Change and the muslim world

viii, 46 hlm.; ilus.; 30 cm

Mechanisms controlling ion channel trafficking in <i>S. macrurus</i> electrocytes.

<p>Ion channel proteins are synthesized in the endoplasmic reticulum and then further processed and inserted into vesicles in the Golgi apparatus. Delayed rectifier potassium channels undergo exocytosis to the cell surface without subsequent endocytosis. Inward rectifier channels and Na⁺ channels are constitutively cycled into and out of the membrane. This process is accelerated when the melanocortin peptide hormone ACTH activates a G-protein coupled melanocortin receptor. The receptor initiates a signaling cascade that elevates cAMP and activates PKA. PKA then upregulates the exocytosis of channels into the membrane increasing the number of Na⁺ and inward rectifier channels present in the electrocyte membrane, thereby increasing the magnitude of both conductances. For simplicity we have illustrated Na⁺ and inward rectifier channels as being in the same vesicles, although we do not know whether they are in the same or different vesicles.</p

Disrupting vesicular trafficking prevents the ACTH-induced increase in the inward rectifier potassium current (IK_IR) but has no effect on the delayed rectifier current (IK_DR).

<p>(A,B) CQ and NEM prevented the ACTH-induced increase in IK_IR magnitude. I-V curves depict normalized steady-state IK at baseline and after 30 min exposure to CQ or NEM, then 30 minutes after addition of ACTH. Potassium currents are isolated in all conditions by blocking INa with 1 µM TTX. The apparent increase in DR current with NEM treatment plus ACTH is not statistically significant when compared at individual membrane voltages. Insets: summary of peak IK_IR at baseline and after 30 min exposure to CQ or NEM, then 30 min after addition of ACTH. Both compounds produced a decrease in current magnitude. Asterisks indicate conditions significantly different from baseline by Tukey's HSD following significant omnibus repeated measures ANOVA (for CQ <i>F</i>_[2,4,8] = 26.6, <i>p</i><0.001; NEM <i>F</i>_[2,5,10] = 6.00, <i>p</i><0.05). (C) The magnitude of IK_DR at peak current is not changed by CQ or CQ with ACTH (<i>F</i>_[2,4,8] = 2.76, <i>p</i>>0.1). (D) The magnitude of IK_DR at peak current was not changed in the presence of NEM or NEM with ACTH (<i>F</i>_[2,5,10] = 0.12, <i>p</i>>0.8).</p

Disrupting vesicular trafficking prevents the ACTH-induced increase in INa.

<p>(A–C) CQ and NEM prevented the ACTH-induced increase in INa magnitude, whereas BFA did not prevent the ACTH-induced current enhancement. Potassium currents were blocked in all conditions with 60 mM TEA. I-V curves depict normalized INa at baseline and after 30 min exposure to CQ, NEM, or BFA, then 30 min after addition of ACTH. Insets: summary of peak INa at baseline and after 30 min exposure to CQ, NEM, or BFA, then 30 min after addition of ACTH. Asterisks indicate conditions significantly different from other conditions by Tukey's HSD following significant omnibus repeated measures ANOVA (for CQ <i>F</i>_[2,4,8] = 17.13, <i>p</i><0.01; NEM <i>F</i>_[2,4,8] = 17.28, <i>p</i><0.01; BFA <i>F</i>_[2,5,10] = 16.16, <i>p</i><0.001).</p

Circadian and social cues increase EOD amplitude.

