PACEMAKER CURRENT (HYPERPOLARIZATION-ACTIVATED INWARD CURRENT) IN ISOLATED SINGLE PREGNANT RAT UTERINE MYOCYTES

Whole-cell patch clamp experiments were performed to examine the underlying currents to generate spontaneous activity in freshly isolated single longitudinal muscle cells of pregnant rat uterus (18-day gestation). Isolated single cells were spindle- or round-shaped (50-700 μm in length and 2-30 μm in diameter). The holding potential was -30 mV. Long-duration (3 sec) hyperpolarizing pulses were applied to -40 to -120 mV, in increments of 10 mV. Experiments were performed at room temperature (22℃). A　hyperpolarization-activated inward current (I_f) was produced. Current density at -120 mV was -1.03±0.31 pA/pF (n=5). The average capacitance was 64.3±2.3 pF (n=8). The threshold potential for activation of I_f was about -50 to -60 mV. The reversal potential was -18.6±2.1 mV (n=4). In the presence of Cs⁺ (3 mM), the I_f current at -120 mV was decreased by 76.5±2.1% (P<0.01, n=5). These results indicate that the Cs⁺⁻sensitive hyperpolarization-activated inward current is present in the longitudinal muscle cells of pregnant rat uterus. This I_f current may contribute somewhat to the electrogenesis of the spontaneous activity

ENHANCEMENT OF ARITHMETIC SKILLS BY COFFEE-DRINKING : DOUBLE-BLIND STUDY FOR CAFFEINE-CONTAINING AND CAFFEINE-FREE COFFEES

Effects of caffeine on arithmetic performance were investigated with 99 university students. A double-blind study for arithmetic skill test was carried out, after drinking caffeine-free or caffeine-containing (180 mg) coffee. Eight rounds for the arithmetic tests were performed ; four rounds before, and four rounds after coffee break. Each round was 1-min arithmetic tests three times with 1-min intervals. The arithmetic skill and the ratio of arithmetic errors per each round were averaged. Simultaneously, heart rate was also measured. The mean values of arithmetic skill and the ratio of errors at the 4th round (n=99) were 81.4±1.7/min and 0.28±0.02%. The heart rate was 76.9±1.5 beats/min. Before and after coffee-drinking, arithmetic skill was enhanced, whereas the heart rate was significantly decreased. As compared with the caffeine-free group, caffeine significantly enhanced the arithmetic skill 45 and 60 min after coffee-drinking. But the ratio of arithmetic errors was not affected. The heart rate was significantly decreased only at 45 min later. These results indicate that caffeine (-containing coffee drinking) can enhance arithmetic skill and decrease the heart rate, presumably resulting from stimulation of the central nervous system

CARDIOPROTECTIVE ACTIONS OF ATP-SENSITIVE K⁺ CHANNEL OPENERS (CROMAKALIM, PINACIDIL AND NICORANDIL) IN CARDIAC PURKINJE FIBERS

Effects of ATP-dependent K⁺ channel openers on the action potentials and the contractile force in canine cardiac Purkinje fibers were examined. Cromakalim and pinacidil (0.3 to 10 μM), and nicorandil (0.3 to 1 mM) shortened both the 50% and 90% action potential durations (APD), and decreased the contractile force, in a concentration and frequency-dependent manner. These responses were reversible. The APD shortening and the negative inotropic effect induced by a switch of stimulation frequency (from 0.5 to 3 Hz) were potentiated by application of the openers. Other action potential parameters were unaffected, but the resting potential of relatively less negative voltage was hyperpolarized. These effects were potently antagonized by glibenclamide (a selective blocker of ATP-sensitive K⁺ channel). Under the calcium overload condition, the K⁺ channel openers abolished a delayed afterdepolarization and recovered the contractile force. These results suggest that the ATP-sensitive K⁺ channel openers increase the K⁺ conductance and simultaneously may possess cardioprotective actions to reduce the cellular Ca²⁺ level in calcium overloaded cells

CARDIOPROTECTIVE ACTION OF AMILORIDE, A POTASSIUM SPARING DIURETIC DRUG, IN CANINE VENTRICULAR MUSCLE

Electrophysiological and mechanical effects and alteration of intracellular Ca²⁺ concentration in canine ventricular muscle by amiloride, a potassium sparing diuretic drug, were examined, using conventional microelectrode technique and fura-2 Ca²⁺-sensitive fluorescent dye. Amiloride (50 μM) depressed the action potential amplitude by 8.2±1.4% (n=8, P<0.05) and the maximum rate of depolarization by 16.2±2.0% (n=8, P<0.01). In addition, the action potential duration was prolonged by 26.7±3.4% (n=6, P<0.05) at 30 μM, and the resting potential was depolarized by 11.8±1.7% (n=6, P<0.05) at 0.5 mM amiloride. In contrast, amiloride (0.5 to 1 mM) significantly increased the contractile force by 8 to 30% (n=8), but tended to decrease it at lower concentrations (30 μM to 0.1 mM). The positive inotropic effect was not affected by propranolol (0.1 μM), a β-adrenoceptor blocker. In fura-2 loaded ventricular myocytes, amiloride (1 mM) initially elevated cellular Ca²⁺ level ([Ca]₁) by 24.5±2.9% (n=6, P<0.01), and during the application, the [Ca]₁ level declined. These results indicate that amiloride possesses complex cardiac (protective) actions : electrical inhibitory and mechanical stimulatory actions, accompanied with the elevation of cellular Ca²⁺ concentration

