Norepinephrine Output Research Articles

Effects of moderate hypothermia on in situ cardiac sympathetic nerve endings

Although hypothermia is known to alter neuronal control of circulation, it has been uncertain whether clinically used hypothermia (moderate hypothermia) affects in situ cardiac sympathetic nerve endings. We examined the effects of moderate hypothermia on cardiac sympathetic nerve ending function in anesthetized cats. By use of a cardiac dialysis technique, we implanted dialysis probes in the midwall of the left ventricle and monitored dialysate norepinephrine (NE) levels as an index of NE output from cardiac sympathetic nerve endings. Hypothermia (27.0±0.5 °C) induced decreases in dialysate NE levels. Dialysate NE levels did not return to the control level at normothermia after rewarming. Dialysate NE response to inferior vena cava occlusion was attenuated at hypothermia but restored at normothermia after rewarming. Dialysate NE response to high K + (100 mM) was attenuated at hypothermia and was not restored at normothermia after rewarming. Hypothermia induced increases in dialysate dihydroxyphenylglycol (DHPG) levels. There were no differences in desipramine (neuronal NE uptake blocker, 10 μM) induced increment in dialysate NE level among control, hypothermia, and normothermia after rewarming. However, hypothermia induced an increase in DHPG/NE ratio. These data suggest that hypothermia impairs vesicle NE mobilization rather than membrane NE uptake. We conclude that moderate hypothermia suppresses exocytotic NE release via central mediated reflex and regional depolarization.

Role of endogenous PACAP in catecholamine secretion from the rat adrenal gland.

We elucidated the contribution of endogenous pituitary adenylate cyclase-activating polypeptide (PACAP) to neurally evoked catecholamine secretion from the isolated perfused rat adrenal gland. Infusion of PACAP (100 nM) increased adrenal epinephrine and norepinephrine output. The PACAP-induced catecholamine output responses were inhibited by the PACAP type I receptor antagonist PACAP- (6-38) (30-3,000 nM) but were resistant to the PACAP type II receptor antagonist [Lys1,Pro2,5,Ara3,4,Tyr6]-vasoactive intestinal peptide (LPAT-VIP; 30-3,000 nM). Transmural electrical stimulation (ES; 1-10 Hz) or infusion of ACh (6-200 nM) increased adrenal epinephrine and norepinephrine output. PACAP-(6-38) (3,000 nM), but not LPAT-VIP, also inhibited the ES-induced catecholamine output responses. However, PACAP-(6-38) did not affect the ACh-induced catecholamine output responses. PACAP at low concentrations (0.3-3 nM), which had no influence on catecholamine output, enhanced the ACh-induced catecholamine output responses, but not the ES-induced catecholamine output responses. These results suggest that PACAP is released from the nerve endings to facilitate the neurally evoked catecholamine secretion through PACAP type I receptors in the rat adrenal gland.

Effect of halving the dose of venlafaxine to adjust for putative pharmacokinetic and pharmacodynamic changes in an animal model of chronic hepatic encephalopathy.

Patients with chronic hepatic encephalopathy display monoaminergic perturbations together with affective symptoms. Thus, these patients belong to a group with a probability of receiving antidepressant drug treatment. The liver impairment may result in pharmacokinetic alterations of the antidepressant drug, which in turn may affect the already perturbed monoaminergic function. Venlafaxine (VEN) was administered as a single subcutaneous challenge to portacaval shunted (experimental hepatic encephalopathy model) rats (5 mg/kg) and sham-operated rats (5 and 10 mg/kg). The aims were to investigate whether a reduced dose in portacaval shunted rats resulted in higher concentrations of VEN and serotonin as compared to control rats, which had been demonstrated earlier when the rats received the same dose (10 mg/kg). A 50% reduction of the dose of VEN administered to portacaval shunted rats resulted in elevated levels of VEN in serum, brain parenchyma, and brain dialysate about 300 minutes after the injection. The VEN challenge increased the serotonin and noradrenaline concentrations in dialysate in portacaval shunted rats and both sham groups, but the three VEN groups did not differ in any major way in serotonin and noradrenaline output. Therefore, when the dose of VEN administered to experimental hepatic encephalopathy was reduced 50% as compared to control rats, mainly pharmacokinetic, and possibly also monoaminergic, alterations were observed.

Effects of ketamine on in vivo cardiac sympathetic nerve endings.

