Lack of Growth Hormone Effect on Insulin-Associated Suppression of Insulinlike Growth Factor Binding Protein 1 in Humans CHERYL A. CONOVER, PETER C. BUTLER, MICHAEL WANG, ROBERT A. RIZZA, AND PHILLIP D.K. LEE
Insulinlike growth factor binding protein 1 (IGFBP-1) has been shown to modulate the metabolic and mitogenic actions of the growth hormone (GH)dependent peptide insulinlike growth factor I. Previous studies showed that levels of IGFBP-1 are regulated by insulin. The relative role of GH in the regulation of IGFBP-1 levels is less well defined and was examined in our study with a contiguous two-part protocol. Overnight (part A) and pre- and post-morning meal (part B) blood samples were obtained from eight healthy adults during a constant infusion of saline (SAL) or 4 (xg • kg 1 • mirr 1 GH. Five of eight subjects were restudied with glucose (GLUC) infused during part B (SAL + GLUC) to match glucose and insulin to levels observed during GH infusion. During SAL infusion, IGFBP-1 levels measured by specific radioimmunoassay showed a marked immediate decline after the evening meal in part A, with a subsequent nocturnal rise of 2.4- to 17.3-fold. GH infusion resulted in a similar meal-induced fall in IGFBP-1 levels but led to a delayed nocturnal rise in IGFBP-1, which was associated with elevated postprandial insulin concentrations. During part B, changes in plasma IGFBP-1 levels showed a similar pattern, with a delayed postprandial increase observed during both GH and SAL + GLUC infusions. The half-life of IGFBP-1 disappearance was calculated at ~2 h for all three infusion groups. Comparison of venous and arterialized blood samples showed no consistent pattern of difference, arguing against peripheral tissue clearance or compartmentalization as the mechanism for the rapid rise and fall in IGFBP-1 levels. Our studies, the first to measure GH, insulin, and glucose in concurrent sampling and under controlled physiological conditions, support previous
From the Endocrine Research Unit, Mayo Clinic, Rochester, Minnesota; and the Diabetes Research Center, Baylor College of Medicine, Houston, Texas. Address correspondence and reprint requests to Dr. Cheryl Conover, Endocrine Research Unit, Room 5-164, West Joseph Building, Mayo Clinic, Rochester, MN 55905. Received for publication 3 November 1989 and accepted in revised form 4 May 1990.
DIABETES, VOL. 39, OCTOBER 1990
investigations suggesting that insulin (not GH or glucose) is the primary regulator of plasma IGFBP-1 levels. Furthermore, we postulate that the observed fluctuations in plasma levels are caused by a direct insulin effect on hepatic IGFBP-1 production. Diabetes 39:1251-56, 1990
I
nsulinlike growth factors (IGFs) are growth hormone (GH)dependent peptides that are structurally related to insulin and have metabolic and mitogenic effects both in vivo and in vitro (1). An important and unique aspect of IGF physiology is the high-affinity association of the IGFs with specific binding proteins that are important determinants of IGF action. One such binding protein, designated IGF binding protein 1 (IGFBP-1), has been purified from human amniotic fluid, HepG2 human hepatoma cell-conditioned medium, and placenta (2,3,4). IGFBP-1 has been cloned and sequenced, and it has a predicted molecular weight of 25,274 (5). (The nomenclature of the IGF binding proteins used in this article follows the consensus of the Workshop on IGF Binding Proteins, Vancouver, Canada, 17-19 June 1989.) Circulating IGFBP-1 levels have been shown to be inversely correlated with insulin levels, with elevated levels in insulin-dependent diabetes mellitus and decreased levels in the immediate postprandial period (6-11). Euglycemic hyperinsulinemic infusion leads to acute suppression of IGFBP1 levels (9,10). Plasma IGFBP-1 concentrations also show an inverse relationship to GH status. IGFBP-1 levels are elevated in GH-deficient patients and diminished with GH administration (7,12,13). It is uncertain whether this represents a direct effect of GH or an indirect effect caused by the associated GH-induced changes in insulin concentration. Furthermore, the regulatory mechanisms responsible for fluctuations in plasma IGFBP-1 levels have not been defined and may involve changes in production, use, or clearance. In this study, we investigated changes in IGFBP-1 levels
1251
REGULATION OF IGFBP-1 IN HUMANS
in relation to physiological and pharmacological changes in GH levels, with an experimental model that allows mealrelated changes in insulin levels to be used as a control variable. We also studied the arteriovenous clearance of IGFBP-1 across a peripheral vascular bed. Our results indicate that GH does not have an independent effect on IGFBP-1 levels in vivo and that the acute insulin-induced changes in IGFBP-1 levels are not primarily caused by peripheral clearance.
