The Properties of Voltage-operated Ca2+-channels in Bovine Isolated Trachealis Cells

Summary: Freshly-dispersed bovine trachealis cells were used for recording by the patch clamp technique of whole-cell and unitary currents through Ca²⁺-channels. Whole-cell Ca²⁺-current (I_Ca) activated at -40 mV and appeared to be carried by a single type of Ca²⁺-channel. Inactivation of I_Ca was increased by increasing the concentration of free Ca²⁺ within the recording pipette but reduced by using Ba²⁺ as the charge carrier. Steady-state inactivation studies showed that the Ca²⁺-channels were half-maximally available following a conditioning depolarization to -35 mV. A two-pulse protocol showed that I_Ca induced by the step to a test potential was inversely related to I_Ca induced by the step to the conditioning potential. Unitary Ba²⁺-currents were activated at a threshold of -30 mV and had a reversal potential of +41 mV. The channel carrying the unitary Ba²⁺-currents had a slope conductance of 23 pS. Steady-state inactivation studies showed that the unitary Ba²⁺-currents were half-maximally available at a holding potential of -28 mV. I_Ca and unitary Ba²⁺-currents were inhibited by nifedipine (10 nM-1 μM) but augmented by Bay K 8644 (10 μM). It is concluded that the plasmalemma of bovine trachealis muscle contains a single population of voltage-dependent Ca²⁺-channels of the L-type. These channels may be subject to inactivation primarily by an increase in the concentration of free Ca²⁺ on the cytosolic side of the plasmalemma and secondarily by a voltage-dependent mechanism. Overlap of the inactivation and activation curves of I_Ca may allow the passage of 'window current' through the Ca²⁺-channels during sustained depolarization.