ISSN:
1432-2013
Keywords:
Photoreceptors
;
Vertebrates
;
Rods and cones
;
Monovalent cations
;
Guanylate cyclase
;
3′,5′-Cyclic GMP
;
Phosphodiesterase
;
Guanosine triphosphate
;
Adenosine triphosphate
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Abstract 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 fromGekko 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) $$[P_{Li} + : _{Na} + : _K + : _{Rb} + _{: guanidinium)} /P_{Na} +$$ = 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+.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00724521
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