RESEARCH DESIGN AND METHODS The experimental design was based on unpublished observations (P.C.B., E. Kryshak, and R.A.R). With written consent and after approval of the Mayo Clinic Institutional Review Board, eight healthy volunteers (5 men, 3 women; mean ± SE age 27 ± 2 yr, weight 65 ± 5 kg, body mass index 21.6 ± 0.7 kg/m2) were admitted to the clinical research center, and intravenous cannulas were placed for infusions and intermittent blood sampling. For part A of the study, infusions of saline (SAL) or SAL plus recombinant DNA-derived methionyl human GH (Genentech, South San Francisco, CA) at 4 |xg • kg" 1 • min~1 were started at 1700. The SAL and GH infusions were administered on separate occasions in random order. At 1750, the subjects ate a standard 680-cal mixed meal. Thereafter, only water was permitted until the meal study on the following day. Blood samples were collected at 2-h intervals during this portion of the study. Part B of the study began the following morning. After the overnight fast and with uninterrupted infusions, subjects ingested a solid 472-cal mixed meal at 1000, which was consumed within 15 min. Blood samples were obtained at 0, 30, 60, 120, 180, 240, 300, and 360 min postprandially. Simultaneous samples of deep venous blood from a retrograde antecubital vein and arterialized venous blood from a dorsal hand vein were collected during this portion of the study to allow estimation of arteriovenous differences across a peripheral vascular bed. The hand was maintained at 55°C by a heated Plexiglas box. Arterialization was confirmed by measurement of O2 saturation (>90%) before and after the meal, and it was comparable in samples from all three infusion groups. Five subjects were studied on a third occasion. In addition to nocturnal saline infusion (part A), glucose (GLUC) was infused (81 ± 48 mg • kg-' • 3 h~1 glucose before the morning meal and 874 ± 145 mg • kg- 1 • 6 hr1 postprandially) to match the increased arterial insulin and glucose concentrations observed during the GH infusion. This group in part B is designated SAL + GLUC. The nonequilibrium radioimmunoassay (RIA) for IGFBP-1 was previously described (14). In brief, a 100-|xl sample or standard was preincubated with 100 |xl rabbit polyclonal anti-IGFBP-1 antiserum (1:1000 initial titer) for 1 h at room temperature. 125l-labeled IGFBP-1, prepared with the lodobead method (Pierce, Rockford, IL), was then added at 5000-10,000 counts/min [cpm]/tube for 16 h at 4°C. Bound counts were separated by agarose-immobilized goat antirabbit Ig (Bio-Rad, Richmond, CA) and measured in an automatic 7-counter. Pure IGFBP-1 from HepG2 cell-conditioned medium or amniotic fluid (D. Powell, Baylor College of Medicine, Houston, TX) was used for standards and ra-
1252
dioligand. Data were analyzed with the IBM-PC RIA data reduction program (M.L. Jaffe, Silver Spring, MD). Interassay and intra-assay coefficients of variation at 7.86 ng/ml were 17.3 ± 1.36 and 13.4 ± 3.8%, respectively. Limits of detection were 1 and 200 ng/ml or 0.1 and 20 ng/tube. Plasma insulin, C-peptide, and GH were measured by RIA as previously described (14). Glucose concentrations were measured with a glucose oxidase method (YSI, Yellow Springs, OH). Data are presented as means ± SE. Correlations were analyzed by linear regression, and areas under the curve were calculated with the trapezoidal rule. Comparisons were made by Student's nest, with statistical significance defined as P < 0.05. RESULTS Plasma GH, insulin, and glucose concentrations. Figure 1 summarizes the mean GH, insulin, and glucose data for parts A and B of the study. Changes in C-peptide concentrations paralleled those of insulin and are not presented. During the overnight sampling (part A), SAL-infused subjects had low GH levels with nocturnal surges of 15 |xg/L. GH infusion caused hypersomatotropinemia, reaching an equilibrium level of 26 ± 3 [xg/L after ~3 h. During part A, insulin and glucose levels increased after the evening meal (1750), with levels of insulin and glucose higher, but not statistically different, for the GH compared with the SAL groups; however, the postprandial decrease in insulin levels was delayed with GH infusion compared with SAL infusion and did not attain fasting levels (40-90 pM) until 0100, ~2 h later than with SAL infusion.
