Atrial natriuretic factor increases cyclic GMP and cyclic AMP levels in a directly photosensitive pineal organ

Atrial natriuretic factor (ANF) stimulates accumulation of cyclic GMP in a photosensitive organ, as evidenced for the first time in cultured trout pineals. Stimulation was rapid (within a few min), dose-dependent, and stronger in organs cultured in darkness than in those cultured under light. After 30 min in the dark, (i) cyclic AMP levels were slightly increased at 10^–7 mole/l of ANF, (ii) cyclic GMP and cyclic AMP increased dramatically after inhibition of the phosphodiesterases by isobu-tylmethylxanthine (IBMX), (iii) ANF and IBMX effects were more than additive on cyclic GMP, (iv) pertussis toxin decreased the cyclic GMP response to ANF. These responses were affected by light. The possibility that cyclic GMP might be a second messenger of both light and chemical (ANF) inputs, in pineal photoreceptor cells, is hypothetized.