Bradykinin activates airway parasympathetic ganglion neurons by inhibiting M-currents.

The action of bradykinin on neurons acutely isolated from airway parasympathetic ganglia of rats and its mechanism were investigated using the nystatin-perforated patch-clamp recording technique. Under current clamp conditions, an application of 0.1 microM bradykinin onto rat airway ganglion neurons induced a depolarization which was accompanied by the action potential firing. Bradykinin elicited inward currents with decreasing the membrane conductance when a ganglion neuron was held at a holding potential of -40 mV. The half-maximum effective concentration was 8.9 nM. The bradykinin response was mimicked by a B(2) receptor agonist, [Hyp(3)]-bradykinin, and was inhibited by HOE-140, a B(2) antagonist, suggesting the contribution of B(2) receptors. The bradykinin-induced inward current reversed at the K(+) equilibrium potential, which shifted 56.5 mV with a 10-fold change in extracellular K(+) concentration. The application of 10(-3) M Ba(2+) induced the inward current, and bradykinin failed to evoke a further inward current in the presence of Ba(2+). Bradykinin also reduced the amplitude of M-current deactivation induced by a hyperpolarizing step from a holding potential of -25 mV to -50 mV with a half-maximum effective concentration of 16 nM. Pretreatment with pertussis toxin had no effect on the bradykinin-induced inhibition of the M-current. From these results we suggest that bradykinin may be able to depolarize the airway parasympathetic ganglion neurons of rats associated with an inhibition of M-type K(+) channels through the B(2) type of bradykinin receptors.

Activation of potassium conductance by ophiopogonin-D in acutely dissociated rat paratracheal neurones.

1. The effect of ophiopogonin-D (OP-D), a steroidal glycoside and an active component of Bakumondo-to, a Chinese herbal antitussive, on neurones acutely dissociated from paratracheal ganglia of 2-week-old Wistar rats was investigated using the nystatin-perforated patch recording configuration. 2. Under current-clamp conditions, OP-D (10 microM) hyperpolarized the paratracheal neurones from a resting membrane potential of -65.7 to -73.5 mV. 3. At the concentration of 1 microM and above, OP-D concentration-dependently activated an outward current accompanied by an increase in the membrane conductance under voltage-clamp conditions at a holding potential of -40 mV. 4. The reversal potential of the OP-D-induced current (I(OP-D)) was -79.4 mV, which is close to the K(+) equilibrium potential of -86.4 mV. The changes in the reversal potential for a 10 fold change in extracellular K(+) concentration was 53.1 mV, indicating that the current was carried by K(+). 5. The I(OP-D) was blocked by an extracellular application of 1 mM Ba2+ by 59.0%, but other K(+) channel blockers, including 4-aminopyridine (3 mM), apamin (1 microM), charybdotoxin (0.3 microM), glibenclamide (1 microM), tolbutamide (0.3 mM) and tetraethylammonium (10 mM), did not inhibit the I(OP-D). 6. OP-D also inhibited the ACh- and bradykinin-induced depolarizing responses which were accompanied with firing of action potentials. 7. The results suggest that OP-D may be of benefit in reducing the excitability of airway parasympathetic ganglion neurones and consequently cholinergic control of airway function and further, that the hyperpolarizing effect of OP-D on paratracheal neurones via an activation of K(+) channels might explain a part of mechanisms of the antitussive action of the agent.