[Met5]-enkephalin-like peptides of the adrenal medulla: release by nerve stimulation and functional implications

The effects of exercise training programs on plasma concentrations of proenkephalin Peptide F and catecholamines

To determine if exercise training alters the pattern and magnitude of plasma concentrations of proenkephalin Peptide F and epinephrine, plasma proenkephalin [107-140] Peptide F(ir) and catecholamines were examined pre-training (T-1), and after 4- (T-2), 8- (T-3), and 12-weeks (T-4) of training. 26 healthy men were matched and randomly assigned to one of three groups: heavy resistance strength training (Strength, n=9), high intensity endurance training (Endurance, n=8), or both training modalities combined (Combined, n=9). Blood was collected using a syringe with a cannula inserted into a superficial arm vein with samples collected at rest, after each 7 min stage and 5 and 15 min into recovery. With training, all groups observed shifted plasma Peptide F responses to graded exercise, where significant increases were observed at lower exercise intensities. Increases in plasma epinephrine with exercise were observed in all groups. The Combined group saw increases at 25% at T-3 and for 50% at T-2, T-3, and T-4 which was higher than T-1. The Endurance group demonstrated increases for 50% at T-1, T-2, T-3 but not at T-4. The plasma epinephrine response to graded exercise was reduced in the Strength group. Increases in plasma norepinephrine above rest were observed starting at 50% . The Strength group demonstrated a significant reduction in norepinephrine observed at 100% at T-3 and T-4. Peptide F and catecholamines responses to graded exercise can be altered by different types of physical exercise training. Simultaneous high intensity training may produce adrenal medulla exhaustion when compared to single mode training.

Decrease of plasma glucose by allantoin, an active principle of yam ( Dioscorea spp.), in streptozotocin-induced diabetic rats

The effect of allantoin, an active component of yam, on plasma glucose of streptozotocin-induced diabetic rats (STZ-diabetic rats) is investigated. Allantoin decreased plasma glucose levels in a dose-related manner, which was reduced by pretreatment with naloxone or naloxonazine. A concomitant increase in plasma β-endorphin, detected by enzyme-linked immunosorbent assay, was observed. Moreover, allantoin enhanced β-endorphin release from the isolated adrenal medulla of STZ-diabetic rat in a dose-related manner. However, its plasma glucose lowering action was reduced but not totally abolished by bilateral adrenalectomy. Furthermore, allantoin directly increased radioactive glucose uptake in isolated skeletal muscle, and repeated administration for 3 days increased GLUT4 mRNA and protein levels in muscle. This effect was markedly reduced in STZ-diabetic rats with bilateral adrenalectomy. This study suggests that allantoin increases GLUT4 gene expression in muscle by increasing β-endorphin secretion from the adrenal gland in STZ-diabetic rats.

Mechanism for blockade of angiotensin subtype 1 receptors to lower plasma glucose in streptozotocin-induced diabetic rats

AIMS

We investigated the mechanism(s) by which valsartan, a selective antagonist of angiotensin subtype 1 (AT(1)) receptor, decreased plasma glucose in streptozotocin (STZ)-induced diabetic rats.

METHODS

The plasma glucose concentration was assessed by the glucose oxidase method. The concentration of beta-endorphin in plasma or medium incubating adrenal medulla was measured using an enzyme-linked immunosorbent assay. The mRNA levels of the subtype 4 form of glucose transporter (GLUT4) in soleus muscle and phosphoenolpyruvate carboxykinase (PEPCK) in the liver were detected by Northern blotting analysis, while the protein levels of GLUT4 in isolated soleus muscle and hepatic PEPCK were investigated using Western blotting analysis.

RESULTS

A single intravenous injection of valsartan dose-dependently increased plasma beta-endorphin-like immunoreactivity (BER) in parallel with the lowering of plasma glucose concentration in STZ-induced diabetic rats. Naloxone and naloxonazine inhibited the plasma glucose-lowering action of valsartan at doses sufficient to block opioid micro-receptors. In contrast to its action in wild-type diabetic mice, valsartan failed to modify plasma glucose in opioid micro-receptor knockout diabetic mice. Bilateral adrenalectomy in STZ-induced diabetic rats eliminated both the plasma glucose-lowering action and the plasma BER-elevating action of valsartan. In the isolated adrenal medulla of STZ-induced diabetic rats, angiotensin II (Ang II) or valsartan did not affect spontaneous BER secretion. Activation of cholinergic receptors by 1.0 micromol/l acetylcholine (ACh) enhanced BER secretion from the isolated adrenal medulla of STZ-induced diabetic rats, but not in the presence of 1.0 nmol/l Ang II, while valsartan reversed this inhibition by Ang II in a concentration-dependent manner. Treatment of STZ-induced diabetic rats with valsartan (0.2 mg/kg) three times daily for 3 days resulted in an increase in gene expression of GLUT4 in soleus muscle and impeded the reduction of elevated mRNA or protein level of hepatic PEPCK. Both of these effects were blocked by opioid micro-receptor antagonist.

