DETECTION OF PARTICULATE GUANYLATE-CYCLASE IN RAT NEUROHYPOPHYSIS AFTER STIMULATION WITH ANF AND BNP - AN ULTRACYTOCHEMICAL STUDY

We investigated the ultracytochemical localization of particulate guan ylate cyclase (GC) in the rat neurohypophysis after activation with ra t atrial natriuretic factor (rANF) or porcine brain natriuretic peptid e (pBNP). Under our experimental conditions, the presence of GC reacti on product indicated that rANF and pBNP were strong activators of part iculate GC since samples incubated in basal conditions without rANF or pBNP did not reveal any GC reaction product. The rANF-stimulated GC w as localized both to pituicytes and to nerve fibers and endings wherea s the pBNP-stimulated GC was present exclusively in nerve fibers and e ndings. Recently, two subtypes of receptors for natriuretic peptides h ave been identified as two isoforms of particulate GC [24,50]. Our dat a indicate that the receptors of the two hormones have a partially dis tinct distribution in the rat neurohypophysis. In pituicytes, GC react ion product was found on plasma membrane of finger-like processes and on the membranes surrounding the lipid droplets. In nerve fibers and e ndings, GC reaction product was associated with intracellular membrane s. This finding suggests that the enzyme could mediate an internal inh ibitory action of these hormones on the release of vasopressin and oxy tocin.