Voltage‐activated and calcium‐activated currents studied in solitary rod inner segments from the salamander retina

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Abstract
Solitary rod inner segments were obtained by enzymatic dissociation of the tiger salamander (A. tigrinum) retina. Their membrane currents were studied with the single-pipette voltage-clamp technique. Individual currents were isolated with the aid of pharmacological agents. Extracellular Cs blocked a current activated by hyperpolarization from -30 mV. Changing external Na and K concentrations altered the value of the reversal potential in a manner consistent with the current being carried equally by both ions. Extracellular tetraethylammonium (TEA) blocked a current activated by depolarization from -70 mV. In normal medium this current had a reversal potential of -72 mV. Changing the external K concentration altered the value of the reversal potential in a manner consistent with the current being carried predominantly by K. Extracellular Co blocked a current activated by depolarization that had an initial inward and a later outward component. After EGTA [ethylene glycol bis(.beta.-amino-ethyl ether)-N,N,N'',N''-tetraacetic acid] was injected into an inner segment the outward component was suppressed. Co then blocked an inward current. This current is believed to be carried predominantly by Ca. The conductance increased with depolarization from -45 mV and reached a maximum at .apprx. 0 mV. Following a step of depolarization the current activated rapidly (< 20 ms) and then remained constant for at least several seconds without evidence of inactivation. Injecting Ce into an inner segment eliminated a Ca activated outward current believed to be carried by P+. After the injection of Ce there remained another Ca-activated current with a reversal potential of -17 mV. Changing extracellular Cl concentration altered the value of the reversal potential in a manner consistent with Cl carrying at least 70% of the current. Another anion may carry the balance. When the 5 currents mentioned in items 2, 3, 5, 6 and 7 were blocked, the membrane resistance between -90 and -25 mV was linear, time-independent, and had a high value (2.1 G.OMEGA.). The 5 identified currents can all be activated in the physiological range of voltage in which salamander rods normally operate.