Using the dialysis technique, we examined the effect of ketamine on dialysate norepinephrine (NE) levels in the myocardial interstitial space in anesthetized cats. Dialysis probes were implanted in the left ventricular myocardium, and we measured the dialysate NE levels serving as an indicator of NE output at the cardiac sympathetic nerve endings. During local administration of ketamine (10 mM), we examined the time-course of the change in dialysate NE levels and the dialysate NE response to coronary occlusion. Dialysate NE levels significantly increased from 39+/-7 pg/ml at control to 133+/-22 pg/ml 30 min after beginning the ketamine administration. Addition of either omega-conotoxin GVIA (N-type calcium channel blocker) at 10 microg/kg intravenously or desipramine (neuronal NE transport blocker) at 100 microM did not inhibit the increment in dialysate NE evoked by ketamine. These findings suggest that the increase in dialysate NE evoked by ketamine is dependent neither on the activity of NE exocytosis nor on the neuronal NE transport. Left descending coronary artery occlusion evoked increments in dialysate NE. The addition of ketamine augmented the dialysate NE response to coronary occlusion. A ketamine-induced increment in dialysate NE might occur as a consequence of NE exocytosis independent or membrane NE transport insensitive efflux of NE.

Central imidazoline and angiotensin II receptors in cardiovascular responses to chronic cold exposure in rats

(1) Chronic exposure to cold (5°C) induced hypertension in rats within one week. (2) Chronic central administration of an I 1 imidazoline receptor agonist, rilmenidine, prevented the development of cold-induced hypertension (CIH). (3) Plasma renin activity and urine norepinephrine output were significantly decreased by central administration of rilmenidine during cold acclimation. (4) Pressor response to central injection of angiotensin II (AII) was inhibited by rilmenidine. (5) Thus, activation of central I1 receptors could prevent the development of CIH and this may be related to inhibition of pressor effect of central AII. This finding reveals possible relationship between central I 1 and AII receptors in cardiovascular responses to chronic cold exposure.

Role of calcium channels and adenylate cyclase in the PACAP-induced adrenal catecholamine secretion.

We elucidated the functional contribution of voltage-dependent calcium channels (VDCCs) and adenylate cyclase to epinephrine (Epi) and norepinephrine (NE) secretion induced by pituitary adenylate cyclase-activating polypeptide (PACAP) in the isolated perfused rat adrenal gland. PACAP increased Epi and NE output, which was inhibited by perfusion with calcium-free solution or by nifedipine, an L-type VDCC blocker. However, the PACAP-induced responses were resistant to omega-conotoxin GVIA, an N-type VDCC blocker, or omega-conotoxin MVIIC, a P/Q-type VDCC blocker. MDL-12330A, an adenylate cyclase inhibitor, inhibited the PACAP-induced increase in Epi, but not NE, output. Treatment with nifedipine and MDL-12330A caused additive inhibition of the PACAP-induced catecholamine responses. These results suggest that opening of L-type VDCCs is responsible for adrenal catecholamine secretion induced by PACAP and that activation of adenylate cyclase is involved in the PACAP-induced Epi, but not NE, secretion. These pathways may act independently of each other.

Nisoxetine infusion into the olfactory bulb enhances the capacity for male rats to identify conspecifics.

In the present report, the norepinephrine uptake inhibitor nisoxetine as well as a cocktail of nisoxetine and the alpha-adrenergic receptor antagonist phentolamine were infused unilaterally into the olfactory bulb during microdialysis to assess their effects upon the capacity of male rats to identify conspecifics. A social discrimination test was conducted while simultaneously measuring olfactory bulb norepinephrine output in the dialysate before, during, and after behavioral testing. Nisoxetine significantly increased norepinephrine levels in the olfactory bulb compared with the Ringer's solution control group. Following such increases in olfactory bulb norepinephrine, identification responses were enhanced compared with that observed in the Ringer's control. In the presence of phentolamine, nisoxetine elevated olfactory bulb norepinephrine to levels similar to that obtained in the nisoxetine alone group, however, investigatory responses directed to the conspecifics indicated an absence of identification capacity similar to that observed in the Ringer's control group. These results reveal a direct link between norepinephrine transmission in the olfactory bulb and enhanced identification via its activation of postsynaptic alpha-adrenergic receptors. These results also show that inhibition of norepinephrine uptake may represent an important mechanism involved with the enhancement of social identification and suggest a possible novel effect for the antidepressant nisoxetine.

