Oral octreotide absorption in human subjects: comparable pharmacokinetics to parenteral octreotide and effective growth hormone suppression.

CONTEXT: Oral administration of a novel octreotide formulation enabled its absorption to the systemic circulation, exhibiting blood concentrations comparable to those observed with injected octreotide and maintaining its biological activity. OBJECTIVES: The aim of the study was to determine oral octreotide absorption and effects on pituitary GH secretion compared to sc octreotide injection. DESIGN: Four single-dose studies were conducted in 75 healthy volunteers. INTERVENTION: Oral doses of 3, 10, or 20 mg octreotide and a single sc injection of 100 µg octreotide were administered. MAIN OUTCOME MEASURE: We measured the pharmacokinetic profile of orally administrated octreotide and the effect of octreotide on basal and stimulated GH secretion. RESULTS: Both oral and sc treatments were well tolerated. Oral octreotide absorption to the circulation was apparent within 1 h after dose administration. Escalating oral octreotide doses resulted in dose-dependent increased plasma octreotide concentrations, with an observed rate of plasma decay similar to parenteral administration. Both 20 mg oral octreotide and injection of 0.1 mg sc octreotide resulted in equivalent pharmacokinetic parameters [mean peak plasma concentration, 3.77 ± 0.25 vs. 3.97 ± 0.19 ng/ml; mean area under the curve, 16.2 ± 1.25 vs. 12.1 ± 0.45 h × ng/ml); and median time ≥ 0.5 ng/ml, 7.67 vs. 5.88 h, respectively). A single dose of 20 mg oral octreotide resulted in basal (P < 0.05) and GHRH-stimulated (P < 0.001) mean GH levels suppressed by 49 and 80%, respectively. CONCLUSIONS: The results support an oral octreotide alternative to parenteral octreotide treatment for patients with acromegaly.

Characterization and regulation of type A endothelin receptor gene expression in bovine luteal cell types.

Our previous studies demonstrated that endothelin-1 (ET-1), a 21-amino acid vasoconstrictor peptide, has a paracrine regulatory role in bovine corpus luteum (CL). The peptide is produced within the gland where it inhibits progesterone production by acting via the selective type A endothelin (ETA) receptors. The present study was designed to characterize ETA receptor gene expression in different ovarian cell types and its hormonal regulation. ETA receptor messenger RNA (mRNA) levels were high in follicular cells as well as in CL during luteal regression. At this latter stage, high ETA receptor expression concurred with low prostaglandin F2alpha receptor mRNA. The ETA receptor gene was expressed by all three major cell populations of the bovine CL; i.e. small and large luteal cells, as well as in luteal endothelial cells. Among these various cell populations, the highest ETA receptor mRNA levels were found in endothelial cells. cAMP elevating agents, forskolin and LH, suppressed ETA receptor mRNA expression in luteinized theca cells (LTC). This inhibition was dose dependent and was evident already after 24 h of incubation. In luteinized granulosa cells (LGC), 10 and 100 ng/ml of insulin-like growth factor I and insulin (only at a concentration of 2000 ng/ml) markedly decreased ETA receptor mRNA levels. In both LGC and LTC there was an inverse relationship between ETA receptor gene expression and progesterone production; insulin (in LGC) and forskolin (in LTC) enhanced progesterone production while inhibiting ETA receptor mRNA levels. Our findings may therefore suggest that, during early stages of luteinization when peak levels of both LH and insulin-like growth factor I exist, the expression of ETA receptors in the gland are suppressed. This study demonstrates physiologically relevant regulatory mechanisms controlling ETA receptor gene expression and further supports the inhibitory role of ET-1 in CL function.

LH receptor mRNA and cytochrome P450 side-chain cleavage expression in bovine theca and granulosa cells luteinized by LH or forskolin.

This study was undertaken to characterize the cDNA sequence encoding bovine LH receptor (LHR), and study the effect of different luteinization protocols on the steroidogenic capacity and LHR mRNA in bovine luteal cells. The bovine LHR cDNA sequence reported here showed high sequence identity with that of other species. Several mRNA splice variants were expressed in bovine corpus luteum (CL). Reverse transcription-polymerase chain reaction conducted with primers spanning exons 2-11 revealed, in addition to the full-length sequence, a shorter fragment homologous to splice variant B reported in porcine and ovine LHR cDNA sequences. Granulosa and theca cells derived from bovine preovulatory follicles were cultured with either forskolin (10 microM, for 8 d culture-Protocol 1) or LH (100 ng/ml, for 12 hr followed by a 7.5-d culture with 2 ng/ml-Protocol 2). LHR mRNA was not detected in luteal granulosa cells (LG); in contrast, luteal theca cells (LT) contained LHR mRNA at similar levels when cultured under Protocol 1 or 2. cAMP responses to a short challenge with LH were in good agreement with LHR mRNA levels. Cytochrome P450 side-chain-cleavage (P450scc) contents were lower in luteal cells cultured with LH as compared with cells cultured with forskolin; however, the difference between the two luteinization protocols was much larger in LT (P < 0.05) than in LG. This study suggests that a) LHR mRNA is mainly present in the theca-derived luteal cell and b) LHR mRNA and cytochrome P450scc expression in each of the luteal cell types seems to be under different control.

Characterization of messenger ribonucleic acid expression for prostaglandin F2 alpha and luteinizing hormone receptors in various bovine luteal cell types.

LH and prostaglandin F2 alpha (PGF2 alpha) control the life span and function of the corpus luteum (CL). Nevertheless, identification of the various cell types (steroidogenic and nonsteroidogenic) expressing the receptors for these hormones remains controversial. In this study we characterized LH and PGF2 alpha receptor (r) expression in the various luteal cell types using quantitative reverse transcription-polymerase chain reaction. We found, in agreement with previously described functions of PGF2 alpha, that the two steroidogenic cell types, as well as luteal endothelial cells, expressed PGFr. In contrast, LHr was mainly expressed by small luteal cells. A similar pattern of PGFr and LHr expression was observed in steroidogenic cells luteinized in vitro and in cells derived from the mature CL. The expression of these two receptors was inversely affected by increased levels of cAMP (achieved by incubating cells with varying doses of forskolin); LHr expression was down-regulated by 50% in the presence of 10 microM forskolin (p < 0.05), while an increase was observed in PGFr expression. In granulosa-derived luteal cells, maximal expression of PGFr was higher (approximately by 3-fold, p < 0.05) than in the theca-derived luteal cells. PGF2 alpha, mimicking its in vivo effect, markedly down-regulated LHr expression in thecaderived luteal cells, abolishing expression at a concentration of 100 ng/ml. In summary, these studies depict cAMP and PGF2 alpha as major regulators of PGFr and LHr expression in the two steroidogenic cell types. All three major cell types of the CL (steroidogenic and endothelial) express PGFr. LHr mRNA, on the other hand, was detected mainly in small luteal cells. Such broad cellular distribution of PGFr may highlight the significant role played by this prostaglandin in the bovine CL.