The increase in oral availability of felodipine and other commonly used medications when taken with grapefruit juice has been assumed to be due to inhibition of CYP3A4, a cytochrome P450 that is present in liver and intestine. To evaluate the effect of repeated grapefruit juice ingestion on CYP3A4 expression, 10 healthy men were given 8 oz of grapefruit juice three times a day for 6 d. Before and after receiving grapefruit juice, small bowel and colon mucosal biopsies were obtained endoscopically, oral felodipine kinetics were determined, and liver CYP3A4 activity was measured with the [ 14 C N -methyl] erythromycin breath test in each subject.Grapefruit juice did not alter liver CYP3A4 activity, colon levels of CYP3A5, or small bowel concentrations of P-glycoprotein, villin, CYP1A1, and CYP2D6. In contrast, the concentration of CYP3A4 in small bowel epithelia (enterocytes) fell 62% ( P ϭ 0.0006) with no corresponding change in CYP3A4 mRNA levels. In addition, enterocyte concentrations of CYP3A4 measured before grapefruit juice consumption correlated with the increase in C max when felodipine was taken with either the 1st or the 16th glass of grapefruit juice relative to water ( r ϭ 0.67, P ϭ 0.043, and r ϭ 0.71, P ϭ 0.022, respectively). We conclude that a mechanism for the effect of grapefruit juice on oral felodipine kinetics is its selective downregulation of CYP3A4 in the small intestine. ( J. Clin. Invest. 1997. 99:2545-2553.)

Interpatient differences in the oral clearance of cyclosporine (INN, ciclosporin) have been partially attributed to variation in the activity of a single liver enzyme termed CYP3A4. Recently it has been shown that small bowel also contains CYP3A4, as well as P-glycoprotein, a protein able to transport cyclosporine. To assess the importance of these intestinal proteins, the oral pharmacokinetics of cyclosporine were measured in 25 kidney transplant recipients who each had their liver CYP3A4 activity quantitated by the intravenous [14C-N-methyl]-erythromycin breath test and who underwent small bowel biopsy for measurement of CYP3A4 and P-glycoprotein. Forward multiple regression revealed that 56% (i.e., r2 = 0.56) and 17% of the variability in apparent oral clearance [log (dose/area under the curve)] were accounted for by variation in liver CYP3A4 activity (p < 0.0001) and intestinal P-glycoprotein concentration (p = 0.0059), respectively. For peak blood concentration, liver CYP3A4 activity accounted for 32% (p = 0.0002) and P-glycoprotein accounted for an additional 30% (p = 0.0024) of the variability. Intestinal levels of CYP3A4, which varied tenfold, did not appear to influence any cyclosporine pharmacokinetic parameter examined. We conclude that intestinal P-glycoprotein plays a significant role in the first-pass elimination of cyclosporine, presumably by being a rate-limiting step in absorption. Drug interactions with cyclosporine previously ascribed to intestinal CYP3A4 may instead be mediated by interactions with intestinal P-glycoprotein.

Sexually dimorphic growth hormone (GH) secretory pattern is important in the determination of gender-specific patterns of growth and metabolism in rats. Whether GH secretion in humans is also sexually dimorphic and the neuroendocrine mechanisms governing this potential difference are not fully established. We have compared pulsatile GH secretion profiles in young men and women in the baseline state and during a continuous intravenous infusion of recombinant human insulin-like growth factor I (rhIGF-I). During the baseline study, men had large nocturnal GH pulses and relatively small pulses during the rest of the day. In contrast, women had more continuous GH secretion and more frequent GH pulses that were of more uniform size. The infusion of rhIGF-I (10 g/kg/h) potently suppressed both spontaneous and growth hormone-releasing hormone (GHRH)-induced GH secretion in men. In women, however, rhIGF-I had less effect on pulsatile GH secretion and did not suppress the GH response to GHRH. These data demonstrate the existence of sexual dimorphism in the regulatory mechanisms involved in GH secretion in humans.

Combined T and GH treatment of men with hypopituitarism for 2 years did not improve the measured structural or mechanical parameters of the distal tibia more than T alone. However, testosterone significantly increased the structural and mechanical properties of trabecular bone but decreased most of these properties of cortical bone, illustrating the potential importance of assessing trabecular and cortical bone separately in future studies of the effect of testosterone on bone.

