Stimulatory role for endogenous opioid peptides on postexercise insulin secretion in rats

Endogenous opioid peptides (EOP) and prior exercise may modulate the stimulatory effect of glucose on insulin secretion. To gain insights into these relationships, we studied male Wistar rats (187-245 g) during sustained hyperglycemia by use of the glucose clamp technique. Four groups of sedentary fed rats (n = 8/group) either ran (Ex) at 24 m/min, 0% grade, or rested (R) for 40 min. Thirty minutes after Ex or R, arterial blood glucose was elevated to and maintained at 11 mM for 2 h by a variable glucose infusion. At the start of Ex or R rats had saline (Sal) or naloxone (Nal, an opioid antagonist) intravenous infusions for 160 min (40 min Ex + 30 min R + 90 min of a 120-min glucose clamp). Steady-state glucose infusion rates (SSGIR) were approximately 55 mg.kg-1.min-1 at the start of the clamp and declined significantly over the 2nd h to approximately 45 mg.kg-1.min-1. No significant differences existed in SSGIR between groups. R-Sal and Ex-Sal groups did not differ in their insulin response to hyperglycemia. In contrast, when all groups were compared at the end of the Nal or Sal infusion, Ex-Nal had the lowest insulin concentration (749 +/- 174 pmol/l), whereas the R-Nal group had the highest (1,581 +/- 216 pmol/l, P less than 0.05). These data suggest a stimulatory role for EOP on insulin secretion that is expressed after a prior stress (Ex). Thus one function of exercise-induced activation of EOP may be to regulate insulin secretion in the immediate postexercise period.     

Possible participation of endogenous opioid peptides on the mechanism involved in analgesia induced by vouacapan.

The involvement of opioid peptides in the mechanism of action of vouacapan, a new experimental compound extracted from seeds of Pterodon poligalaeflorus Benth, was investigated both in mice utilizing acetic acid writhing response and in rats utilizing inflammatory hyperalgesia induced by carrageenan and modified Randall-Selitto method. Vouacapan, in both models, caused a dose-dependent analgesia when injected p.o., s.c. and i.p. The analgesic effect was partially blocked by naloxone, nalorphine and n-methyl-nalorphine. Significant tolerance to analgesic effect was observed following repeated administration of vouacapan or morphine. On the last day of treatment, cross administration revealed symmetrical and asymmetrical cross-tolerance between vouacapan and morphine, in rats and mice, respectively. We conclude that a release of endorphins could be involved in the analgesic mechanism of vouacapan in both models tudied.     

Prostaglandin F2alpha (PGF2alpha) and prolactin secretion in rats.

Plasma prolactin and F-prostaglandins (PGF) were measured anesthetized male Sprague-Dawley rats before and at 15, 30, 45 and 60 minutes following i.v. injection of either PGF2alpha (4 mg/kg), chlorpromazine, 1 mg/kg or chlorpormazine (1 mg/kg) after pretreatment with i.p. indomethacin (2 mg/kg). Following PGF2alpha administration, plasma prolactin levels increased significantly only at 15 and 30 minutes in spite of extremely high PGF levels throughout 60 minutes. Besides the expected rise in plasma prolactin, chlorpromazine caused a transient but statistically significant increase in PGF. Indomethacin blocked the chlorpormazine-induced PGF rise but not prolactin increase. Animals stressed with ether anesthesia showed elevation of plasma prolactin, which was not blocked by indomethacin although PGF concentration fell. Theese results indicate that PGF2alpha can stimulate prolactin release. This effect does not appear to be physiologic since very high PGF levels are required. Furthermore, blockade of prostaglandin synthesis by indomethacin does not prevent the release of prolactin in response to chlorpormazine or stress. Our findings do not support a possible role of PGFs as intermediaries in prolactin release. However, it is possible that PGFs may work through other mechanisms not investigated in our study.     

Effect of insulin on alpha1-adrenergic actions in hepatocytes from euthyroid and hypothyroid rats. Possible involvement of two pathways in alpha1-adrenergic actions

The effect of insulin on the alpha1-adrenergic stimulation of glycogenolysis and ureogenesis, which is very small or undetectable in hepatocytes from control animals, is marked in hepatocytes from hypothyroid rats; the metabolic actions due to alpha1-adrenergic activation, but not those due to glucagon, were nearly blocked by insulin in cells from hypothyroid rats. The alpha1-adrenergic-mediated stimulation of phosphatidylinositol labelling was not affected by insulin in cells from either control or hypothyroid rats. The data suggest that the alpha1-adrenergic action proceeds through two pathways, one of which is very sensitive to insulin and predominates in cells from hypothyroid rats.     

