CALCIUM-SENSITIVE CALCIUM INFLUX IN PHOTORECEPTOR INNER SEGMENTS

The effect of external calcium concentration([Ca2+](o)) on membrane po tential-dependent calcium signals in isolated tiger salamander rod and cone photoreceptor inner segments was investigated with patch-clamp a nd calcium imaging techniques. Mild depolarizations led to increases i n 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 Ca 2+ conductance increased 30% with the increase in [Ca2+](o). When exte rnal calcium was lowered to 1 mM [Ca2+](o), maximum Ca2+ conductance w as reduced, as expected, but the mild depolarization-induced increase in [Ca2+](i) was larger than in 3 mM [Ca2+](o). In contrast, when phot oreceptors were strongly depolarized, the increase in [Ca2+](i) was le ss when [Ca2+](o) was reduced. An explanation for these observations c omes from an assessment of Ca2+ channel gating in voltage-clamped phot oreceptors under changing conditions of [Ca2+](o). Although Ca2+ condu ctance increased with increasing [Ca2+](o), surface charge effects dic tated large shifts in the voltage dependence of Ca2+ channel gating. R elative to the control condition (3 mM [Ca2+](o)), 10 mM [Ca2+](o) shi fted Ca2+ channel activation 8 mV positive, reducing channel open prob ability over a broad range of potentials. Reducing [Ca2+](o) to 1 mM r educed Ca2+ conductance but shifted Ca2+ channel activation negative b y 6 mV. Thus the intracellular calcium signals reflect a balance betwe en competing changes in gating and permeation of Ca2+ channels mediate d by [Ca2+](o). In mildly depolarized cells, the [Ca2+](o)-induced cha nges in Ca2+ channel activation proved stronger than the [Ca2+](o)-ind uced changes in conductance. In response to the larger depolarizations caused by 80 mM [K+](o), the opposite is true, with conductance chang es dominating the effects on channel activation.