Adaptation of pancreatic islet B-cells during the last third of pregnancy: regulation of B-cell function and proliferation by lactogenic hormones in rats.

In rodents, placental lactogen (PL)-I is considered to be the first trigger to enhance pancreatic islet B-cell function, and after its secretion is diminished at mid-pregnancy, PL-II takes over this role. However, little information is available on the regulation of islet B-cell function and proliferation by lactogenic hormones during the last third of pregnancy. This was the focus of the present study using rats in which pregnancy was forcibly prolonged. This rat possesses unique characteristics in that PL-I is re-secreted during the prolonged period of pregnancy and the peak concentrations in maternal circulation are comparable with those observed during mid-pregnancy in normal-pregnancy rats. Pregnancy was prolonged by successive administration of pregnant mare's serum gonadotropin (30IU/rat, s.c. on day 12) and human chorionic gonadotropin (10IU/rat, i.v. on day 14). When the insulin secretory responses to 10mmol/l glucose in islets obtained from normal-pregnancy and prolonged-pregnancy rats were tested, each insulin secretory response correlated well with the values of plasma lactogenic activity throughout the period of pregnancy and lactation. Examination of B-cell proliferation in normal-pregnancy rats showed that 5-bromo-2'-deoxyuridine (BrdU) incorporation into dividing B-cells reached a maximum on day 15 and then decreased markedly towards term. No increase in B-cell proliferation was observed on day 19 when plasma lactogenic activity reached the maximum. In prolonged-pregnancy rats, BrdU incorporation also continued to decrease as observed in normal-pregnancy rats after day 15, and then no enhancement in B-cell proliferation was observed even when the plasma lactogenic activity, including re-secreted PL-I, reached maximum. These results suggest that, in the last third of pregnancy, B-cell proliferation is no longer stimulated by lactogenic hormones in contrast to the insulin secretory response which is sustained.