The regulation of DNA replication by growth hormone and the production of somatomedin C/insulin-like growth factor I (SM-C/IGF-I) and insulin by fetal rat islets in culture has been studied. Islets were cultured for 3 days in medium containing 2.7 or 16.7 mM glucose, various concentrations of fetal calf serum (FCS), and 100-1000 ng/ml human growth hormone (GH). DNA replication was determined by incorporation of [3H]thymidine into islet DNA; SM-C/IGF-I and insulin secreted into the medium were measured by specific radioimmunoassays.

Glucose caused a twofold stimulation of islet DNA replication in medium containing ≥ 1% FCS but failed to stimulate DNA replication at lower serum concentrations. In the presence of 16.7 mM glucose, GH (100–1000 ng/ml) stimulated DNA replication at all serum concentrations. In medium containing 2.7 mM glucose, GH was stimulatory only in the presence of 1% FCS.

Somatomedin C/IGF-I release into the culture medium could be detected in all experimental groups. Glucose alone did not affect SM-C/IGF-I release, and in serum concentrations <0.1% FCS, GH also failed to increase the release of the peptide. In medium containing 1% FCS and 16.7 mM glucose, 100–1000 ng/ml GH caused a 50-100% increase in SM-C/IGF-I release into the medium.

Addition of 100 ng/ml exogenous SM-C/IGF-I to medium containing 16.7 mM glucose and 0.1–1.0% FCS caused a twofold stimulation of the islet DNA replication. This effect could be abolished by the addition of an antibody to SM-C/IGF-I. This antibody also significantly reduced GH-stimulated DNA replication, although some stimulation remained in its presence.

Glucose stimulated insulin secretion at all serum concentrations. However, with < 1% FCS in the medium, insulin secretion was markedly reduced and could not be stimulated by the addition of GH. In medium containing 1% FCS and 16.7 mM glucose, 1000 ng/ml GH augmented insulin release almost twofold. We conclude that GH, but not glucose, can stimulate islet cell DNA replication by inducing the local production of SM-C/IGF-I. Apparently, however, part of the stimulation of DNA replication by GH is independent of SM-C/IGF-I and may be related to a direct effect of GH on the islets. Whether the insulin production is augmented via the mediation of somatomedins or by a direct effect of GH remains to be resolved.

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