K. Starke

Alpha sympathomimetic inhibition of adrenergic and cholinergic transmission in the rabbit heart

SummaryExperiments were performed in isolated rabbit hearts to investigate the mode of action of α-receptor stimulants and α-adrenolytic drugs on the release of noradrenaline by adrenergic nerve impulses. In addition, the effect of α-receptor stimulants on parasympathetic transmission has been studied. 1.Dihydroergotamine enhanced the overflow of noradrenaline in response to sympathetic nerve stimulation at a concentration (0.19 μg/ml) which did not interfere with the removal of exogenous noradrenaline from the perfusion fluid by the heart.2.Phenylephrine (in the presence of cocaine), oxymetazoline, and naphazoline caused a dose-dependent decrease of the stimulation-induced overflow of noradrenaline. During perfusion with oxymetazoline and naphazoline, also the mechanical response to stimulation was diminished. Since the inactivation of noradrenaline by the cardiac tissue was not enhanced by any of the three drugs, it can be concluded that overflow was diminished because of a decreased liberation of noradrenaline from the nerve endings.3.The degree of inhibition caused by oxymetazoline was inversely related to the absolute overflow of noradrenaline in response to nerve stimulation before the infusion of oxymetazoline.4.Oxymetazoline and naphazoline at a concentration (100 ng/ml) which maximally inhibited the liberation of noradrenaline were devoid of myocardial effects. In contrast, all the concentrations of phenylephrine (1 μg/ml and above) which diminished the secretion of noradrenaline greatly increased the force of contraction. During perfusion with phenylephrine, but not with oxymetazoline or naphazoline, noradrenaline was released into the venous effluent. The myocardial effects of phenylephrine were diminished by phentolamine, but not by propranolol or cocaine, though cocaine prevented the release of noradrenaline by phenylephrine.5.The effects of oxymetazoline and cocaine on the overflow of noradrenaline were independent of each other. In contrast, pretreatment with oxymetazoline prevented the increase of transmitter overflow by small concentrations of phentolamine (0.28 μg/ml) or phenoxybenzamine (0.034 μg/ml). The increase of overflow caused by a high concentration of phentolamine (2.8 μg/ml) which, in addition to blocking α-receptors, has cocaine-like effects, was only partly counteracted by oxymetazoline.6.Surface anaesthesia on the rabbit cornea was induced by oxymetazoline and naphazoline only at very high and systemically toxic concentrations.7.The results indicate that α-receptors are involved in the regulation of the release of noradrenaline from adrenergic nerve terminals in response to nerve impulses. Stimulation of these receptors by liberated noradrenaline triggers a restriction of transmitter discharge. Receptor occupation by α-receptor-blocking drugs prevents the feed-back inhibition of noradrenaline secretion. α-Stimulants mimic the inhibitory action of noradrenaline. The receptors influencing the secretion of noradrenaline are not identical with the myocardial phenylephrine-receptors; a prejunctional, neuronal localization seems probable.8.Oxymetazoline and naphazoline inhibit the myocardial response to vagal stimulation, presumably by decreasing the release of acetylcholine.

Influence of extracellular noradrenaline on the stimulation-evoked secretion of noradrenaline from sympathetic nerves: evidence for an -receptor-mediated feed-back inhibition of noradrenaline release.

SummaryThe influence of infusions of unlabelled (−)-noradrenaline on the release of previously stored (±)-14C-noradrenaline was investigated in the isolated perfused rabbit heart.1.The spontaneous outflow of 14C-noradrenaline and total radioactivity was accelerated by unlabelled noradrenaline in concentrations of 10 and 100 ng/ml. The acceleration was prevented by cocaine (25 μg/ml) and was diminished by phenoxybenzamine (1 μg/ml).2.In the presence of cocaine, unlabelled noradrenaline caused a concentrationdependent decrease of the stimulation-induced outflow of 14C-noradrenaline and total radioactivity, 10 ng/ml being significantly effective. In contrast, the unlabelled amine did not influence the response to stimulation in the presence of phenoxybenzamine.3.It is concluded that extracellular noradrenaline lowers the liberation of intraneuronal noradrenaline evoked by nerve impulses. The inhibition is mediated by α-receptors which may be localized in the neuronal membrane. An analogous action of endogenous, previously liberated noradrenaline will result in a feed-back inhibition of the secretory response to stimulation.