0
1700
0100
Hour
0900
120 240 Min
360
1600
FIG. 1. Plasma growth hormone (GH), glucose, and insulin concentrations for overnight fast after standard meal (A) and during meal study next day (B). Subjects were infused with saline (O and • ) or 4 (j.g • kg~ 1 • min" 1 GH (A) starting at 1700. Infusions were continued until 1600 next day. At 0700 next day, glucose infusion was started in 1 overnight saline-infused group ( • ) . Blood samples were taken at designated time points. Arrows, meals.
DIABETES, VOL. 39, OCTOBER 1990
C.A. CONOVER AND ASSOCIATES
150
i
130 -
110 -
90 I
y
QL
(D 50 -
30 \
10
/ l"
I
i
SAL infusions at 0500 (79 ± 16 and 131 ± 2 7 |xg/L, respectively). From 2100 to 0500, the area under the curve (the response above the baseline) was 285% greater for SAL compared with the GH infusion (P < 0.05); however, the rate of increase was not significantly different between GH- and SAL-infused groups. At the start of part B, plasma IGFBP-1 levels were 115 ± 14, 64 ± 8, and 101 ± 2 2 u.g/L for the SAL, GH, and SAL + GLUC infusion groups, respectively (Fig. 3). Plasma concentrations decreased rapidly after the meal at 1000 (designated as 0 min) for all three infusion conditions. The to,5 of plasma IGFBP-1 disappearance was 125 ± 8, 108 ± 8, and 133 ± 23 min for SAL, GH, and SAL + GLUC, respectively. These half-lives were not significantly different. After 180 min, IGFBP-1 levels in the SAL-infusion group began to rise immediately, returning to preprandial levels of 105 ± 9 M-g/L at 360 min. Postprandial increases in IGFBP1 were delayed for GH and SAL + GLUC infusions, with significant increases noted only after 360 min for both groups. Composite mean insulin and IGFBP-1 levels are shown for each infusion condition in Fig. 4. In every instance, regardless of whether GH or GLUC was infused, IGFBP-1 levels began to rise when insulin levels fell below a threshold concentration of —90 pM. Plasma insulin levels >90 pM led to a rapid decrease in IGFBP-1 levels. GH infusion resulted in
1700 1900 2100 2300 0100 0300 0500
Hour FIG. 2. Plasma insulinlike growth factor binding protein 1 (IGFBP-1) concentrations during overnight fast after standard meal at 1750. Subjects were continually infused with saline (O and • ) or growth hormone (A) starting at 1700. • , Subjects given glucose during meal study next day. Values are means ± SE.
