Adrenal cortical and medullary responses to acetylcholine and vasoactive intestinal peptide in conscious calves

1. Adrenal responses to intra-aortic infusions of acetylcholine and vasoactive intestinal peptide (VIP) have been investigated in functionally hypophysectomized calves given exogenous adrenocorticotrophic hormone (ACTH, 2 ng min-1 kg-1 I.V.). 2. Infusions of VIP at a dose of 0.13 micrograms min-1 kg-1 caused a small, but significant increase in adrenaline and noradrenaline output which was, however, far below the level recorded previously in response to acetylcholine (0.7 micrograms min-1 kg-1). In contrast, these doses of the two agonists produced closely similar rises in adrenal cortisol output. 3. The steroidogenic effects of acetylcholine and VIP were found to be strictly additive and no evidence of potentiation was obtained in relation to either cortical or medullary responses or in the case of any of the cardiovascular responses which were monitored. 4. Intra-aortic infusions of VIP, at a dose which produced a substantial increase in adrenal steroidogenesis (0.065 micrograms min-1 kg-1), had no effect on the output of catecholamines, enkephalin-like immunoreactivity or corticotrophin-releasing factor, either in the presence or absence of acetylcholine. 5. It is concluded that VIP is unlikely to modulate adrenal medullary responses to muscarinic stimulation in this species as it has been claimed to do in the rat and does not potentiate adrenal steroidogenesis in response to acetylcholine as it does to ACTH.

Screening Alzheimer's disease patients for mutations in the amyloid precursor protein gene

Over the past two years a series of different mutations has been discovered in the amyloid precursor protein (APP) gene in patients with Alzheimer's disease. All have been clustered in exons 16 and 17, a region encoding the beta A4 peptide found in the amyloid deposits of neuritic plaques and cerebral blood vessels. We have used the powerful technique of denaturing gradient gel electrophoresis to screen for mutations in exons 7, 16 and 17 of the APP gene in a cohort of 105 patients with presenile dementia of the Alzheimer type and 71 patients with autopsy-confirmed senile Alzheimer's disease. No new mutations were found, confirming earlier suggestions that APP mutations account for only a small proportion of cases of familial and sporadic Alzheimer's disease.

Changes in uterine G-protein content during pregnancy in the guinea pig

G-protein content (G(i) alpha, Gs alpha, Gq/11 alpha G(o) alpha and beta subunits) has been measured in membranes prepared from guinea pig uterus at different stages of pregnancy using SDS-PAGE and immunoblotting. Quantification using HRP- or 125I-labelled IgG as second antibody showed a good correlation between added membrane protein and measured G-protein content. Gs alpha appears as two bands of 45 kDa and 52 kDa respectively, the content of both were comparatively high in the non-pregnant uterus and fell about 4-fold close to term (60-67 days). G(i) alpha showed the converse with low level in membranes from the non-pregnant uterus with level approximately 6-fold higher by term. G(o) alpha exhibited changes similar to G(i) alpha. The changes in the content of Gq/11 alpha where biphasic, with comparatively high levels in membranes from the non-pregnant uterus, a sharp fall early in pregnancy followed by a 3-fold rise by near term. The uterine membrane content of the common beta subunit exhibited changes comparable to that of G(i) alpha and G(o) alpha with a 6-fold rise in content between non- and late-pregnant. Measurement of the effect of GTP gamma S action on phosphatidylinositol phospholipase C activity in uterine membranes with exogenous substrate showed pregnancy-dependent effects. In membranes from the non-pregnant uterus 0.1 microM GTP gamma S caused a modest stimulation of activity of 16 +/- 1.9%, whilst at 100 microM it inhibited the enzyme by 25 +/- 6.48%. In membranes from the late-pregnant guinea pig uterus GTP gamma S at both concentrations caused stimulation of enzyme activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipase A2 and arachidonic acid release from permeabilised myometrial cells from guinea pig uterus