Control of breathing by endogenous opioid peptides: possible involvement in sudden infant death syndrome.

Experiments have been performed in order to evaluate the respiratory consequences of a suppression or accumulation of endogenous opioid peptides, in the neuronal network which generates the motor respiratory activity. Iontophoretic application of naloxone onto respiratory neurons increases their firing activity and increases their respiratory modulation. On the other hand the local injection of kelatorphan (an enkephalinase inhibitor) decreases the firing of respiratory neurons and thus reduces the respiratory modulation. This effect of kelatorphan mimics the effect on respiratory neuron of an iontophoretic application of met-enkephalin. Furthermore the local injection of kelatorphan reduces the frequency of the respiratory output recorded from the phrenic nerve. This effect is reversed by systemic administration of naloxone. The results demonstrate the involvement of endogenous opioid peptides in the control of breathing suggesting that in Sudden Infant Death Syndrome a possible dysregulation in opioidergic system could occur.     

Protection against alloxan-induced diabetes in mice by stimulation of alpha 2-adrenergic receptors

A single intravenous injection of alloxan in mice induced hyperglycemia in a dose dependent fashion. This diabetogenic action of alloxan was prevented by a single intraperitoneal injection of the alpha 2-adrenergic agonists, i.e. oxymetazoline, clonidine or epinephrine 40 min prior to the injection of alloxan. The alpha 1-adrenergic agonists, i.e. methoxamine and phenylephrine, and a beta-adrenergic agonist, isoproterenol, failed to prevent the diabetogenic action of alloxan. The inhibitory effect of clonidine on alloxan-induced diabetes was antagonized by yohimbine or phentolamine, but not by prazosin. Although alpha 2-adrenergic agonists caused a transient hyperglycemia at the time of alloxan administration (40 min after the administration of alpha 2-adrenergic agonists), the plasma glucose level at the time of alloxan injection did not correlate with the anti-diabetogenic effect of alpha 2-adrenergic agents. These results clearly demonstrate that the alpha 2-adrenergic mechanism which inhibits insulin release from pancreatic B cells prevented the diabetogenic action of alloxan in mice.     

Ovarian steroid involvement in endogenous opioid modulation of LH secretion in seasonally anoestrous mature ewes

In June, 16 mature ewes were ovariectomized and allocated to four groups: 1, saline; 2, naloxone; 3, progesterone implant plus naloxone; 4, oestrogen implant plus naloxone. Steroids were implanted at the time of ovariectomy. At 5 days after ovariectomy, the animals were intravenously infused with saline for 8 h and naloxone (50 mg/h) in saline for 8 h the following day. Three intact ewes were given naloxone in a similar way. During infusions and for 8 h on the day after naloxone, jugular venous blood samples were taken every 15 min and assayed for LH. Naloxone resulted in significant increases in mean LH concentration (P less than 0.01), LH episode frequency and episode height (P less than 0.05) in Group 3 ewes, but was without effect in any other group. These results provide evidence that the progesterone status of the ewe affects its response to naloxone, that progesterone negative feedback on LH release may be mediated by an opioid system, and that increased oestradiol negative feedback during seasonal anoestrus is unlikely to work via increased opioid inhibition of LH.     

A role for the endogenous opioid peptides in the regulation of gonadotropin secretion in the primate

This short review discusses the gonadotropin-releasing hormone 'pulse generator' in the female rhesus monkey and the important modulatory role which the endogenous opioid peptides exert in the control of its activity. Also discussed is the modulation of opioid peptide release by ovarian steroids and corticotropin-releasing hormone.     

阿片肽对胰岛素释放的影响

阿片肽作为一类重要的神经活性物质发挥着许多生物学效应,近来已有研究证明类阿片物质有影响胰岛素释放的作用.胰岛素是由胰岛β细胞分泌的一种重要激素,它可以调节机体的血糖稳定.因此这些结果将可能为糖尿病治疗开辟新天地,但其具体作用机制目前尚不清楚.本文根据国内外研究成果及最新研究进展,主要介绍了阿片肽及其受体的生理功能,阿片受体介导的信号转导以及阿片肽对胰岛素释放的调节机制.     