CONCLUSIONS

The results suggest that blockade of AT(1) receptor by valsartan may enhance the adrenal beta-endorphin secretion induced by ACh, activating the opioid micro-receptors to increase glucose utilization and/or to decrease hepatic gluconeogenesis, resulting in the reduction of plasma glucose in STZ-induced diabetic rats.

Mediation of beta-endorphin by myricetin to lower plasma glucose in streptozotocin-induced diabetic rats

Streptozotocin-induced diabetic (STZ-diabetic) rats were employed to investigate the mechanism(s) whereby myricetin, the active principle of Abelmoschus moschatus (Malvaceae), exerts its glucose-lowering effects. Myricetin was purified from the aerial portion of the plant and administered intravenously. A dose-dependent decrease in plasma glucose concentration was observed 30 min following injection, in parallel with increased plasma beta-endorphin-like immunoreactivity (BER). Myricetin enhanced BER release similarly from isolated adrenal medulla. Plasma glucose-lowering and BER-elevating effects of myricetin were both eliminated after bilateral adrenalectomy. Myricetin failed to lower plasma glucose after treatment with opioid mu-receptor antagonists and in opioid mu-receptor knockout diabetic mice. Injection of myricetin three times daily for three consecutive days resulted in increased expression of the glucose transporter subtype 4 (GLUT 4) in soleus muscle and in reduced expression of phosphoenolpyruvate carboxykinase (PEPCK) in liver; these inductions were preventable by opioid mu-receptor blockade. Findings support the conclusion that the plasma glucose-lowering action of myricetin in insulin-deficient animals is mediated by activation of opioid mu-receptors of peripheral tissues in response to increased beta-endorphin secretion. Opioid mu-receptor activation is held responsible for the enhancement of muscle GLUT 4 gene expression and the attenuation of hepatic PEPCK gene expression observed in these myricetin-treated diabetic animals.

Novel mechanism for plasma glucose-lowering action of metformin in streptozotocin-induced diabetic rats

To better understand the insulin-independent plasma glucose-lowering action of metformin, we used streptozotocin (STZ)-induced diabetic rats to investigate the possible mechanisms. Oral intake of metformin decreased the plasma glucose of STZ-induced diabetic rats with a parallel increase of plasma beta-endorphin-like immunoreactivity (BER). Mediation of opioid mu-receptors in the action of metformin was identified by the blockade of receptors with antagonist in STZ-induced diabetic rats and the failure of action in opioid mu-receptor knockout diabetic mice. Release of BER from adrenal glands by metformin was characterized, using bilateral adrenalectomy and the release of BER from isolated adrenal medulla of STZ-induced diabetic rats. Repeated treatment with metformin in STZ-induced diabetic rats increased the mRNA and protein levels of GLUT-4 in soleus muscle that was blocked by naloxonazine. Reduction of the mRNA or protein levels of hepatic PEPCK was also impeded in the same group of STZ-induced diabetic rats. In conclusion, our results provide novel mechanisms for the plasma glucose-lowering action of metformin, via an increase of beta-endorphin secretion from adrenal glands to stimulate opioid mu-receptor linkage, leading to an increase of GLUT-4 gene expression and an attenuation of hepatic PEPCK gene expression in STZ-induced diabetic rats.

Activation of imidazoline receptors in adrenal gland to lower plasma glucose in streptozotocin-induced diabetic rats

AIMS/HYPOTHESIS

The present study investigated the effect of agmatine, an endogenous ligand of imidazoline receptors, on plasma glucose in streptozotocin-induced diabetic rats (STZ-diabetic rats).

METHODS

Plasma glucose was assessed by the glucose oxidase method. Plasma insulin and beta-endorphin-like immunoreactivity in plasma or adrenal medulla were measured by enzyme-linked immunosorbent assay. Systolic blood pressure was determined by the tail-cuff method. The mRNA levels of glucose transporter subtype 4 (GLUT4) in soleus muscle and phosphoenolpyruvate carboxykinase (PEPCK) in liver were detected by northern blotting. Protein levels of GLUT4 in soleus muscle and hepatic PEPCK were estimated using western blotting analysis.

