Nicotine receptors do not modulate the 3H-noradrenaline release from the isolated rat heart evoked by sympathetic nerve stimulation

Modulation by nicotine of the ionic currents in guinea pig ventricular cardiomyocytes. Relatively higher sensitivity to IKr and IKl

The effects of nicotine on the ionic currents in guinea pig cardiomyocytes were investigated using a whole-cell voltage-clamp technique. Nicotine (30 microM to 1 mM) inhibited the ionic currents in a concentration-dependent manner. Nicotine at 30 microM did not affect the Ca2+ current (ICa), but at 300 microM inhibited ICa at 10 mV by 29.3 +/- 2.4% (n = 6, P < .01) and at 1 mM almost blocked the ICa (by approximately 90%, n = 5, P < .001). After 5- to 10-min washout, these responses had 50-70% recovery. The fast time constant (tau f) of the inactivation phase for ICa at 10 mV was not affected, but the slow one (tau s) increased from 35.7 +/- 2.8 to 39.5 +/- 2.4 ms (n = 7) at 300 microM nicotine. Nicotine at 100 microM also inhibited the delayed rectifier K+ current (IK) at 60 mV by 42.7 +/- 3.0% (n = 7, P < .01), and at 30 microM inhibited the inwardly rectifying K+ current (IKl) at -110 mV by 43.0 +/- 2.5% (n = 7, P < .01). The responses to nicotine were not significantly modified by atropine, hexamethonium, and nicotine receptor antagonists (d-tubocurarine and benzoquinonium). The IK is composed of two components for rapidly and slowly activated currents (IKr and IKs). Nicotine markedly decreased the tail current of IKr, but had less or no effect on that of IKs. However, the activation and inactivation kinetics (d infinity and f infinity) for ICa and its activation kinetics (P infinity) for IKr and IKs were not modified. These results suggest that nicotine inhibits the ionic currents with relatively higher sensitivity to IKl and IKr, resulting in modulation of the cardiac functions.

Effects of nicotine on spontaneous activity and underlying ionic currents in rabbit sinoatrial nodal cells

1. Effects of nicotine on the spontaneous action potentials and the underlying ionic currents in rabbit sinoatrial (SA) nodal cells were investigated using current-clamp and whole-cell voltage-clamp modes. 2. Nicotine (30 microM to 1 mM) produced a negative chronotropic effect in a concentration-dependent manner (at 1 mM by 10.6 +/- 2.8%, n = 9, p < 0.01). Nicotine at 300 microM significantly decreased the maximum rate of depolarization by 9.8 +/- 1.3% (n = 9, p < 0.05). Other action potential parameters were not affected to any significant extent. 3. Pretreatment with atropine (1 microM) and hexamethonium (1 mM) did not modify the nicotine-induced effects. After washout, these responses were reversible. 4. Isoprenaline decreased the responses induced by nicotine, but ACh increased them. 5. Nicotine at 100 microM did not affect the L-type Ca2+ current (ICa), but at 300 microM inhibited it at + 10 mV by 21.6 +/- 2.9% (n = 6, p < 0.05). The fast time constant (tau f) of the inactivation phase for ICa was not affected, but the slow one (tau s) significantly increased from 36.8 +/- 1.9 ms to 41.2 +/- 2.8 ms (n = 6) at 300 microM nicotine. The activation and inactivation kinetics (d infinity and f infinity) for ICa were not modified. 6. Nicotine also did not affect the delayed rectifier K+ current (IK) and its activation kinetic (P infinity). 7. These results suggest that nicotine depresses the action potentials and causes a negative chronotropic effect due to inhibitions of the ionic currents in the SA nodal cells.

Involvement of alpha 2-adrenoceptors and protein kinase C on nicotine-induced facilitation of noradrenaline release in bovine cerebral arteries

1. Incubation of bovine cerebral vessels (previously exposed to [3H]-noradrenaline) with nicotine for 30 sec produced a facilitation of the electrically-induced noradrenaline release, which was antagonized by hexamethonium, a blocker of nicotinic receptors. This facilitation was not observed when the incubation time was increased to 20 or 75 min. 2. Rauwolscine, an alpha 2-adrenoceptor blocker, enhanced and phorbol 12, 13-dibutyrate reduced the facilitator effect produced by 30 sec exposure to nicotine. 3. These data suggest: (1) presynaptic nicotinic receptors produce a facilitation of stimulated noradrenaline release; these receptors are easily desensitized by increasing the incubation time with nicotine; (2) protein kinase C and alpha 2-adrenoceptors appear to be involved in this process.

