Light-mediated cyclic GMP hydrolysis controls important aspects of kinetics of retinal rod voltage response

Brief, intracellularly injected pulses of cyclic GMP transiently depolarized toad retinal rod outer segments (ROS). The depolarization is antagonized by light, perhaps by the activation of phosphodiesterase (PDE), as shown in the biochemical studies of others. As measured by the antagonism of cyclic GMP pulses by light, PDE activity peaks after the peak of the receptor potential and has approximately the same recovery time as the membrane voltage after weak illumination, but recovers more slowly than the membrane potential after strong illumination, as sensitivity does in other preparations. A cyclic GMP pulse delivered just after the hyperpolarizing phase of the receptor potential tends to turn off the light response. The kinetics of recovery from this turnoff are similar to those of the initial phase of the receptor potential. This similarity suggests that the initial phase of the receptor potential is controlled by light-activated PDE. Both EGTA and saturating doses of cyclic GMP block the light response, but only cyclic GMP increases response latency, which suggests that if calcium is involved in transduction, it is controlled by the hydrolysis of cyclic GMP. After brief pulses of cyclic AMP, a new steady state of increased depolarization occasionally develops. The effects described above also occur under these conditions. The results are consistent with the hypothesis that light-activated hydrolysis of cGMP is an intermediary process in transduction.

Download Full-text

Effects of magnesium on cyclic GMP hydrolysis by the bovine retinal rod cyclic GMP phosphodiesterase

Biochemical Journal ◽

10.1042/bj3080653 ◽

1995 ◽

Vol 308 (2) ◽

pp. 653-658 ◽

Cited By ~ 19

Author(s):

D Srivastava ◽

D A Fox ◽

R L Hurwitz

Keyword(s):

Cyclic Gmp ◽

Ternary Complex ◽

Light Response ◽

Competitive Inhibitor ◽

Velocity Versus ◽

Retinal Rod ◽

The Press ◽

Rapid Equilibrium ◽

Kinetics Of ◽

Dissociation Constant Kd

Knowledge of the kinetics of the rod cyclic GMP phosphodiesterase is essential for understanding the kinetics and gain of the light response. Therefore, the interactions between Mg2+, cyclic GMP, and purified, trypsin-activated bovine rod cyclic GMP phosphodiesterase (EC 3.1.4.17) were examined. The effects of Mg2+ and of cyclic GMP on the rod phosphodiesterase activity were mutually concentration-dependent. Formation of a free Mg-cyclic GMP complex is unlikely due to its high dissociation constant (Kd = 19 mM). Plots of 1/velocity versus 1/[cyclic GMP] as a function of [Mg2+] and 1/velocity versus 1/[Mg2+] as a function of [cyclic GMP] intersected to the left of the 1/velocity axis. This is consistent with the formation of a ternary complex between the phosphodiesterase, Mg2+, and cyclic GMP. A competitive inhibitor of the phosphodiesterase relative to cyclic GMP, 3-isobutyl-1-methylxanthine, non-competitively inhibited the enzyme relative to Mg2+, Pb2+, a competitive inhibitor of the phosphodiesterase relative to Mg2+ [D. Srivastava, R.L. Hurwitz and D. A. Fox (1995) Toxicol. Appl. Pharmacol, in the press] non-competitively inhibited the enzyme relative to cyclic GMP. Collectively these results are suggestive of a rapid equilibrium random binding order of Mg2+ and cyclic GMP to the rod phosphodiesterase.

Download Full-text

Extracellular ATP selectively modulates a high-voltage-activated calcium conductance in salivary gland cells of the leech Haementeria ghilianii

Journal of Experimental Biology ◽

10.1242/jeb.181.1.313 ◽

1993 ◽

Vol 181 (1) ◽

pp. 313-319

Author(s):

WA Wuttke ◽

MS Berry

Keyword(s):

Salivary Gland ◽

High Voltage ◽

Cyclic Gmp ◽

Cyclic Nucleotides ◽

Extracellular Atp ◽

Salivary Gland Cells ◽

Selective Modulation ◽

Voltage Dependent ◽

Kinetics Of ◽

Higher Doses

Extracellular ATP appears to have a widespread role as a neurotransmitter or neuromodulator in mammals (Gordon, 1986; Burnstock, 1990), but little is known about any similar functions in invertebrates. During studies of the effects of cyclic nucleotides on electrically excitable salivary cells of the leech, we found that cyclic GMP produced a rapid (less than 1min) reduction of spike duration, suggesting an extracellular effect (Wuttke and Berry, 1991). We now show that micromolar concentrations of ATP (and higher doses of other nucleotides) also reduce spike duration, and that this is caused by depression of a specific voltage-dependent Ca2+ conductance. Selective modulation of Ca2+ current by external ATP has rarely been found, and the effect is also unusual because it changes the kinetics of inactivation rather than those of activation.

Download Full-text