Possible involvement of prolactin in endocrine-resistant metastatic prostate cancer

The hormone resistance of prostate cancer has been proved to depend at least in part on enhanced neuroendocrine activity and the resultant increase in blood concentrations of chromogranin A. Other experimental observations have suggested the involvement of prolactin (PRL), which appears to be a potential growth factor for prostate cancer. Abnormally high levels of PRL have been detected in metastatic prostate cancer, but the clinical significance of this finding has still to be clarified. In an attempt to explain the prognostic significance of serum PRL levels in prostate cancer, in this preliminary study we have analyzed the PRL levels in a group of metastatic prostate cancer patients with hormone-dependent or hormone-resistant cancer. The study included 50 patients with metastatic prostate cancer, 15 of whom had hormone-resistant tumors. The serum levels of PRL were measured by the RIA method. Abnormally high concentrations of PRL were found in 11/50 (22%) patients. Moreover, the percent of patients with cancer-related hyperprolactinemia was significantly higher in the hormone-resistant group than in the hormone-dependent group (8/15 vs 3/35, p < 0.01). This study confirms the possible existence of a hyperprolactinemic state in metastatic prostate cancer, as previously reported by other authors. Moreover, it appears to demonstrate that the occurrence of hyperprolactinemia is more frequent in hormone-resistant neoplasms, suggesting the possible involvement of PRL in hormone independence. Further studies concomitantly evaluating PRL and chromogranin A blood concentrations will be necessary to establish whether the hyperprolactinemia precedes and promotes the onset of hormone resistance in prostate cancer, or whether it is simply a consequence of the hormone independence.     

The stimulation of glycogenolysis in isolated hepatocytes by opioid peptides.

The opioid peptides [Leu]enkephalin and dynorphin-(1-13) were shown to enhance glycogen breakdown when added directly to hepatocytes. This was the result of a concerted effect on the enzymes of glycogen metabolism, with a stimulation of glycogen phosphorylase activity and a simultaneous decrease in glycogen synthase I activity. The latter only became significant when the enzyme was activated by incubating the cells in presence of 20 mM- or 40 mM-glucose. The effect of the opioid peptides was independent of an increase in cyclic AMP or any change in the activity ratio of the cyclic AMP-dependent protein kinase and was abolished by depleting the cells of Ca2+. Both [Leu]enkephalin and dynorphin-(1-13) produced a significant decrease in cyclic AMP formation, suggesting that in liver, as in neuronal tissue, they may act by inhibiting adenylate cyclase activity.     

Possible involvement of two mechanisms of signal transduction in alpha 1-adrenergic action. Selective effect of cycloheximide

We have previously suggested that the effects of alpha 1-adrenergic agents on hepatocyte metabolism involve two pathways: (a) a calcium-independent, insulin-sensitive process which is modulated by glucocorticoids; and (b) a calcium-dependent, insulin-insensitive process which is modulated by thyroid hormones. Cycloheximide stimulated ureogenesis through a prazosin-sensitive mechanism in liver cells (alpha 1-adrenergic). The effect of cycloheximide was insulin-insensitive and calcium-dependent. Furthermore, a clear effect of cycloheximide was observed in hepatocytes obtained from adrenalectomized animals, whereas no effect was observed in cell from hypothyroid rats. The effects of epinephrine and cycloheximide were blocked by phorbol esters in all the conditions tested. Binding competition experiments indicated that cycloheximide interacts with only a fraction of the alpha 1-adrenergic sites labeled with [3H]prazosin. It is suggested that cycloheximide activates preferentially one of the pathways involved in the alpha 1-adrenergic action in liver cells.     

