In order to determine the effect of lactogen on insulin secretion and junctional coupling among islet beta cells, ovine prolactin (oPRL) was infused by Alzet minipumps into female rats for 4 days. This treatment produced an oPRL level of 994 ± 122 ng/ml which, combined with residual rat PRL (rPRL) (12 ± 2 ng/ml), represented nearly a 20-fold increase from control (rPRL: 53 ± 17 ng/ml). In addition, plasma insulin was increased nearly 50% (control: 21.9 ± 3 μU/ml; experimental: 30.3 ± 3 μU7ml; p < 0.05). When pancreata from lactogen-treated and control animals were perfused with linear 30-200 mg/dl glucose gradients, the apparent glucose threshold for insulin secretion in the experimental group was nearly 33% lower than that of the controls (i.e., 70 ± 4.6 mg/dl vs. 104 ± 7.5 mg/dl; p < 0.01). The oPRL treatment also increased dye coupling among beta cells. Central cells in islets isolated from lactogen-treated and control animals were injected with Lucifer Yellow CH to estimate the extent of gap junctional coupling. There was nearly a twofold increase in the projected area of dye transfer per injection in the experimental vs. the controls: 4,607 ± 575 μm2vs. 2,302 ± 474 μm2, respectively; p < 0.02. The effects of oPRL decreased the apparent glucose threshold for insulin release, increased the above-threshold glucose-induced insulin secretion, and increased the extent of dye coupling among beta cells. These changes in insulin secretion and dye coupling closely resemble those observed in islets from pregnant rats.
- Insulin secretion
- Junctional coupling