Guidelines for the diagnosis and treatment of acromegaly: a Canadian perspective.

Acromegaly is a chronic, debilitating condition caused by excessive secretion of growth hormone (GH). In the majority of cases the condition results from benign pituitary adenomas or, rarely, from ectopic production of GH-releasing hormone. Regardless of the cause, excess GH results in physical disfigurement associated with arthropathy, diabetes, hypertension, cardiac dysfunction, obstructive sleep apnea and colonic neoplasia. The death rate for acromegalic patients is 2 to 3 times higher than that of the general population, but with appropriate reduction of GH hypersecretion it tends to shift into the normal range. Treatment is thus aimed at normalizing GH secretion; eradicating or stabilizing the pituitary tumour while preserving normal pituitary function, and managing the associated complications. The treatment modalities available to achieve these objectives include transsphenoidal surgery, pharmacotherapy and radiation, or various combinations of these. This review provides an update on our current understanding of the pathophysiology of GH hypersecretion in acromegaly, the newly defined diagnostic criteria and the end point for a cure for acromegaly, and on new developments in drug treatment with the advent of slow-release forms of somatostatin analogues and the longer-acting dopamine receptor agonists, as well as in the area of radiotherapy. Its main purpose is to guide any physician involved in the diagnosis and management of patients with acromegaly.

Structure-activity relationships for relaxation of smooth muscle by VIP.

Utilizing VIP and five VIP analogues, concentration-response curves for relaxation of rat mesenteric artery and rat gastric longitudinal muscle were determined for comparison with our previously published radioligand binding data on rat smooth muscle and other tissues. The biological potency of the VIP analogues in the present study compared more closely with their potency for VIP receptor binding in smooth muscle tissue (arteries) vs. other tissues (pituitary, brain, liver).

Comparative study of vascular relaxation and receptor binding by PACAP and VIP.

The pharmacological properties of the pituitary adenylate cyclase activating peptides (PACAPs) and vasoactive intestinal peptide (VIP) were compared using: (i) relaxation of vascular and gastric smooth muscle in vitro, and (ii) radioligand binding to membrane preparations of a variety of tissues. Vasoactive intestinal peptide and PACAP-27 were similarly potent in relaxing rat mesenteric arteries, porcine coronary arteries, and rat gastric smooth muscle, whereas PACAP-38 was either more or less potent than the other two peptides depending on the tissue model. Cross-desensitization to relaxation and radioligand binding studies of porcine coronary arteries suggested that VIP and the PACAPs interact with a common receptor in this tissue. A PACAP-preferring receptor with low affinity for VIP was identified in radioligand binding studies of rat brain and anterior pituitary. A second, nonselective, receptor that binds VIP and both PACAPs with high affinity was observed in preparations of rat and porcine arteries and rat lung, liver, brain, and anterior pituitary.

Selective effect of castration on the anterior pituitary VIP receptor of male rats.

We investigated the effect of surgical castration of male rats on the binding of [Tyr(125I)10]VIP to receptors on the anterior pituitary gland, superior mesenteric artery, brain, liver, and prostate gland. In anterior pituitary membranes the maximum number of VIP binding sites was increased whereas binding affinity was decreased 24 hours following castration. In particular, the high affinity equilibrium dissociation constant (KD) increased from 0.13 +/- 0.02 nM (mean +/- SEM) to 0.67 +/- 0.07 nM and the maximum number of high affinity binding sites (Bmax) increased from 71 +/- 9 to 470 +/- 112 fmol/mg protein. No significant change was observed in the other tissues. Anesthesia or sham operation did not alter the anterior pituitary VIP receptor binding parameters. The changes in the VIP receptor 24 hours after castration were prevented by prior injection of testosterone. These findings demonstrate tissue-selective alterations to the anterior pituitary VIP receptor by castration that are likely mediated by withdrawal of testosterone.

PHI preferentially binds to VIP receptors in normal rat tissues.

Vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI) are homologous neuropeptides with parallel biological actions. These similarities raise the question whether VIP and PHI have common or distinct mechanisms of action, including receptors. The present study attempted to distinguish specific binding sites for VIP and PHI in normal rat tissues using the homologous radioligands [Tyr(125I)10]VIP and [Tyr(125I)10]rat PHI. In rat brain, anterior pituitary, and liver membranes both radioligands identified a VIP-preferring receptor. Rat PHI had less than 10% the binding potency of VIP in these tissues irrespective of which radioligand was used. In rat uterine membranes [Tyr(125I)10]VIP bound to a receptor with approximately 100 times greater affinity for VIP over PHI. No specific binding of [Tyr(125I)10]rat PHI to rat uterus could be demonstrated. In conclusion, these results support the predominance of VIP-preferring receptors as opposed to PHI-preferring receptors in normal rat brain, anterior pituitary, liver and uterus.

