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 p ig. 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 ob tained 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 a ge (r=0.41; P<0.02). In female fetuses no significant correlation coul d be calculated. Mean GH concentrations fell significantly in male fet uses 3 to 4 days before delivery (P<0.05) and the same tendency was ob served in females (P<0.06). Between fetal days 94 and 98 GH pulse ampl itude and GH and IGF-I concentrations were higher in males than in fem ales (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 c oncentrations increased with progressing gestation (r=0.46; P<0.001). (II) 24-h profiles. Eight male and four female late-gestational fetuse s (fetal days 104-108) were studied. Blood samples were taken at 30-mi n intervals over 24 h. Darns and fetuses showed an episodic GH secreti on over the 24-h period but no diurnal rhythm was observed, Whereas ma ternal IGF-I secretion was constant, fetal IGF-I release was character ized by marked fluctuations over the 24 h. In half of the fetuses (n=6 ) the fluctuations appeared at regular intervals. Again no diurnal rhy thm existed. These data demonstrated that: (1) porcine fetal GH secret ion is pulsatile and decreases shortly before birth; (2) a sex differe nce 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 s ecretion are regulated independently.