CGMP, vision

The Gt- and Gg-proteins are also classed as Gi-proteins, based on sequence homologies. The Gt- and Gg-proteins are involved in transmitting sensory signals. Signal transmission in the vision process is mediated via G-proteins known as transducins (Gt). The Gt-proteins are activated by the photoreceptor rhodopsin and are located in the rods and cones of the retina. The sequential effector molecules of the Gt-proteins are cGMP-specific phosphdiesterases (see Fig. 17.9). 

In the vision process, cGMP has the role of regulating Ca influx via cation chaimels. 

Cyclic GMP has a second mode of action in the vertebrate eye it causes ion-specific channels to open in the retinal rod and cone cells. We return to this role of cGMP in the discussion of vision in Section 12.7. 

Figure 6.7 The rhodopsin-transducin cascade resulting in cGMP hydrolysis. R is rho-dopsin R is "activated rhodopsin" is the transducin ffy-subunit complex Ta is the transducin a subunit. Inset A rod cell showing entry of Na+ ions into the outer segment of the cell through cGMP-sensitive channels and existing from the inner segment via an ATPase pump. (Based, in part, on Lolley RN, Lee RH. Cyclic GMP and photoreceptor function. FASEB J 4 3001-3008, 1990, and partly on Zurer PS. Chemistry of vision. Chem Eng News November, 1983, p. 34.)...

In theory, it should be possible to further augment the effect of sildenafil with nitrate drugs, shouldn t it However, this cannot be recommended - it has been tried and resulted in heart attacks. Sildenafil may also cause disturbances of colour vision this is due to the role of cGMP-gated channels in the excitation of the sensory cells of the retina. 

Gti.Gt2 (two genes) Opsins (vision) cGMP phosphodiesterase activation... 

Naturally occurring mutations In the polypeptide sequence of rhodopsin, in cGMP phosphodiesterase (an enzyme used in the rods and cones), and other proteiirs have been shown to be responsible for various types of vision defects and blindness (Nathans ct a ., 1992). 

Atropine is the classic anticholinergic bronchodila-tor. It antagonizes acetylcholine, resulting in reduced intracellular cyclic guanosine monophosphate (cGMP) and smooth muscle relaxation. In horses, the therapeutic index of atropine is narrow and the duration of action is short (0.5-2.0 h). Adverse systemic effects associated with parenteral atropine administration include mydriasis, ileus, dry mucous membranes, blurred vision, excitement and tachycardia. Atropine is not suitable for routine administration to horses with recurrent airway obstruction. 

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