Effects of nociceptin on cardiac norepinephrine and acetylcholine release evoked by ouabain

We investigated whether the novel peptide, nociceptin, modulates neuronal transmission at autonomic nerve endings. Using a cardiac dialysis technique, the effects of locally applied nociceptin on cardiac acetylcholine (ACh) and norepinephrine (NE) release were examined in anesthetized cats. Dialysis probes were implanted in the left ventricular wall, with the concentration of dialysate NE or ACh serving as an indicator of NE or ACh output at cardiac sympathetic or parasympathetic nerve endings. Locally applied ouabain evoked increases in NE and ACh output. Nociceptin suppressed the ouabain induced ACh increment. The ouabain induced NE release was not altered by nociceptin. However, in the presence of desipramine (a NE uptake inhibitor), nociceptin suppressed the ouabain-induced NE release. Inhibition by nociceptin of ouabain-induced release of NE or ACh was blocked by pretreatment with nocistatin (a nociceptin action blocking peptide). Nociceptin-induced inhibition of ACh or NE release is attributable to pre-synaptic modulation rather than a reversal of the ouabain effect. These findings demonstrate that nociceptin inhibits cardiac autonomic neurotransmission via a presynaptic opioid receptor-like1(ORL1) receptor.

Hyperinsulinemia and autonomic nervous system dysfunction in obesity: effects of weight loss.

Because hyperinsulinemia acutely stimulates adrenergic activity, it has been postulated that chronic hyperinsulinemia may lead to enhanced sympathetic tone and cardiovascular risk. In 21 obese (body mass index, 35+/-1 kg/m(2)) and 17 lean subjects, we measured resting cardiac output (by 2-dimensional echocardiography), plasma concentrations and timed (diurnal versus nocturnal) urinary excretion of catecholamines, and 24-hour heart rate variability (by spectral analysis of ECG). In the obese versus lean subjects, cardiac output was increased by 22% (P:<0.03), and the nocturnal drop in urinary norepinephrine output was blunted (P:=0.01). Spectral power in the low-frequency range was depressed throughout 24 hours (P:<0.04). During the afternoon and early night, ie, the postprandial phase, high-frequency power was lower, heart rate was higher; and the ratio of low to high frequency, an index of sympathovagal balance, was increased in direct proportion to the degree of hyperinsulinemia independent of body mass index (partial r=0.43, P:=0.01). In 9 obese subjects who lost 10% to 18% of their body weight, cardiac output decreased and low-frequency power returned toward normal (P:<0.05). In free-living subjects with uncomplicated obesity, chronic hyperinsulinemia is associated with a high-output, low-resistance hemodynamic state, persistent baroreflex downregulation, and episodic (postprandial) sympathetic dominance. Reversal of these changes by weight loss suggests a causal role for insulin.

Interaction of SK(Ca) channels and L-type Ca(2+) channels in catecholamine secretion in the rat adrenal gland.

We elucidated the interaction of small-conductance Ca(2+)-activated K(+) (SK(Ca)) channels and L-type Ca(2+) channels in muscarinic receptor-mediated control of catecholamine secretion in the isolated perfused rat adrenal gland. The muscarinic agonist methacholine (10-300 microM) produced concentration-dependent increases in adrenal output of epinephrine and norepinephrine. The SK(Ca) channel blocker apamin (1 microM) enhanced the methacholine-induced catecholamine responses. The facilitatory effect of apamin on the methacholine-induced catecholamine responses was not observed during treatment with the L-type Ca(2+) channel blocker nifedipine (3 microM) or Ca(2+)-free solution. Nifedipine did not affect the methacholine-induced catecholamine responses, but it inhibited the responses during treatment with apamin. The L-type Ca(2+) channel activator Bay k 8644 (1 microM) enhanced the methacholine-induced catecholamine responses, whereas the enhancement of the methacholine-induced epinephrine and norepinephrine responses were prevented and attenuated by apamin, respectively. These results suggest that SK(Ca) channels are activated by muscarinic receptor stimulation, which inhibits the opening of L-type Ca(2+) channels and thereby attenuates adrenal catecholamine secretion.

Special feature for the Olympics: effects of exercise on the immune system: overtraining effects on immunity and performance in athletes.