The release of arachidonic acid and inositol polyphosphates from permeabilised myocytes derived from guinea pig uterus has been studied. Both are enhanced by free calcium at 100 nM and 10 microM and particularly by 50 microM GTP gamma S. To distinguish between the contributions of phospholipase C and A2 to the release of arachidonic acid the phospholipase C inhibitor neomycin was used. At 1 and 10 mM, but not at 0.1 mM, neomycin caused effective inhibition of inositol polyphosphate release of over 95%. Neomycin (1 mM) also reversed GTP gamma S-stimulated, but not calcium-stimulated release of arachidonic acid. This action was reflected in changes in [3H]arachidonic acid labelling of the membrane phosphatidylinositol and phosphatidylcholine pools, which were depressed by over 20% on the addition of 50 microM GTP gamma S, an effect completely reversed by 1 mM neomycin. The effects of neomycin were much more pronounced on inositol phosphate than on arachidonic acid release. The ability of 1 mM neomycin to inhibit arachidonic acid release was reversed by addition of 1 microM phorbol 12-myristate 13-acetate, implying a role for protein kinase C activation in stimulation of arachidonic acid release. Measurement of phospholipase A2 activity with 1-stearoyl 2-arachidonoyl phosphatidylcholine as exogenous substrate demonstrated the ability of 1 and 10 mM neomycin to inhibit the enzyme particularly when it was maximally activated with 1 mM free calcium.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation by protein kinase C of arachidonic acid release from permeabilised myometrial cells of guinea pig uterus

The effects of protein kinase C activation on phospholipase A2 and phospholipase C activity in permeabilised cultured myometrial cells from guinea pig uterus have been studied. Phospholipase A2 activity was followed by measurement of [3H]arachidonic acid release from [3H]arachidonic acid-prelabelled membrane lipids. [3H]Arachidonic acid release was stimulated by Ca2+ at 1-10 microM and by GTP gamma S at 1 microM to 1 mM in the presence of 10 microM Ca2+. The activation by calcium was enhanced 89.5 +/- 12.7% (P < 0.01) in the presence of 1 microM phorbol 12-myristate 13-acetate (PMA) and that by 1 microM GTP gamma S by 65.4 +/- 4.4% (P < 0.001). The PMA enhancement of arachidonic acid release was completely blocked by 3 microM staurosporine. Phospholipase C activation was followed by measurement of [3H]inositol polyphosphate production from [3H]inositol-prelabelled membrane lipids. This was stimulated by Ca2+ at 0.1 and 10 microM and by 1 and 50 microM GTP gamma S. PMA at 1 microM caused a consistent reduction in the extent of Ca2+ and GTP gamma S-stimulated inositol polyphosphate production and 3 microM reversed the inhibitory action of PMA. The data are consistent with arachidonic acid release in permeabilised myometrial cells from guinea pigs reflecting in large part phospholipase A2 activation and with that pathway being stimulated by protein kinase C activation. They are also consistent with protein kinase C activation causing reduction in phospholipase C pathways in uterine myocytes, at least as measured by inositol polyphosphate release.

Measurement of arachidonic acid release from permeabilised myometrial cells of guinea pig uterus

A technique has been developed for prelabelling and permeabilisation of guinea pig uterine myocytes to enable measurement of arachidonic acid release/phospholipase A2 activity in cells with intact membranes. Intact cells were prelabelled with [3H]inositol or [3H]arachidonic acid for measurement of phospholipase C and A2 respectively. In intact cells 10 nM endothelin-1 or 1 microM bradykinin stimulated both inositol polyphosphate and arachidonic acid release, whilst 1 microM oxytocin, arginine vasopressin or histamine were without effect. In Streptolysin-O permeabilised myometrial cells calcium-stimulation of inositol polyphosphate and arachidonic acid release was detected between 10 microM and 1 mM free calcium. The patterns of inositol polyphosphate and arachidonic acid release were broadly similar. Responses to 1 mM calcium were not detected in intact cells not treated with Streptolysin-O. For arachidonic acid release the K0.5 for calcium activation was about 7 microM, a level above that normally likely to be found in the uterine myocyte. Hence it is concluded that unless there are high local concentrations of calcium close to the plasma membrane, calcium is unlikely alone to be the primary regulator of arachidonic acid release and phospholipase A2.

