.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.

Calcium influx is often an essential intermediate step for the release of neurotransmitter. However, some retinal neurons appear to release transmitter by a mechanism that does not require calcium influx. It was uncertain whether depolarization released calcium from an intracellular store or released transmitter by a mechanism that does not require calcium. The possibility that voltage, and not calcium, can regulate the release of transmitter was studied with pairs of solitary retinal neurons. Horizontal and bipolar cells were isolated from fish retinas and juxtaposed in culture. Communication between them was studied with electrophysiological methods. A horizontal cell released its neurotransmitter, gamma-aminobutyric acid, when depolarized during conditions that buffered the internal calcium concentration and prohibited calcium entry. The speed and amount of material released were sufficient for a contribution to synaptic transmission.

Since a nonmotile, primary (9 + 0) cilium projects from most mammalian kidney epithelial cells into the tubule lumen, where it is exposed to fluid motion, the present study examined primary cilium response to fluid shear stress. The reversible, large-angle bending of the primary cilium upon exposure to fluid shear forces (10(-11)-10(-10) N.m2 = 10(-8)-10(-7) dyn/cm) was characterized in vitro using videomicroscopic side views of PtK1 cells, and the cilium was then mathematically modeled as a cantilevered beam. The flexural rigidity of the primary cilium was calculated to be 3.1 +/- 0.8 x 10(-23) N.m2 with a corrected quadruple integration approach and 1.4-1.6 x 10(-23) N.m2 with the "heavy elastica" theory. Comparison of theoretical profiles to the experimental bending responses of cilia established the validity of the "heavy elastica" model; this model, in turn, was used to predict primary cilium bending behavior under representative conditions in the rat nephron. The results of the study are consistent with the hypothesis that primary cilia serve a mechanosensory function in kidney epithelial cells.

SUMMARY1. Retinas from channel catfish were dissociated and the cells maintained in culture. Horizontal cells that normally receive input from cone photoreceptors were identified. The conductance of the electrical junction formed between a pair of 'cone' horizontal cells was measured by controlling the membrane voltage of each cell with a voltage clamp maintained through either a micropipette or a patch pipette. The two techniques yielded similar results.2. Transjunctional current was measured while transjunctional voltage was stepped to values between+60 mV. The current (measured 5 ms after a step) was proportional to voltage over the range tested. For steps to voltages greater than + 45 mV, the current exhibited a slight time-dependent decline.3. Dopamine decreased junctional conductance in a dose-dependent fashion. A 50% reduction was obtained with 10 nM-dopamine. The D1 agonist fenoldopam (100 nM) also decreased junctional conductance. The uncoupling produced by either agent was rapid and reversible.

1. We have studied exocytosis and endocytosis in the synaptic terminal of salamander rods using a combination of Ca2+ imaging, capacitance measurement and the photolysis of Ca2+ buffers. 2. The average cytoplasmic Ca2+ concentration at the dark resting potential was 2‐4 microM. 3. An average cytoplasmic Ca2+ concentration of 2‐4 microM maintained a high rate of continuous exocytosis and endocytosis. 4. Changes in the rate of exocytosis were followed in less than 0.7 s by compensatory changes in the rate of endocytosis. 5. Vesicle cycling in the rod synapse is specialized for graded transmission and differs from that previously described for synapses that release synchronized bursts of transmitter.