Developmental and lactational changes in the rat anterior pituitary VIP receptor.

The regulation of female rat anterior pituitary vasoactive intestinal peptide (VIP) receptors was examined during postnatal development and lactation. VIP receptor binding to anterior pituitary membranes from 1- to 60-week-old rats and lactating rats was examined using HPLC purified [Tyr(125I)10]VIP. Nonlinear regression of competitive binding studies indicated the presence of 2 VIP binding sites in 3-week and older animals, whereas only 1 site was identified in 1- and 2-week-old rats. The single site did not differ significantly in affinity or number when compared to the low affinity site of older animals. The guanine nucleotide, GTP-gamma-S, decreased the specific binding of VIP by 60-80% in 3-week and older animals, but not in younger animals. Compared with adult nonlactating animals, the number of high affinity binding sites decreased significantly during lactation, with no change in receptor binding affinity.

Relaxation of isolated bovine coronary arteries by vasoactive intestinal peptide.

The relaxant action of vasoactive intestinal peptide (VIP) was investigated using helical strips of four major branches of bovine coronary arteries. The concentration of VIP causing 50 percent of maximal relaxation ranged from 23 to 90 nM. Preincubation of arterial strips with VIP shifted the concentration-response curves for contractions elicited by potassium chloride or prostaglandin F2 alpha to the right. The relaxant effect of VIP was retained following removal of the vascular endothelium or in the absence of extracellular calcium. The structurally homologous peptides porcine and human peptide histidine isoleucine (PHI) were less potent than was VIP. It is concluded that there are functional receptors for VIP in bovine coronary arteries.

Selectivity for binding of peptide analogs to vascular receptors for vasoactive intestinal peptide.

The structure-activity relationships for vasoactive intestinal peptide (VIP) receptor binding were studied using N-terminally modified VIP analogs. VIP fragments, and VIP receptor antagonists. Tissue sources included bovine coronary artery, rat mesenteric artery, rat pituitary, rat brain synaptosomes, and rat liver. Experimental conditions for receptor binding were maintained as near to identical as possible. The competitive binding curves for VIP analogs were similar in the bovine and rat vascular preparations. However, appreciable differences were observed between the vascular and other preparations. The vascular receptors discriminated between [D-His1]VIP and [Phe1]VIP, whereas the receptors in other tissues did not. The greatest selectivity was found for [D-Ala4]VIP, which was among the lowest affinity analogs tested on the vasculature but among the highest affinity analogs in the other preparations. The rank orders of analog potencies were comparable for the rat brain and pituitary receptors. The rat liver VIP receptor differed from its counterpart in brain and pituitary predominantly by discriminating between [D-Phe2]VIP and [D-Arg2]VIP. The two VIP receptor antagonists bound weakly and nonselectively to all receptor preparations. Integrity of the full VIP molecule was necessary for full potency of binding to the vascular receptor. We conclude that the vascular VIP receptor possesses recognition properties that are distinct from those for VIP receptors in liver, pituitary, or brain.

Receptors for vasoactive intestinal peptide in rat anterior pituitary glands: localization of binding to lactotropes.

Vasoactive intestinal peptide (VIP) has been implicated as a physiological PRL-releasing factor; however, characterization of VIP receptors on normal pituitaries using radioligand-binding methods has been problematic. In this study we demonstrated specific receptors for VIP in anterior pituitary glands of female rats using HPLC-purified monoiodinated [Tyr(125I)10]VIP. Binding of VIP was reversible, saturable to receptor and radioligand, regulated by guanine nucleotides, and dependent on time and temperature. Scatchard analysis of competitive binding studies indicated high and low affinity binding sites, with equilibrium dissociation constants (Kd) of 0.19 +/- 0.03 and 28 +/- 16 nM, respectively. The corresponding maximum numbers of binding sites were 158 +/- 34 fmol/mg and 11.7 +/- 6.9 pmol/mg. Binding was specific, as peptides with structural homology to VIP were less than 100th as potent as VIP. The rank order of potency of the peptides tested was VIP greater than rat (r) peptide histidine isoleucine = human (h) PHI greater than rGRF greater than bovine GRF = porcine PHI = VIP-(10-28) greater than hGRF greater than secretin greater than apamin greater than glucagon. Radioligand binding was associated primarily with lactotrope-enriched fractions prepared by unit gravity sedimentation of dispersed anterior pituitary cells. VIP stimulated PRL release from cultured rat anterior pituitary cells, with an ED50 of 1 nM. These results, comprising the first identification of specific VIP receptors in normal rat anterior pituitary tissue using radioligand-binding methods, provide additional support for a biological role of VIP in lactotrope function.

