Injection of Guanosine and Adenosine Nucleotides into Limulus Ventral Photoreceptor Cells

1. Several nucleotide and nucleotide analogues had striking effects when pressure-injected into Limulus ventral photoreceptor cells. The poorly hydrolysable GTP analogues guanosine 5'-0-(3-thiotriphosphate) (GTPgammaS), guanylyl imidodiphosphate (Gpp[NH]p) and guanylyl (beta, gamma methylene) diphosphonate (Gpp[CH(2)]p) produced large increases in the frequency of ;discrete events' that were recorded from photoreceptors in darkness. This effect was only observed after the injected cell was exposed to light. Injection of the ATP analogue ATPgammaS had effects similar to those of the GTP analogues.2. We conclude that GTPgammaS, Gpp[NH]p, Gpp[CH(2)]p and ATPgammaS act at a common site to cause a light-dependent, long-term activation of the excitation mechanism of the photoreceptor.3. Injection of GTP or GDP at pH 4.8 was followed by a smooth, transient depolarization that was observed neither when GTP at pH 7.5 was injected nor when ATP, 5'GMP or 2-[N-morpholino] ethane sulphonic acid (MES) were injected at pH 4.8. The reversal potential of the current induced by GTP injection was significantly more positive than the reversal potential of the light-induced current.4. We conclude that GTP injection induces changes of membrane conductance either in addition to, or different from, the light-induced change of membrane conductance.5. Injection of the ATP analogue adenylyl imidodiphosphate (App[NH]p), and the pyrophosphate analogue imidodiphosphate (p[NH]p) produced a drastic decrease in the sensitivity of photoreceptors to light. This decrease in sensitivity was partially reversed when the concentration of calcium ions in the bathing medium was reduced.6. We suggest that App[NH]p and p[NH]p injections act by increasing the cytoplasmic concentration of calcium ions.