During part B, the postprandial glycemic response was significantly higher for GH than for SAL infusion during the first 2 h after meal ingestion. To control for differences in glucose and insulin concentrations between GH and SAL groups, five of the subjects were studied with the SAL + GLUC protocol. At the time of meal ingestion, plasma glucose concentrations were comparable, whereas plasma insulin concentrations were slightly but significantly lower in the SAL + GLUC group than in the GH group (P < 0.05). After meal ingestion, integrated plasma glucose, insulin, and C-peptide concentrations did not differ between the GH and SAL + GLUC infusions, but they were significantly higher compared with the SAL group. During part B, GH levels in the SAL and SAL + GLUC groups increased to - 1 1 |xg/L when glucose concentrations were rapidly falling toward the basal level, 4-6 h after the morning meal (1000). IGFBP-1 levels. Mean plasma IGFBP-1 at admission for part A (1700) was 72 ± 15 jtg/L. During SAL and GH infusions (Fig. 2), IGFBP-1 levels fell rapidly after the evening meal to reach 16-27 |xg/L (22-38% of initial levels) at 2100. IGFBP-1 levels then began to rise. During SAL infusion, IGFBP-1 levels increased 2.4- to 17.3-fold in individual subjects between 2100 and 0500. The nocturnal rise in plasma IGFBP-1 concentrations seen in the GH-infused group was delayed by ~2 h, reaching 60% of the levels attained during
DIABETES, VOL. 39, OCTOBER 1990
300
360
Minutes FIG. 3. Plasma insulinlike growth factor binding protein 1 (IGFBP-1) concentrations after meal ingestion at 1000 (0 min). Groups as in Fig. 2. • , Glucose infusion started at 0700 (see RESEARCH DESIGN AND METHODS). Values are means ± SE.
1253
REGULATION OF IGFBP-1 IN HUMANS
SAL
SAL.
c"
d. CO
c
u_ O
1700 2100 0100 0500
1700 2100 0100 0500
1700 2100 0100 0500
Hour
SAL
GH
SAL+GLUC
a c"
"5 w c
0
120 240 360
0
120 240 360
120 240 360
Minutes FIG. 4. Mean plasma insulin ( • ) and insulinlike growth factor binding protein 1 (IGFBP-1; O) concentrations during overnight fast (A) and meal study (B). SAL, saline; GH, growth hormone; SAL2, salineinfused subjects who received glucose during meal study; GLUC, glucose.
higher basal and exaggerated meal-induced insulin excursions; hence, there was a lag in reaching insulin concentrations of
Insulinlike growth factor binding protein 1 (IGFBP-1) has been shown to modulate the metabolic and mitogenic actions of the growth hormone (GH)dependent peptide insulinlike growth factor I. Previous studies showed that levels of IGFBP-1 are regulated by insulin. The relative role of GH in the regulation of IGFBP-1 levels is less well defined and was examined in our study with a contiguous two-part protocol. Overnight (part A) and pre- and post-morning meal (part B) blood samples were obtained from eight healthy adults during a constant infusion of saline (SAL) or 4 (xg • kg 1 • mirr 1 GH. Five of eight subjects were restudied with glucose (GLUC) infused during part B (SAL + GLUC) to match glucose and insulin to levels observed during GH infusion. During SAL infusion, IGFBP-1 levels measured by specific radioimmunoassay showed a marked immediate decline after the evening meal in part A, with a subsequent nocturnal rise of 2.4- to 17.3-fold. GH infusion resulted in a similar meal-induced fall in IGFBP-1 levels but led to a delayed nocturnal rise in IGFBP-1, which was associated with elevated postprandial insulin concentrations. During part B, changes in plasma IGFBP-1 levels showed a similar pattern, with a delayed postprandial increase observed during both GH and SAL + GLUC infusions. The half-life of IGFBP-1 disappearance was calculated at ~2 h for all three infusion groups. Comparison of venous and arterialized blood samples showed no consistent pattern of difference, arguing against peripheral tissue clearance or compartmentalization as the mechanism for the rapid rise and fall in IGFBP-1 levels. Our studies, the first to measure GH, insulin, and glucose in concurrent sampling and under controlled physiological conditions, support previous
From the Endocrine Research Unit, Mayo Clinic, Rochester, Minnesota; and the Diabetes Research Center, Baylor College of Medicine, Houston, Texas. Address correspondence and reprint requests to Dr. Cheryl Conover, Endocrine Research Unit, Room 5-164, West Joseph Building, Mayo Clinic, Rochester, MN 55905. Received for publication 3 November 1989 and accepted in revised form 4 May 1990.
