Presynaptic modulation by VIP, secretin and isoproterenol of somatostatin release from enriched enteric synaptosomes: role of cAMP

Abstract

The release of somatostatin-like immunoreactivity was studied in isolated enteric synaptosomes. A significant release of somatostatin-like immunoreactivity was observed in the presence of vasoactive intestinal polypeptide (VIP) (10 −6 M: 53.0 ± 12.4 pg/mg, basal: 14.3 ± 1.7 pg/mg, n = 5, P < 0.05), secretin (10 −6 M: 56.1 ± 3.8 pg/mg, basal: 25.8 ± 1.6 pg/mg, n = 6, P < 0.01) and isoproterenol (10 −5 M: 54.0 ± 13.4 pg/mg, basal: 20.0 ± 3.4 pg/mg, n = 8, P < 0.05). Forskolin, an unspecific activator of the adenylate cyclase, caused a significant release of somatostatin-like immunoreactivity (10 −6 M: 57.3 ± 13.2 pg/mg, basal: 30.0 ± 5.8 pg/mg, n = 13, P < 0.01) which was further augmented in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX 10 −4 M) (77.0 ± 17.8 pg/mg, n = 13, P < 0.01). 3-Isobutyl-1-methylxanthine and N 6,2′- O-dibutyryladenosine-3′5′-cyclic monophosphate mimicked the effect of forskolin and VIP. The release of somatostatin was paralleled by an increase of cAMP immunoreactivity in the presence of VIP (10 −6 M: 37.1 ± 9.4 pmol/mg, basal: 19.8 ± 4.2 pmol/mg, n = 10, P < 0.05), isoproterenol (10 −5 M: 42.4 ± 9.8 pmol/mg, basal: 16.7 ± 2.4 pmol/mg, n = 12, P < 0.01) and forskolin (10 −6 M: 47.1 ± 12.4 pmol/mg, basal: 19.8 ± 4.2 pmol/mg, n = 10, P < 0.01). The effect of nitric oxide (NO) which acts as an inhibitory neurotransmitter in the enteric nervous system was studied. NO is known to activate soluble guanylate cyclase to induce transmitter release. The NO-generating compound sodium nitroprusside and bromoguanosine-3′,5′-cyclic monophosphate (8-Br-cGMP) had no effect on the release of somatostatin-like immunoreactivity. These data demonstrate the stimulatory effect of VIP, secretin and isoproterenol on release of somatostatin-like immunoreactivity from enteric synaptosomes, which is presumably mediated by cAMP-dependent mechanisms. cGMP-dependent mechanisms seem to be of minor relevance.