Quantitative estimation of secretion and reuptake of adrenergic transmitter in the rabbit heart

Effects of cocaine on several adrenergic system parameters

The effect of acute and chronic administration of cocaine on the sympathetic nervous system in the cardiac tissue and the adrenal glands of rats were studied. Both with acute and chronic administration of the drug, the concentration-response curve to (-)-norepinephrine on the isolated rat atria was shifted to the right, with a lower pD2. The endogenous ventricular norepinephrine levels remain unchanged after acute injections of cocaine, but when the drug was administered chronically, the endogenous neurotransmitter content were augmented about 50%. The tyrosine hydroxylase activity in atria and adrenal homogenates decreased after chronic injections of cocaine, while it was not observed any modifications in the enzyme activity after acute administrations of the drug.

Inotropic effect of prostacyclin (PGI2) on isolated rat atria at different contraction frequencies

The effects of a wide range of Prostacyclin (PGI2) concentrations on the Isometric Developed Tension (IDT) of isolated left auricles driven at different frequencies (0.8, 1.6 or 3.3 Hz) were explored. A comparison of the positive inotropism of PGI2, norepinephrine (NE), tyramine and phenylephrine (Phenyl) indicated that PGI2, NE and tyramine enhanced peak tension significantly less at slow (0.8 and 1.6 Hz) than at higher rates (3.3 Hz); whereas Phenyl augmented equally the IDT at all of these three driving frequencies. The positive inotropism evoked by PGI2 was inhibited by (-)-propranolol and also by a treatment with 6-OHDA. Cocaine, normetanephrine and U-0521, augmented the positive inotropic influence of PGI2 on atria paced at a slow rate (0.8 Hz) but not at a faster one (3.3 Hz). These results suggest that the action of PGI2 on atrial contractions is apparently indirect and mediated by the release of tissue catecholamines. In addition the effect of PGI2 and NE at slow and fast rates appears associated with a different relative relevance of the processes which terminate the action of added sympathomimetic agonists, namely, neuronal or extraneuronal uptake as well as metabolization via COMT. These mechanisms seen to be more prominent at slower driving frequencies and could explain the diminished effect of PGI2 and NE on slowly paced atria.

The effects of cocaine, chlorpheniramine and tripelennamine on the cardiac responses to sympathetic nerve stimulation

The effects of cocaine (COC), chlorpheniramine (CHL), and tripelennamine (TRI) on the cardiac responses to sympathetic neural stimulation were compared in open-chest, anesthetized dogs. The amplitudes of the chronotropic responses were not significantly altered by any of these drugs. Similarly, the amplitudes of the inotropic responses were not affected significantly by either COC or TRI, but they were increased moderately by CHL. All three drugs prolonged the inotropic and especially the chronotropic responses. CHL and TRI, in addition to being antihistaminics, are inhibitors of the neuronal uptake of norepinephrine. Therefor, the similarity of the effects of CHL, TRI, and COC on the cardiac responses to sympathetic stimulation is probably ascribable to their inhibitory activities on the neuronal uptake mechanism.

The effects of cocaine and metanephrine on the cardiac responses to sympathetic nerve stimulation in dogs

The effects of infusions of cocaine (COC) and metanephrine (MET) on the inotropic and chronotropic responses to cardiac sympathetic nerve stimulation were studied in open-chest, anesthetized dogs. COC blocks the neuronal uptake of norepinephrine (NE), whereas MET blocks the extraneuronal uptake. Both blocking agents slightly enhanced the inotropic but not the chronotropic responses. COC prolonged the cardiac responses significantly, particularly the chronotropic responses, whereas MET had no appreciable effect on the durations of these responses. Hence, it appears that the neuronal uptake mechanism plays a major role in the dissipation of neurally released NE in the heart, but that the extra-neuronal uptake mechanism plays only a minor role in its dissipation. In contrast to the results in certain other tissues, the combined effects of COC and MET on the cardiac responses were no greater or more prolonged than the sum of the effects produced by each agent acting alone.

The influence of cocaine and desipramine on the cardiac responses to exogenous and endogenous norepinephrine

In open-chest, anesthetized dogs, cocaine and desipramine potentiated the pressor, chronotropic, inotropic and coronary sinus blood flow responses to norepinephrine (NE) infusions. The chronotropic and inotropic responses were also prolonged, the former more markedly than the later and the extraction of exogenous NE from the coronary blood stream was diminished. Cocaine and desipramine also potentiated the pressor and coronary sinus blood flow responses, but not the chronotropic or inotropic responses, to stimulation of the left ansa subclavia. The inotropic response was slightly prolonged, however, and the chronotropic response was markedly prolonged. The overflow of NE into the coronary sinus blood was not increased by either neuronal uptake blocking agent. It is proposed that cocaine and desipramine, at the doses employed, diminish the release of NE from the cardiac nerve endings at the same time that they inhibit reuptake of the neurotransmitter. Their mechanisms of action and their side effects on the circulatory system do not appear to differ significantly.

The effect of the pattern of cardiac sympathetic activity on myocardial contractile force and norepinephrine overflow in the dog heart

The left of right cardiac sympathetic nerves in open-chest, anesthetized dogs were stimulated at mean frequencies of 2 or 4 Hz. The stimuli were applied intermittently, in patterns with repetition rates or either 60/min or 15/min, to simulate the spontaneous patterns of sympathetic neural activity that occur synchronously with the cardiac or respiratory cycles, respectively. With either repetition rate, intermittent stimulation of the left sympathetic nerves was about 10-20% less effective in enhancing myocardial contractile force (CF) and about 10% less effective in increasing coronary sinus blood flow than was steady stimulation at the same mean frequency. With right-sided stimulation, there was no appreciable difference between steady and intermettent stimulation patterns with respect to the effect on heart rate. With either left-or right-sided stimulation, the rate of norepinephrine (NE) overflow into the coronary sinus blood was 20-40% less with intermittent than with steady stimulation. Cocaine administration did not materially affect this difference in NE overflow. It was concluded that the higher instantaneous frequencies that prevail during intermittent stimulation result in a reduction in the rate of NE release at the sympathetic postganglionic nerve endings in the heart.