DIABETES, VOL. 39, OCTOBER 1990
investigations suggesting that insulin (not GH or glucose) is the primary regulator of plasma IGFBP-1 levels. Furthermore, we postulate that the observed fluctuations in plasma levels are caused by a direct insulin effect on hepatic IGFBP-1 production. Diabetes 39:1251-56, 1990
I
nsulinlike growth factors (IGFs) are growth hormone (GH)dependent peptides that are structurally related to insulin and have metabolic and mitogenic effects both in vivo and in vitro (1). An important and unique aspect of IGF physiology is the high-affinity association of the IGFs with specific binding proteins that are important determinants of IGF action. One such binding protein, designated IGF binding protein 1 (IGFBP-1), has been purified from human amniotic fluid, HepG2 human hepatoma cell-conditioned medium, and placenta (2,3,4). IGFBP-1 has been cloned and sequenced, and it has a predicted molecular weight of 25,274 (5). (The nomenclature of the IGF binding proteins used in this article follows the consensus of the Workshop on IGF Binding Proteins, Vancouver, Canada, 17-19 June 1989.) Circulating IGFBP-1 levels have been shown to be inversely correlated with insulin levels, with elevated levels in insulin-dependent diabetes mellitus and decreased levels in the immediate postprandial period (6-11). Euglycemic hyperinsulinemic infusion leads to acute suppression of IGFBP1 levels (9,10). Plasma IGFBP-1 concentrations also show an inverse relationship to GH status. IGFBP-1 levels are elevated in GH-deficient patients and diminished with GH administration (7,12,13). It is uncertain whether this represents a direct effect of GH or an indirect effect caused by the associated GH-induced changes in insulin concentration. Furthermore, the regulatory mechanisms responsible for fluctuations in plasma IGFBP-1 levels have not been defined and may involve changes in production, use, or clearance. In this study, we investigated changes in IGFBP-1 levels
1251
REGULATION OF IGFBP-1 IN HUMANS
in relation to physiological and pharmacological changes in GH levels, with an experimental model that allows mealrelated changes in insulin levels to be used as a control variable. We also studied the arteriovenous clearance of IGFBP-1 across a peripheral vascular bed. Our results indicate that GH does not have an independent effect on IGFBP-1 levels in vivo and that the acute insulin-induced changes in IGFBP-1 levels are not primarily caused by peripheral clearance.
RESEARCH DESIGN AND METHODS The experimental design was based on unpublished observations (P.C.B., E. Kryshak, and R.A.R). With written consent and after approval of the Mayo Clinic Institutional Review Board, eight healthy volunteers (5 men, 3 women; mean ± SE age 27 ± 2 yr, weight 65 ± 5 kg, body mass index 21.6 ± 0.7 kg/m2) were admitted to the clinical research center, and intravenous cannulas were placed for infusions and intermittent blood sampling. For part A of the study, infusions of saline (SAL) or SAL plus recombinant DNA-derived methionyl human GH (Genentech, South San Francisco, CA) at 4 |xg • kg" 1 • min~1 were started at 1700. The SAL and GH infusions were administered on separate occasions in random order. At 1750, the subjects ate a standard 680-cal mixed meal. Thereafter, only water was permitted until the meal study on the following day. Blood samples were collected at 2-h intervals during this portion of the study. Part B of the study began the following morning. After the overnight fast and with uninterrupted infusions, subjects ingested a solid 472-cal mixed meal at 1000, which was consumed within 15 min. Blood samples were obtained