<p>(A) Experimental tank used to record calibrated EODs of free-swimming fish. EODs were digitized from nichrome recording electrodes at the ends of the tank only when circuitry detected that the fish was centered within an unglazed ceramic tube, equidistant from the recording electrodes at the ends of the tank. ADC, analog-to-digital-converter. (B) EOD amplitudes of a representative fish recorded approximately every 60 s over 3 d. Signal amplitude shows a clear day-night rhythm increasing to maximum during lights-out and decreasing to a minimum at midday. Inset: superimposed EOD waveforms taken from the same fish at nighttime maximum and daytime minimum. (C) EOD amplitudes were significantly higher at nighttime peak than at daytime minimum (<i>n</i> = 8, <i>t</i> = 3.91, df = 7, <i>p</i><0.01). Bars show means, and error bars indicate SEM. (D) Adding a second fish into the center compartment for 1 h at midday (arrowheads) caused transient increases in EOD amplitudes of four fish. All voltages are referenced to a point 10 cm from the center of a 5 cm calibration dipole <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000203#pbio.1000203-Franchina1" target="_blank">[38]</a>.</p

Rundown of sodium current and washout of CQ effect.

<p>(A) Decrease in peak Na⁺ current following 30 min exposure to Saline, CQ, NEM, and BFA. (B) Washing out CQ while maintaining ACTH in the bath saline led to increased INa magnitude (<i>n</i> = 4). (C) Washing out CQ with normal saline for 60 min did not produce recovery of INa, but subsequent addition of ACTH increased INa magnitude (<i>n</i> = 4).</p

ACTH increases the inward rectifier potassium current (IK_IR) via a cAMP/PKA pathway; the delayed rectifier current (IK_DR) is stable across all conditions.

<p>(A) Representative family of electrocyte potassium currents recorded with INa blocked by 1 µM TTX. To generate the IV curves in Panel B, current amplitude was measured at steady state (open square in A). (B) Normalized IV curves at baseline and after 20 min exposure to saline (control) or ACTH. (C) Both ACTH and the cAMP analog 8-Br-cAMP increase the magnitude of IK_IR. The PKA blocker H89 had no effect alone, but pretreatment with H89 blocked the ACTH- and 8-Br-cAMP-induced increase in IK_IR magnitude. Asterisks indicate conditions different from saline controls (ANOVA <i>F</i>_{[5, 34]} = 16.22, <i>p</i><0.0001; pairwise comparisons by Tukey's HSD). (D) The delayed rectifier was stable across all experimental conditions (ANOVA <i>F</i>_{[5, 34]} <1, <i>p</i>>0.5).</p

Generation of the EOD.

<p>(A) The EOD is produced by the coordinated APs of the electric organ cells, called electrocytes. A medullary pacemaker nucleus controls the electrocyte APs via spinal electromotor neurons which innervate the electrocytes. (B) Electrocytes are innervated on the posterior end of the cell, where the spinal nerve forms a large cholinergic synapse. The electrically excitable region of the cell membrane, populated by Na⁺ and K⁺ channels, is localized to the posterior most region of the cell, extending approximately 150 µm toward the anterior of the cell. The remainder of the cell membrane is electrically passive. APs in the electrocytes cause current to move along the rostral-caudal body axis and out into the surrounding water. (C) A section of electric organ from the tail, with skin removed to expose the electrocytes, which are densely packed within the electric organ. A single electrocyte is outlined in red. (D) The EOD waveform recorded from <i>S. macrurus</i> is a sinusoidal wave emitted at a steady frequency by each fish. The EOD frequency among fish has a range of approximately 70 to 150 Hz.</p

ACTH increases EOD amplitude in vivo and electrocyte AP amplitude in vitro.

<p>(A) Injections of ACTH increased EOD amplitude whereas saline injections have little or no effect. Plots show EODs recorded from two fish that received counterbalanced injections of saline or ACTH at midday. Inset: superimposed representative EOD waveforms taken at baseline and 1 h after ACTH injection. (B, C) Percentage increase in EOD amplitude and half width following injections of saline or ACTH injections. Bars show means, and error bars indicate SEM. (D) Representative traces of APs recorded at baseline and after 20 min exposure to saline (control) or ACTH. (E) In cells exposed to ACTH, AP amplitude and half width increased compared to saline controls. Both experimental and control cells showed slight but similar increases in input resistance (unpublished data). Asterisks indicate conditions different from saline control (unpaired <i>t</i> test, <i>p</i><0.05). (F) Time course of ACTH-induced increase in AP amplitude.</p