Changes in phospholipase A2 in myometrium of the guinea pig uterus during pregnancy

Phospholipase A2 activity has been measured in membrane and cytosolic fractions from non-pregnant and pregnant guinea pig myometrium has been studied. Enzyme activity was measured with 1-stearoyl-2- [3H]arachidonoyl-phosphatidylcholine exhibiting Michaelis-Menton kinetics with Km of 83.8 +/- 21.6 and 53.2 +/- 14.1 for membrane and cytosolic enzymes respectively. Fractionation of the myometrium from non-pregnant guinea pigs suggested that 35% of the activity was membrane associated compared with 20% (P < 0.01) in tissue from pregnant animals. In the presence of 1 mM calcium total activity rose from 3.03 +/- 0.41 to 1737 +/- 368 nmol/h per uterus between non-pregnant and late pregnancy. Calcium activated the membrane enzyme, but the effect was greater late in pregnancy with almost a 6-fold increase in activity at 1 mM calcium compared with a doubling in membrane from non-pregnant guinea pigs. The K0.5 for calcium activation was about 150 microM. Immunoblotting with anti-human-110 KDa phospholipase A2 showed in guinea pig uterus a 34 KDa form of the enzyme that, consistent with changes in activity, showed a fifteen-fold increase in quantity between non-pregnant and late pregnancy. The data are consistent with dramatic increases in the capacity for arachidonic acid release and prostaglandin production in the guinea pig myometrium late in pregnancy.

GTP gamma S effects on phosphatidylinositol phospholipase C alpha isoenzyme activity isolated from guinea pig uterine smooth muscle at different stages of pregnancy

The ability of GTP gamma S to activate release of inositol polyphosphates from isolated permeabilised guinea pig uterine smooth muscle cells and from partially purified PI-PLC alpha has been studied. Streptolysin O permeabilised and [3H]inositol prelabelled cells show a time dependent release of inositol polyphosphates, predominantly inositol 4-phosphate. Ca2+ stimulated IP release with a Ka of 161 +/- 1.1 nM and this was further enhanced in an additive manner by GTP gamma S between 1-100 microM; the Ka for Ca2+ in the presence of 0.1 mM GTP gamma S was 117 +/- 0.7 nM. GTP gamma S activation of IP production did not require Ca2+ in the medium. Permeabilisation of the uterine smooth muscle cells with Streptolysin O readily released PI-PLC activity into the medium. However, unlike studies with isolated membranes 63.4 +/- 6.4% of the enzyme activity remained associated with membranes and/or particulate fractions of the cell. Studies were undertaken with PI-PLC alpha, the predominant isoenzyme form, partially purified from uterine smooth muscle at different stages of pregnancy by Q-Sepharose and Heparin-Agarose chromatography. The enzyme co-purifies with firmly associated GTP-binding activity. Enzyme prepared from near-term uterus is activated by 0.1 mM GTP gamma S, up to 100% when Ca2+ is between 0.1-1 microM, while 10 microM AlF4- under those conditions caused complete inhibition of the enzymes. Responses for enzymes prepared from non-pregnant uteri were broadly similar. In contrast enzyme preparations from guinea pig uteri at 20-60 days of pregnancy show an inhibition of activity in response to 0.1 mM GTP gamma S addition.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in phosphatidylinositol phospholipase C isoenzymes and in their association with GTP gamma S-binding activity in guinea pig uterine smooth muscle during pregnancy

