Injection of guanosine and adenosine nucleotides into Limulus ventral photoreceptor cells

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Abstract
Several nucleotides and nucleotide analogs had striking effects when pressure-injected into Limulus ventral photoreceptor cells. The poorly hydrolysable GTP analoges guanosine 5''-O-(3-thiotriphosphate) (GTP.gamma.S), guanylyl imidodiphosphate (Gpp[NH]p) and guanylyl (.beta.,.gamma. methylene) diphosphonate (Gpp[CH2]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 analog ATP.gamma.S had effects similar to those of the GTP analogs. GTP.gamma.S, Gpp[NH]p, Gpp[CH2]p and ATP.gamma.S act at a common site to cause a light-dependent, long-term activation of the excitation mechanism of the photoreceptor. 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 sulfonic 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. GTP injection induces changes of membrane conductance either in addition to, or different from, the light-induced change of membrane conductance. Injection of the ATP analog adenylyl imidodiphosphate (App[NH]p), and the pyrophosphate analog 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 Ca ions in the bathing medium was reduced. App[NH]p and p[NH]p injections probably act by increasing the cytoplasmic concentration of Ca ions.