Release of proinsulin from the human fetal beta cell.

beta cells in the human fetal pancreas are immature in that they release little or no insulin in response to nutrients, such as glucose. The aim of this study was to examine further the immaturity of these cells, specifically regarding the storage and release of the precursor of insulin, proinsulin. Explants of human fetal pancreas were cultured in vitro for 3 weeks. Levels of proinsulin remained relatively constant throughout at 0.04 +/- 0.002 (S.E.M.) pmol/mg per day with a molar ratio of proinsulin to insulin of 2.2 +/- 0.11%. This low ratio was slightly greater than that observed in culture medium conditioned by adult human islets (0.3 +/- 0.1%), but similar to that found in acid-ethanol extracts of cultured explants (1.4 +/- 0.3%). Passaging of human fetal pancreas for 3 months in diabetic nude mice, which should have caused some maturation of the fetal beta cell, did not change the proportion of proinsulin present. Culture of explants in the presence of 12-O-tetra-decanoylphorbol-13-acetate resulted in some inhibition of proinsulin release, but much less than that for insulin, so that the molar ratio increased to 15.4 +/- 1.6% from the control 3.5 +/- 0.3%. Static stimulation of cultured explants with 10 mmol Ca2+/l, 10 mmol theophylline/l, and these two agents together caused 15-, 4- and 10-fold enhancement respectively of proinsulin release; glucose, leucine, arginine and KCl had no effect. In contrast, all these agents caused significant insulin release, the last four to a much smaller extent (less than or equal to three fold) than the first three (10-, 19- and 65-fold respectively).(ABSTRACT TRUNCATED AT 250 WORDS)