The nature distribution and associated GTP gamma S binding activity of phosphatidylinositol phospholipase C (PI-PLC) has been studied in non-pregnant and pregnant guinea pig uterine smooth muscle. Cytosolic fractions partially purified by Q-Sepharose and heparin-Agarose chromatography show two isoenzyme forms, one with an apparent molecular weight of 58 kD that crossreacts with PI-PLC alpha and a has Km for phosphatidylinositol of 292 +/- 72.6 microM, designated alpha, and a form that has an apparent molecular weight of 86 kD and a substrate Km of 54 +/- 20 microM designated delta. Approximately 80% of the total PI-PLC activity was recovered in the cytosolic fraction and this increased 8-10 fold for both isoenzymes from the non-pregnant to the late pregnant uterus and the proportion of the alpha isoenzyme increased from approximately 40% to 55% of the total. PI-PLC alpha but not delta activity had GTP gamma S binding activity associated with it after Q-Sepharose or heparin-Agarose chromatography. This associated activity accounted for 2% of the total GTP gamma S-binding activity in the non-pregnant uterus and 31% of that in the near-term uterus. On separation of the PI-PLCa-GTP gamma S-binding complex by gel filtration on Sephacryl S200 gave two peaks one of 118 kD accounting for two-thirds of all the binding and two-thirds of the enzyme activity and a 58 kD peak. The 118 kD peak could not be separated by treatment with 0.5% cholate, but in this form enzyme activity was protected from detergent inactivation found with the 58 kD form. In sodium dodecyl sulphate polyacrylamide-gel electrophoresis PI-PLC alpha was released from the 118 kD complex and showed an apparent molecular weight of 61.5 kD. All the activity in the residual membrane fraction could be released by washing with buffer followed by, 2 M KCl and then 2 M KCl plus 0.5% cholate. This released isoenzyme forms that appeared identical to those in the cytosolic fraction and with GTP gamma S-binding activity associated with PI-PLC alpha. It is concluded that in the near term guinea pig uterus there is a dramatic increase in the capacity for inositol polyphosphate production. Moreover the dramatic increase in GTP gamma S-binding activity associated with PI-PLC alpha implies large changes in the extent and possibly nature of the putative G-protein activation of this pathway.(ABSTRACT TRUNCATED AT 400 WORDS)

Mutation in codon 713 of the beta amyloid precursor protein gene presenting with schizophrenia

Following reports of mutations of codon 717 in exon 17 of the amyloid precursor protein (APP) gene in early-onset familial Alzheimer's disease, we screened exon 17 for new mutations in presenile dementia. The majority of the 105 patients screened had definite or probable Alzheimer's disease, but we also included atypical cases and some chronic schizophrenics. We identified a single abnormal case--a chronic schizophrenic with cognitive defects. Sequencing revealed a C to T nucleotide substitution which produces an alanine to valine change at codon 713. We were unable to detect the mutation in the remaining members of the original cohort nor in a further 100 chronic schizophrenics and 100 non-demented controls. Nonetheless, the position of the mutation in a critical portion of the APP gene suggests that it may well prove to be pathogenic.

Enhancement of noradrenaline-induced inositol polyphosphate formation by glucocorticoids in rat vascular smooth muscle cells

Glucocorticoids are known to regulate the contractility of vascular smooth muscle by increasing its response to noradrenaline. The molecular mechanisms for achieving this remain unclear. Recent results in our laboratory have demonstrated that glucocorticoids affect both alpha 1-adrenoceptor number and coupling to G proteins. Whether this leads to an increase in second-messenger production has to be established. The present experiments, therefore, report the effects of dexamethasone on inositol polyphosphate production in vascular smooth muscle cells in culture. Noradrenaline induced the release of inositol polyphosphates from prelabelled [3H]inositol phosphoinositides in the membrane in a dose-dependent manner. The concentration of noradrenaline which caused half-maximal response was 1.26 mumol/l. Prazosin inhibited noradrenaline-induced inositol monophosphate formation to 10.26 +/- 3.67% (mean +/- S.E.M.; P less than 0.01, n = 5) of control value whereas yohimbine reduced it to only 61.74 +/- 11.82% (P less than 0.05, n = 5), suggesting an action primarily through alpha 1-adrenergic receptors. Dexamethasone (100 nmol/l, 48 h) enhanced noradrenaline-induced inositol monophosphate, bisphosphate and trisphosphate formation up to twofold (P less than 0.001, n = 5). The enhancement of the response occurred despite the fact that dexamethasone reduced [3H]inositol prelabelling of membrane phosphoinositides by 49.5 +/- 9.9% (P less than 0.05, n = 3). The present results suggest that the potential action of glucocorticoids on vascular smooth muscle contractility is, at least in part, through controlling alpha 1-adrenoceptor-mediated second-messenger production.