P2-purinoceptor-mediated inhibition of noradrenaline release in rat atria

1. We looked for P2-purinoceptors modulating noradrenaline release in rat heart atria. Segments of the atria were preincubated with [3H]-noradrenaline and then superfused with medium containing desipramine (1 microM) and yohimbine (1 microM) and stimulated electrically, by 30 pulses/1 Hz unless stated otherwise. 2. The adenosine A1-receptor agonist, N6-cyclopentyl-adenosine (CPA; EC50 9.7 nM) and the nucleotides, ATP (EC50 6.6 microM) and adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S; EC50 4.8 microM), decreased the evoked overflow of tritium. The adenosine A2a-agonist, 2-p-(2-carbonylethyl)-phenethylamino-5'-N-ethylcarboxamido-a denosine (CGS-21680; 0.03-0.3 microM) and the P2x-purinoceptor agonist beta, gamma-methylene-L-ATP (30 microM) caused no change. 3. The concentration-response curve of CPA was shifted to the right by the adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropyl-xanthine (DPCPX; 3 nM; apparent pKB value 9.7) but hardly affected by the P2-purinoceptor antagonist, cibacron blue 3GA (30 microM). In contrast, the concentration-response curves of ATP and ATP gamma S were shifted to the right by DPCPX (3 nM; apparent pKB values 9.3 and 9.4, respectively) as well as by cibacron blue 3GA (30 microM; apparent pKB values 5.0 and 5.1, respectively). Combined administration of DPCPX and cibacron blue 3GA caused a much greater shift of the concentration-response curve of ATP than either antagonist alone. The concentration-response curve of ATP was not changed by indomethacin, atropine or the 5'-nucleotidase blocker alpha, beta-methylene-ADP. 4. Cibacron blue 3GA (30 microM) increased the evoked overflow of tritium by about 70%. The increase was smaller when the slices were stimulated by 9 pulses/O00 Hz instead of 30 pulses/I Hz.5. The results indicate that the postganglionic sympathetic axons in rat atria possess P2-purinoceptors in addition to the known adenosine Al-receptor. Both mediate inhibition of noradrenaline release. Some adenine nucleotides such as ATP and ATP gamma S act at both receptors. The presynaptic P2-purinoceptor seems to be activated by an endogenous ligand, presumably ATP, under the condition of these experiments. This is the first evidence for presynaptic P2-purinoceptors at cardiac postganglionic sympathetic axons.

Negative chronotropic and inotropic responses to nicotine in rat right and left atria

1. Comparison between the chronotropic and inotropic effects induced by nicotine in the right and left atrial muscles of rat was investigated. 2. Nicotine (300 microM and 1 mM) significantly produced negative chronotropic and inotropic effects in the right atrium. The effects were concentration-dependent, and reached a maximum about 30 sec after nicotine administration (1 mM). The responses were attenuated with time even during exposure to nicotine (desensitization). 3. In the left atrium, nicotine (300 microM and 1 mM) caused a negative inotropic effect. The depression was decreased with an increase in the frequencies of stimulation (1-3 Hz). The negative inotropic effect reached a maximum at approx. 60 sec later. 4. Nicotine (1 mM) caused a greater negative inotropic response in the right atrium than in the left atrium by 23% (stimulated at 3 Hz). The effects were not modified by atropine (1 microM) and hexamethonium (1 mM). 5. There was no difference between single and cumulative administrations of nicotine in both right and left atrial muscles. 6. These results suggest that the negative chronotropic and inotropic effects of nicotine may be due mainly to a direct action on the cell membrane of rat atria, accompanied with down regulation of nicotinic ACh receptors. The difference between the right and left atria could result from a different innervation of the autonomic nervous system.

