To survive the nutritional transition at birth, the neonate must have functional pancreatic β cells capable of secreting insulin in response to glucose (Fowden et al. 1982). In newborn horses, regulation of glycaemia is poor compared to other species (Madigan, 1991). Hence, this study examined insulin and proinsulin secretion in newborn foals during the first 10 days after birth when enteral nutrition is being established.

Over a 48 h period at 3 time intervals after birth (Days 1-2, 5-6 and 9-10 post partum), pancreatic β cell responses to glucose (0.5 g/kg, iv, 40 % glucose, Days 1, 5 & 9) or saline (20 ml 0.9 % (w/v) NaCl, Days 2, 6 and 10) were measured in 7 Pony foals. Blood samples were taken from the jugular vein via an indwelling catheter (inserted under local anaesthesia) at 5-15 min intervals for 30 before and 60 min after administration. Foals were muzzled throughout this period. Plasma glucose concentrations were determined enzymatically. Plasma insulin and total proinsulin concentrations were measured by immunoassay validated for equine plasma (Hemmila et al. 1984). Statistical analyses were made by ANOVA and paired and unpaired t tests. Mean ± S.E.M. are presented throughout.

Basal plasma glucose and insulin concentrations did not alter significantly between Days 1 and 10, whereas basal proinsulin levels increased significantly from 7.0 ± 1.1 pmol/l on Day 1 to 14.9 ± 3.3 pmo/l on day 5 (P < 0.05, n = 7) and then remained stable. Glucose administration evoked β cell responses at all ages studied. The maximum increment in plasma insulin in response to glucose did not vary with age. In contrast, the maximum proinsulin increment increased significantly from 5.7 ± pmol/l on Day 1 to 12.4 ± 2.9 pmol/l on Day 9 (P < 0.05, n = 7). The maximum increment in plasma glucose was similar at the 3 ages but the half time for glucose disappearance was greater on Day 1 than at older ages (P < 0.004). Proinsulin and glucose, but not insulin, levels fell significantly in response to saline administration at all 3 ages. When the data from each time period were combined, there were significant linear correlations between the plasma insulin and proinsulin concentrations (Days 1-2 y = 0.043x + 3.679; Days 5-6 y = 0.081x + 2.562; Days 9-10 y = 0.079x + 5.177). The slope of this relationship was significantly shallower on Days 1-2 than at Days 5-6 or Days 9-10 (P < 0.05).

These results demonstrate that pancreatic β cells of the neonatal foal undergo developmental changes during the first 10 days of life as suckling is established. The ontogenic increase in proinsulin secretion suggests that there are either changes in insulin processing or an increased demand for insulin with postnatal age.

This work was supported by the Horserace Betting Levy Board.