Comparative Effects of Phosphodiesterase Inhibitors on Detached Rod Outer Segment Function

  • G. Rispoli
  • P. G. Gillespie
  • P. B. Detwiler
Part of the NATO ASI Series book series (NSSA, volume 194)


In vertebrate retinal rods light reduces a standing inward current that flows into the outer segment in darkness. Current is carried primarily by sodium ( 70%) and calcium ( 15%) ions which pass through channels in the outer segment surface membrane that are opened by cGMP (Yau and Baylor, 1989). A photoproduct (Rh *) formed by isomerization of rhodopsin catalyzes GTP-GDP exchange to activate a G protein (transducin, T) which stimulates cGMP-specific phosphodiesterase (PDE) causing cGMP hydrolysis, channel closure and a decrease in dark current. The recovery of current following light exposure involves inactivation of the light stimulated PDE and an increase in cGMP synthesis by activation of guanylate cyclase (Hodgkin and Nunn, 1988; Detwiler et al., 1989). The sequential activation and inactivation of PDE in the onset and recovery of rod light responses has been probed using IBMX, a membrane permeant phosphodiesterase inhibitor (Beavo et al., 1970; Capovilla et al., 1983; Hodgkin and Nunn, 1988). While such studies have provided useful information about the orchestration of transduction IBMX is not an ideal inhibitor. It is not very potent (K i = 1 × 10−5M) which makes it practically impossible to use to fully inhibit light-stimulated PDE. It is also non-specific in that it inhibits many kinds of cyclic nucleotide phosphodiesterases which could potentially complicate the interpretation of results. Assays on isolated enzymes have shown two compounds (dipyridamole and M&B:22,948) selectively inhibit cGMP phosphodiesterases (Beavo, 1988; Gillespie and Beavo, 1989) and are 20 to 100 times more potent than IBMX (K t = 3.8 × 10−7M and 1 × 10−7M respectively). This short paper compares the effects of IBMX, dipyridamole (FitzGerald, 1987) and M&B:22,948 (Broughton et al., 1974) on dark current and light responses recorded from detached rod outer segments during whole-cell voltage clamp. Our results show that there are significant differences in the action of the three inhibitors, which suggests that they may affect more than PDE. In the course of these experiments we also discovered that in nucleoside triphosphate depleted outer segments steps of bright light cause an increase rather than a decrease in dark current. Since these cells contain little or no GTP and dark current is maintained by an exogenous source of cGMP, the production of inward or inverted light responses suggests that steps of strong light can inhibit PDE possibly through a G protein independent pathway.


Dark Current Outer Segment Guanylate Cyclase Light Response Dialysis Solution 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. Beavo, J. A., 1988, Multiple isotymes of cyclic nucleotide phosphodiesterase,in“Advances in Second Messenger and Phosphoprotein Research,” 22:1–38.PubMedGoogle Scholar
  2. Beavo, J. A., Rogers, N. L., Crofford, O. B., Hardman, J. G., Sutherland, E. V., and Newman, E. V., 1970, Effects of xanthine derivatives on lipolysis and on adenosine 3’,5’ monophosphate activity, Mol. Pharm., 6:597–603.Google Scholar
  3. Broughton, B. J., Chaplen, P., Knowles, P., Lunt, E., Pain, D. L., Wooldridge, K. R. H., Ford, R., Marshall, S., Walker, J. L., and Maxwell, D. R., 1974, New inhibitor of reagin-mediated anaphylaxis, Nature, 251:650–652.PubMedCrossRefGoogle Scholar
  4. Capovilla, M., Cervetto, L., and Torre, V., 1983, The effects of phosphodiesterase inhibitors on the electrical activity of toad rods, J. Physiol., 343:277–295.PubMedGoogle Scholar
  5. Detwiler, P. B., Rispoli, G., and Sather, W.A., 1989, Manuscript in preparation.Google Scholar
  6. Fitzgerald, G. A., 1987, Medical intelligence. Drug therapy:dipyridamole, N. Engl. J. Med., 316:1247–1256.PubMedCrossRefGoogle Scholar
  7. Gillespie, P. G., and Beavo, J. A., 1989, (Manuscript submitted).Google Scholar
  8. Goldberg, N. D., Ames, A., III, Grander, J. E., and Walseth, T. F., 1983, Magnitude of increase in retinal cGMP metabolic flux determined by 18O incorporation into nucleotide a-phosphoryls corresponds with intensity of photic stimulation, J. Biol. Chem., 258:9213–9219.PubMedGoogle Scholar
  9. Hodgkin, A. L., and Nunn, B. J., 1988, Control of light-sensitive current in salamander rods, J. Physiol., London, 403:439–471.PubMedGoogle Scholar
  10. Karpen, J. W., Zimmerman, A. L., Stryer, L., and Baylor, D. A., 1988, Gating kinetics of the cyclic GMP-activated channel of retinal rods: flash photolysis and voltage-jump studies, Proc. Natl. Acad. Sci., USA, 85:1287–1291.PubMedCrossRefGoogle Scholar
  11. Koch, K. W., and Stryer, L., 1988, Highly cooperative feedback control of retinal rod guanylate cyclase by calcium ion, Nature, 334:64–66.PubMedCrossRefGoogle Scholar
  12. Rispoli, G., and Detwiler, P. B., 1989, Light adaptation in Gecko rods may involve changes in both the initial and terminal stage of the transduction cascade, Biophys. J., 55:380P.Google Scholar
  13. Rispoli, G., Sather, W. A., and Detwiler, P. B., 1988, Effects of triphosphate nucleotides on the response of detached outer segments to low external Ca++, Biophys. J., 53:390PGoogle Scholar
  14. Sakmann, B., and Neher, E., eds., 1983, “Single channel recording”, Plenum Press, New York.Google Scholar
  15. Sather, W. A., 1988, Phototransduction in detached rod outer segments, Doctoral thesis, University of Washington, Seattle, WA.Google Scholar
  16. Sather, W. A., and Detwiler, P. B., 1987, Intracellular biochemical manipulation of phototransduction in detached outer segments, Proc. Natl. Acad. Sci., USA, 84:9290–9294.PubMedCrossRefGoogle Scholar
  17. Sather, W. A., Rispoli, G., and Detwiler, P.B., 1988, Effect of calcium on light adaptation in detached Gecko rod outer segments, Biophys. J., 53:390P.Google Scholar
  18. Yau, K. W. and Baylor, D. A., 1989, Cyclic GMP-activated conductance of retinal photoreceptor cells, Ann. Rev. Neurosci., 12:289–327.PubMedCrossRefGoogle Scholar

Copyright information

© Plenum Press, New York 1990

Authors and Affiliations

  • G. Rispoli
    • 1
  • P. G. Gillespie
    • 1
    • 2
  • P. B. Detwiler
    • 1
  1. 1.Department of Physiology and BiophysicsUniversity of Washington School of MedicineSeattleUSA
  2. 2.Department of PharmacologyUniversity of Washington School of MedicineSeattleUSA

Personalised recommendations