Effect of phentolamine on noradrenaline uptake and release

SummaryThe influence of phentolamine on the uptake of exogenous noradrenaline infused into the aortic cannula and on the overflow of endogenous noradrenaline caused by sympathetic nerve stimulation was investigated in the isolated perfused rabbit heart. 10−6 M phentolamine doubled the overflow of endogenous noradrenaline, but did not change noradrenaline uptake. 10−5 M phentolamine increased the stimulation-induced overflow of noradrenaline 4-fold and inhibited amine uptake by about 50%. 10−4 M phentolamine elevated the overflow of noradrenaline less than 10−5 and 3×10−5 M did. The augmentation of transmitter overflow was only partly reversed by 13 min perfusion with drug-free medium.Pretreatment of hearts with 1.5×10−5 M cocaine or with 10−7 or 10−6 M desipramine did not change the effect of phentolamine on the overflow of noradrenaline evoked by nerve impulses. Pretreatment of hearts with 10−5 M, but not with 10−6 M, phentolamine prevented the increase of transmitter overflow by cocaine.It is concluded that low concentrations of phentolamine potentiate the overflow of noradrenaline during nerve stimulation by a mechanism different from that of cocaine, i.e. different from inhibition of neuronal re-uptake. The nature of this mechanism is discussed.

Wirkung von Angiotensin auf Funktion und Noradrenalinabgabe isolierter Kaninchenherzen in Ruhe und bei Sympathicusreizung

Summary1.The influence of angiotensin on the isolated rabbit heart perfused at constant pressure or constant flow was investigated at rest and during electrical stimulation of the postganglionic sympathetic nerves.2.In low concentrations (32–100ng/min or 1.4–5.3 ng/ml) angiotensin had considerable positive inotropic effects, while its chronotropic effects were weak and variable. High concentrations decreased the rate of beat, and an increase in the height of contraction was preceded by a transient phase of inhibition. In hearts perfused at constant pressure, angiotensin diminished coronary flow. The positive inotropic effect of angiotensin was not accompanied by the appearance of increased amounts of noradrenaline in the venous effluent.3.Angiotensin caused a dose-dependent increase in the output of noradrenaline induced by sympathetic nerve stimulation. The peptide was most potent in hearts perfused at constant flow; the threshhold dose of angiotensin for this procedure was 3.2 ng/min (128 pg/ml), and maximal effects (i.e., an increase by 81%) were observed with 32 ng/min (1.28 ng/ml). Doses about three times higher were required to obtain similar effects in hearts perfused at constant pressure. The reduction of coronary flow as seen under these conditions may be responsible for the weaker effect of angiotensin. Continuous infusion increased the output of noradrenaline during several successive stimulation periods.4.In most cases, the increase in output of noradrenaline induced by angiotensin did not lead to an increase in the response of the pacemaker or of the myocardium to sympathetic stimulation. Only in hearts perfused at constant flow during infusion of 32 ng/min of angiotensin the positive inotropic effect of sympathetic stimulation was significantly augmented.5.The results indicate that angiotensin may modulate the activity of the autonomic nervous system by influencing peripheral sympathetic nerves.

Interactions of angiotensin, phenoxybenzamine and propranolol on noradrenaline release during sympathetic nerve stimulation

Abstract In isolated rabbit hearts, angiotensin, phenoxybenzamine, cocaine and propranolol increased the overflow of noradrenaline caused by sympathetic nerve stimulation. After the outflow of transmitter had been raised by propranolol, angiotensin caused a further increase indistinguishable from that observed in non-pretreated hearts. The potentiation of overflow by cocaine was prevented in propranolol-treated hearts. After the outflow of noradrenaline had been augmented by phenoxybenzamine, addition of angiotensin did not result in a further increase. It is proposed that phenoxybenzamine and angiotensin share a common mechanism of action, i.e. an increase of the amount of the adrenergic transmitter liberated per stimulus from the nerve terminals.

Interactions of inhibitors of noradrenaline uptake and angiotensin on the sympathetic nerves of the isolated rabbit heart.