at 0, 30, 60, 120, 180, 240, 300, and 360 min postprandially. Simultaneous samples of deep venous blood from a retrograde antecubital vein and arterialized venous blood from a dorsal hand vein were collected during this portion of the study to allow estimation of arteriovenous differences across a peripheral vascular bed. The hand was maintained at 55°C by a heated Plexiglas box. Arterialization was confirmed by measurement of O2 saturation (>90%) before and after the meal, and it was comparable in samples from all three infusion groups. Five subjects were studied on a third occasion. In addition to nocturnal saline infusion (part A), glucose (GLUC) was infused (81 ± 48 mg • kg-' • 3 h~1 glucose before the morning meal and 874 ± 145 mg • kg- 1 • 6 hr1 postprandially) to match the increased arterial insulin and glucose concentrations observed during the GH infusion. This group in part B is designated SAL + GLUC. The nonequilibrium radioimmunoassay (RIA) for IGFBP-1 was previously described (14). In brief, a 100-|xl sample or standard was preincubated with 100 |xl rabbit polyclonal anti-IGFBP-1 antiserum (1:1000 initial titer) for 1 h at room temperature. 125l-labeled IGFBP-1, prepared with the lodobead method (Pierce, Rockford, IL), was then added at 5000-10,000 counts/min [cpm]/tube for 16 h at 4°C. Bound counts were separated by agarose-immobilized goat antirabbit Ig (Bio-Rad, Richmond, CA) and measured in an automatic 7-counter. Pure IGFBP-1 from HepG2 cell-conditioned medium or amniotic fluid (D. Powell, Baylor College of Medicine, Houston, TX) was used for standards and ra-
1252
dioligand. Data were analyzed with the IBM-PC RIA data reduction program (M.L. Jaffe, Silver Spring, MD). Interassay and intra-assay coefficients of variation at 7.86 ng/ml were 17.3 ± 1.36 and 13.4 ± 3.8%, respectively. Limits of detection were 1 and 200 ng/ml or 0.1 and 20 ng/tube. Plasma insulin, C-peptide, and GH were measured by RIA as previously described (14). Glucose concentrations were measured with a glucose oxidase method (YSI, Yellow Springs, OH). Data are presented as means ± SE. Correlations were analyzed by linear regression, and areas under the curve were calculated with the trapezoidal rule. Comparisons were made by Student's nest, with statistical significance defined as P < 0.05. RESULTS Plasma GH, insulin, and glucose concentrations. Figure 1 summarizes the mean GH, insulin, and glucose data for parts A and B of the study. Changes in C-peptide concentrations paralleled those of insulin and are not presented. During the overnight sampling (part A), SAL-infused subjects had low GH levels with nocturnal surges of 15 |xg/L. GH infusion caused hypersomatotropinemia, reaching an equilibrium level of 26 ± 3 [xg/L after ~3 h. During part A, insulin and glucose levels increased after the evening meal (1750), with levels of insulin and glucose higher, but not statistically different, for the GH compared with the SAL groups; however, the postprandial decrease in insulin levels was delayed with GH infusion compared with SAL infusion and did not attain fasting levels (40-90 pM) until 0100, ~2 h later than with SAL infusion.
0
1700
0100
Hour
0900
120 240 Min
360
1600
FIG. 1. Plasma growth hormone (GH), glucose, and insulin concentrations for overnight fast after standard meal (A) and during meal study next day (B). Subjects were infused with saline (O and • ) or 4 (j.g • kg~ 1 • min" 1 GH (A) starting at 1700. Infusions were continued until 1600 next day. At 0700 next day, glucose infusion was started in 1 overnight saline-infused group ( • ) . Blood samples were taken at designated time points. Arrows, meals.
