EFFECTS OF TRICYCLIC ANTIDEPRESSANTS ON UPTAKE AND RELEASE OF 3H-NORADRENALINE IN ISOLATED GUINEA-PIG ATRIA

Sympathomimetic actions of methylenedioxymethamphetamine in rat and rabbit isolated cardiovascular tissues

The aim of this study was to investigate the actions of methylenedioxymethamphetamine (MDMA) in several isolated cardiovascular tissues. In spontaneously beating rat atria, concentration-dependent positive chronotropic responses to MDMA and amphetamine were blocked by the neuronal-uptake inhibitor desipramine (1 microM) and the beta-adrenoceptor antagonist propranolol (1 microM). In atria incubated with [3H]noradrenaline to label transmitter stores, 10 microM MDMA and 1 microM amphetamine increased the resting outflow of radioactivity, while 1 microM desipramine had no effect on resting outflow. The MDMA- and amphetamine-induced release of radioactivity were blocked by 1 microM desipramine. MDMA, amphetamine and desipramine each enhanced the electrical stimulation-induced (2 Hz, 30-s train) release of radioactivity; the enhancing effects of MDMA and amphetamine were blocked by 1 microM desipramine. In rat isolated perfused hearts, MDMA (1 and 10 microM) increased heart rate by a similar amount to the increase caused by noradrenaline (10 and 50 nM). MDMA also induced dysrhythmias in 7 out of 11 rat isolated perfused heart preparations. In rabbit isolated perfused and superfused ear arteries preloaded with [3H]noradrenaline, MDMA increased the resting release of radioactivity by 230 +/- 18% (n = 6) of control resting release; the increase was accompanied by a rise in perfusion pressure of 17 +/- 7 mmHg (n = 6). MDMA also facilitated the vasoconstrictor responses to noradrenaline (3-9 ng) and perivascular nerve stimulation (1-5 Hz, 10-s train). MDMA-induced vasoconstriction and the facilitation of vasoconstrictor responses to noradrenaline and electrical stimulation were blocked by 1 microM desipramine.(ABSTRACT TRUNCATED AT 250 WORDS)

Disadvantages of cocaine as a neuronal uptake blocking agent: comparison with desipramine in guinea-pig right atrium

Cocaine and desipramine (DMI) are widely used as neuronal uptake blocking agents in studies of cardiac sympathetic transmission in isolated tissue preparations. It is generally assumed that these pharmacological tools do not alter transmitter release or postjunctional effector response. To test this assumption, we have compared the effects of cocaine and DMI on rate responses to sympathetic nerve stimulation and exogenous noradrenaline in guinea-pig isolated right atria. Right atria were equilibrated with the irreversible alpha-adrenoreceptor antagonist benextramine to prevent any effect of presynaptic alpha-adrenoreceptors. Cumulative (-) noradrenaline concentration-response curves were shifted to the left by DMI (0.01-1 microM) without significant change in the resting or maximum rates. Cocaine (1-100 microM) also caused sensitisation to noradrenaline but caused a biphasic change in resting atrial rate. In addition there was a small but significant depression of the maximum rate at cocaine 10 and 100 microM. Sympathetic nerve stimulation was achieved by applying trains of 1, 2 and 4 electrical field pulses delivered during one atrial refractory period. DMI caused a concentration dependent potentiation of responses to field stimulation. Cocaine (1 microM) caused significant enhancement of peak responses to field stimulation but no further enhancement and indeed depressed peak responses were observed at cocaine 10 and 100 microM respectively. The time for atrial period to return halfway to baseline after field stimulation (t 1/2) was enhanced by cocaine in a concentration dependent manner as was observed with DMI. We conclude that cocaine (but not DMI) decreases the maximum response to exogenous noradrenaline (postjunctional depression). The reduction of the peak response to sympathetic nerve stimulation in the presence of cocaine to below control responses suggests that cocaine also depresses the release of transmitter. These additional depressant properties of cocaine, which occur in a concentration range of neuronal uptake block, are important disadvantages and should discourage its use in experiments on sympathetic transmission.

The effect of some antidepressants on prejunctional muscarinic receptors on the sympathetic nerves of the isolated rabbit ear artery

The antimuscarinic activity of amitriptyline, desipramine, iprindole, mianserin and viloxazine on prejunctional sympathetic nerve endings were compared in the isolated rabbit ear artery. In the presence of cocaine (10 micro M) and yohimbine (1 micro M), amitriptyline (0.5-1 micro M), desipramine (1-3 micro M) and iprindole (5-10 micro M) produced parallel rightward shifts of the concentration-response curve for the inhibitory effect of carbachol (CCh) on responses to electrical stimulation of the preparation at 3 Hz. Mianserin (3 micro M) produced some inhibition but altered the slope of the concentration-responses curve to CCh while viloxazine (less than or equal to 10 micro M) produced no inhibition. The depression of tritium efflux by CCh from arteries preincubated in 3H-noradrenaline was inhibited significantly (P less than 0.05) by amitriptyline (0.1 micro M) and desipramine (1 micro M) and not by iprindole (17 micro M), mianserin (3 micro M) or viloxazine (10 micro M). Amitriptyline was 10-fold more active than desipramine and at least 30-fold more active than the other antidepressants as a muscarine receptor blocking drug in this preparation. Thus, mianserin, viloxazine and iprindole exhibit much weaker antimuscarinic activity relative to amitriptyline on prejunctional muscarine receptors on sympathetic nerve endings compared with that observed by others for excitatory muscarine receptors in sympathetic ganglia. The findings support an earlier suggest that these receptors differ.

Effects of pimozide on noradrenergic transmission in rabbit isolated ear arteries

In the rabbit ear artery both dopamine and noradrenaline inhibit stimulation-induced (S-I) transmitter noradrenaline efflux. Pimozide, which is reported to be a specific dopamine receptor antagonist, was used to further study the effects of dopamine on transmitter efflux. In a concentration of 0.2 micrometer pimozide blocked the inhibition of S-I efflux produced by 0.5 micrometer dopamine but not that produced by 0.5 micrometer noradrenaline. In a concentration of 10 nM, pimozide enhances transmitter release and vasoconstrictor responses to sympathetic nerve stimulation; this may be due to blockade of feedback inhibition of transmitter release by endogenous dopamine. In a concentration of 1 micrometer, pimozide reduced transmitter release and vasoconstrictor responses to sympathetic nerve stimulation. Vasoconstrictor responses to noradrenaline and histamine are antagonized by pimozide in a noncompetitive manner.