Abstract
The effects of VIP and related-peptides (PHI, secretin, glucagon) on cyclic AMP formation were investigated in intact pieces of rabbit retina. VIP and PHI increased cyclic AMP levels with EC
50 of 160 nM and 300 nM respectively. At 5 μM the peptides increased cyclic AMP 46 fold (VIP) and 38 fold (PHI). Secretin was much less potent and glucagon was totally inactive. VIP was also tested for its effects on glycogen levels under similar experimental conditions. In contrast to its pronounced glycogenolytic action in mouse cerebral cortical slices, VIP at 1 μM decreased only moderately (38.3%)
3H-glycogen newly synthesized from
3H-glucose by pieces of rabbit retina. Furthermore a discrepancy between the efficacy of VIP in increasing cyclic AMP and in promoting glycogenolysis appears to exist. A similar dissociation between these two cellular events was also observed with other neuroactive substances. Thus the pronounced increase in cyclic AMP induced by dopamine and forskolin was accompanied by only a moderate decrease in
3H-glycogen levels. Conversely 50 mM potassium induced a 79.9% decrease in
3H-glycogen levels without any significant increase in cyclic AMP.