Three novel mutations in the cystic fibrosis gene detected by chemical cleavage: analysis of variant splicing and a nonsense mutation

We have used the chemical cleavage mismatch technique to screen for mutations in the cystic fibrosis gene. Analysis of exons 10 and 11 in the first nucleotide binding fold led to the detection of several described mutations and two novel mutations, V520F and C524X. V520F results from a G-->T nucleotide substitution changing a valine to a phenylalanine residue, while C524X (a nonsense mutation), results from a C-->A transversion. A third novel mutation, Q1291H (G-->C), at the last nucleotide of exon 20, would substitute a histidine residue for glutamine. Further study, involving RNA based PCR, revealed that Q1291H was also a splice mutation. Both correctly and aberrantly spliced mRNAs are produced from the Q1291H allele. The incorrectly spliced product results from the use of a nearby cryptic splice site 29 bases into the adjacent intron.

The role of corticotrophin releasing factor in relation to the neural control of adrenal function in conscious calves

1. Adrenal responses to intra-aortic infusions of pure synthetic ovine corticotrophin releasing factor (CRF), and to electrical stimulation of the preganglionic sympathetic innervation, have been investigated in functionally hypophysectomized conscious calves, in the presence and absence of a specific CRF antagonist. 2. CRF exerted a substantial steroidogenic effect on the adrenal gland of functionally hypophysectomized calves when infused intra-aortically at a dose (1.3 ng min-1 kg-1) below that which caused any fall in the arterial blood pressure. This response was significantly reduced, but not abolished by a concomitant infusion of CRF-antagonist into the aorta. 3. The steroidogenic effect of CRF was significantly reduced in the presence of exogenous adrenocorticotrophic hormone (ACTH) (2 ng min-1 kg-1, I.V.) and the surviving response was completely abolished by CRF-antagonist. 4. Stimulation of the peripheral end of the splanchnic nerve at 4 Hz in functionally hypophysectomized calves given exogenous ACTH produced a rise in mean adrenal output of the same order of magnitude as did exogenous CRF under the same conditions. The response to splanchnic nerve stimulation was apparently unaffected by CRF-antagonist although release of endogenous CRF from the gland was significantly increased thereby. 5. These results indicate that release of CRF from the adrenal gland during splanchnic nerve stimulation in the calf does not contribute significantly to the steroidogenic response thereto.

Inhibitory effects of proopiomelanocortin on cortical and medullary activity in the calf adrenal

The effects of the infusion of ACTH1-24 and proopiomelanocortin on the denervated adrenal gland have been investigated in conscious 3-6 weeks-old calves by means of the adrenal-clamp technique. To prevent variation in the release of endogenous ACTH the pituitary stalk was cauterized during preparatory surgery. ACTH1-24 (5 ng/min per kg, i.v.) increased the output of cortisol from the adrenal by about 500 ng/min per kg body weight and this effect was rapidly reduced by simultaneous infusion of ovine proopiomelanocortin at 5 ng/min per kg. Release of met5-enkephalin and leu-enkephalin from the adrenal was reduced by ACTH1-24 (P less than 0.05) and this effect was enhanced significantly by additional infusion of proopiomelanocortin (P less than 0.02). However met5-enkephalin represented less than one-third of the met-enkephalin containing peptide released and the output of this pool was unaffected by infusion of ACTH. Proopiomelanocortin had no effect on met5-enkephalin production nor that of the total pool of met5-enkephalin containing peptides. It is concluded that ACTH reduced the quantity of pro-enkephalin processed to met5-enkephalin.