1 The interaction of angiotensin and several inhibitors of the uptake of noradrenaline across the neuronal membrane (cocaine, desipramine, protriptyline, and pronethalol) on the output of noradrenaline produced by sympathetic nerve stimulation has been studied in the isolated perfused rabbit heart. 2 Most of these drugs increased noradrenaline outflow—angiotensin, for example, by 175%. Cocaine (10−4m) did not change the amine overflow, probably because this very high concentration inhibited not only the re‐uptake but also the liberation of noradrenaline. 3 Desipramine, protriptyline, and pronethalol, although infused in concentrations which enhanced the noradrenaline output, were not able to impair the angiotensin‐induced increase of transmitter overflow. In the presence of cocaine (10−4m) the increase elicited by angiotensin was slightly reduced, though lower concentrations of cocaine, as previously described, do not interfere with the effect of angiotensin. 4 In contrast to the interaction between uptake inhibitors and angiotensin, the augmented output of noradrenaline caused by an uptake inhibitor could not be increased further by infusion of a second uptake inhibitor. 5 It is concluded that the increase of the outflow of noradrenaline during sympathetic nerve stimulation by small doses of angiotensin is not caused by an inhibition of re‐uptake. On the contrary, the transmitter liberation seems to be facilitated. This is a novel principle of drug action on the sympathetic nerve terminals.

The influence of angiotensin on the uptake of noradrenaline by the isolated heart of the rabbit

The influence of angiotensin on the removal of noradrenaline (10 ng/ml) from the perfusion medium and on the net‐uptake of noradrenaline (5 and 20 μg/ml) was examined in the rabbit isolated heart. Concentrations of angiotensin, known to augment the output of noradrenaline from rabbit heart during sympathetic nerve stimulation, did not inhibit the removal of infused noradrenaline from the perfusion fluid. Only very high concentrations of angiotensin (13 μg/ml) significantly diminished the loss of noradrenaline. The positive inotropic effect of noradrenaline was not potentiated by angiotensin. On the other hand, cocaine, in doses which enhance the output of noradrenaline during sympathetic stimulation greatly reduced the loss of noradrenaline from the perfusion fluid. Neither angiotensin (130 ng/ml) nor metanephrine (5 and 20 μg/ml) significantly influenced the net‐uptake of noradrenaline from high concentrations. The observations made support the assumption that an increase of transmitter liberation rather than inhibition of transmitter inactivation is responsible for the increase caused by angiotensin in the output of noradrenaline during stimulation of sympathetic nerves.

Interactions of angiotensin and cocaine on the output of noradrenaline from isolated rabbit hearts

SummaryThe output of noradrenaline from isolated rabbit hearts during sympathetic nerve stimulation is increased by angiotensin (1.3 ng/ml) to 176% of the preceding control stimulation period. During inhibition of noradrenaline re-uptake by cocaine (5 or 15 μg/ml), the augmentation caused by the peptide is unchanged (181 and 171%, respectively). The result favours the assumption that angiotensin enhances the output of noradrenaline by an increase of the amount of transmitter liberated from the nerve terminals rather than by interfering with transmitter inactivation.

Influence of peptides on the output of noradrenaline from isolated rabbit hearts

Abstract In the isolated rabbit heart, the effects of angiotensin, vasopressin and mechanical reduction of coronary flow on the output of noradrenaline caused by sympathetic nerve stimulation were investigated. Both mechanical diminution of flow by 25–75% and infusion of Arg 8 -or Lys 8 -vasopressin (1–25 mU/min) decreased the output of noradrenaline. However, angiotensin (10 – 100 ng/min) augmented the output of transmitter. The regression equations relating changes of coronary flow to those of noradrenaline output were calculated for the three groups of hearts. Positive correlations with similar slopes were found for vasopressin and mechanical reduction of flow, while there was a negative correlation for angiotensin. - If coronary flow was diminished then the percentage of infused noradrenaline eliminated from the perfusion medium by the heart was significantly augmented. Vasopressin probably decreases the output of noradrenaline by reducing the coronary flow and consequently favouring the retention of the transmitter. Although the same mechanism may contribute to the effect of angiotensin, the output of noradrenaline is not reduced, but is enhanced by some specific interaction with adrenergic nerve function.