Transient calcium current of retinal bipolar cells of the mouse.

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Abstract
Isolated bipolar cells were obtained by enzymic (papain) dissociation of the adult mouse retina. The membrane voltage was clamped and the membrane currents were measured by the whole-cell version of the patch-clamp technique. Isolated bipolar cells and horizontal cells of the goldfish retina were also studied for comparison. Hyperpolarization from the holding voltage, Vh, of -46 mV evoked a slowly activating, Cs+-sensitive, inward current (probably an h-current), and depolarization evoked a TEA- and Cs+-sensitive outward current (probably a combination of K+ currents). Depolarization from a more negative Vh (e.g. -96 mV) evoked a transient inward current that had maximal amplitude between -40 and -20 mV. This current was identified as a Ca2+ current (ICa): its amplitude was increased with elevated [Ca2+]O and was decreased with reduced [Ca2+]o, and its was blocked by 4 nM-Co2+, but not by 5 .mu.M-TTX. Both the perikaryon and the axon terminal generated ICa with similar properties. The plot of Ca2+ conductance (gCa) against membrane voltage (activation curve) was sigmoidal: in 10 mM [Ca2+]o, gCa increased for membrane voltages more positive than -65 mV, was half-maximal at about -25 mV, and reached saturation at about +30 mV. The plot of inactivation of gCa against membrane voltage was also sigmoidal: with 1 s conditioning depolarization in 10 mM [Ca2+]o, gCa decreased for membrane voltages more positive than -80 mV, was half-maximal at about -50 mV, and was fully suppressed for voltages greater than -30 mV. ICa in the mouse bipolar cells was insensitive to 50 .mu.M-Cd2+, 10 .mu.M-nifedipine and 10 .mu.M-Bay K 8644. In contrast, the calcium currents of bipolar and horizontal cells of the goldfish retina were markedly suppressed by 50 .mu.M-Cd2+ and 10 .mu.M-nifedipine, and were augmented several fold by 10 .mu.M-Bay K 8644. The calcium currents of goldfish bipolar and horizontal cells were sustained, and were activated in a more positive range of potentials than the ICa of mouse bipolar cells. The voltage range at which the ICa of mouse bipolar cells is activated includes the presumed range of membrane potentials spanned during light-evoked responses; thus, this current may participate in synaptic transmission. The transient character of ICa may also help to shape transient responses of ganglion cells.