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 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⁺+++. Without IBMX, a dark current develops with K⁺ but not with Na⁺. These effects are not due to altered channel permeability (P) = 0.841.00 1.011.090.42], 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⁺.