Decrease in bradycardic effect of AQ-A 39 and alinidine in guinea-pig sinoatrial node depolarized by high external K+-concentration

Pharmacological tests of the mechanism of the periodic rhythm caused by veratramine in the sinoatrial node of the guinea pig

1. We investigated the effects of several drugs and extracellular ions on the periodic sinoatrial node rhythm caused by high concentrations of veratramine (>2 microM) in isolated guinea pig sinus atria. 2. During the active phase of this rhythm, pacemaker activity appeared to be due to transient afterdepolarizations resembling the delayed afterdepolarizations attributed to Ca++-induced Ca++ release in cardiac tissue. 3. Ryanodine (200-2200 nM) did not decrease the transient afterdepolarizations, and instead increased the heart rate during the active phase, prolonged the active phase, and sometimes caused conversion to regular rhythm. 4. Dichlorobenzamyl (10-110 microM), a blocker of electrogenic Na+-Ca++ exchange, did not slow or stop beating during periodic rhythm, but rather increased average heart rate and, at a higher concentration, caused conversion to regular rhythm. 5. Ouabain (0.1 microM), an inhibitor of the sodium pump and electrogenic Na+-K+ exchange, had little effect on veratramine periodic rhythm, but at higher concentrations it caused increased average heart rate and conversion to regular rhythm. 6. The chronotropic effect of Ca++ was normally weakly positive; however, in the presence of veratramine, and before the appearance of periodic rhythm, the chronotropic effect of Ca++ was weakly negative, and was associated with destabilization of the heart rate, leading to frequency oscillations or periodic rhythm. 7. Veratramine changed the chronotropic effect of K+ from weakly negative to moderately positive. 8. When half the Na+ or Cl- in the bathing medium was replaced by an impermeant ion, in the absence of veratramine the average heart rate was slightly decreased, whereas, in the presence of veratramine and periodic rhythm the average rate was increased, although the increase was not statistically significant in the case of low Na+. 9. These observations indicate that Ca++-induced Ca++ release, Na+-Ca++ exchange, and probably electrogenic Na+-K+ exchange play no important role in generation of periodic rhythm. The increased K+ dependence suggests an altered pacemaker mechanism.

Modulation of EPSP shape and efficacy by intrinsic membrane conductances in rat neocortical pyramidal neurons in vitro

1. Intracellular recordings were made from pyramidal neurons in layers II/III and V of rat visual cortical slices. Distal and proximal excitatory postsynaptic potentials (EPSPs) were evoked using extracellular bipolar electrodes placed on the slice horizontal to each cell, near the apical and basal dendrites respectively. Experiments were conducted in the presence of 2-amino-5-phosphonopentanoate, picrotoxin and, in most cases, 2-hydroxy-saclofen. 2. For layer II/III pyramidal neurons, voltage undershoots following distal and proximal EPSPs (n = 7 pairs) and injected somatic pulses were rarely apparent. In layer V pyramidal neurons substantial voltage undershoots were recorded following distal and proximal EPSPs (n = 27 pairs) and injected somatic pulses, with undershoot being greatest for apical inputs (P = 0.001). The greater undershoots following apical EPSPs were also apparent in semilogarithmic plots of voltage decay where the slope of decay for apical EPSPs was quicker than the voltage decay following pulses of current injected at the soma. There was no significant difference in the shapes of distal and proximal EPSPs in layer II/III or layer V pyramidal cells under control conditions. 3. Pharmacological agents were used to reduce voltage undershoots. The most successful of these was alinidine, a putative blocker of the slow inward rectifier (IH) conductance. In the presence of bath-applied 100 microM alinidine, undershoots were significantly reduced and it became possible to distinguish the relative origins of EPSPs on the basis of their shape. Distally generated EPSPs (n = 14) had rise times and half-widths that were 2.8 and 1.5 times longer respectively than those evoked proximally (n = 10; P = 0.001 for both parameters). 4. These results confirm previous theoretical simulations of somatic recordings in passive model neurons where distal EPSPs display slower rise times and longer half-widths than proximal EPSPs. The present results suggest that, at least in pyramidal neurons of layer V, distal synaptic inputs can be specifically modulated by intrinsic membrane conductances.

Cardiovascular effects of E4080, a novel bradycardiac agent with coronary vasodilating properties, in anesthetized dogs

The cardiohemodynamic effects of E4080, a novel bradycardiac agent with a coronary vasodilating feature, were studied in anesthetized open-chest dogs. E4080 (0.3 and 1 mg/kg i.v.) decreased heart rate (HR), mean aortic pressure (MAP) and total peripheral resistance, and increased coronary blood flow (CBF) without affecting cardiac output and the electrocardiogram. The maximum rate of rise in left ventricular pressure decreased at 1 mg/kg. In addition, E4080 (0.3 and 1 mg/kg i.v.) decreased myocardial oxygen consumption. On administration in sinus node artery, E4080 (10 and 30 micrograms) selectively decreased HR. Glibenclamide, an ATP-sensitive K+ channel blocker (5 mg/kg i.v.), inhibited both the increase in CBF and the decrease in MAP caused by E4080 (1 mg/kg i.v.) but did not inhibit the bradycardia. These results suggested that E4080 has both bradycardiac and coronary vasodilating effects, and that activation of ATP-sensitive K+ channel contributes to the vasodilating action of E4080 but not to the bradycardiac action.

On the mechanism of the "specific bradycardic action" of the verapamil derivative UL-FS 49

Membrane currents were measured in single sino-atrial node cells of guinea pig and rabbit hearts as well as in guinea pig ventricular myocytes using the patch-clamp technique. UL-FS 49 blocked the L-type calcium current (ICa) in sino-atrial node cells at drug concentrations which had little or no effect on the amplitude of the hyperpolarization-activated current ih(f). In guinea pig ventricular myocytes UL-FS 49 also blocked ICa but not as strongly as in sino-atrial node cells. In a computer simulation of the sino-atrial node action potential the extent of rate reduction by block of either ih(f) or ICa was estimated. From the data obtained by single cell measurements and the computations we concluded that rate reduction in primary pacemaker cells by application of UL-FS 49 is mainly due to a use dependent block of the L-type calcium current. Voltage dependent unblock of iCa at potentials more negative than -50 mV together with the lower drug sensitivity of ventricular cells can explain the "specific bradycardic action" of UL-FS 49.

Effects of alinidine on exercise-induced regional contractile dysfunction in dogs

The effects of the bradycardiac agent alinidine on hemodynamic parameters and regional contractile function were investigated in 6 chronically instrumented dogs trained to submit to 5 consecutive treadmill exercise runs. The experiments were performed during stenosis of the circumflex branch of the left coronary artery (LCX). After 2 control runs which had induced regional contractile dysfunction of comparable intensity, alinidine was infused intravenously at a dosage of 1 mg/kg per 5 min. The drug significantly reduced the resting function of both the LCX area (-16%) and the area perfused by the unstenosed anterior descending branch of the left coronary artery (LAD, -3%). However, the exercise-induced dysfunction of the LCX area was markedly improved in the 1st post-drug run and completely abolished during the 2nd and 3rd post-drug runs. As indicated by the reduction of heart rate (-18%) and positive dp/dtmax (-24%) during peak exercise, this improvement may be attributed to a bradycardiac and a negative inotropic effect of this drug. Further benefit may be ascribed to a decreased in arterial blood pressure after alinidine.