Enkephalin peptides in fetal sheep, changes with gestation, origin and production by the placenta

Enkephalin-containing peptides have been followed in the circulation of fetal sheep between 118-143 days gestation. Using a combination of radioimmunoassay and hplc met5-enkephalin was found in the concentration range 60-500 pg/ml and proenkephalins containing met5-enkephalin had a concentration of 150-4000 pg/ml. The concentration of both increased towards term. The sources of the enkephalin peptides was investigated by measurement of differences across the umbilical circulation and by studying the effects of fetal adrenal demedullation and chemical sympathectomy. The placenta showed a continuous net output of enkephalin peptides which increased close to term. This placental output was increased sharply by reduction of uterine blood flow either using compression of the uterine artery or through infusion of adrenaline at 35 micrograms/min into the maternal circulation. Maternal hypoxia caused by breathing 9% O2 plus 3% CO2 also increased fetal plasma enkephalin levels, although not output from the placenta. Adrenal demedullation, particularly if accompanied by chemical sympathectomy depressed fetal plasma enkephalin concentrations and sharply suppressed the fetal peptide responses to maternal hypoxia. It is concluded that the placenta and the fetal adrenal are important sources of met5-enkephalin-containing peptides in the fetal circulation. The placental production appears to be closely tied to changes in uterine perfusion and adrenal output changes in response to fetal oxygenation.

Muscarinic adrenal responses to acetylcholine in conscious calves

1. Adrenal responses to intra-aortic infusions of acetylcholine (4.5 nmol min-1 kg-1 for 10 min) have been investigated in hypophysectomized conscious calves given exogenous adrenocorticotrophic hormone (ACTH) (2 ng min-1 kg-1 I.V.) in the presence and absence of hexamethonium. 2. Acetylcholine produced a significant increase in adrenal cortisol output and plasma cortisol concentration. In the absence of nicotinic blockade with hexamethonium this was apparently accounted for by an increase in adrenal ACTH presentation secondary to increased adrenal blood flow. However, administration of hexamethonium revealed a direct steroidogenic action of acetylcholine in the presence of exogenous ACTH. 3. Adrenal medullary responses to acetylcholine, including the release of catecholamines, enkephalins and corticotrophin-releasing factor (CRF), were not significantly reduced by nicotinic blockade. 4. It is concluded that both adrenal medullary and cortical responses to intraaortic infusions of acetylcholine at a low dose are mediated mainly by muscarinic receptors, as it has previously been shown that they are substantially reduced in the presence of atropine.

Endocrine responses to intra-aortic infusions of acetylcholine in conscious calves

1. Adrenal responses to intra-aortic infusions of acetylcholine (4.5 nmol min-1 kg-1 for 10 min) have been investigated in conscious, functionally hypophysectomized, 3- to 6-week-old calves, in the presence and absence of exogenous ACTH (2 ng min-1 kg-1, I.V.). 2. Acetylcholine produced a substantial fall in adrenal vascular resistance, which was significantly reduced in the presence of exogenous ACTH, while producing minimal changes in aortic blood pressure and heart rate. 3. There was also a significant rise in right adrenal cortisol output which was sufficient to produce a measurable rise in plasma cortisol concentration. The effect could be accounted for by the increase in adrenal ACTH presentation. It was abolished by pre-treatment with atropine (0.2 mg kg-1). A small but significant rise in aldosterone output during acetylcholine infusions was also abolished in the presence of ACTH. 4. Both adrenaline and noradrenaline were released during intra-aortic acetylcholine infusions and these responses were substantially reduced, but not abolished, by pre-treatment with atropine. 5. Acetylcholine also stimulated the release of corticotrophin-releasing factor (CRF) and [Met5]enkephalins from the gland. The output of CRF was enhanced and that of free [Met5]enkephalin was significantly reduced in the presence of exogenous ACTH. All these responses were largely, but not completely, suppressed by atropine. 6. Acetylcholine also promoted the release of the pancreatic hormones glucagon, insulin and pancreatic polypeptide (PP). The amounts of pancreatic glucagon and insulin that were released were highly dependent on the concentration of glucose in the circulating plasma and all these responses were abolished by atropine. 7. It is concluded that acetylcholine is capable of stimulating the release of a wide variety of agonists from the adrenal gland when infused intra-aortically at a dose of 4.5 nmol min-1 kg-1. The increase in cortisol output appears to be secondary to an increase in blood flow whereas the adrenal medullary responses are not, and appear to be due largely, but not entirely, to activation of muscarinic receptors.