RESULTS

After intravenous injection into fasting STZ-diabetic rats for 30 min, agmatine decreased plasma glucose in a dose-dependent manner without changing systolic blood pressure. At the same time, plasma beta-endorphin-like immunoreactivity also increased in STZ-diabetic rats receiving the same treatment. Plasma glucose was significantly elevated in STZ-diabetic rats by an intravenous injection of clonidine at a dose sufficient to decrease systolic blood pressure. Involvement of I(1)-imidazoline receptors and/or alpha2-adrenoceptors in this effect of agmatine was thus unlikely. The lowering of plasma glucose and increase of plasma beta-endorphin-like immunoreactivity by agmatine were abolished by pretreating the rats with BU-224 at a dose sufficient to block I(2)-imidazoline receptors. Both effects of agmatine were also abolished in adrenalectomised STZ-diabetic rats. Moreover, agmatine enhanced beta-endorphin-like immunoreactivity release from the isolated adrenal medulla of STZ-diabetic rats, an effect also blocked by BU-224. Release of beta-endorphin from the adrenal glands by I(2)-imidazoline receptor activation seems responsible for the plasma glucose-lowering action of agmatine. This was supported by the fact that intravenous injection of naloxone or naloxonazine at doses sufficient to block opioid mu-receptors inhibited the action of agmatine. In addition to lowering plasma glucose, repeated intravenous injection of agmatine into STZ-diabetic rats for 4 days also increased mRNA and protein levels of GLUT4 in soleus muscle. The same treatment also reversed the higher mRNA and protein levels of PEPCK in liver of STZ-diabetic rats.

CONCLUSIONS/INTERPRETATION

Our results suggest that agmatine may activate I(2)-imidazoline receptors in the adrenal gland. This enhances secretion of beta-endorphin, which can activate opioid mu-receptors to increase GLUT4 gene expression and/or suppress hepatic PEPCK gene expression, resulting in a lowering of plasma glucose in diabetic rats lacking insulin. The results provide a potential new target for intervention in type 1 diabetes.

Mediation of Endogenous beta-endorphin by Tetrandrine to Lower Plasma Glucose in Streptozotocin-induced Diabetic Rats

The role of beta-endorphin in the plasma glucose-lowering action of tetrandrine in streptozotocin-induced diabetic rats (STZ-diabetic rats) was investigated. The plasma glucose concentration was assessed by the glucose oxidase method. The enzyme-linked immunosorbent assay was used to determine the plasma level of beta-endorphin-like immunoreactivity (BER). The mRNA levels of glucose transporter subtype 4 (GLUT4) in soleus muscle and phosphoenolpyruvate carboxykinase (PEPCK) in the liver of STZ-diabetic rats were detected by Northern blotting analysis. The expressed protein of GLUT4 or PEPCK was characterized by Western blotting analysis. Tetrandrine dose-dependently increased plasma BER in a manner parallel to the decrease of plasma glucose in STZ-diabetic rats. Moreover, the plasma glucose-lowering effect of tetrandrine was inhibited by naloxone and naloxonazine at doses sufficient to block opioid μ-receptors. Further, tetrandrine failed to produce plasma glucose-lowering action in opioid μ-receptor knockout diabetic mice. Bilateral adrenalectomy eliminated the plasma glucose-lowering effect and plasma BER-elevating effect of tetrandrine in STZ-diabetic rats. Both effects were abolished by treatment with hexamethonium or pentolinium at doses sufficient to block nicotinic receptors. Tetrandrine enhanced BER release directly from the isolated adrenal medulla of STZ-diabetic rats and this action was abolished by the blockade of nicotinic receptors. Repeated intravenous administration of tetrandrine (1.0 mg/kg) to STZ-diabetic rats for 3 days resulted in an increase in the mRNA and protein levels of the GLUT4 in soleus muscle, in addition to the lowering of plasma glucose. Similar treatment with tetrandrine reversed the elevated mRNA and protein levels of PEPCK in the liver of STZ-diabetic rats. The obtained results suggest that tetrandrine may induce the activation of nicotinic receptors in adrenal medulla to enhance the secretion of beta-endorphin, which could stimulate opioid μ-receptors to increase glucose utilization or/and reduce hepatic gluconeogenesis to lower plasma glucose levels in STZ-diabetic rats.

Dexamethasone modulates hypotension induced by opioids in anaesthetised rats

The effect of dexamethasone on hypotension induced by mu-, kappa- and delta-opioid receptor agonists was investigated in pentobarbital-anaesthetised rats. Morphine (nonselective opioid receptor agonist), DAGO (D-Ala2-N-methyl-[Phe4-Gly5-ol]enkephalin; mu-opioid receptor-selective agonist), U50-488H (trans(+/-)-3,4-dichloro-N-methyl-N-(2[1pyrrolidynyl]cyclohexyl)-benzeneacetamide; kappa-opioid receptor-selective agonist) and deltorphin II (delta-opioid receptor-selective agonist), given intravenously, 5 micromol/kg, induced hypotension in rats. This hypotension was characterised by a fall in mean arterial blood pressure in 1-2 min that recovered in 30 min for morphine and U50-488H and in 5 or 20 min for DAGO and deltorphin II, respectively. Dexamethasone per se at a dose of 7.5 micromol/kg, i.v. did not significantly modify the mean arterial blood pressure of animals. Dexamethasone administration 90 min, but not 30 or 60 min, before the opioid agonists injection, prevented the hypotension induced by morphine or U50-488H, but not that induced by DAGO or deltorphin II. Pretreatment with RU-38486 (mifepristone; 7.5 micromol/kg, i.v.), a glucocorticoid receptor antagonist, 15 min before the steroid, prevented dexamethasone inhibition of hypotension induced by morphine and U50-488H. Furthermore, pretreatment with cycloheximide, a protein synthesis inhibitor (3.5 micromol/kg, i.v.), was also able to abolish the effects of dexamethasone on morphine- and U50-488H-induced hypotension. Results of the present study indicate that dexamethasone inhibited kappa-opioid receptor-mediated hypotension in rats, indicating a further important functional interaction between corticosteroids and the opioid system at kappa receptors. The ability of cycloheximide and RU-38486 to block dexamethasone effects indicates that steroid interference with kappa-opioid receptor-mediated hypotension involves a protein synthesis-dependent mechanism via glucocorticoid receptors.

