Ion channel compartments in photoreceptors: Evidence from salamander rods with intact and ablated terminals

Vertebrate photoreceptors are highly polarized sensory cells in which several different ionic currents have been characterized. In the present study we used whole cell voltage-clamp and optical imaging techniques, the former combined with microsurgical manipulations, and simultaneous recording of membrane current and intracellular calcium signals to investigate the spatial distribution of ion channels within isolated salamander rods. In recordings from intact rods with visible terminals, evidence for five previously identified ionic currents was obtained. These include two Ca2+ -dependent, i.e., a Ca2+ -dependent chloride current [I-Cl(Ca)] and a large-conductance Ca2+ - and voltage-dependent K+ or BK current [I-K(Ca)], and three voltage-dependent currents, i.e., a delayed-rectifier type current [I-K(V)], a hyperpolarization-activated cation current (I-h), and a dihydropyridine-sensitive L-type calcium current (I-Ca). Of these, I-Cl(Ca) was highly correlated with the presence of a terminal; rods with visible terminals expressed I-Cl(Ca) without exception (n = 125), whereas approximately 71% of rods (40/56) without visible terminals lacked I-Cl(Ca). More significantly, I-Cl(Ca) was absent from all rods (n = 33) that had their terminals ablated, and recordings from the same cell before and after terminal ablation led, in all cases (n = 10), to the loss of I-Cl(Ca). In contrast, I-K(Ca), I-K(V), and I h remained largely intact after terminal ablation, suggesting that they arose principally from ion channels located in the soma and/or inner segment. The outward I-K(Ca) in terminal-ablated rods was reversibly suppressed on puffing a Ca2+ - free extracellular solution over the soma and was appreciably enhanced by the L-type Ca2+ channel agonist, Bay K 8644 (0.1-2 mu M). These data indicate that rod photoreceptors possess discrete targeting mechanisms that preferentially sort ion channels mediating I-Cl(Ca) to the terminal.