GTP-regulatory proteins, in particular transducin, mediate light- induced stimulation of phospholipase A2 and C (PLA2, PLC) in rod outer segments (ROS) of bovine retina. Studies using G protein-specific agents, cholera toxin and pertussis toxin, in both light and dark-adapted ROS suggest a dual role for G proteins in both activation and inhibition of PLA2 and PLC. Beta-gamma subunits of transducin stimulated PLA2 while alpha subunits were inhibitory. Both alpha and beta-gamma subunits of transducin stimulated PLC, possibly through the inhibition of a inhibitory G protein. In addition to light, stimulation of PLA2 and PLC by other retinal neurotransmitters, in particular dopamine and somatostatin has been characterized. Isolated alpha subunits from Gs, Gi, Go and transducin were phosphorylated in vitro by cAMP-dependent protein kinase and protein kinase C. Phosphorylated subunits were altered in their ability to stimulate phospholipase activity and this may represent a biochemical mechanism of regulation of G protein- mediated pathways. In retina from Xenopus laevis, both somatostatin and dopamine inhibit the circadian rise in N-acetyl transferase (NAT), the rate limiting enzyme in melatonin synthesis. These effects are mediated by G proteins although through different transduction mechanisms. Somatostatin, acts through a non-cAMP dependent mechanism. Receptors for somatostatin are regulated by Na+ and GTP and co-localize with G proteins, in particular Go, by autoradiographic and immunocytochemical analyses respectively. In addition, the ability to immunocytochemically detect G protein subunits is markedly altered during peak periods of circadian activity. Activation of PLA2 and PLC by bradykinin has been shown to occur through distinct G proteins in Swiss 3T3 cells. Activation of PLA2 occurs via a pertussis toxin-insensitive G protein. Evidence to support the existence of two distinct bradykinin receptor subtypes coupled to the activation of phospholipases was obtained.