Peripheral modulation of learning and memory: enkephalins as a model system

Extensive research on the effects of enkephalins on conditioning is reviewed and used as the basis for a model of peripheral modulation of learning and memory. An overall theme emphasized throughout our discussion is that these peptides can influence the strength with which a memory is acquired and stored by acting outside the blood-brain barrier. This assertion is supported by research on the behavioral effects of systemically administered enkephalins and opioid antagonists, the rapid hydrolysis of circulating enkephalins in vivo, and the limited ability of these peptides to penetrate the blood-brain barrier. A consideration of the extensive distribution of enkephalins throughout peripheral autonomic systems leads to the proposal that enkephalins may act to modulate learning and memory by altering peripheral autonomic function; autonomic afferents may then communicate with the memory trace in the CNS through a central modulatory pathway outlined herein. Evidence that some stressful experiences may lead to increases in circulating enkephalins also is discussed. The sites of action of these circulating enkephalins may involve peripheral autonomic sites, or additionally may involve the circumventricular organs. As a further regulatory mechanism, circulating enkephalin levels may be controlled by experience-dependent alterations of the activity of enzyme systems that participate in their breakdown. Finally, it is emphasized that the mechanisms of enkephalin action postulated herein may be applicable to the actions of other peripheral hormones, peptides, and neurotransmitters that participate in the modulation of learning and memory storage processes.

Exercise and recovery responses of adrenal medullary neurohormones to heavy resistance exercise

PURPOSE

The purpose of this study was to examine the effect of dynamic resistance exercise on the response patterns of adrenal medullary neurohormones in strength-trained men.

METHODS

Ten strength-trained men (21.7+/-0.58 yr) gave informed consent and performed two resistance exercise protocols, high force and high power, of equal total work in a randomized order separated by 1 wk. Blood samples were obtained pre-exercise (baseline), 0 (R-0), 15 (R-15), and 240 (R-240) min postexercise and under resting control conditions for each time point.

RESULTS

There were no significant differences in control concentrations for each time point and no difference in pre-exercise values between the two resistance exercise protocols for plasma lactate, epinephrine, plasma peptide F (P-F), or norepinephrine (NE). Plasma lactate significantly (P< or = 0.05) increased from baseline for both protocols; however, concentrations were higher in response to the high force protocol. Plasma epinephrine was significantly increased from baseline at R-0 and returned to baseline at R-15 for both protocols. In contrast, plasma P-F was significantly decreased at R-0 from baseline; however, at R-240 P-F had significantly increased to >80% baseline for both protocols.

CONCLUSIONS

These results indicate that the adrenal medulla was activated in response to the acute stress of both types of heavy resistance exercise. Furthermore, during longer recovery periods, the adrenal medulla was also active above baseline conditions as increased concentrations of proenkephalin fragments (i.e., P-F) were detected in the circulation.

Opioid peptide modulation of circulatory and endocrine response to mental stress in humans

Healthy subjects were classified according to their percent increase in systolic blood pressure (SBP) after mental arithmetic test (MAT) as low (delta SBP 9.3-15.1%, n = 15) and high (delta SBP 35.1-45.4%, n = 15) responders. During MAT, low responders showed significantly (p < 0.01) increased plasma levels of beta-endorphin, cortisol, catecholamines, and atrial natriuretic factor (ANF) and decreased levels of endothelin-1, whereas high responders showed increased (p < 0.01) levels of Metenkephalin, dynorphin B, and catecholamines. Pretreatment with naloxone hydrochloride enhanced (p < 0.01) SBP, heart rate, noradrenaline, cortisol, and endothelin-1 levels, and reduced (p < 0.01) ANF in low responders in response to MAT, whereas it decreased (p < 0.01) hemodynamic parameters, noradrenaline, and endothelin-1 in high responders. The individual differences in hemodynamic and endocrine responses to MAT may depend on a different activation of the endogenous opioid system.

