RELEASE OF SOMATOSTATIN-LIKE IMMUNOREACTIVITY FROM ENRICHED ENTERIC NERVE VARICOSITIES OF RAT ILEUM

Synaptosomes were isolated from rat ileum by various steps of differen tial centrifugation. The peptide content for somatostatin-like immunor eactivity was used as marker for neuronal membranes. The enriched syna ptosomal fraction (P2) showed a good enrichment of somatostatin conten t (4-fold) in comparison to the post-nuclear supernatant. The basal re lease of somatostatin-like immunoreactivity was 26 +/- 3 pg/mg tissue protein. KCl-evoked depolarization (65 mM) caused a significant increa se of somatostatin-like immunoreactivity release (72 +/- 11 pg/mg, n = 12, P < 0.001) compared to basal release. In Ca2+-free medium the evo ked release of somatostatin-like immunoreactivity was abolished. A sub stantial increase of somatostatin-like immunoreactivity release (52 +/ - 7 pg/mg, n = 12, P < 0.05) was also observed in the presence of the Ca2+ ionophore A-23187. The cholinergic agonist carbachol elicited a d ose-dependent release of somatostatin-like immunoreactivity (10(-7) M: 54 +/- 8 pg/mg, 10(-6) M: 63 +/- 6 pg/mg, 10(-5) M: 53 +/- 5 pg/mg, n = 12, P < 0.001), which was blocked by atropine (10(-6) M: 35 +/- 6 p g/mg, n = 12, P < 0.001), but not by hexamethonium. Other presynaptic modulating substances such as serotonin, the selective neurokinin-B ag onist [beta Asp(4),MePhe(7)]neurokinin B-(4-10), neurotensin, cholecys tokinin-8, caerulein and pentagastrin had no stimulatory effect on rel ease of somatostatin-like immunoreactivity. In summary, somatostatin-l ike immunoreactivity can be released from enteric synaptosomes by both depolarization with KCI and cholinergic stimulation via a muscarinic mechanism. The synaptosomes of intrinsic nerves offer an approach to s tudy release of neuronal somatostatin on the subcellular level.