Control of glucose metabolism in the perinatal period

The central importance of glucose as a fuel for energy metabolism and growth of the fetus is clear as is the role of insulin in coordinating its utilisation by many fetal tissues. What is less clear is the qunatitative nature of the interaction between the fetus and placenta in organising glucose metabolism. Increasingly there is evidence that the fetus coordinates some of the supply of glucose to the placenta and that this is particularly important when uterine blood flow is reduced. It is unclear how this is regulated, but substrate cycles of glucose and lactate appear to make a significant contribution to carbohydrate metabolism in fetus and placenta. Another area as yet unresolved in the control of fetal glucose metabolism is the coordination of the changes that occur around the time of birth. Notable of these is the activation of glycogen mobilisation and of glucose synthesis and changes in the setting of glucose regulatory mechanism. These are briefly reviewed.

Adrenal responses to corticotrophin-releasing factor in conscious hypophysectomized calves

1. Adrenal responses to intra-aortic infusions of pure synthetic ovine corticotrophin-releasing factor (CRF) have been investigated in functionally hypophysectomized calves previously fitted with an adrenal clamp. 2. CRF caused an increase in the output of cortisol from the adrenal gland, which was dose related over the range 4-8 pmol min-1 and maximal at the higher of these doses; this response was observed at a dose below that which produced any change in adrenal vascular resistance. Cortisol output was also found to be related linearly to the rate at which CRF was estimated to be presented to the gland during these infusions. 3. The infusions of CRF also provoked the release of small, but readily detectable, amounts of adrenocorticotrophin-like peptides (ACTH) from the gland. This was mainly in the form of ACTH1-39 with some pro-opiomelanocortin (POMC) also being released. 4. Comparison of the adrenal steroidogenic response to exogenous CRF with that to synthetic ACTH1-24 showed that CRF was the more potent; in each case cortisol output was related linearly to the presentation rate of the peptide. 5. It is concluded that the adrenal cortex in the calf is capable of releasing cortisol in response to exogenous CRF at low concentrations and is even more sensitive to CRF than it is to exogenous ACTH over the dose range that was employed.

The effect of changes in adrenal blood flow on adrenal cortical responses to adrenocorticotrophin in conscious calves

1. The effect of varying adrenal blood flow on the rate at which it was estimated that adrenocorticotrophin (ACTH) was presented to the adrenal gland was related to right adrenal cortisol output in conscious calves fitted with 'adrenal clamps'. 2. Intra-aortic infusions of endothelin at either 15.0 or 7.5 pmol min-1 kg-1 produced a substantial fall in right adrenal blood flow which was dose-related over this range. There was an associated fall in right adrenal cortisol output and cortisol output was linearly related to estimated ACTH presentation to the gland over the whole range investigated. The changes in adrenal cortisol output were reflected by changes in the concentration of cortisol in the peripheral plasma, which could be attributed entirely to the fluctuations in adrenal cortisol output. 3. It is concluded that delivery of ACTH to the adrenal gland is flow dependent over the physiological range in these animals and that changes in adrenal cortical blood flow can therefore be expected to result in changes in adrenal output due to variations in the presentation rate of ACTH.

Glucocorticoids regulate the amount of G proteins in rat aorta

Glucocorticoids are known to influence cardiovascular sensitivity to catecholamines but the molecular mechanisms are undefined. We recently showed that glucocorticoids control the coupling of adrenergic receptors to G protein. Alterations in the amount of G protein is one mechanism by which receptor-G protein coupling may be controlled. Therefore, we set out to measure the levels of G proteins in aorta from normal, adrenalectomized and dexamethasone-treated adrenalectomized rats. G proteins were measured in plasma membrane preparations by immunoblotting and horseradish peroxidase staining. After adrenalectomy there was 53% (n = 5) decrease in the density of staining for Gi (ANOVA; P less than 0.05 compared to controls). Conversely, there was a 210% (n = 5) increase in the density of staining for Gs. The levels of Go and the beta-subunit of G proteins were not changed by adrenalectomy. Dexamethasone-replacement treatment after adrenalectomy returned Gi and Gs close to control values. Go remained unaltered compared to controls but was 24% (n = 3) less than the adrenalectomized values (ANOVA; P less than 0.05). The levels of beta-subunit after dexamethasone replacement were significantly greater (ANOVA; P less than 0.05) than both the controls and adrenalectomized values. These results show that glucocorticoids can differentially regulate the amounts of G proteins in rat aorta as in other tissues. This may be an important mechanism by which steroids control receptor-G protein coupling and hence transmembrane signalling pathways in vascular smooth muscle.