Endogenous opioids may modulate the activity of the hypothalamus-pituitary-adrenocortical axis in domestic fowl

The activity of the hypothalamus-pituitary-adrenocortical (HPA) axis is under complex neuronal, hormonal and peptidergic control. In order to determine the role of the endogenous opioids in the modulation of the HPA axis in hens we have examined the changes of the plasma levels of Met-enkephalin, alpha-neo-endorphin, catecholamines and corticosterone during rest, stress (30 min of overcrowding) and after naltrexone pretreatment. Short overcrowding induced an increase of all parameters, but the time of response and duration of elevation was different. Plasma Met-enkephalin showed a biphasic response: decrease followed by increase at the end of stress. Catecholamines and corticosterone peaked at 30 min of overcrowding and the highest level of alpha-neo-endorphin has been noticed at 20 min after the experiment was started. Naltrexone (2 mg/kg b.w., i.v) diminished the responses to stress of all parameters and did not change their basal levels, except for corticosterone. These results indicate that the endogenous opioids may take part in the mediation of HPA activity in hens during resting and stressful situations.

Stereospecific blocking effects of naloxone against hemodynamic compromise and ventricular dysfunction due to myocardial ischemia and reperfusion

Endogenous opioid peptides subserve regulatory roles in cardiovascular function and are released upon myocardial ischemia contributing to the development of ischemic arrhythmias and cardiogenic shock, which are reversed by the opioid antagonist naloxone. Since the hallmark of myocardial infarction is the impairment of hemodynamics and ventricular function, we evaluated further if blockade of opioids reverses the ischemia induced hemodynamic compromise, and if the effects are mediated by opioid receptors. Thirty-two mongrel dogs were anesthetized and artificially ventilated. Median thoracotomy was performed, the heart exposed, and the left anterior descending coronary artery isolated for subsequent occlusion and reperfusion. All cardiac parameters were recorded on an Electronics for Medicine recorder through the intracardiac catheters advanced from femoral vessels. Results indicate that naloxone significantly reversed the ischemic and reperfusion induced reduction in aortic, left ventricular and pulmonary arterial pressures, and left ventricular dp/dt. The inactive (+) stereoisomer of naloxone was without effect. These data demonstrate that opioids may have a role in the pathophysiology of myocardial infarction, mediated by opioid receptors, and provide new insight and strategies for the understanding and treatment of ischemic heart disease.

Nicotine-induced alterations in peripheral tissue concentrations of native and cryptic Met- and Leu-enkephalin

This study examined the peripheral tissue distribution of native and cryptic Met- and Leu-enkephalin, and regulation of tissue enkephalins by nicotine. Met- and Leu-enkephalin concentrations showed widespread variation in tissue concentration and degree of processing. HPLC characterization of homogenate of spleen revealed that both native and cryptic immunoreactive Met-enkephalin are comprised of two peaks, one representing authentic Met-enkephalin pentapeptide and the other its sulfoxide. Subacute repeated administration of nicotine 0.1 mg/kg ip, six times at 30 min intervals, increased native Met- and Leu-enkephalin in adrenal medulla without affecting cryptic Met- and Leu-enkephalin concentrations, consistent with increased processing of larger peptides to Met- and Leu-enkephalin. Subacute nicotine decreased splenic concentrations of native and cryptic Met-enkephalin and native Leu-enkephalin, consistent with increased release of Met- and Leu-enkephalin from spleen and decreased synthesis of proenkephalin A or inadequate processing of larger peptides to enkephalin pentapeptides in spleen to compensate for the increased release during this period. HPLC characterization revealed that nicotine-induced decrease in native Met-enkephalin in spleen resulted from reductions in both pentapeptide and its sulfoxide. Nicotine also increased native Met-enkephalin in jejunum, decreased cryptic Met-enkephalin in heart atrium, increased native Leu-enkephalin in anterior pituitary and decreased cryptic Leu-enkephalin in jejunum. Nicotine may produce some of its effects through alterations in release of enkephalins from peripheral tissues.

Effect of captopril on plasma levels of neuropeptide Y (NPY)-like immunoreactivity in the pithed guinea pig

The effect of exogenous angiotensin II (A II) and the angiotensin converting enzyme (ACE)-inhibitor captopril on plasma levels of neuropeptide Y-like immunoreactivity (NPY-LI) has been studied in the pithed guinea pig. Four periods of pre-ganglionic nerve stimulation (PNS, 8 Hz for 30s with 20 min intervals) were applied and the increases of mean arterial blood pressure (delta BP), heart rate (delta HR) and plasma NPY-LI (delta NPY-LI) in response to PNS were analysed in non-pre-treated and captopril pre-treated animals. Captopril (5 mg x kg-1 i.v.) reduced basal blood pressure and delta BP by 20% and 11%, respectively. Infusion of A II (0.5 microgram x kg-1 = min-1 i.v.) caused a significant increase in basal and PNS-induced maximal blood pressure response but reduced delta BP in captopril and non-pre-treated animals by 40% and 16%, respectively. A II elicited a long-lasting increase of basal heart rate by 12% but reduced delta HR by 36% in non-pre-treated animals. However, neither captopril alone nor A II infusion to captopril pre-treated animals significantly changed heart-rate values. The effects of exogenous A II on the cardiovascular responses were abolished by the A II-antagonist saralasin (a bolus injection, 40 micrograms x kg-1 i.v. followed by an infusion, 30 micrograms x kg-1 x min-1), which per se had no significant effect. Captopril pre-treatment reduced basal plasma NPY-LI levels by 38% and delta NPY-LI by 46% in response to PNS 1. However, neither in non-pre-treated nor in captopril pre-treated animals did infusion of A II significantly change the plasma NPY-LI level.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonal and metabolic response to operative stress in the neonate

