Interaction of hydrolysis-resistant analogs of cyclic GMP with the phosphodiesterase and light-sensitive channel of retinal rod outer segments.
cGMP opens cation-selective channels when applied to the cytoplasmic side of excised patches of membrane from retinal rod outer segments (ROS). If the light-sensitive channel in intact rods is gated only by cGMP, it should be possible to find a hydrolysis-resistant analog of cGMP that blocks the normal response to light by holding the channel open independent of the degree of illumination. We have studied the interaction of 8-bromo-cGMP (8-Br-cGMP) and the SP and RP phosphorothioate derivatives of cGMP [(Sp)-cGMP[S] and (RP)-cGMP[S]) with the cGMP phosphodiesterase (PDEase) of ROS, the cGMP-sensitive channel of excised ROS patches, and the light-sensitive channel of intact rods. All three analogs were hydrolyzed by PDEase much more slowly than was cGMP. The maximal rates of hydrolysis of 8-Br-cGMP, (SP)-cGMP[S], and (RP)-cGMP[S] were 7.3, 3.7, and less than 0.2 s-1, respectively, compared with 4000 s-1 for cGMP. These analogs are effective competitive inhibitors of the PDEase, with Ki values of 48, 25, and 90 microM, respectively. The nucleotide-activated conductances of excised patches were half-maximal at concentrations of 1.6, 210, and 1200 microM, respectively, compared with 17 microM for cGMP. Thus, 8-Br-cGMP is a highly potent channel agonist. The effects of these analogs on the dark current and photoresponses of intact rod cells were also measured. A suction electrode monitored membrane current across the ROS, while a patch electrode sealed on the inner segment was used to introduce a cGMP analog and to control membrane potential. All three analogs increased the dark current and markedly slowed the response to light flashes. 8-Br-cGMP increased the dark current of the outer segment as much as 48-fold. After the concentration of this analog had risen sufficiently, little of the current could be shut off by light, as expected of a direct effect on the light-sensitive channel of the plasma membrane. These results are consistent with the notions that (i) the light-sensitive channel of rods is controlled solely by the instantaneous concentration of cGMP and (ii) the cGMP-sensitive channel of excised patches is identical to the light-sensitive channel of intact rods.
Zimmerman
A L
AL
Yamanaka
G
G
Eckstein
F
F
Baylor
D A
DA
Stryer
L
L
D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000378:metabolism; D000818:Animals; D002417:Cattle; D002462:Cell Membrane / Q000502:physiology; D006152:Cyclic GMP / Q000031:analogs & derivatives* / Q000494:pharmacology; D004553:Electric Conductivity; D004789:Enzyme Activation / Q000187:drug effects; D006868:Hydrolysis; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000528:radiation effects; D007700:Kinetics; D008027:Light; D008565:Membrane Proteins / Q000378:metabolism; D010786:Photoreceptor Cells / Q000502:physiology*; D012374:Rod Cell Outer Segment / Q000502:physiology*; D013329:Structure-Activity Relationship; D015293:Transducin; D014562:Caudata
D007473:Ion Channels; D008565:Membrane Proteins; C016276:8-bromocyclic GMP; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D015293:Transducin; D006152:Cyclic GMP