Effects of GTP analogs and dithiothreitol on the binding properties of the vascular vasoactive intestinal peptide receptor.

Previous studies have demonstrated a specific vascular receptor for the neurotransmitter peptide, vasoactive intestinal peptide (VIP), and have suggested that the receptor is positively coupled to vascular adenylate cyclase. The present study addressed the questions whether the vascular VIP receptor is subject to regulation by guanine nucleotides and whether a disulfide reducing agent, dithiothreitol, would perturb the binding function of the vascular VIP receptor. Guanosine triphosphate (GTP) and its non-hydrolyzable analogs, guanylyl imidodiphosphate (Gpp(NH)p) and guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S), increased the rate of dissociation of radiolabeled VIP from arterial receptors in a concentration-dependent manner. GTP-gamma-S increased the equilibrium dissociation constant (KD) of the high affinity vascular VIP binding site, a result consistent with decreased high affinity binding of VIP induced by GTP-gamma-S. These results are consistent with a regulatory role for guanine nucleotides in the function of the vascular VIP receptor. The disulfide reducing agent, dithiothreitol, caused a decrease in specific binding of radiolabeled VIP. Upon Scatchard analysis the effect of dithiothreitol was characterized by an increase in the KD and a decrease in the maximum number of binding sites (Bmax) of the high affinity binding site. These results suggest that disulfide bonds are important for ligand binding to vascular VIP receptors. The sulfhydryl alkylating agents, N-ethylmaleimide and iodoacetamide, had minimal effects on radioligand binding.

The role of adenylate cyclase in relaxation of brain arteries: studies with forskolin.

The natural product, forskolin, which stimulates adenylate cyclase by a direct, non-receptor-mediated mechanism, was studied for its effect on the tension of isolated brain arteries and adenylate cyclase activity of cerebral arteries. Helical strips of bovine and porcine basilar arteries and bovine middle cerebral arteries, which had been precontracted with prostaglandin F2 alpha (PGF2 alpha) or KCl, relaxed potently to administration of forskolin with ED50 values, ranging from 22 to 69 nM. Incubation of forskolin with a broken cell preparation of bovine cerebral arteries resulted in an efficacious stimulation of adenylate cyclase, approximating 5 times basal activity at a forskolin concentration of 1 microM. The metal salts nickel chloride and manganese chloride decreased the potency of vasorelaxation by vasoactive intestinal peptide (VIP), which stimulates adenylate cyclase via the VIP receptor. In contrast, nickel chloride had little effect on vasorelaxation by forskolin. The endogenous nucleoside, adenosine, which acts via the adenosine receptor and adenylate cyclase, relaxed bovine basilar and middle cerebral arteries with ED50 values ranging from 0.26 to 0.94 microM. The data presented support a role for adenylate cyclase in mediating vasodilation of brain blood vessels.

VIP receptors in mesenteric and coronary arteries: a radioligand binding study.

Using a biologically active radioligand, [Tyr(125I)10]VIP, we have identified and characterized receptors for vasoactive intestinal peptide (VIP) on membranes prepared from the rat superior mesenteric artery and bovine coronary arteries. Binding was specific, saturable, reversible and dependent on time and temperature. Scatchard analysis suggested the presence of a high and a low affinity binding site in each arterial system with the following binding constants: the rat mesenteric artery, KD = 0.22 +/- 0.02 and 13.6 +/- 7.8 nM (corresponding maximum number of binding sites, RO = 606 +/- 44 fmol/mg protein and 2.1 +/- 0.2 pmol/mg protein); bovine circumflex coronary artery, KD = 0.10 +/- 0.01 and 37.8 +/- 16.1 nM (corresponding RO = 369 +/- 65 fmol/mg protein and 2.0 +/- 0.7 pmol/mg protein); bovine left and right descending coronary arteries, KD = 0.12 +/- 0.03 and 21.3 +/- 6.4 nM (corresponding RO = 472 +/- 7 fmol/mg protein and 2.2 +/- 0.3 pmol/mg protein). The arterial VIP receptors did not recognize secretin, glucagon, apamin or bovine parathyroid hormone, and had reduced affinity for PHI, PHM and growth hormone releasing factors (GRF). These recognition properties were, by and large, similar to those seen in the bovine cerebral arteries although a between-species heterogeneity of recognition function could be deduced from the differences in the competitive binding of rat and bovine vascular VIP receptors with the corresponding species-specific GRFs.