Influence of pregnancy on G-protein coupling to adenylate cyclase activation in guinea-pig myometrium

The regulatory factors controlling uterine activity during pregnancy remain unclear in many species. Since myometrial relaxants raise intracellular cyclic AMP, modulation of signalling pathways coupling cell-surface receptors to adenylate cyclase activation could be an important site for control. To assess the functional activity of the stimulatory GTP-binding protein Gs we have measured adenylate cyclase activation by GTP, its non-hydrolysable analogue guanosine 5'-(beta-gamma-imido)triphosphate (Gpp(NH)p), fluoride, forskolin and manganese in a 50,000 g membrane fraction prepared from the myometrium of non-pregnant, mid-pregnant (30-32 days) and late-pregnant (62-66 days) guinea-pigs (full term 67 +/- 2 days). While forskolin- and manganese-dependent enzyme activation was unaltered by pregnancy, maximal stimulation by Gpp(NH)p and fluoride was enhanced by up to 200%. Recovery of adenylate cyclase activity in the 50,000 g fraction was essentially constant at 20-24% of the total activity throughout pregnancy, and thus cannot explain the increases observed. Since guanine nucleotides and fluoride stimulate adenylate cyclase through activating Gs, and forskolin and manganese act at the level of the catalytic unit, these data are consistent with a pregnancy-related increase in Gs functional coupling while adenylate cyclase activity is unaltered. These observations suggest a physiological regulation of myometrial Gs activity during pregnancy which could facilitate hormonal stimulation of adenylate cyclase and contribute to uterine quiescence by increasing uterine sensitivity to relaxants.

Effect of glucocorticoids on alpha 1-adrenergic receptor binding in rat vascular smooth muscle

Glucocorticoids are known to have marked effects on blood pressure regulation, predominantly through altering cardiovascular sensitivity to noradrenaline. However, the molecular mechanisms underlying this action remain unclear. As part of our studies into these we have measured alpha 1-adrenergic receptor binding using the ligand [3H]prazosin in plasma membrane fractions of aortas prepared from control, adrenalectomized and dexamethasone-treated adrenalectomized rats. In controls there were 50 +/- 8 (S.E.M.; n = 6) fmol alpha 1-adrenergic receptors/mg membrane protein (Bmax) with a dissociation constant (Kd) of 0.52 +/- 0.10 nM (n = 6). Adrenalectomy 8 days before tissue preparation caused a 40% decrease in Bmax and a 60% decrease in Kd. Dexamethasone replacement after adrenalectomy returned these values close to those of controls. Noradrenaline competed for the [3H]prazosin-binding sites. Computer analysis by a non-linear curve-fitting program (LIGAND) showed that noradrenaline binding was to a heterogeneous population of high- and low-affinity receptors with Kd values of 1.87 +/- 0.73 microM and 0.48 +/- 0.12 mM (n = 5) respectively. Guanosine thiotriphosphate (GTP[S]) caused the conversion of high-affinity to low-affinity binding, consistent with the model of the high-affinity sites being coupled to a G protein. After adrenalectomy, noradrenaline binding was to a homogeneous population of low-affinity receptors; hence, the effect of GTP[S] was no longer apparent, suggesting that under these conditions the alpha 1-adrenergic receptors were unable to couple to a G protein. The two-site model of binding and GTP[S] effect was returned by dexamethasone treatment.(ABSTRACT TRUNCATED AT 250 WORDS)