Pulsatile and diurnal secretion of GH and IGF-I in the chronically catheterized pig fetus.

The ontogeny of GH and IGF-I secretion was investigated in the fetal pig. Pulse studies were performed to describe the pattern of GH release. Twenty-four-hour profiles were recorded to examine possible diurnal variations in these hormones. (I) PULSE STUDIES: Blood samples were obtained at 15-min intervals for 2-h periods from 24 male and 20 female fetuses at various gestational ages (fetal day 89-113; term 113 +/- 1 S.D.). Fetuses revealed a pulsatile GH release. The GH pulse frequency did not vary with gestational age in either sex (0.95 +/- 0.19 pulses/h). In males the GH pulse amplitude decreased with increasing fetal age (r = -0.41; P < 0.02). In female fetuses no significant correlation could be calculated. Mean GH concentrations fell significantly in male fetuses 3 to 4 days before delivery (P < 0.05) and the same tendency was observed in females (P < 0.06). Between fetal days 94 and 98 GH pulse amplitude and GH and IGF-I concentrations were higher in males than in females (P < 0.01, P < 0.001 and P < 0.02 respectively). Fetal IGF-I secretion showed no ontogenetic changes in both sexes. However, maternal IGF-I concentrations increased with progressing gestation (r = 0.46; P < 0.001). (II) 24-H PROFILES: Eight male and four female late-gestational fetuses (fetal days 104-108) were studied. Blood samples were taken at 30-min intervals over 24 h. Dams and fetuses showed an episodic GH secretion over the 24-h period but no diurnal rhythm was observed. Whereas maternal IGF-I secretion was constant, fetal IGF-I release was characterized by marked fluctuations over the 24 h. In half of the fetuses (n = 6) the fluctuations appeared at regular intervals. Again no diurnal rhythm existed. These data demonstrated that: (1) porcine fetal GH secretion is pulsatile and decreases shortly before birth; (2) a sex difference in GH and IGF-I concentrations exists between fetal days 94 and 98, suggesting that IGF-I is at least partially under the control of GH before birth; (3) fetal GH and IGF-I secretion is episodic over 24 h, but does not vary diurnally; and (4) fetal and maternal GH and IGF-I secretion are regulated independently.