Possible role of an endogenous opioid in the antihypertensive action of propranolol in spontaneously hypertensive rats

The effect on systolic blood pressure and heart rate of the acute and chronic intraperitoneal (i.p.) administration of d- and dl-propranolol was investigated on unanesthetised spontaneously hypertensive rats. The effect of naloxone on the propranolol induced hypotension was also studied to test the hypothesis that the antihypertensive effect of propranolol involves the release of an endogenous opiate. On i.p. administration, 3 mg/kg d-propranolol was inactive; 3 and 30 mg/kg dl-propranolol decreased blood pressure and heart rate in a dose-dependent manner. When the rats were pretreated with 2 mg/kg naloxone i.p., the effect of propranolol on the blood pressure was nearly completely abolished, while that on the heart rate was only partially blocked. Chronic administration of dl-propranolol (30 mg/kg b.i.d.) to spontaneously hypertensive rats from the age of 6 weeks (prehypertensive phase) for 29 days prevented the development of hypertension while the rats treated with physiological saline for 29 days (control group) developed hypertension. Naloxone (2 mg/kg i.p.) administered on the 29th day to chronically treated rats induced a reversal of the propranolol action on systolic blood pressure and heart rate, i.e., blood pressure and heart rate increased. Naloxone had no such effect in the control group. We suggest that the release of an endogenous opioid contributes to the acute and chronic antihypertensive action of i.p. propranolol in spontaneously hypertensive rats and that the secretion of endogenous opioids participating in the control of cardiovascular functions is influenced by adrenergic mechanisms.     

Modulation of ICa-L by alpha1-adrenergic stimulation in rat ventricular myocytes

We found when L-type calcium current (ICa-L) was recorded with the perforated patch-clamp method in rat ventricular myocytes that bath application of phenylephrine (with propranolol) evoked a biphasic response characterized by an initial transient suppression followed by a sustained potentiation. The transient suppression occurred 30-60 s after phenylephrine perfusion and reached peak inhibition at approximately 2 min. The biphasic modulation of ICa-L was also elicited by methoxamine, and the effects of phenylephrine were blocked by prazosin, indicating that the responses were mediated through alpha1-adrenoceptors. Pretreatment of cells with H7 (100 micromol/L), a broad-spectrum protein kinase inhibitor that inhibits both protein kinase C and A, eliminated potentiation but did not affect transient suppression. The transient suppression occurred concurrently with the acceleration of the fast component of ICa-L inactivation. Depletion of intracellular Ca2+ stores by ryanodine plus caffeine or thapsigargin eliminated the transient suppression. When ICa-L was recorded with whole-cell patch-clamp and with 0.05 mmol/L EGTA in the pipette solution to allow intracellular Ca2+ to fluctuate, phenylephrine evoked a transient suppression as in the perforated patch recordings. Heparin, a specific blocker of IP3 (inositol 1,4,5-trisphosphate) receptors, eliminated the phenylephrine-induced transient suppression of ICa-L when added to the pipette solution. Intensive chelation of intracellular Ca2+ by 5 mmol/L BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) in the pipette solution also eliminated the phenylephrine-induced transient suppression of ICa-L. We conclude that transient increase in the concentration of intracellular calcium ([Ca2+]i) caused by Ca2+ release from intracellular stores underlies the transient suppression of ICa-L, whereas the potentiation of ICa-L is a result of activation of protein kinases.     

Hormonal regulation of biliary calcium excretion in rats: inhibition of calcitonin action by alpha 1-adrenergic stimulation

The effect of synthetic [Asu1,7] eel calcitonin (CT) and other hormones on biliary calcium excretion was investigated in rats cannulated bile duct. Administration of CT (80 mU/100 g body weight) produced a significant increase in liver calcium and a corresponding elevation of bile calcium content. The increase in bile calcium content was also caused by administration of insulin (0.1 U/100 g), epidermal growth factor (10 micrograms/100 g), glucagon (10 micrograms/100 g), epinephrine (10 micrograms/100 g), norepinephrine (10 micrograms/100 g), 4 beta-phorbol 12-myristate-13-acetate (10 micrograms/100 g) and ATP (1.0 mg/100 g), suggesting that this increase may be a receptors-mediated response. Of these hormones and drugs, norepinephrine, a alpha-receptor mediator, clearly prevented CT effect on biliary calcium excretion. Moreover, phenylephrine, a alpha 1-receptor agonist, caused an inhibition of the CT effect, while the agonist significantly increased biliary calcium excretion. The present study clearly demonstrates that biliary calcium excretion is stimulated by various hormones which increase calcium influx into liver cells, and suggests that the CT action may be inhibited by alpha 1-adrenergic stimulation.     

