitrendipine whereas the response to phenylephrine was not. It is concluded that spasmogens modulate the responsiveness of α-receptors to certain agonists, possibly by causing a depolarization of the cell membrane and, thereby, sensitization of a mechanism involved in excitation-contraction coupling, conceivably a calcium gating mechanism.

Notizen: Summary The steady state levels of various phosphate esters were measured in the compound eyes of two species of decapod crustaceans (Astacus andEupagurus) and of the blowfly (Calliphora). In the crustaceans, free nucleotides—especially ATP—contribute only little to the soluble phosphate fraction. The adenylate charges of all three species are in the range of about 0.8, though the proportion of adenine nucleotides to other organic phosphates differs considerably. The main energy rich phosphate compound in crustacean eyes was found to be arginine phosphate; the ratio ATP/Arg-P is 1:10 inAstacus, and 1:4 inEupagurus; in the blowfly it is near 1:1. Arg-P is considered to play a role as a phosphagen.