Modulation of guanylate cyclase and phosphodiesterase by monovalent cations and nucleoside triphosphates in light-sensitive excised patches of rod outer segments.
Excised inside-out patches of vertebrate rod outer segment can support phototransduction. I have examined how ionic and metabolic conditions influence the functional properties of light-sensitive patches from Gekko gekko. I find that such patches retain a variable level of basal phosphodiesterase activity, which lowers the cyclic guanosine monophosphate (cGMP) concentration reaching the channels and reduces the dark current. The dose/response relationship for channel opening by cGMP varies among patches and this variability is only reduced by working in darkness with the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX), suggesting that it is only partially due to phosphodiesterase activity. MgATP or MgGTP, but not Mg or ATP separately, increase this activity but a kinase does not appear to be involved. Intracellular monovalent cations also influence dark current intensity and light response kinetics. With 5 mM MgGTP, 1 mM IBMX, and 144 mM Li+, Na+, K+, or Rb+, dark current intensity and recovery time follow the respective sequences K+ > Rb+ > Na+ > Li+ and K+ < Rb+ < Li+ < Na+. Without IBMX, a dark current develops with K+ but not with Na+. These effects are not due to altered channel permeability (P) [PLi+:Na+:K+:Rb+:guanidinium)/PNa+ = 0.84:1.00:1.01:1.09:0.42], or differential Mg2+ block, but to modulation of guanylate cyclase, which overcomes phosphodiesterase when the major cation is K+ but not when it is Na+.
Ertel
E A
EA
Department of Physiology and Biophysics, University of Washington, Seattle 98195.
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002414:Cations, Monovalent / Q000494:pharmacology*; D006152:Cyclic GMP / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000494:pharmacology*; D006162:Guanylate Cyclase / Q000378:metabolism*; D007473:Ion Channels / Q000502:physiology; D008027:Light; D008116:Lizards; D010477:Perfusion; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D012374:Rod Cell Outer Segment / Q000187:drug effects* / Q000378:metabolism* / Q000528:radiation effects; D012964:Sodium / Q000494:pharmacology; D012996:Solutions
D002414:Cations, Monovalent; D007473:Ion Channels; D012996:Solutions; C036852:magnesium GTP; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D012964:Sodium; D010727:Phosphoric Diester Hydrolases; D006162:Guanylate Cyclase; D006152:Cyclic GMP; D011188:Potassium