The constitutive secretory pathway is a major route for islet amyloid polypeptide secretion in neonatal but not adult rat islet cells

Islet; amyloid polypeptide (IAPP or amylin) is a normal secretory product o f the pancreatic beta-cell that; is cosecreted with insulin and is the majo r constituent of islet amyloid deposits in individuals with type 2 diabetes or insulinomas, We have previously reported that glucose stimulates IAPP, but not insulin secretion, from neonatal rat 3-cells when regulated secreti on is prevented by use of calcium-free media, suggesting that IAPP secretio n occurs via a constitutive secretory pathway. To directly test this hypoth esis, we examined the effects of 2 substances-brefeldin A (BFA) and cyclohe ximide (CHX)-that are predicted to selectively block constitutive secretion on the release of IAPP-like immunoreactivity (IAPP-LI) and immunoreactive insulin (IRI) from neonatal rat islet; cell monolayer cultures, When regula ted release was prevented by use of calcium-free media, glucose-stimulated IAPP-LI release was nearly abolished by blocking constitutive release with 10 mu g/ml BFA (mean +/- SD: 8.7 +/- 7.7 vs, 29.3 +/- 14.3 pmol/l; n = 5; P < 0.05), an inhibitor of constitutive vesicle formation. Similarly, calciu m-independent, glucose-stimulated IAPP-LI secretion was markedly suppressed when new protein synthesis was blocked by administration of 20 mu g/ml CHX (4.6 +/- 2.1 vs. 29.5 +/- 14.0 pmol/l; n = 5; P < 0.005), Secretion of IRI was low in the absence of calcium, and neither BFA nor CHX had any further effect. When calcium was added to the incubation media to allow regulated secretion of both TRI and IAPP-LI, both BFA ( 47.7 +/-: 8.7 vs. 80.7 +/- 10 .3 pmol/l; P < 0.001) and CHX (37.3 +/- 5.8 vs. 73.3 +/- 6.2 pmol/l; n = 5; P < 0.0001) inhibited glucose-stimulated IAPP-LI secretion by 40%, but aga in had no Inhibitory effect on IRI secretion, These data indicate that simi lar to 40% of glucose-stimulated IAPP-LI release occurs via a constitutive secretory pathway in neonatal rat islet cells, By contrast, in adult rat is lets, glucose-stimulated IAPP-LI release was almost abolished in the? absen ce of calcium (86 +/- 3% inhibition; P < 0.05) and unaffected by addition o f BFA (275 +/-28 vs. 205 +/- 89 pmol/l; NS) or CHX (160 +/- 205 +/- 89 pmol /l; NS), suggesting that constitutive secretion of IAPP does not occur in m ature beta-cells. collectively, these data suggest that a significant propo rtion of glucose-stimulated IAPP secretion from neonatal, but not adult, ra t islet cells occurs via a constitutive secretory pathway.