.SUMMARY 1. Solitary rod inner segments were obtained by enzymatic dissociation of the tiger salamander (Ambystoma tigrinum) retina. Their membrane currents were studied with the single-pipette voltage-clamp technique. Individual currents were isolated with the aid of pharmacological agents.2. Extracellular caesium blocked a current activated by hyperpolarization from -30 mV. Changing external sodium and potassium concentrations altered the value of the reversal potential in a manner consistent with the current being carried equally by both ions.3. 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 potassium concentration altered the value of the reversal potential in a manner consistent with the current being carried predominantly by potassium.4. Extracellular cobalt blocked a current activated by depolarization that had an initial inward and a later outward component. 5. After EGTA was injected into an inner segment the outward component was suppressed. Cobalt then blocked an inward current. This current is believed to be carried predominantly by calcium. The conductance increased with depolarization from -45 mV and reached a maximum at approximately 0 mV. Following a step of depolarization the current activated rapidly (< 20 msec) and then remained constant for at least several seconds without evidence of inactivation.6. Injecting caesium into an inner segment eliminated a calcium-activated outward current believed to be carried by potassium ions.7. After the injection of caesium there remained another calcium-activated current with a reversal potential of -17 mV. Changing extracellular chloride concentration altered the value of the reversal potential in a manner consistent with chloride carrying at least 70 % of the current. Another anion may carry the balance.8. When the five currents mentioned in items 2, 3, 5, 6 and 7 were blocked, the C. R. BADER, D. BERTRAND AND E. A. SCHWARTZ membrane resistance between -90 and -25 mV was linear, time-independent, and had a high value (2-1 Ge).9. The five identified currents can all be activated in the physiological range of voltage in which salamander rods normally operate.

Mechanisms underlying Ca 2؉ signaling during human myoblast terminal differentiation were studied using cell cultures. We found that T-type Ca 2؉ channels (T-channels) are expressed in myoblasts just before fusion. Their inhibition by amiloride or Ni 2؉ suppresses fusion and prevents an intracellular Ca 2؉ concentration increase normally observed at the onset of fusion. The use of antisense oligonucleotides indicates that the functional T-channels are formed by ␣1H subunits. At hyperpolarized potentials, these channels allow a window current sufficient to increase [Ca 2؉ ]i. As hyperpolarization is a prerequisite to myoblast fusion, we conclude that the Ca 2؉ signal required for fusion is produced when the resting potential enters the T-channel window. A similar mechanism could operate in other cell types of which differentiation implicates membrane hyperpolarization.

1. Single, isolated, rod photoreceptors were obtained by enzymatic dissociation of the tiger salamander (Ambystoma tigrinum) retina. These solitary cells retained the morphological features of rods of the intact retina and could be maintained in culture for several days. Solitary cells were penetrated with one or two micropipettes and their electrophysiology was studied by the voltage-clamp technique. 2. Intracellular recording with two micropipettes demonstrated that the inner segment of a solitary rod was effectively isopotential with the outer segment. 3. The time course of the voltage response to a flash resembled that of responses observed in rods in the intact retina. At low light intensities the response reached a peak in approximately 0.7 sec and then slowly declined. At high light intensities the time to peak response decreased and an initial transient arose as the response, after reaching the peak, quickly decreased to a less polarized plateau. 4. The normal voltage response could be compared with the current observed during a voltage clamp. At low light intensities the time course of the current response resembled the time course of the voltage response. When light intensity was increased the time course of the current response differed from the voltage response in that the time to peak amplitude remained relatively constant and an initial transient did not occur. It was possible to predict the current response produced by any intensity of light by using (i) an empirical equation which reproduced the time course of a dim response and (ii) the Michaelis-Menten equation. 5. The time course of the voltage-clamp current produced by a flash was the same at different values of maintained voltage. 6. The maximum amplitude of the voltage-clamp current produced by a flash or step of light was a non-linear function of membrane potential. It was relatively constant within the physiological range, decreased as the membrane potential was moved toward 0 mV, reversed polarity between 0 and 10 mV, and rapidly increased in magnitude as membrane potential was made more positive. Although this current was voltage dependent, no time dependence was evident (recording resolution greater than or equal to 5 msec). 7. Voltage-clamp experiments demonstrated an inward current which slowly developed after a hyperpolarizing voltage step. The effect of this voltage and time dependent current was to reduce, after a delay, the polarization initiated by light.

A newly identified splice variant of STIM1 called STIM1L forms constitutive clusters that interact with actin and Orai1 and allows fast repetitive Ca2+ release.