USE-DEPENDENT BLOCK AND RECOVERY OF NA⁺ CHANNELS BY CLASS IC ANTIARRHYTHMIC DRUGS (FLECAINIDE AND ETHACIZIN) IN CANINE VENTRICULAR MUSCLE

Electrophysiological effects of flecainide and ethacizin (class Ic antiarrhythmic drugs) were examined using conventional microelectrode techniques. Flecainide significantly depressed the maximum rate of depolarization (⩒max) at 3x10⁻⁶M, and depolarized the resting potential (RP) at 10⁻⁵M, in a concentration-dependent manner. Ethacizin depressed ⩒max at 10⁻⁶M, and depolarized RP at 10⁻⁵M, significantly. However, both drugs did not affect the effective refractory period (ERP) nor the action potential duration (75% repolarization, APD₇₅). Both also had no effect on the action potential amplitude (APA). On the other hand, the drugs caused a use (or rate)-dependent block of ⩒max, and their time constants of onset of inhibition (at 3 Hz) were slow ; 6.3±1.2　msec (n=10) in the presence of flecainide (10⁻⁵M), and 6.0±1.6 msec (n=6) in the presence of ethacizin (10⁻⁵M). The time constants of the recovery were also so late : 12.2±2.5 sec (n=3) for flecainide (10⁻⁵M), and 27.1±13.3 sec (n=3) for ethacizin (2x10⁻⁶M), These results indicate that both antiarrhythmic drugs, flecainide and ethacizin, have no effect on APD₇₅ and ERP, but possess the characteristics for very slow kinetics of the use-dependent block and the recovery for fast Na⁺ channels of cardiac muscles. Ethacizin produces slower kinetics for the Na⁺ channels than flecainide

PHARMACOLOGY WITH A CUP OF COFFEE : DOUBLE-BLIND ANALYSIS FOR ARITHMETIC SKILL AND HEMODYNAMIC EFFECTS

Effects of caffeine on aritlmetic skill test and hemodynamic effects were investigated after drinking caffeine-free or caffeine-containing (180mg) coffee. The development of tolerance in habitual coffee-drinkers was also examined. A double-blind study of 417 healthy medical university students (22.4±2.1 years old) was carried out. Ten rounds of the arithmetic tests were performed; the first four rounds before, and the subsequent six rounds after a coffee-break. Each round was composed of three 1-min arithmetic tests and two 1-min rests between the tests. The arittmetic skill per round was averaged. Simultaneously, heart rate and blood pressure were also measured once a round. The mean value of arithmetic skill at the 4th round before coffee-break was 86.7±3.9/ min (n=417). The heart rate and mean blood pressure were 73.7±3.3 beats/min and 85.7 ±3.8 mmHg. As compared with caffeine-free group (n=205), caffeine-containing groups had a significant enhancement of arithmetic skill at 45 to 60 min later. The mean blood pressure increased at 15 to 90 min, but the heart rate was unaffected. Simultaneously, the serum concentration of caffeine increased from 0.9±0.3 to 4.3±1.6 ptg/ml (n=12, P<0.01) at 45-60min after coffee (180mg caffeine)-drinking. Habitual coffee-drinkers reduced the stimulations of arithmetic skill and mean blood pressure, as compared with non- habitual coffee-drinkers. These results indicate that a cup of coffee (containing 180 mg caffeine) enhances the arithmetic skill and modulates the hemodynamic actions, presumably resulting from stimulation of the central and sympathomimetic nervous systems. A tolerance to caffeine's actions also develops in habitual coffee-drinkers

Analgesic action of nicotine on tibial nerve transection (TNT)-induced mechanical allodynia through enhancement of the glycinergic inhibitory system in spinal cord

The activation of cholinergic pathways by nicotine elicits various physiological and pharmacological effects in mammals. For example, the stimulation of nicotinic acetylcholine receptors (nAChRs) leads to an antinociceptive effect. However, it remains to be elucidated which subtypes of nAChR are involved in the antinociceptive effect of nicotine on nerve injury-induced allodynia and the underlying cascades of the nAChR-mediated antiallodynic effect. In this study, we attempted to characterize the actions of nicotine at the spinal level against mechanical allodynia in an animal model of neuropathic pain, tibial nerve transection (TNT) in rats. It was found that the intrathecal injection of nicotine, RJR-2403, a selective 4 2 nAChR agonist, and choline, a selective 7 nAChR agonist, produced an antinociceptive effect on the TNT-induced allodynia. The actions of nicotine were almost completely suppressed by pretreatment with mecamylamine, a non-selective nicotinic antagonist, or dihydro- -erythroidine, a selective 4 2 nAChR antagonist, and partially reversed by pretreatment with methyllycaconitine, a selective 7 nAChR antagonist. Furthermore, pretreatment with strychnine, a glycine receptor antagonist, blocked the antinociception induced by nicotine, RJR-2403, and choline. On the other hand, the GABAA antagonist bicuculline did not reverse the antiallodynic effect of nicotine. Together, these results indicate that the 4 2 and 7 nAChR system, by enhancing the activities of glycinergic neurons at the spinal level, exerts a suppressive effect on the nociceptive transduction in neuropathic pain