Autoradiographic localization of binding sites for vasoactive intestinal peptide (VIP) in bovine cerebral arteries.

A substantial body of published evidence indicates that vasoactive intestinal peptide (VIP) may function as a vasodilatory neurotransmitter to cerebral blood vessels via a specific VIP receptor. In the present study in vitro autoradiography utilizing monoiodinated [125I-Tyr10]-VIP demonstrated VIP binding sites in the medial layer of bovine anterior, middle, and posterior cerebral arteries. This observation is consistent with the VIP receptor being localized in vascular smooth muscle components.

Fatal Aspergillus pneumonia in chronic granulomatous disease.

A 20-year-old man died from Aspergillus pneumonia three weeks after heavy exposure to grain dust. Lung biopsy and autopsy demonstrated a distinctive form of suppurating granulomatous bronchopneumonia caused by Aspergillus fumigatus, which was the sole agent cultured from the tissue. The liver and lymph nodes contained pigmented lipid histiocytes characteristic of chronic granulomatous disease, and subsequently both of the patient's brothers were found to have X-linked chronic granulomatous disease. The authors suggest that this morphologic expression of Aspergillus pneumonia should raise a suspicion of neutrophil dysfunction or chronic granulomatous disease.

Characterization of functional receptors for vasoactive intestinal peptide in bovine cerebral arteries.

This study reports the characterization of receptors for vasoactive intestinal peptide (VIP) on membranes prepared from bovine cerebral arteries. By use of HPLC we prepared two purified monoiodinated VIP radioligands with nearly equivalent cerebral vasorelaxant potency as native VIP, [Tyr(125I)10 )VIP and [Tyr(125I)22]VIP. The former resulted in a higher proportion of specific binding to arterial membranes than the latter and was therefore thought to be the superior radioligand for receptor characterization. The binding of [Tyr(125I)10]VIP to cerebral arterial membranes was saturable, specific, reversible, and dependent on time and temperature. Scatchard analysis suggested the presence of a high- and a low-affinity binding site with KD values of 0.2 and 11 nM and receptor concentrations of 79 and 737 fmol/mg of protein, respectively. The dose-response curves for binding to the VIP receptor by the VIP-homologous peptides PHI, PHM, and rat growth hormone-releasing factor (GRF) were very similar to their dose-response curves for relaxation of cerebral arteries. The order of potency was VIP greater than PHM greater than PHI greater than rat GRF. It is suggested that the characteristics of the vascular VIP binding sites and the close correlation between the binding and vasorelaxant properties of VIP and its related peptides argue for the vascular binding sites being functional receptors for VIP.

Localization, chromatographic characterization, and development of somatostatin-like immunoreactivity in the guinea pig retina.

We have studied the anatomical localization, chromatographic properties, and development of somatostatin-like immunoreactivity (SLI) in the guinea pig retina. The majority of guinea pig retinal SLI in the adult and fetus eluted similarly to somatostatin-28 by gel filtration and high pressure liquid chromatography. This represents a higher ratio of somatostatin-28-like material to total SLI than had been observed in retinal extracts from most other animal species. Somatostatin-like-immunoreactive cells and fibers were localized using two antisera in cryostat-sectioned and flat mounted retinae. Distribution of cells and fibers differed uniquely from that in other species previously reported. Reactive perikarya were located only in the far peripheral region: in the innermost layer of the inner nuclear layer (INL), in the inner plexiform layer (IPL), and in the ganglion cell layer. Reactive fibers were prominent in the IPL and nerve fiber layer (NFL) in both the peripheral and central retina. Less frequently, processes were observed between the NFL and IPL, between the IPL and INL, and, rarely, in the outer plexiform layer and outer nuclear layer. Small numbers of reactive fibers were found in the optic nerve and disc. These observations suggest that processes of intrinsic (amacrine or associational ganglion cells) and projection neurons (true ganglion cells or efferent fibers) containing SLI are intermingled in the guinea pig retina. SLI, quantified by radioimmunoassay, was present in the developing retina as early as the 6th week of gestation (full term is 10 weeks). Immunohistochemically detected somatostatin-like-immunoreactive elements were seen first at 2 weeks before birth, coincident with the onset of the period of most rapid increase in levels of assayed SLI. Somatostatin-like-immunoreactive peptides reached two-thirds of adult levels by birth.

