Extra-vesicular binding of noradrenaline and guanethidine in the adrenergic neurones of the rat heart: a proposed site of action of adrenergic neurone blocking agents.
1 The binding and efflux characteristics of [14C]-guanethidine and [3H]-noradrenaline were studied in heart slices from rats which were pretreated with reserpine and nialamide. 2 Binding of both compounds occurred at extra-vesicular sites within the adrenergic neurone. After a brief period of rapid washout, the efflux of [14C]-guanethidine and [3H]-noradrenaline proceeded at a steady rate. The efflux of both compounds appeared to occur from a single intraneuronal compartment. 3 (+)-Amphetamine accelerated the efflux of [14C]-noradrenaline; this effect was inhibited by desipramine. 4 Unlabelled guanethidine and amantadine also increased the efflux of labelled compounds. Cocaine in high concentrations increased slightly the efflux of [14C]-guanethidine but not that of [3H]-noradrenaline. 5 Heart slices labelled with [3H]-noradrenaline became refractory to successive exposures to releasing agents although an appreciable amount of labelled compound was still present in in these slices. 6 It is suggested that [14C]-guanethidine and [3H]-noradrenaline are bound at a common extravesicular site within the adrenergic neurone. Binding of guanethidine to the extra-vesicular site may be relevant to its pharmacological action, i.e., the blockade of adrenergic transmission. (+info)
Trigeminal and carotid body inputs controlling vascular resistance in muscle during post-contraction hyperaemia in cats.
1. In anaesthetized cats, the effects of stimulation of the receptors in the nasal mucosa and carotid body chemoreceptors on vascular resistance in hindlimb skeletal muscle were studied to see whether the responses were the same in active as in resting muscle. The measurements of vascular resistance were taken, first, in resting muscle, and second, in the immediate post-contraction hyperaemic phase that followed a 30 s period of isometric contractions. 2. Stimulation of the receptors in the nasal mucosa caused reflex apnoea and vasoconstriction in muscle. The latter response was attenuated when the test was repeated during post-contraction hyperaemia. 3. Stimulations of the carotid bodies were made during a period of apnoea evoked reflexly by electrical stimulation of both superior laryngeal nerves. This apnoea prevented any effects of changes in respiration on the carotid body reflex vascular responses. Stimulation of the carotid bodies evoked hindlimb muscle vasoconstriction. In the post-contraction hyperaemic period, the response was reduced or abolished. A similar attenuation of the reflex vasoconstrictor responses occurred in decentralized muscles stimulated through their motor roots in the cauda equina. 4. Evidence is presented that the attenuation of the vasoconstrictor responses evoked by the two reflexes is a phenomenon localized to the contracting muscles themselves resulting from an interaction between sympathetic neuronal activity and the local production of metabolites. 5. The results are discussed in relation to the metabolic needs of tissues in relation to asphyxial defence mechanisms such as occur in the diving response. (+info)
Sympathetic neuroeffector transmission in the rat anococcygeus muscle.
1. When intracellular recordings were made from preparations of rat anococcygeus muscle, transmural nerve stimulation evoked noradrenergic excitatory junction potentials (EJPs) made up of two distinct components. Both components were abolished by either guanethidine or alpha-adrenoceptor antagonists, indicating that they resulted from the release of transmitter from sympathetic nerves and the subsequent activation of alpha-adrenoceptors. 2. The first component was associated with a transient increase in the intracellular concentration of calcium ions ([Ca2+]i) and a contraction. Although the second component was often associated with a long lasting increase in [Ca2+]i it was not associated with a contraction unless the second component initiated an action potential. 3. The increase in [Ca2+]i associated with the first component resulted from Ca2+ release from an intracellular store and from entry of Ca2+ through voltage-dependent Ca2+ channels. The increase in [Ca2+]i associated with the second component resulted only from the entry of Ca2+ through L-type Ca2+ channels (CaL channels). The depolarization associated with the initial increase in [Ca2+]i was abolished by reducing the external concentration of chloride ions ([Cl-]o), suggesting that it involved the activation of a Cl- conductance. 4. When the relationships between changes in [Ca2+]i, membrane depolarization and contraction produced by an increasing number of sympathetic nerve stimuli were determined in control, and caffeine- and nifedipine-containing solutions, it was found that an increase in [Ca2+]i recorded in nifedipine produced a larger contraction and larger membrane depolarization than did a similar increase in [Ca2+]i recorded in either control or caffeine-containing solutions. These observations indicate that Ca2+ released from stores more readily triggers contraction and membrane depolarization than does Ca2+ entry via CaL channels. (+info)
Trigeminal nerve ganglion stimulation-induced neurovascular reflexes in the anaesthetized cat: role of endothelin(B) receptors in carotid vasodilatation.
