Comparative mechanical and electrical actions of A23187 and X-537A in canine Purkinje fibers

Accidental poisoning of 17 dogs with lasalocid

Over a period of 10 days, 17 dogs became weak and developed neurological deficits of different degrees of severity. About 12 hours before these clinical signs appeared they had all eaten a particular brand of commercial dog food from a recently opened bag. They were all quadriparetic and hyporeflexic, and some of them also showed additional systemic or neurological signs, including dyspnoea, a high body temperature, tongue laxity, hyperaesthesia and anisochoria. Serum biochemical abnormalities included high activities of creatine kinase, lactate dehydrogenase and aspartate aminotransferase. Analysis of the suspect food revealed high concentrations of the ionophore lasalocid. Fifteen of the dogs were given supportive treatment at home and two were hospitalised. Five of the dogs died, but the others improved gradually and had fully recovered by one to four days after the appearance of the clinical signs.

Pharmacological actions of monovalent ionophores on spontaneously beating rabbit sino-atrial nodal cells

The effects of sodium (monensin) and potassium (nigericin and lonomycin A) ionophores on the spontaneous activity in rabbit sino-atrial (SA) nodal cells were investigated using microelectrode and whole-cell voltage-clamp techniques. In the multicellular preparations, the ionophores produced a negative chronotropic effect in a concentration-dependent manner, and at 3x10(-5) M significantly decreased the amplitude and duration of action potentials and enhanced the maximum rate of depolarization. The ionophores elicited dysrhythmias and then a sinus arrest often occurred. These responses were reversible. In whole-cell clamp experiments, monensin enhanced the L-type Ca2+ current (ICa), whereas lonomycin A and nigericin inhibited ICa. The fast component of the inactivation phase for ICa was decreased by the ionophores, but the slow component was unaffected. The activation and inactivation kinetics (d infinity and f infinity) were not altered. The ionophores did not affect the hyperpolarization-activated inward current. Monensin inhibited the delayed rectifier K+ current (I(K)), but lonomycin A and nigericin increased I(K). Its activation kinetics shifted in the depolarizing direction. The effects on the ionic currents were irreversible. Monensin (30 microM) increased cellular Ca2+ concentration ([Ca2+]i), using Ca2+ -sensitive fluorescent dye (fura-2). These results indicate that the monovalent ionophores depress the action potentials and produce a negative chronotropic effect due to direct and indirect modulations of the ionic currents and the [Ca2+]i level in rabbit 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.