The alpha2-adrenergic receptor agonist UK 14,304 inhibits secretin-stimulated ductal secretion by downregulation of the cAMP system in bile duct-ligated rats

Secretin stimulates ductal secretion by activation of cAMP PKA CFTR Cl^–/HCO₃^– exchanger in cholangiocytes. We evaluated the expression of _2A-, _2B-, and _2C-adrenergic receptors in cholangiocytes and the effects of the selective ₂-adrenergic agonist UK 14,304, on basal and secretin-stimulated ductal secretion. In normal rats, we evaluated the effect of UK 14,304 on bile and bicarbonate secretion. In bile duct-ligated (BDL) rats, we evaluated the effect of UK 14,304 on basal and secretin-stimulated 1) bile and bicarbonate secretion; 2) duct secretion in intrahepatic bile duct units (IBDU) in the absence or presence of 5-(N-ethyl-N-isopropyl)amiloride (EIPA), an inhibitor of the Na⁺/H⁺ exchanger isoform NHE3; and 3) cAMP levels, PKA activity, Cl^– efflux, and Cl^–/HCO₃^– exchanger activity in purified cholangiocytes. ₂-Adrenergic receptors were expressed by all cholangiocytes in normal and BDL liver sections. UK 14,304 did not change bile and bicarbonate secretion of normal rats. In BDL rats, UK 14,304 inhibited secretin-stimulated 1) bile and bicarbonate secretion, 2) expansion of IBDU luminal spaces, and 3) cAMP levels, PKA activity, Cl^– efflux, and Cl^–/HCO₃^– exchanger activity in cholangiocytes. There was decreased lumen size after removal of secretin in IBDU pretreated with UK 14,304. In IBDU pretreated with EIPA, there was no significant decrease in luminal space after removal of secretin in either the absence or presence of UK 14,304. The inhibitory effect of UK 14,304 on ductal secretion is not mediated by the apical cholangiocyte NHE3. ₂-Adrenergic receptors play a role in counterregulating enhanced ductal secretion associated with cholangiocyte proliferation in chronic cholestatic liver diseases. bicarbonate secretion; chloride efflux; gastrointestinal hormones; intrahepatic biliary epithelium; protein kinase A     

Involvement of peptide histidine isoleucine (PHI) in prolactin secretion induced by serotonin in rats

The possible role of hypothalamic peptide histidine isoleucine (PHI) in prolactin (PRL) secretion induced by serotoninergic mechanisms was investigated in male rats using a passive immunization technique. Intracerebroventricular injection of serotonin (5HT, 10 micrograms/rat) raised plasma PRL levels both in urethane-anesthetized rats and in conscious rats pretreated with normal rabbit serum (0.5 ml/rat, iv, 30 min before). Plasma PRL responses to 5HT were blunted in these animals when they were pretreated with rabbit antiserum specific for PHI (0.5 ml/rat, iv, 30 min before) (mean +/- SE peak plasma PRL: anesthetized rats 271.3 +/- 38.3 ng/ml vs 150.0 +/- 12.6 ng/ml, p less than 0.01, conscious rats 54.3 +/- 6.8 ng/ml vs 30.7 +/- 4.1 ng/ml, p less than 0.025). These results suggest that hypothalamic PHI is involved, at least in part, in PRL secretion induced by central serotoninergic stimulation in the rat.     

Presence of a Ca2+-dependent K+ channel in brown adipocytes. Possible role in maintenance of alpha 1-adrenergic stimulation

We have previously demonstrated mobilization of Ca2+ in and efflux of Rb+ (K+) from isolated hamster brown adipocytes as a consequence of norepinephrine stimulation. We have now investigated the adrenoceptor subtype specificity of these responses and found them both to be of the alpha 1-subtype. Further, we have found that the Rb+ (K+) efflux was dependent upon a primary Ca2+ mobilization event in response to the alpha 1-adrenergic stimulation, since the Rb+ efflux could also be demonstrated by the addition of the Ca2+ ionophore A23187 to the cells. The norepinephrine- and A23187-stimulated Rb+ effluxes were both inhibited by the Ca2+-dependent K+-channel blocker apamin. Apamin also significantly attenuated Ca2+ mobilization in cells in response to a submaximal concentration of norepinephrine. We conclude that alpha 1-adrenergic stimulation of brown fat cells leads to a mobilization of intracellular Ca2+ which, in itself or via other mechanisms, leads to an increase in cytosolic Ca2+ concentration which, in turn, activates a Ca2+-dependent K+ channel, leading to a K+ release from these cells. A possible role for this channel to sustain and augment the response to alpha 1-adrenergic stimulation is discussed.