The Journal of Neurophysiology Vol. 82 No. 3 September 1999, pp. 1317-1325
Copyright ©1999 by the American Physiological Society

Large-Conductance Ca²⁺-Activated Potassium Channels in Secretory Neurons

Centro de Investigaciones Biomédicas, Universidad de Colima, Colima 28000, Mexico

Lara, Jesús, Juan José Acevedo, and Carlos G. Onetti. Large-Conductance Ca²⁺-Activated Potassium Channels in Secretory Neurons. J. Neurophysiol. 82: 1317-1325, 1999. Large-conductance Ca²⁺-activated K⁺ channels (BK) are believed to underlie interburst intervals and contribute to the control of hormone release in several secretory cells. In crustacean neurosecretory cells, Ca²⁺ entry associated with electrical activity could act as a modulator of membrane K⁺ conductance. Therefore we studied the contribution of BK channels to the macroscopic outward current in the X-organ of crayfish, and their participation in electrophysiological activity, as well as their sensitivity toward intracellular Ca²⁺, ATP, and voltage, by using the patch-clamp technique. The BK channels had a conductance of 223 pS and rectified inwardly in symmetrical K⁺. These channels were highly selective to K⁺ ions; potassium permeability (P_K) value was 2.3 × 10¹³ cm³ s¹. The BK channels were sensitive to internal Ca²⁺ concentration, voltage dependent, and activated by intracellular MgATP. Voltage sensitivity (k) was ~13 mV, and the half-activation membrane potentials depended on the internal Ca²⁺ concentration. Calcium ions (0.3-3 µM) applied to the internal membrane surface caused an enhancement of the channel activity. This activation of BK channels by internal calcium had a K_D(0) of 0.22 µM and was probably due to the binding of only one or two Ca²⁺ ions to the channel. Addition of MgATP (0.01-3 mM) to the internal solution increased steady state-open probability. The dissociation constant for MgATP (K_D) was 119 µM, and the Hill coefficient (h) was 0.6, according to the Hill analysis. Ca²⁺-activated K⁺ currents recorded from whole cells were suppressed by either adding Cd²⁺ (0.4 mM) or removing Ca²⁺ ions from the external solution. TEA (1 mM) or charybdotoxin (100 nM) blocked these currents. Our results showed that both BK and K_ATP channels are present in the same cell. Even when BK and K_ATP channels were voltage dependent and modulated by internal Ca²⁺ and ATP, the profile of sensitivity was quite different for each kind of channel. It is tempting to suggest that BK and K_ATP channels contribute independently to the regulation of spontaneous discharge patterns in crayfish neurosecretory cells.