1 Quiz 03 Results Question 1: Multiple Choice Average Score 0.40506 points Imagine a light-sensitive (photoreceptor) cell in the eye of a particular invertebrate. The cell membrane contains cGMP-gated K+ channels. In the dark, this cell has low levels of cGMP in its cytoplasm, such that the cGMP-gated K+ channels are in the closed state. When light strikes the cell, a sequence of events is triggered that results in the activation of guanylyl cyclase. Which one of the following is most likely to happen when the cell, which had been in the dark, is suddenly illuminated? Correct Percent Answered The membrane potential of the cell will become less negative (depolarization). 6.962% K+ ions will flow out of the cell through the activated K+ channels. 81.013% K+ ions will flow out of the cell but Na+ ions will flow in at a greater rate, resulting in a net inward current. 9.494% Insufficient information is given to answer this question. 2.532% Unanswered 0% Question 2: Multiple Choice Average Score 0.41139 points Imagine that you treat a typical neuron with a drug that blocks Cl- channels, and you observe that the cell depolarizes slightly. Which one of the following statements about the chloride equilibrium potential (ECl) is likely to be true? Correct Percent Answered ECl is the same as the resting potential. 2.532% ECl is more positive than the resting potential. 14.557% ECl is more negative than the resting potential. 82.278% ECl is the same as the potassium equilibrium potential. 0% Unanswered 0.633% Question 3: Multiple Choice Average Score 0.4019 points Years ago, when I was a graduate student at UCLA (sorry), I helped lead a field trip for an invertebrate physiology class I was TAing. This was a 3-day camping trip to the intertidal rocks and mudflat at Puerto Penasco, Mexico. Field trips were fun back then! Our professor, who worked on bioluminescent animals, took us out at low tide on a moonless night to search for bioluminescent species. We were armed with squirt guns filled with 0.5 M KCl. Why would squirting 0.5 M KCl on the rocks help us find our quarry? In case you were wondering, the osmolarity of seawater is about 1,000 mOsM. Correct Percent Answered The hypoosmolarity of the KCl solution stimulated the animals’ nervous systems, leading them to produce light and thus become visible. 1.266%2 The composition of the squirted solution didn’t matter because it was the mechanical impact of the stream that stimulated the animals to luminesce. Squirting seawater would probably have worked just as well. 1.266% Squirting KCl onto an animal lowered Na+ concentration in its immediate environment, which shifted ENa (the sodium equilibrium potential) in a positive direction, leading to depolarization and stimulation of the nervous system. 17.089% When a luminescent animal was squirted, the local increase in K+ concentration shifted EK (the potassium equilibrium potential) in a positive direction and, given their typically high K+ permeability at rest, this depolarized and excited neurons. 80.38% Unanswered 0% Question 4: Essay Average Score 0 points Take another look at the slide entitled “NaCl and H2O Transport in the Lung (& Colon)” from the Membrane Dynamics lecture. Imagine that the cell shown is in the epithelium lining the intestine and the space above the apical membrane is the lumen of the intestine. All other mechanisms are the same. Now, consider these two facts: a) The alpha-subunit of the G-protein trimer has an intrinsic GTPase activity, which is important in terminating the action of G-protein coupled systems. b) The CFTR is activated by cAMP. Here’s the question: If you introduced a toxin into the intestinal cell that blocks the intrinsic GTPase activity of a stimulatory G-protein (Gs), what would be the most significant effect on intestinal function? Briefly explain the mechanism underlying your answer, including the most important steps in the signal transduction pathway. Given Answers If a toxin that blocks the intrinsic GTPase activity of a stimulatory G-protein was introduced into the intestinal cell, the G protein pathway that converts ATP to cAMP would not be able to inactivate and cAMP would be continually produced. cAMP activates the CFTR so this chloride ion channel would be indefinitely open for Cl- to diffuse into the intestinal lumen. This subsequently higher ion concentration in the lumen would lead to more water entering into the lumen through osmosis via aquaporins. This could lead to dehydration and diarrhea. The diarrhea resulting from cholera is produced by a similar increase in cAMP concentration. If the GTPase activity of the G-protein was blocked, the pathway would no longer have its terminating action. Therefore, the entire pathway would be overactivated. Overactivation of the G-protein coupled system would lead to excess amounts of cAMP, and - as stated in point (b) - cAMP activates the CFTR. Excess cAMP would likely cause excess CFTR activation which would then allow excess chloride to exist the cell. When chloride exits the cell, sodium will follow due to the electrical potential difference. When these ions exit the cell in excess, they create an osmotic gradient which then drives water across the apical membrane. Overactivation of CFTRs will cause excess water to enter the lumen of the intestine. This would have effects remarkably similar to those of cholera: diarrhea and extreme water loss leading to dehydration and possibly death. If there was a toxin in the intestinal cell that blocked the GTPase activity of a stimulatory G-protein, ion channels in the affected area would not close (G-protein coupled systems could not terminate because the GTPase would not remove the GTP from the alpha-subunit, leaving the system in the "activated" state, where the GCPR is separated from the alpha-subunit). The activated GPCR converts ATP to cAMP, which, as stated above, activates the CFTR. This would lead to an oversecretion of the Cl- ions, and H20 and Na+3 would follow the Cl-. This would result in dehydration and excessive loss of electrolytes through the mucus of the intestine. A GTP molecule activates cAMP, which then activates the CFTR channel through phosphorylation. If a toxin inhibits the GTPase activity of the G-protein, GTP will never get hydrolyzed and cAMP will remain active. This will result in Cl- being constantly pumped into the lumen of the