Overtraining is a process of excessive exercise training in high-performance athletes that may lead to overtraining syndrome. Overtraining syndrome is a neuroendocrine disorder characterized by poor performance in competition, inability to maintain training loads, persistent fatigue, reduced catecholamine excretion, frequent illness, disturbed sleep and alterations in mood state. Although high-performance athletes are generally not clinically immune deficient, there is evidence that several immune parameters are suppressed during prolonged periods of intense exercise training. These include decreases in neutrophil function, serum and salivary immunoglobulin concentrations and natural killer cell number and possibly cytotoxic activity in peripheral blood. Moreover, the incidence of symptoms of upper respiratory tract infection increases during periods of endurance training. However, all of these changes appear to result from prolonged periods of intense exercise training, rather than from the effects of overtraining syndrome itself. At present, there is no single objective marker to identify overtraining syndrome. It is best identified by a combination of markers, such as decreases in urinary norepinephrine output, maximal heart rate and blood lactate levels, impaired sport performance and work output at 110% of individual anaerobic threshold, and daily self-analysis by the athlete (e.g. high fatigue and stress ratings). The mechanisms underlying overtraining syndrome have not been clearly identified, but are likely to involve autonomic dysfunction and possibly increased cytokine production resulting from the physical stress of intense daily training with inadequate recovery.

Role of endogenous endothelins in catecholamine secretion in the rat adrenal gland

We investigated the role of endogenous endothelins in catecholamine secretion in response to transmural electrical stimulation in the retrogradely perfused rat adrenal gland. ( R)2-[( R)-2-[( S)-2-[[1-(hexahydro-1 H-azepinyl)]carbonyl]amino-4-methyl-pentanoyl]amino-3-[3-(1-methyl-1 H-indoyl)]propionyl]amino-3-(2-pyridyl) propionic acid (FR139317; 0.03–3 μM), an endothelin ET A receptor antagonist, inhibited the electrical stimulation-induced epinephrine and norepinephrine output. Neither N- cis-2,6-dimethylpiperidinocarbonyl- l-γ-methylleucyl- d-1-methoxycarbonyltryptophanyl- d-norleucine (BQ-788; 0.03–3 μM), an endothelin ET B receptor antagonist, nor phosphoramidon (1–100 mM), an endothelin-converting enzyme inhibitor, affected the catecholamine output responses. However, the inhibition by FR139317 of the catecholamine output responses was abolished by pretreatment with phosphoramidon (100 mM) or BQ-788 (3 μM). These results indicate that activation of endothelin ET B receptors by endogenous endothelins inhibits the catecholamine output responses under the condition in which endothelin ET A receptors are blocked. Exogenous endothelin-1 (1–100 nM) did not affect the catecholamine output responses, but it inhibited the responses under treatment with phosphoramidon and FR139317. Activation of endothelin ET A receptors may interfere with the endothelin ET B receptor-mediated inhibitory action on the neuronally evoked secretion of adrenal catecholamines.

Role of potassium channels in catecholamine secretion in the rat adrenal gland.

We elucidated the functional contribution of K(+) channels to cholinergic control of catecholamine secretion in the perfused rat adrenal gland. The small-conductance Ca(2+)-activated K(+) (SK(Ca))-channel blocker apamin (10-100 nM) enhanced the transmural electrical stimulation (ES; 1-10 Hz)- and 1, 1-dimethyl-4-phenyl-piperazinium (DMPP; 5-40 microM)-induced increases in norepinephrine (NE) output, whereas it did not affect the epinephrine (Epi) responses. Apamin enhanced the catecholamine responses induced by acetylcholine (6-200 microM) and methacholine (10-300 microM). The putative large-conductance Ca(2+)-activated K(+) channel blocker charybdotoxin (10-100 nM) enhanced the catecholamine responses induced by ES, but not the responses induced by cholinergic agonists. Neither the K(A) channel blocker mast cell degranulating peptide (100-1000 nM) nor the K(V) channel blocker margatoxin (10-100 nM) affected the catecholamine responses. These results suggest that SK(Ca) channels play an inhibitory role in adrenal catecholamine secretion mediated by muscarinic receptors and also in the nicotinic receptor-mediated secretion of NE, but not of Epi. Charybdotoxin-sensitive Ca(2+)-activated K(+) channels may control the secretion at the presynaptic site.

Tyramine-induced endogenous noradrenaline efflux from in situ cardiac sympathetic nerve ending in cats.