Prejunctional nicotinic receptors involved in facilitation of stimulation-evoked noradrenaline release from the vas deferens of the guinea-pig

In guinea-pig prostatic vas deferens loaded with [3H]-noradrenaline ([3H]-NA), nicotinic receptor agonists, nicotine and dimethylphenylpiperazinium (DMPP) enhanced the resting and facilitated the stimulation-evoked release of [3H]-NA in a concentration-dependent fashion. The effect of nicotine on both contraction of vas deferens and release of NA in response to field stimulation was stereospecific in favour of the naturally occurring (-)-enantiomer. Prolonged (15 min) exposure to (-)-nicotine resulted in a cessation of the facilitatory effect on NA release and on responses of the vas deferens to field stimulation. 2 The rank order of agonist potency in facilitating NA release was DMPP = (-)-nicotine greater than (+)-nicotine. Cytisine had no agonistic activity. The dissociation constants (KD) of antagonists were 9.3 +/- 0.6 and 31.4 +/- 2.4 microM for (+)-tubocurarine and hexamethonium, respectively, when (-)-nicotine was used as agonist. alpha-Bungarotoxin had no antagonistic activity. These findings suggest that nicotinic receptors located on noradrenergic axon terminals are different from those located postsynaptically in striated muscle or ganglia but seem similar to those present on cholinergic axon terminals at the neuromuscular junction. 3. Cotinine, the breakdown product of nicotine failed to have any agonistic activity indicating that nicotine itself is responsible for the effects observed on axon terminals. 4 Stimulation of presynaptic muscarinic receptors by oxotremorine prevented the nicotine-induced facilitation of [3H]-NA release, indicating the presence of both inhibitory muscarinic and facilitatory nicotinic receptors on noradrenergic axon terminals.

Cholinesterase activity and exposure time to acetylcholine as factors influencing the muscarinic inhibition of [3H]-noradrenaline overflow from guinea-pig isolated atria

Guinea-pig isolated atria were incubated and loaded with [3H]-noradrenaline. The release of 3H and of [3H]-noradrenaline was induced by field stimulation (6-9 trains of 150 pulses at 5 Hz). The stimulation-evoked overflows of 3H and of [3H]-noradrenaline were determined. In the absence of an inhibitor of acetylcholinesterase, acetylcholine (12 min preincubation before nerve stimulation, up to 10 microM) failed to inhibit the evoked [3H]-noradrenaline overflow. In the presence of atropine, an increase by acetylcholine of evoked release was observed in the same atria. In contrast, the selective muscarinic agonist methacholine significantly decreased the evoked overflow. The inhibition was antagonized by atropine. Methacholine did not enhance release in the presence of atropine. When present for only 2 min, acetylcholine 10 microM inhibited the evoked overflow and no facilitation of release was observed in the presence of atropine. In the presence of physostigmine, acetylcholine (12 min preincubation, 1 and 10 microM) inhibited evoked [3H]-noradrenaline overflow, but the overflow was increased by acetylcholine 10 microM in the presence of atropine. In the presence of cocaine, corticosterone, phentolamine, propranolol and hexamethonium together, acetylcholine 1 microM inhibited the evoked [3H]-noradrenaline overflow. The inhibition was significantly enhanced in the presence of physostigmine. It decreased with preincubation time of the agonist, despite the presence of physostigmine and constant replacement by new drug. Neither inhibition nor facilitation of evoked release was observed in the presence of atropine. It is concluded that a muscarinic inhibition by acetylcholine (upon prolonged exposure time) may be masked by a concomitant facilitation of release and/or desensitization of the muscarinic inhibitory mechanism. Furthermore, degradation by acetylcholinesterase contributes in part to the ineffectiveness of acetylcholine as a presynaptic inhibitor. When a distortion of the overflow/release ratio was excluded, adrenergic and nicotinic effects were prevented, and acetylcholinesterase was inhibited, the fading of muscarinic inhibition by acetylcholine may have been exclusively due to a slow and moderate desensitization of the presynaptic muscarinic mechanism.

Interaction between the inhibitory action of acetylcholine and the alpha-adrenoceptor autoinhibitory feedback system on release of [3H]-noradrenaline from rat atria and rabbit ear artery

