Article 7

Question 1

how were the samples prepared for the experiments?

Answer 1

retinas were treated with trypsin and shaken gently in a small volume of saline to obtain a solution containing detached rod outer segments (ROS)s and whole photoreceptor cells

Question 2

what was the purpose of the recording electrode?

Answer 2

was used to make contact with the lateral surface of the outer segment of a rod or an isolated ROS to measure membrane conductance

Question 3

how was a patch of membrane excised?

Answer 3

sharp shift of the pipette

Question 4

what effect did cyclic GMP have on the conductance of the membrane patches?

Answer 4

increased conductance in a reversible manner

Question 5

how long did the sensitivity to cGMP last after patch was isolated?

Answer 5

remained constant for 10-15 min

Question 6

did the experiments involve the use of nucleoside triphosphates?

Answer 6

cGMP-dependent increase in conductance occued in absense of these, addition of ATP and GTP had no effect

Question 7

what was the conclusion about the action of GMP on the channels in the membrane?

Answer 7

was found to directly act on the channels, rather than through protein phosphorylation

Question 8

why were single channel activities not detected in the experiment?

Answer 8

were not detected, so the conductance properties of the membrane patches were determined from integral V-C relations

Question 9

how did the presence of CA ions affect the conductance of the system?

Answer 9

had no effect on conductance in absense of cGMP, but cGMP-dependent conductance decreased as [Ca] was reduced

Question 10

what was the purpose of testing the ion selectivity of cGMP-dependent conductance?

Answer 10

to determine whether cGMP induces cationic or anionic conductance

Question 11

how did they measure the reversal potentials for the cGMP dependent current

Answer 11

varied the ionic composition of the media bathing the intracellular surface of the membrane in different patches and compared the data

Question 12

what ion selectivity was observed for the channels controlled by cGMP?

Answer 12

selective for monovalent cations, Na+> Li+ > K+ > Rb + > Cs+ > Cl-

Question 13

how did they express cGMP-induced conductance in a normalized form?

Answer 13

taking conductance at 300um cGMP as unity

Question 14

what was the hill coeff for the dependence of conductance on cGMP concentration?

Answer 14

-1.8, indicating cooperativity in the response to cGMP

Question 15

what effect did ca2+ ions, 3’,5’-cyclic AMP, 2’,3’-cyclic GMP, and 5’ GMP have on the cyclic GMP dependence of conductance?

Answer 15

none had any effect, confirming that cGMP is a specific agent for inducing the membran conductance increase

Question 16

what was the amplitude of the estimated unit event for the cGMP-induced conductance?

Answer 16

-3fA in standard ringer solution at a membrane voltage of -30mv

Question 17

how was the time constant for teh cGMP-dependent currents power density spectrum estimated?

Answer 17

approx 0.5ms, corresponding to a cut-off frequency of 300hz

Question 18

how do they suggest cGMP affects the conductance of the plasma membrane in photoreceptor cells?

Answer 18

cGMP is the messanger modulating the conductance, not ca2+, and that it is fast response time which is essential in photoreceptors

Question 19

what was the significance of the similarities between the propertie of the light dependent conductance of the ROS membrane and the cGMP dependent conductance of the rod membrane patches?

Answer 19

suggest that cGMP activated conductance may be responsible for the light-dependent conductance of the ROS membrane, and this mechanism could be important in photoreceptor function