Calcium-sensitive calcium influx in photoreceptor inner segments

The effect of external calcium concentration([Ca2+](o)) on membrane potential-dependent calcium signals in isolated tiger salamander rod and cone photoreceptor inner segments was investigated with patch-clamp and calcium imaging techniques. Mild depolarizations led to increases in intracellular Ca2+ levels ([Ca2+](i)) that were smaller when [Ca2+](o) was elevated to 10 mM than when it was 3 mM, even though maximum Ca2+ conductance increased 30% with the increase in [Ca2+](o). When external calcium was lowered to 1 mM [Ca2+](o), maximum Ca2+ conductance was reduced, as expected, but the mild depolarization-induced increase in [Ca2+](i) was larger than in 3 mM [Ca2+](o). In contrast, when photoreceptors were strongly depolarized, the increase in [Ca2+](i) was less when [Ca2+](o) was reduced. An explanation for these observations comes from an assessment of Ca2+ channel gating in voltage-clamped photoreceptors under changing conditions of [Ca2+](o). Although Ca2+ conductance increased with increasing [Ca2+](o), surface charge effects dictated large shifts in the voltage dependence of Ca2+ channel gating. Relative to the control condition (3 mM [Ca2+](o)), 10 mM [Ca2+](o) shifted Ca2+ channel activation 8 mV positive, reducing channel open probability over a broad range of potentials. Reducing [Ca2+](o) to 1 mM reduced Ca2+ conductance but shifted Ca2+ channel activation negative by 6 mV. Thus the intracellular calcium signals reflect a balance between competing changes in gating and permeation of Ca2+ channels mediated by [Ca2+](o). In mildly depolarized cells, the [Ca2+](o)-induced changes in Ca2+ channel activation proved stronger than the [Ca2+](o)-induced changes in conductance. In response to the larger depolarizations caused by 80 mM [K+](o), the opposite is true, with conductance changes dominating the effects on channel activation.