With the use of dialysis technique, the effects of tyramine on in situ cardiac sympathetic nerve endings were examined in anaesthetized cats. Dialysis probes were implanted in the left ventricular myocardium, and the concentration of dialysate noradrenaline (NA) served as an indicator of NA output at the cardiac sympathetic nerve ending. Locally applied tyramine (600 microM) increased dialysate NA levels from 17 +/- 1 (pg mL-1) to 3466 +/- 209 (pg mL-1). Pretreatment with reserpine (vesicle transport NA blocker 1 microM) did not affect tyramine-induced NA efflux. The tyramine-induced NA efflux was augmented by pretreatment with pargyline (1 mM) but suppressed by pargyline (10 mM). Pretreatment with alpha-methyl-tyrosine suppressed NA efflux evoked by tyramine. These pretreatments did not affect the time course of NA efflux but only altered peak height of NA efflux. The efflux of NA evoked by tyramine was not associated with any reduction of dihydroxyphenylglycol (DHPG). In contrast, in the pretreatment with reserpine, the efflux of NA was associated with a reduction of DHPG. This result suggests that NA graduation between axoplasm and stored vesicle contributes to maintaining the axoplasmic NA level during carrier-mediated outward NA transport. The tyramine-induced NA efflux provides a close reflection of the NA content at the nerve ending. With the use of dialysis, this experimental model is suitable for studying the mechanism of sympathomimetic amine-induced neurotransmitter efflux.

Cognitive–behavioral stress management intervention effects on anxiety, 24-hr urinary norepinephrine output, and T-cytotoxic/suppressor cells over time among symptomatic HIV-infected gay men.

The present study tested the effects of a multimodal cognitive-behavioral stress management (CBSM) intervention on anxious mood, perceived stress, 24-hr urinary catecholamine levels, and changes in T-lymphocyte subpopulations over time in symptomatic HIV+ gay men. Seventy-three men were randomized to either a group-based CBSM intervention (n = 47) or a wait-list control (WLC) condition (n = 26). Men assigned to CBSM showed significantly lower posttreatment levels of self-reported anxiety, anger, total mood disturbance, and perceived stress and less norepinephrine (NE) output as compared with men in the WLC group. At the individual level, anxiety decreases paralleled NE reductions. Significantly greater numbers of T-cytotoxic/suppressor (CD3+CD8+) lymphocytes were found 6 to 12 months later in those assigned to CBSM. Moreover, greater decreases in NE output and a greater frequency of relaxation home practice during the 10-week CBSM intervention period predicted higher CD3+CD8+ cell counts at follow-up.

AT-1 receptor and phospholipase C are involved in angiotensin III modulation of hypothalamic noradrenergic transmission.

1. We previously reported that angiotensin III modulates noradrenergic neurotransmission in the hypothalamus of the rat. In the present work we studied the effects of angiotensin III on norepinephrine release and tyrosine hydroxylase activity. We also investigated the receptors and intracellular pathways involved in angiotensin III modulation of noradrenergic transmission. 2. In rat hypothalamic tissue labeled with [3H]norepinephrine 1, 10, and 100 nM and 1 microM losartan (AT1 receptor antagonist) had no effect on basal neuronal norepinephrine release, whereas 10 and 100 nM and 1 microM losartan partially diminished norepinephrine secretion evoked by 25 mM KCl. The AT2 receptor antagonist PD 123319 showed no effect either on basal or evoked norepinephrine release. The increase in both basal and evoked norepinephrine output induced by 1 microM angiotensin III was blocked by 1 microM losartan, but not by 1 microM PD 123319. 3. The phospholipase C inhibitor 5 microM neomicin inhibited the increase in basal and evoked norepinephrine release produced by 1 microM angiotensin III. 4. Tyrosine hydroxylase activity was increased by 1 microM angiotensin III and this effect was blocked by 1 microM LST and 5 microM neomicin, but not by PD 123319. On the other hand, 1 microM angiotensin III enhanced phosphatidyl inositol hydrolysis that was blocked by 1 microM losartan and 5 microM neomicin. PD 123319 (1 microM) did not affect ANG III-induced phosphatidyl inositol hydrolysis enhancement. 5. Our results confirm that angiotensin III acts as a modulator of noradrenergic transmission at the hypothalamic level through the AT1-phospholipase C pathway. This enhancement of hypothalamic noradrenergic activity suggests that angiotensin III may act as a central modulator of several biological processes regulated at this level by catecholamines, such as cardiovascular, endocrine, and autonomic functions as well as water and saline homeostasis.

The effects of noradrenergic activation of the nucleus tractus solitarius on memory and in potentiating norepinephrine release in the amygdala.