It is evident from this review that newborns, even those born prematurely, are capable of mounting an endocrine and metabolic response to operative stress. Unfortunately, many of the areas for which a relatively well-characterized response exists in adults are poorly documented in neonates. As is the case in adults, the response seems to be primarily catabolic in nature because the combined hormonal changes include an increased release of catabolic hormones such as catecholamines, glucagon, and corticosteroids coupled with a suppression of and peripheral resistance to the effects of the primary anabolic hormone, insulin.

Neuronal and adrenal enkephalins and catecholamines in response to acute CNS ischemia and reserpine in pig

Co-storage of enkephalins and catecholamines in coronary artery, mesenteric artery and vein, middle cerebral artery, vas deferens and adrenal medulla was studied in domestic pig (Sus scrofa). Responses to acute CNS ischemia were correlated with time to peak plasma levels of central venous and adrenal vein outflow samples in controls, during reserpine treatment and after drug withdrawal. Endogenous enkephalins are co-stored in chromaffin granules of adrenal epinephrine-type cells and large dense cored vesicles of noradrenergic terminals. After a lag period, reserpine at near 'therapeutic' doses caused an apparent induction of opioid peptide precursor synthesis accompanied by processing to enkephalins in adrenal medulla up to 8-fold by 30 days and in mesenteric vein up to 4.5-fold by 14 days. Upon 14 days recovery from reserpine, elevated adrenal enkephalins were maintained and depleted catecholamines were largely replenished. Acute CNS ischemia produced rises in MAP (approx. 80 mmHg), marked net depletions of noradrenergic enkephalin stores, and net increases in adrenal vein outflow and central venous levels of enkephalins and catecholamines. Noradrenergic terminals contributed significantly to circulating enkephalins as well as norepinephrine. Reserpine for 7 days nearly abolished all tested responses to acute CNS ischemia, but immediate net 200-400% elevations of endogenous enkephalin stores occurred in coronary artery and mesenteric artery and vein (apparent processing of reserpine-induced neuronal precursor stores). Thus, induction of new synthesis of precursor opioid peptides by reserpine, with or without parallel processing to enkephalins, occurs in noradrenergic terminals in many tissues. All effects of reserpine on endogenous enkephalins implicate a central mechanism to inhibit sympathoadrenal outflow to the periphery. At 14 days recovery from reserpine, when near normal cardiovascular responses to acute CNS ischemia were regained, there was increased net release of the elevated adrenal enkephalins, exaggerated peak plasma enkephalin concentrations, but only minimal depletions of enkephalins from noradrenergic terminals.

Plasma native and peptidase-derivable Met-enkephalin responses to restraint stress in rats. Adaptation to repeated restraint

Met-enkephalin and related proenkephalin A-derived peptides circulate in plasma at picomolar concentration as free, native pentapeptide and at nanomolar concentration in cryptic forms. We have optimized conditions for measurement of immunoreactive Met-enkephalin in plasma and for generation by trypsin and carboxypeptidase B of much greater amounts of total peptidase-derivable Met-enkephalin in plasma of rats, dogs, and humans. Free Met-enkephalin (11 pM) is constituted by native pentapeptide and its sulfoxide. Characterization of plasma total Met-enkephalin derived by peptidic hydrolysis revealed a small amount (38 pM) of Met-enkephalin associated with peptides of molecular mass less than 30,000 D, and probably derived from proenkephalin A, but much larger amounts of Met-enkephalin associated with albumin (1.2 nM) and with a globulin-sized protein (2.8 nM). Thus, plasma protein precursors for peptidase-derivable Met-enkephalin differ structurally and chemically from proenkephalin A. Met-enkephalin generated from plasma by peptidic hydrolysis showed naloxone-reversible bioactivity comparable to synthetic Met-enkephalin. Prolonged exposure of adult, male rats to restraint stress produced biphasic plasma responses, with peaks occurring at 30 s and 30 min in both free native and total peptidase-derivable Met-enkephalin. Repeated daily exposure to this 30-min stress resulted in adaptive loss of responses of both forms to acute restraint. Initial plasma responses of Met-enkephalin paralleled those of epinephrine and norepinephrine, but subsequently showed divergence of response. In conclusion, Met-enkephalin circulates in several forms, some of which may be derived from proteins other than proenkephalin A, and plasma levels of both free native, and peptidase-derivable Met-enkephalin are modulated physiologically.