Characterization of the relaxant effects of vasoactive intestinal peptide (VIP) and PHI on isolated brain arteries.

We have studied the vasorelaxant properties of vasoactive intestinal peptide (VIP) using helical strips of bovine, porcine and human brain arteries in vitro. The resting tension of the arterial strips was increased during experiments by prostaglandin F2 alpha or KCl so as to increase the magnitude of the relaxant response to VIP. Arteries supplying different regions of the bovine brain responded potently to VIP with ED50 values of 1.8 nM, 2.3 nM, 6.8 nM and 9.0 nM for the middle, anterior and posterior cerebral arteries and the basilar artery, respectively. The porcine basilar artery and branches of the human middle cerebral artery responded to VIP with ED50 values of 4.2 nM and 1.6 nM, respectively. The homologous neuropeptide, PHI, relaxed the bovine middle cerebral and porcine basilar arteries less potently than did VIP, with ED50 values for PHI being 11 nM and 43 nM, respectively. However, PHI elicited in the two arteries a maximal vasodilatory response of similar magnitude as did VIP. The other homologous peptides, human pancreatic growth hormone releasing factor 1-40 [hpGRF 1-40], secretin, and glucagon, and the VIP fragments, VIP 1-12 and VIP 10-28, were completely inactive. In contrast, VIP, which had been oxidized to VIP-(Met17 sulfoxide) or VIP-(Met17 sulfone), retained full activity. These structure-activity relationships for relaxation of brain arteries are consistent with previous studies of other biological responses to VIP.

Cerebral vascular adenylate cyclase: evidence for coupling to receptors for vasoactive intestinal peptide and parathyroid hormone.

We have studied the responsiveness of vascular adenylate cyclase to vasoactive intestinal peptide (VIP) and parathyroid hormone (PTH) using preparations of cerebral microvessels and arteries. Cerebral microvessels obtained from rats, guinea-pigs, cattle, and pigs all responded potently to bovine (b) PTH-(1-34), whereas considerable between-species variability was observed in the responsiveness to VIP. The homologous peptide to VIP, PHI (porcine heptacosapeptide), stimulated adenylate cyclase in both rat microvessels and a broken-cell preparation of bovine arteries. The ED50 values for activation of bovine arterial adenylate cyclase by VIP, PHI, and bPTH-(1-34) were 6.9 nM, 10 nM, and 100 nM, respectively, with the following order of efficacy: VIP = PHI greater than bPTH-(1-34). The other related peptides, hpGRF (human pancreatic growth hormone releasing factor), secretin, and glucagon, and the fragment VIP-(10-28) were inactive. The PTH antagonist, [Nle8, Nle18, Tyr34]bPTH-(3-34) amide, inhibited bPTH-(1-34) activation of vascular adenylate cyclase but did not affect activation by VIP using either microvessels or arteries. VIP or PHI demonstrated an additive effect with bPTH-(1-34) on vascular adenylate cyclase activity. However, the effects of VIP and PHI were nonadditive with each other. These data suggest that VIP and bPTH-(1-34) activate cerebral vascular adenylate cyclase by interacting with pharmacologically distinct receptors, whereas PHI and VIP likely interact with a common receptor.

Relaxation of bovine, porcine and human brain arteries by parathyroid hormone.

We have studied the relaxant effect of bovine parathyroid hormone (bPTH) on helical strips of branches of bovine and human middle cerebral arteries and bovine and porcine basilar arteries. All arteries were studied after contraction with prostaglandin (PG) F2 alpha or KCl. In the case of all arteries contracted with PGF2 alpha, the ED50 of PTH vasorelaxation related to maximal vasorelaxation induced by papaverine ranged from 9 to 14 nM for bPTH-(1-34) and 100 to 220 ng/ml for native bPTH-(1-84). The PTH inhibitor, [Nle8, Nle18, Tyr34]bPTH-(3-34) amide, attenuated the vasorelaxant effect of both bPTH-(1-34) and bPTH-(1-84). The vasorelaxant effects of PTH which we have observed in this study are consistent with the stimulatory effects of PTH on vascular adenylate cyclase which we had previously reported.