1. The effects of intravenous administration of endothelin (ET) receptor antagonists SB-209670 (0.001-10.0 mg kg(-1)), SB-217242, SB-234551 (0.01-10.0 mg kg(-1)) and BQ-788 (0.001-1.0 mg kg(-1)) were investigated on trigeminal nerve ganglion stimulation-induced neurovascular reflexes in the carotid vasculature of the anaesthetized cat. Comparisons were made with sumatriptan (0.003-3.0 mg kg(-1)) and alpha-CGRP8-37 (0.001-0.1 mg kg(-1)). 2. Trigeminal nerve ganglion stimulation produced frequency related increases in carotid blood flow, reductions in carotid vascular resistance and non-frequency related increases in blood pressure. Guanethidine (3 mg kg(-1), i.v.) blocked trigeminal nerve ganglion-induced increases in blood pressure but had no effect on changes in carotid flow or resistance. Maximal reductions in carotid vascular resistance was observed at 10 Hz, and this frequency was selected to investigate the effects of drugs on trigeminal nerve ganglion stimulation-induced responses in guanethidine treated cats. 3. Saline, alpha-CGRP8-37 SB-209670 and BQ-788 had little or no effect on resting haemodynamic parameters. SB-217242 (10 mg kg(-1), n=3) produced a 56% reduction in arterial blood pressure whereas SB-233451 (10 mg kg(-1), n=3) produced a 30% reduction in carotid vascular resistance. Sumatriptan produced dose-related reductions in resting carotid flow and increases (max. 104% at 0.3 mg kg(-1), n = 5) in vascular resistance. 4. SB-209670 (n=6-7), SB-217242 (n=3) and BQ-788 (n=3) produced inhibition of trigeminal nerve ganglion stimulation-induced reductions in carotid vascular resistance. Saline, SB-234551, alpha-CGRP8-37 and sumatriptan had no effect. 5. These data demonstrate ET(B) receptor blockade attenuates the vasodilator effects of trigeminal nerve ganglion stimulation in the carotid vascular bed of guanethidine pretreated anaesthetized cats. (+info)
Investigating feed-forward neural regulation of circulation from analysis of spontaneous arterial pressure and heart rate fluctuations.
BACKGROUND: Analysis of spontaneous fluctuations in systolic arterial pressure (SAP) and pulse interval (PI) reveals the occurrence of sequences of consecutive beats characterized by SAP and PI changing in the same (+PI/+SAP and -PI/-SAP) or opposite (-PI/+SAP and +PI/-SAP) direction. Although the former reflects baroreflex regulatory mechanisms, the physiological meaning of -PI/+SAP and +PI/-SAP is unclear. We tested the hypothesis that -PI/+SAP and +PI/-SAP "nonbaroreflex" sequences represent a phenomenon modulated by the autonomic nervous system reflecting a feed-forward mechanism of cardiovascular regulation. METHODS AND RESULTS: We studied anesthetized rabbits before and after (1) complete autonomic blockade (guanethidine+propranolol+atropine, n=13; CAB), (2) sympathetic blockade (guanethidine+propranolol, n=15; SB), (3) parasympathetic blockade (atropine, n=16), (4) sinoaortic denervation (n=10; SAD), and (5) controlled respiration (n=10; CR). Nonbaroreflex sequences were defined as >/=3 beats in which SAP and PI of the following beat changed in the opposite direction. CAB reduced the number of nonbaroreflex sequences (19. 1+/-12.3 versus 88.7+/-36.6, P<0.05), as did SB (25.3+/-11.7 versus 84.6+/-23.9, P<0.001) and atropine (11.2+/-6.8 versus 94.1+/-32.4, P<0.05). SB concomitantly increased baroreflex sensitivity (1.18+/-0. 11 versus 0.47+/-0.09 ms/mm Hg, P<0.01). SAD and CR did not significantly affect their occurrence. CONCLUSIONS: These results suggest that nonbaroreflex sequences represent the expression of an integrated, neurally mediated, feed-forward type of short-term cardiovascular regulation able to interact dynamically with the feedback mechanisms of baroreflex origin in the control of heart period. (+info)
Role of the sympathetic nervous system and insulin in enhancing glucose uptake in peripheral tissues after intrahypothalamic injection of leptin in rats.
