Identification of a low-threshold T-type calcium channel in bovine ciliary epithelial cells

The characteristics of an inward current in single pigmented cells from the bovine ciliary body were examined using the whole cell recording technique. The inward current appeared to activate at potentials 30 mV positive to a holding potential of -80 mV, although activation studies revealed that from more negative holding potentials the current activated at about -75 mV, peaked at around -10 mV, and reversed at a potential of +40 mV. The current was small, peak value -72 +/- 24 pA (n = 13), and exhibited rapid activation and slower inactivation kinetics, both processes being voltage sensitive. Inactivation occurred at holding potentials more positive than -120 mV and was complete at -50 mV with a half-maximal membrane potential of -88 mV. The overlap of the inactivation and activation curves between -75 and -50 mV means that, in the steady state, entry of calcium will be greatest in this range and calcium will thus enter the cell at the resting membrane potential. The current was resistant to tetrodotoxin and nifedipine. It was partially blocked by Cd2+ and almost completely blocked by Ni2+. It is concluded that this current is a low-threshold T-type calcium current.