Plasma leucine enkephalin is increased in liver disease

Plasma methionine enkephalin is increased in liver disease and may contribute to some of the clinical manifestations of hepatic failure. To determine if another 'small' opioid peptide is increased in the plasma of patients with liver disease, leucine enkephalin was measured by radioimmunoassay. Its plasma concentration was raised approximately five-fold in patients with acute liver disease (median 1490 pmol/l, range 830-2420) and three-fold in patients with cirrhosis with ascites (960 pmol/l, 470-2900), compared with disease controls (325 pmol/l, 180-740) and healthy controls (305 pmol/l, 180-560). The increase in plasma leucine enkephalin was proportional to the degree of liver damage, as judged in the patients with acute liver disease by its correlation with the prothrombin time (r = 0.691, p less than 0.01) and alanine aminotransferase (r = 0.502, p less than 0.05), and in the patients with cirrhosis by its negative correlation with the plasma albumin (r = -0.743, p less than 0.001). It is unclear whether the raised plasma leucine enkephalin in liver disease is a consequence of diminished hepatic inactivation, increased secretion from sympathetic nerves and adrenal glands, or both.

Electron microscopic localization of enkephalin-like immunoreactivity in the human adrenal medulla

Enkephalin-like immunoreactivity in human adrenomedullary cells was studied at the light and electron microscopic levels. Enkephalin immunostaining was associated with chromaffin granules and, in a few cells, with the rough endoplasmic reticulum as well. The relative number of stained granules varied from cell to cell, and a correlation with a particular granular population was not noted. Both large and small granules were labelled. It is concluded that in the human the ability to store enkephalin immunoreactive peptides is a general property of chromaffin granules and, furthermore, is not correlated with specific granular subpopulations or the particular type of catecholamine stored within the cell.

The physiology of stress and its relationship to mechanisms of disease and therapeutics

Although stress reactions are organized to protect the homeostatic state of animals, they contain elements that may either enhance or diminish susceptibility to disease processes; in many instances, however, stress reactions themselves may induce pathologic change. It is important, therefore, that the veterinary clinician recognize the elements of a stress reaction and understand the mechanisms of disease with which they interact. This article provides a classification of stress stimuli that can be applied when considering interactions between stress reactions and disease processes.

Neonatal adaptation to stress of parturition and dystocia

The fetal animal undergoes a tremendous transition from intrauterine to extrauterine life at parturition. In this article, the maternal-fetal interactions of parturition are discussed with the aim of examining the normal stress reactions of parturition. Dystocia is discussed from the standpoint of additional distress of the newborn, with an aim toward the development of rational therapeutic support.

Is ascites caused by impaired hepatic inactivation of blood borne endogenous opioid peptides?

Methionine enkephalin and catecholamines were measured in carefully collected plasma samples from 25 patients with cirrhosis and ascites, and 25 with cirrhosis without ascites, 15 disease and 15 healthy controls. Methionine enkephalin was invariably raised in the ascites group, the median value being 4.6-6.9 times that of the other three groups. Similarly, in the ascites group, median noradrenaline was increased 2.5-4.2 and median adrenaline 1.8-2.5 times that of the other groups. Plasma methionine enkephalin is considerably raised in patients with cirrhotic ascites and has actions which could enable it to be an initiating factor of ascites formation.

Immunohistochemical and biochemical evidence for the presence of serotonin in amphibian adrenal chromaffin cells

The presence of serotonin (5-HT) in chromaffin cells of the frog adrenal (inter-renal) gland has been demonstrated both by immunocytochemical and biochemical techniques. Using antisera against 5-HT and tyrosine hydroxylase (TH) on consecutive sections, we found by means of the indirect immunofluorescence technique that a majority of chromaffin cells were also immunopositive for 5-HT. When antibodies to 5-HT and phenylethanolamine-N-methyltransferase (PNMT) were applied on consecutive sections, 5-HT-like immunoreactivity was observed in almost all epinephrine-producing cells which represented about 90% of the total chromaffin cells. No 5-HT-containing fibres could be detected. At the ultrastructural level, using a pre-embedding procedure associated with gold-silver intensification of the immunoperoxidase reaction, 5-HT-immunoreactivity was visualized in secretory vesicles essentially located in the periphery of epinephrine cells. Combination of high performance liquid chromatography and electrochemical detection showed the presence of both 5-HT and its metabolite 5-hydroxyindolacetic acid (5-HIAA) in frog adrenal extracts. Transection of the splanchnic nerve enhanced 5-HT immunoreactivity and augmented the amount of 5-HT in adrenal extracts. Taken together, these results indicate that epinephrine-producing cells of the frog adrenal contain significant amounts of serotonin. The observation of the storage of 5-HT in secretory vesicles of epinephrine cells suggests that serotonin may be released with catecholamines under stress conditions.