DIABETES, VOL. 39, OCTOBER 1990
C.A. CONOVER AND ASSOCIATES
150
i
130 -
110 -
90 I
y
QL
(D 50 -
30 \
10
/ l"
I
i
SAL infusions at 0500 (79 ± 16 and 131 ± 2 7 |xg/L, respectively). From 2100 to 0500, the area under the curve (the response above the baseline) was 285% greater for SAL compared with the GH infusion (P < 0.05); however, the rate of increase was not significantly different between GH- and SAL-infused groups. At the start of part B, plasma IGFBP-1 levels were 115 ± 14, 64 ± 8, and 101 ± 2 2 u.g/L for the SAL, GH, and SAL + GLUC infusion groups, respectively (Fig. 3). Plasma concentrations decreased rapidly after the meal at 1000 (designated as 0 min) for all three infusion conditions. The to,5 of plasma IGFBP-1 disappearance was 125 ± 8, 108 ± 8, and 133 ± 23 min for SAL, GH, and SAL + GLUC, respectively. These half-lives were not significantly different. After 180 min, IGFBP-1 levels in the SAL-infusion group began to rise immediately, returning to preprandial levels of 105 ± 9 M-g/L at 360 min. Postprandial increases in IGFBP1 were delayed for GH and SAL + GLUC infusions, with significant increases noted only after 360 min for both groups. Composite mean insulin and IGFBP-1 levels are shown for each infusion condition in Fig. 4. In every instance, regardless of whether GH or GLUC was infused, IGFBP-1 levels began to rise when insulin levels fell below a threshold concentration of —90 pM. Plasma insulin levels >90 pM led to a rapid decrease in IGFBP-1 levels. GH infusion resulted in
1700 1900 2100 2300 0100 0300 0500
Hour FIG. 2. Plasma insulinlike growth factor binding protein 1 (IGFBP-1) concentrations during overnight fast after standard meal at 1750. Subjects were continually infused with saline (O and • ) or growth hormone (A) starting at 1700. • , Subjects given glucose during meal study next day. Values are means ± SE.
During part B, the postprandial glycemic response was significantly higher for GH than for SAL infusion during the first 2 h after meal ingestion. To control for differences in glucose and insulin concentrations between GH and SAL groups, five of the subjects were studied with the SAL + GLUC protocol. At the time of meal ingestion, plasma glucose concentrations were comparable, whereas plasma insulin concentrations were slightly but significantly lower in the SAL + GLUC group than in the GH group (P < 0.05). After meal ingestion, integrated plasma glucose, insulin, and C-peptide concentrations did not differ between the GH and SAL + GLUC infusions, but they were significantly higher compared with the SAL group. During part B, GH levels in the SAL and SAL + GLUC groups increased to - 1 1 |xg/L when glucose concentrations were rapidly falling toward the basal level, 4-6 h after the morning meal (1000). IGFBP-1 levels. Mean plasma IGFBP-1 at admission for part A (1700) was 72 ± 15 jtg/L. During SAL and GH infusions (Fig. 2), IGFBP-1 levels fell rapidly after the evening meal to reach 16-27 |xg/L (22-38% of initial levels) at 2100. IGFBP-1 levels then began to rise. During SAL infusion, IGFBP-1 levels increased 2.4- to 17.3-fold in individual subjects between 2100 and 0500. The nocturnal rise in plasma IGFBP-1 concentrations seen in the GH-infused group was delayed by ~2 h, reaching 60% of the levels attained during
DIABETES, VOL. 39, OCTOBER 1990
300
360
Minutes FIG. 3. Plasma insulinlike growth factor binding protein 1 (IGFBP-1) concentrations after meal ingestion at 1000 (0 min). Groups as in Fig. 2. • , Glucose infusion started at 0700 (see RESEARCH DESIGN AND METHODS). Values are means ± SE.
1253
REGULATION OF IGFBP-1 IN HUMANS
SAL
SAL.
c"
d. CO
c
u_ O
1700 2100 0100 0500
1700 2100 0100 0500
1700 2100 0100 0500
Hour
SAL
GH
SAL+GLUC
a c"
"5 w c
0
120 240 360
0
120 240 360
120 240 360
Minutes FIG. 4. Mean plasma insulin ( • ) and insulinlike growth factor binding protein 1 (IGFBP-1; O) concentrations during overnight fast (A) and meal study (B). SAL, saline; GH, growth hormone; SAL2, salineinfused subjects who received glucose during meal study; GLUC, glucose.
higher basal and exaggerated meal-induced insulin excursions; hence, there was a lag in reaching insulin concentrations of