Abstract.Whether GH secretion in women varies over the menstrual cycle is uncertain. Previous investigations have led to conflicting conclusions; some studies suggested that there is an estrogen mediated rise in GH during the periovulatory (P0) and luteal (L) phases whereas others indicated no change in GH axis over the cycle. Differences in conclusions could relate to heterogeneity of the study populations, GH sampling paradigms or sensitivity of the GH assays used. In order to investigate whether GH secretion varied over the cycle, 24-h GH profiles using every 10-min sampling were obtained in 6 ovulatory women during the early follicular (EF), PO and L phases of the cycle. The TSH response to TRH, GH response to GRH and fasting plasma IGF-I were measured on each occasion. There was a trend toward higher integrated GH concentration (IGHC) during the PO phase, although this difference was not statistically significant (3284±721 vs 4542±872 vs 4071 ±699 pg/min/L; EF vs PO vs L; p=0.09).Similarly, deconvolution estimated GH secretion did not vary over the cycle (p=0.56).There were no differences in GH pulse amplitude or frequency.There were no correlations between IGHC and sex steroids. Serum IGF-I was constant over the cycle (272±38 vs 277±31 vs 265±38 pg/L; p=0.89).The TSH response to TRH and GH response to GRH did not vary over the cycle. We concluded that the effect of changes in the ovarian steroid milieu on the GH axis during spontaneous menstrual cycles is minimal. THE effects of sex steroids and gender on GH secretion have been extensively investigated. Several groups have reported that women have higher GH secretion than do men [1, 2]. In contrast to these conclusions, in a carefully matched group of young men and women in the early follicular phase of the cycle, we were not able to demonstrate differences in either mean daily GH or daily GH secretion [3]. Although the mean daily GH concentration tended to be higher in women, the within-gender variability of GH secretion obscured any potential small difference between genders from being statistically significant. This study did, however demonstrate that insulin-like growth factor I (IGF-I) was less effective in suppressing GH secretion in women, suggesting genderspecific differences in GH regulation. Moreover, the pattern of GH secretion differs across genders, with women secreting GH more continuously throughout the day than do men. Sex steroids could potentially regulate differences in GH neuroendocrine regulation, but the data on the effect of endogenous sex steroids on GH secretion are contradictory.Faria et al. [4] reported that GH concentration varied over the menstrual cycle, with a two-fold increase of mean GH during the late follicular stage over early follicular levels.GH concentrations during the luteal stage were intermediate

The neuroendocrine mechanisms underlying the generation of pulsatile GH secretion in humans are poorly understood. GH secretory pulses are likely to result from acute GHRH secretory episodes, acute decreases in hypothalamic somatostatin secretion, or a combination of these mechanisms. In earlier studies we demonstrated that a single i.v. bolus of a competitive GHRH antagonist [N-Ac-Tyr1,D-Arg2)GHRH-(1-29); GHRH-Ant] blocked 40% of the nocturnal GH release. Failure to more completely eliminate nocturnal GH secretion could be due to either incomplete antagonism of endogenous GHRH action by GHRH-Ant or a non-GHRH component of GH release. We subsequently investigated whether a continuous infusion of GHRH-Ant would more completely eliminate nocturnal GH secretion. Eight men were given a 400 micrograms/kg i.v. bolus of GHRH-Ant at 2300 h, followed by a 50 micrograms/kg.h i.v. infusion of GHRH-Ant between 2300-0700 h or a saline bolus followed by a saline infusion. An i.v. bolus of GHRH (1 microgram/kg) was given at 0500 h on both occasions. Blood was sampled every 10 min between 2300-0700 h. As measured by the area under the curve (AUC) from 2400-0500 h, GHRH-Ant suppressed GH secretion by an average of 89% (1795 +/- 412 vs. 164 +/- 46 micrograms/min.L; P = 0.004). The response to GHRH was suppressed by 79% (484 +/- 140 vs. 64 +/- 19 micrograms/min.L; P = 0.02). These data demonstrate that the previously observed nonsuppressible GH secretion was probably due to incomplete blockade of pituitary GHRH receptors and that all or nearly all of nocturnal GH pulsatility can be attributed to the effect of hypothalamic GHRH.

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Estradiol (E2) negative feedback on LH secretion was examined in 10 pubertal girls, testing the hypothesis that E2 suppresses LH pulse frequency and amplitude through opioid pathways. At 1000 h, a 32-h saline infusion was given, followed 1 week later by an E2 infusion at 13.8 nmol/m2 x h. During both infusions, four iv boluses of saline were given hourly beginning at 1200 h, and four naloxone iv boluses (0.1 mg/kg each) were given hourly beginning at 1200 h on the following day. Blood was obtained every 15 min for LH determination and every 60 min for E2 determination from 1200 h to the end of the infusion. E2 infusion increased the mean serum E2 concentration from 44+/-17 to 112+/-26 pmol/L (P < 0.01). The mean LH concentration between 2200-1200 h decreased from 3.19+/-0.89 to 1.99+/-0.65 IU/L (P = 0.014), and LH pulse amplitude decreased from 3.4+/-0.6 to 2.6+/-0.5 IU/L (P = 0.0076). Although there were 1.2 fewer pulses during E2 infusion compared to saline infusion, differences did not reach significance (P = 0.1; 95% confidence interval for the difference, -3.5, 1.1). Pituitary responsiveness to GnRH, assessed at the end of the infusion by administering 250 ng/kg GnRH iv, did not change during E2 infusion. The effect of naloxone blockade of opioid activity on LH secretion was determined by assessing the area under the curve (AUC) from 1200-1600 h. During saline infusion, the LH AUC was 1122+/-375 IU/L during saline boluses and 1575+/-403 IU/L during naloxone boluses (P = 0.39). When E2 was infused, the LH AUCs during saline and naloxone boluses were 865+/-249 and 866+/-250 IU/L, respectively. Thus, in pubertal girls: 1) E2 decreases the LH concentration and LH pulse amplitude; 2) the main site of negative feedback effect of E2 appears to be at the level of the hypothalamus; 3) an increase in LH secretion after naloxone administration could not be demonstrated in these girls and may depend on the maturity of the hypothalamic-pituitary-gonadal axis; and 4) opioid receptor blockade does not reverse the E2 inhibition of LH secretion even in the most mature girls. Thus, E2 suppression of LH secretion in pubertal girls appears to be mediated by a decrease in hypothalamic GnRH secretion that is independent of opioid pathways.