Effect of low temperatures on the responses of guinea-pig isolated atria to nicotine and to sympathetic and parasympathetic 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.

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.

Effect of variations in temperature on antimuscarinic activity in guinea-pig atria

The characteristics of the antimuscarinic activity of homatropine, gallamine and stercuronium in guinea-pig atria remained constant over the temperature range 22--37 degrees C in that a linear Arunlakshana-Schild plot was obtained with homatropine and nonlinear plots occurred with gallamine or stercuronium. A trend towards higher dose-ratios with reduction in temperature was only significant for gallamine.

Interactions between acetylcholine, 5-hydroxytryptamine, nicotine and morphine on isolated rabbit atria

1. The effects of 5-hydroxytryptamine (5-HT) and morphine on the responses to acetylcholine and nicotine of isolated rabbit atria were studied.2. 5-Hydroxytryptamine (10 mug/ml.) and morphine (20 mug/ml.) blocked the negative chronotropic and inotropic actions of acetylcholine.3. Nicotine (20 mug/ml.) produced stimulation of the atria, which was blocked by dichlorisoprenaline, morphine, 5-HT, bretylium and hemicholinium. Hemicholinium block was reversed by choline.4. In reserpinized preparations, nicotine produced inhibition of atria and this action was also blocked by atropine, 5-HT and morphine. Inhibition induced by nicotine was potentiated by physostigmine.5. 5-Hydroxytryptamine (20 mug/ml.) produced stimulation of atria. This was blocked by bretylium and reduced by hemicholinium. Hemicholinium block was reversed by choline.6. It is concluded that 5-HT in low concentrations acts as a weak agonist at the cholinoceptive receptors and therefore blocks the action of acetylcholine. Furthermore, nicotine and larger doses of 5-HT have actions on ganglionic structures and liberate acetylcholine, which in turn releases catecholamines.