Abstract
Abstract: :
Purpose: Voltage-gated potassium channels play many roles in regulation of retinal neurons. One type of potassium channel that has been reported by not extensively studied in retina is the large conductance potassium channel (BK). The channel is gated by both voltage and internal calcium, imbuing it with a number of unique properties. We have initiated a study of the BK channel to determine its distribution among retinal neurons, along with its voltage, and calcium sensitivity. Methods: Single channel recordings were performed on cell-attached patches and excised, inside-out patches in isolated salamander retinal neurons. Drugs were applied to the whole cell or to the cytoplasmic side of the patch. Both whole-cell and single channel currents were sometimes measured from the same cell. Results: BK channels were found on almost all retinal neurons. The channels were both voltage and calcium dependent. They were blocked by 1 mM TEA and by 200 nM iberiotoxin. In the intact, cell-attached patch the BK channels were often very active at the resting state of the neuron. BK activity could be observed concomitantly with spontaneous action potentials in some third order cells. Experiments in excised patches indicated that the probability of channel opening was very low until cytoplasmic calcium approached 1 micromolar. Thus, there are localized, high levels of free internal calcium in unstimulated neurons. In cell-attached mode, the BK channels was active for long periods, indicating that they are not rapidly inactivating. In excised patches exposed to 1.8 mM calcium, the BK channels would run through open and closed transitions for prolonged periods, often lasting tens of minutes. Again, this indicates the channels are not rapidly inactivating. Often, when no activity was observed in cell-attached mode, very significant BK activity was observed as soon as the patch was excised. The conditions that promote activity of only some channels in the intact cell have not yet been determined. On average, we observed one channel in a patch, although we detected as many as five. Thus, there are approximately one hundred channels on the soma of an isolated neuron, each of which can produce a 3-4 mV hyperpolarization in a resting cell. Conclusions: The BK channels are surprisingly numerous and ubiquitous. The BK channels are not only active when the cell is depolarized, but can be active at resting potentials due to localized elevations in internal calcium. Our previous studies in a subset of amacrine cells demonstrated a very transient BK current. This is likely to be due to the temporal properties of internal regulators, perhaps the sites of internal calcium release.
Keywords: calcium • ion channels • retina: proximal(bipolar, amacrine, and gangli