Although it is known that norepinephrine (NE) modulates memory by acting on limbic areas, few studies describe how structures supplying NE to the limbic system, such as the nucleus tractus solitarius (NTS) contribute to this process. The present study examined the effects on memory of activating the NE pathway between the NTS and the amygdala (AMYG). Rats received buffer or the beta-noradrenergic agonist clenbuterol (CLN; 10, 50, or 100 ng/0.5 microl) into the NTS after footshock training in a Y-maze discrimination task. Infusion of 100 ng CLN significantly improved memory when retention was tested in the absence or presence of cues associated with the footshock. Experiment 2 used in vivo microdialysis to determine whether the mnemonic effects of CLN are mediated by influencing NE output in the AMYG. Subjects were given an intra-NTS infusion of CLN or phosphate buffered saline, footshock (0.8 mA, 1 s), and injected with epinephrine (EPI; 0.3 mg/kg ip) or saline. CLN or EPI injection produced a significant increase in NE sampled from the AMYG. These findings indicate that activation of NTS neurons that project to and release NE in the AMYG modulates memory storage processing.

Effects of ouabain on in situ cardiac sympathetic nerve endings

Using dialysis technique, the effects of ouabain on in situ cardiac sympathetic nerve endings were examined in anesthetized cats. Dialysis probes were implanted in the left ventricular myocardium, and the concentration of dialysate norepinephrine (NE) was used as an indicator of NE output at the cardiac sympathetic nerve ending. Locally applied ouabain dose-dependently (1, 10, 100 μM) increased dialysate NE levels. This finding suggested that ouabain causes an increase in NE efflux without any requirement for prior mobilization of NE from vesicular stores. Transection of sympathetic nerves innervating the heart, was without effect on the ouabain (100 μM)-induced increase in NE efflux. Pretreatment with a Ca 2+-channel blocker, ω-conotoxin GVIA (10 μg/kg iv) suppressed the ouabain-induced NE efflux. These data suggested that ouabain opened N-type calcium channels coupled to NE release without centrally mediated neural transmission. Furthermore, ouabain-induced NE efflux was suppressed by pretreatment with desipramine (neuronal NE uptake inhibitor, 100 μM). Our data suggest that the two mechanisms (exocytosis and carrier-mediated outward transport), to the same extent, contributed to the amount of NE efflux evoked by ouabain in in situ cardiac sympathetic nerve endings.

Role of cholinergic receptors in adrenal catecholamine secretion in spontaneously hypertensive rats.

We investigated the role of nicotinic and muscarinic receptors in secretion of catecholamines induced by transmural electrical stimulation (ES) from isolated perfused adrenal glands of spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto (WKY) rats. ES (1-10 Hz) produced frequency-dependent increases in epinephrine (Epi) and norepinephrine (NE) output as measured in perfusate. The ES-induced increases in NE output, but not Epi output, were significantly greater in adrenal glands of SHRs than in those of WKY rats. Hexamethonium (10-100 microM) markedly inhibited the ES-induced increases in Epi and NE output from adrenal glands of SHRs and WKY rats. Atropine (0.3-3 microM) inhibited the ES-induced increases in Epi and NE output from adrenal glands of SHRs, but not from those of WKY rats. These results suggest that endogenous acetylcholine-induced secretion of adrenal catecholamines is predominantly mediated by nicotinic receptors in SHRs and WKY rats and that the contribution of muscarinic receptors may be different between these two strains.

Role of calcium channels in catecholamine secretion in the rat adrenal gland.

1. We elucidated the contribution of voltage-dependent Ca2+ channels to cholinergic control of catecholamine secretion in the isolated perfused rat adrenal gland. 2. Nifedipine (0.3-3 microM) inhibited increases in noradrenaline output induced by transmural electrical stimulation (1-10 Hz) and acetylcholine (6-200 microM), whereas it only slightly inhibited the adrenaline output responses. Nifedipine also inhibited the catecholamine output response induced by 1, 1-dimethyl-4-phenyl-piperazinium (DMPP; 5-40 microM) but not by methacholine (10-300 microM). 3. omega-Conotoxin MVIIC (10-1000 nM) inhibited the catecholamine output responses induced by electrical stimulation but not by acetylcholine, DMPP and methacholine. 4. omega-Conotoxin GVIA (50-500 nM) had no inhibitory effect on the catecholamine output responses. 5. These results suggest that L-type Ca2+ channels are responsible for adrenal catecholamine secretion mediated by nicotinic receptors but not by muscarinic receptors, and that their contribution to noradrenaline secretion may be greater than that to adrenaline secretion. P/Q-type Ca2+ channels may control the secretion at a presynaptic site.