Stimulation-induced increases in the efflux of radioactivity (S-I efflux) were measured in the bathing medium following labelling of the noradrenergic transmitter pools of rat atria and rabbit artery preparations with [3H]-noradrenaline. In atria stimulated with trains of 16 or 60 pulses at 2 Hz, phentolamine enhanced, whereas acetylcholine inhibited S-I efflux. With trains of 16 pulses phentolamine had a smaller enhancing effect than with trains of 60 pulses, whereas the inhibitory effect of acetylcholine was more pronounced with 16 pulses of stimulation. The inhibitory effect of acetylcholine was markedly enhanced by phentolamine when stimulation was with 60 pulses. With 16 pulses of stimulation the effect of acetylcholine was unaltered by phentolamine and abolished by the alpha 2-adrenoceptor agonist 3,4-dihydroxyphenylimino-2-imidazolidine (DPI). Phentolamine had no effect on the negative inotropic effect of acetylcholine on driven left atrial preparations. In arterial preparations stimulated with trains of 30 pulses at 1 Hz, both acetylcholine and clonidine inhibited S-I efflux, whereas yohimbine and idazoxan enhanced S-I efflux. Combining acetylcholine with clonidine did not alter the inhibitory effect of clonidine but the combination of acetylcholine with yohimbine or idazoxan abolished the marked enhancing effects of yohimbine or idazoxan on S-I efflux. These findings indicate that there may be a reciprocal interaction between prejunctional alpha-adrenoceptors and prejunctional muscarinic cholinoceptors.

Autoinhibition of noradrenaline release from the rat heart as a function of the biophase concentration. Effects of exogenous alpha-adrenoceptor agonists, cocaine, and perfusion rate

Rat isolated perfused hearts with the right sympathetic nerves intact were loaded with 3H-(-)-noradrenaline. The nerves were stimulated with trains of 180 pulses at 3 Hz and at 10 min intervals. The overflow of 3H-noradrenaline and 3H-metabolites was determined by liquid scintillation spectrometry. Clonidine (IC50 17 nM), oxymetazoline (IC50 63 nM), and alpha-methylnoradrenaline (apparent IC50 35 nM, determined in the presence of cocaine and propranolol) decreased the stimulation-evoked overflow of 3H-noradrenaline by 26, 49, and 78%, respectively, but not methoxamine up to 100 microM (propranolol present). Oxymetazoline and alpha-methyl-noradrenaline did not cause desensitization of the presynaptic adrenoceptors when present at their IC80 for 33 min. At a perfusion rate of 7 ml/min, yohimbine 1 microM enhanced the stimulation-evoked 3H-noradrenaline overflow by 26% in the absence, and by 58% in the presence of cocaine. Phentolamine 1 microM increased it by 69% when the neuronal reuptake was blocked. The increase by the antagonists faded with successive period of nerve stimulations, and was positively correlated with the biophase concentration of noradrenaline as reflected by the amount of 3H-noradrenaline released into the perfusate per nerve stimulation. At a perfusion rate of 1.8 ml/min (neuronal reuptake blocked), yohimbine 1 microM increased the overflow by 127%. The results indicate that the alpha 2-adrenoceptor-mediated autoinhibition in the rat perfused heart depends on the clearance of transmitter from the biophase via neuronal reuptake and diffusion into the vascular space. Reduction of either elimination pathway enhances the biophase concentration of noradrenaline, thus increasing the autoinhibition of release.

The determination of presynaptic pA2 values of yohimbine and phentolamine on the perfused rat heart under conditions of negligible autoinhibition

1 Rat isolated perfused hearts with the right sympathetic nerves attached were loaded with [3H]-(-)-noradrenaline. The nerves were stimulated with up to 40 trains of 10 pulses every min at 1 Hz, and the evoked increases of [3H-]noradrenaline overflow into the perfusate, of right atrial tension development and ventricular beating frequency were measured. 2 Oxymetazoline inhibited the evoked transmitter overflow (IC50: 10 nM) and decreased the postsynaptic responses in a concentration-dependent manner. It behaved as a full against in abolishing the evoked transmitter overflow. 3 Yohimbine up to 1 microM neither enhanced the evoked [3H]-noradrenaline overflow nor the postsynaptic parameters. Phentolamine (1 microM) caused a transient, minor (less than 30%) increase in [3H]-noradrenaline overflow. 4 Yohimbine (0.03-1.0 microM) and phentolamine (0.1-5.0 microM) shifted to the right the concentration-response curve of oxymetazoline for the inhibition of [3H]-noradrenaline overflow in response to nerve stimulation without depressing the maxima. The pA2 values were 7.82 and 7.52, respectively. 5 Yohimbine (0.1 microM) also antagonized the decrease induced by oxymetazoline in the postsynaptic responses to nerve stimulation. 6 The results confirm the existence of presynaptic inhibitory alpha 2-adrenoceptors at the adrenergic nerve fibres of the rat heart in vitro. Under the stimulation and perfusion conditions selected, the released endogenous transmitter apparently does not activate a negative feedback mechanism, thus permitting the determination of pA2 values.