Our previous study demonstrated that microinjection of leptin into the ventromedial hypothalamus (VMH) dramatically increased glucose uptake in the heart, brown adipose tissue (BAT), and skeletal muscles, but not in white adipose tissue (WAT) in conscious unrestrained rats, as assessed in vivo by the 2-[3H]deoxyglucose method. Here we examined the role of the sympathetic nervous system and insulin in enhanced glucose uptake by tissues after hypothalamic leptin injection. Pretreatment with guanethidine significantly suppressed the increased glucose uptake by the tissues in response to leptin injected into the VMH, whereas bilateral adrenal demedullation had no significant effect. Treatment with propranolol but not phenoxybenzamine also decreased significantly enhanced glucose uptake by the tissues. We further examined the interaction of the effects of hypothalamic leptin and insulin administered peripherally by clamping the glucose concentrations at a constant level. When leptin was injected into the VMH and a maximal dose of insulin was administered intravenously, the rates of glucose uptake by the heart, BAT, and skeletal muscles, but not by WAT, markedly increased beyond the values reached by insulin stimulation alone. Surgical sympathetic denervation of BAT abolished the enhancement of glucose uptake in this tissue, decreasing to the level stimulated by insulin alone. These results appear to indicate that leptin in the hypothalamus enhances glucose uptake in certain peripheral tissues through mediation of a beta-adrenergic mechanism for the sympathetic nerves innervating the tissues and that central leptin and peripheral insulin have a synergistic role in augmenting tissue glucose uptake. (+info)
Nicotinic acetylcholine receptor blocking effect of guanethidine in the rat gastric fundus.
1 Guanethidine is commonly used as a drug to investigate adrenergic neurotransmission and, in combination with atropine, to realize non-adrenergic non-cholinergic (NANC) conditions. Previous studies suggested a nicotinic acetylcholine receptor blocking effect of guanethidine. Therefore, we investigated the effect of increasing concentrations of guanethidine (0.1-100 microM) on nicotine-induced relaxations of longitudinal muscle strips of rat gastric fundus. 2 In the presence of 1 microM atropine and 3 microM guanethidine, nicotine (30 microM) induces a fast and sustained relaxation which is partly inhibited by the nitric oxide synthase inhibitors Nomega-nitro-L-arginine (L-NOARG) and Nomega-nitro-L-arginine methyl ester (L-NAME) (both 30 and 100 microM). One microM tetrodotoxin (TTX) completely blocks this nicotine-induced relaxation. 3 High concentrations of guanethidine (> or =10 microM), but not adrenoceptor blockade by the alpha-adrenoceptor antagonist phentolamine in combination with the beta-adrenoceptor antagonist nadolol (both 3 microM), inhibit the nicotine-induced relaxation. 4 Guanethidine (0.1-100 microM) has no effect on relaxations induced by electrical field stimulation (EFS; 1-8 Hz), nitric oxide (NO; 0.01-1 microM), vasoactive intestinal polypeptide (VIP; 0.1-10 nM) or isoprenaline (1-10 nM). 5 We conclude that high concentrations of guanethidine (> or =10 microM) block nicotine-induced NANC relaxations of longitudinal muscle strips of the rat gastric fundus most likely at the level of the nicotinic acetylcholine receptor. (+info)
Effect of guanethidine-induced adrenergic blockade on the different proteolytic systems in rat skeletal muscle.
Overall proteolysis and the activity of skeletal muscle proteolytic systems were investigated in rats submitted to guanethidine-induced adrenergic blockade for 4 days. In soleus, overall proteolysis increased by 15-20% during the first 2 days of guanethidine treatment but decreased to levels below control values after 4 days. Extensor digitorum longus (EDL) did not show the initial increase in total proteolysis, which was already reduced after 2 days of guanethidine treatment. The initial rise in the rate of protein degradation in soleus was accompanied by an increased activity of the Ca(2+)-dependent proteolytic pathway. In both soleus and EDL, the reduction in overall proteolysis was paralleled by decreased activities of the Ca(2+)-dependent and ATP-dependent proteolytic processes. No change was observed in the activity of the lysosomal proteolytic system. Overall proteolysis in soleus and EDL from nontreated rats was partially inhibited by isoproterenol, in vitro. The data suggest an acute inhibitory control of skeletal muscle proteolysis by the adrenergic system, well evident in the oxidative muscle, with an important participation of the Ca(2+)-dependent pathway. (+info)