Abstract
Abstract: :
Purpose: Physiological fluctuation in extracellular calcium ([Ca2+]e) regulates retinal function in many aspects, including photoreceptor transduction, regulation of transmitter release and the voltage–dependent ion channel activity. In this study, we sought to examine effect of low [Ca2+]e on A–type potassium channels of retinal horizontal cells. Methods: Experiments were performed on cultured bass horizontal cells using the conventional whole cell voltage patch–clamp technique. Results: When [Ca2+]e was lowered from 3 mM to 0.3 mM, peak outward currents elicited by voltage pulses that ranged from –45 mV to 90 mV from a resting membrane potential of –70 mV were decreased by approximately 50%, whereas the corresponding sustained currents remained unchanged (n=5). This effect was reversible and rapid. The inhibition of peak outward currents of horizontal cells in 0.3 mM [Ca2+]e was blocked by 30 µM zinc (n=2). Next, we investigated the effect of low [Ca2+]e on the biophysical properties of A–type potassium channels of horizontal cells in detail. Both steady–state activation and inactivation curves for A–type potassium currents were analyzed. The half–activation voltages of A–type potassium currents in 0.3 mM [Ca2+]e (8.7 ± 14.6 mV, mean ± SD, n=4) were not significantly changed compared to those in 3 mM [Ca2+]e (13.0 ± 3.3 mV, n=4, P>0.05). However, low [Ca2+]e profoundly shifted the steady–state inactivation curves toward hyperpolarized potentials. Mean half–inactivation potentials for A–type K currents was –56.3 ± 4.7 mV in 3 mM [Ca2+]e, whereas it was –76.4 ± 3.9 mV in 0.3 mM [Ca2+]e (P<0.001, n=7). Furthermore, the 20–mV negative shift of the steady–state inactivation curves in 0.3 mM [Ca2+]e was completely blocked by 30 µM zinc (n=2). Conclusions: The results implicate that [Ca2+]e is required for maintaining the activity of A–type potassium channels in retinal horizontal cells through gating the channel steady–state inactivation rather than steady–state activation. Zinc, which is co–released with glutamate from photoreceptors, is able to protect the function of A–type potassium channels of horizontal cells at low [Ca2+]e in the intact retina
Keywords: ion channels • horizontal cells • electrophysiology: non-clinical