Neuronal and paracrine regulation of adrenal steroidogenesis: interactions between acetylcholine, serotonin and vasoactive intestinal peptide (VIP) on corticosteroid production by frog interrenal tissue

The adrenocortical cells of frog interrenal (adrenal) tissue are controlled by multiple factors. Recently, we have shown that corticosteroidogenesis is stimulated by acetylcholine released from splanchnic nerve terminals as well as by serotonin and vasoactive intestinal peptide (VIP) which are both contained in chromaffin cells. Since these 3 putative neuroregulators are known to interact with each other on various target organs, we have investigated possible coordinate actions of acetylcholine, serotonin and VIP on adrenal steroid production, using a perifusion system technique for frog interrenal tissue. Simultaneous infusion of submaximal doses of VIP (10(-5) M) and acetylcholine (5 X 10(-5) M) induced stimulations of corticosteroids (corticosterone and aldosterone) which were strictly additive. When VIP (10(-5) M) and serotonin (5 X 10(-6) M) were infused together, a potentiation of the individual responses was observed. In contrast, concomitant infusion of acetylcholine (5 X 10(-5) M) and serotonin (5 X 10(-6) M) caused a total blockage of the stimulatory effect of serotonin. Muscarine (10(-5) M) caused a similar blockade of the response of adrenocortical cells to serotonin while nicotine (5 X 10(-5) M) did not alter the stimulatory effect of serotonin. The inhibitory effect of acetylcholine on serotonin-induced steroidogenesis was antagonized by atropine (10(-5) M). Thus, acetylcholine appears to block the corticotropic action of serotonin by interacting with typical muscarinic receptors. Taken together our results indicate that 3 of the neuroregulators which participate in the control of adrenal steroidogenesis, namely acetylcholine, serotonin and VIP, may interact on their target cell to modulate the activity of their congeners.(ABSTRACT TRUNCATED AT 250 WORDS)

Enkephalins and learning and memory: a review of evidence for a site of action outside the blood-brain barrier

A series of studies indicate that enkephalins exert dramatic influences on learning and memory in rats and mice, when studied with conditioning tasks that are both negatively and positively motivated. Pharmacological analysis of these enkephalin actions on conditioning suggests that the [leu]enkephalin acts through a delta opioid receptor which is located outside the blood-brain barrier. Control studies indicate that enkephalins do not simply affect the performance of a conditioned response through actions on shock sensitivity or locomotor activity. Characterization of the peripheral enkephalin mechanism that affects behavior suggests an action through an enzymatic system that controls the concentrations of enkephalin present at its receptors in the periphery. This enzymatic mechanism is sensitive to experience, since its activity changes following conditioning, which suggests that it may be a regulatory mechanism for behavior.

Proenkephalin-A derived peptides do not modulate cardiovascular effects of epinephrine on the isolated rat atrial preparations

The catecholamines and the opioid peptides are found to be co-localized in the adrenomedullary chromaffin cells. They are co-secreted from the chromaffin granules in response to various stimuli. The stress-induced released of epinephrine is known to exert its effect on the cardiovascular system resulting in the changes in heart rate and blood pressure. However, the role of the co-released proenkephalin-A derived peptides has not been extensively characterized. Previous work from several investigators suggested that the peptides modulate cardiac functions of the catecholamines. There is considerable conflicting results among these reports. Results from the isolated rat atrial preparation indicated that enkephalins attenuated the increase in atrial rate induced by norepinephrine through restriction of the calcium fluxes. Nonetheless, others reported insensitivity of the enkephalins in similar or different test systems. We further re-examined these discrepancies using the isolated rat atrial preparation to investigate the opioid peptide modulatory effect on the cardiovascular changes induced by exogenous epinephrine. Alterations in rate and force of contraction resulting from epinephrine and the peptides were both studied in parallel. The opioid peptides used in this study were [Met5]-enkephalin (ME), [Leu5]-enkephalin (LE), FMRFamide, [Met5]-enkephalyl-Arg6-Phe7 (MEAP), peptide E, and the non-selective opioid agonist, etorphine. We report here that none of the opioid peptides were effective in alleviating or attenuating the increase in heart rate and developed tension caused by epinephrine. The peptides did not affect the basal beating rate nor the force of contraction. Thus, the present results clearly demonstrate the insensitivity of the enkephalins in modulating the cardiac effects of epinephrine. They further indirectly support the prejunctional synaptic nerve endings as the potential peripheral site of action of the peptides.

Target tissue distribution of the proenkephalin peptides F, E, and B

The adrenal medulla is a rich source of endogenous opioid peptides. These peptides exist predominantly in the form of larger (25-34 amino acid long) enkephalin containing peptides, whose biological roles have yet to be elucidated. We report here the tissue binding distribution of three iodinated enkephalin containing peptides, Peptides F, E, and B, in rats, rabbits, and guinea pigs following intravenous injection. Each [125I]peptide has a unique distribution profile but all three are found to bind to the pituitary in each species of animal examined. A number of lines of evidence point toward the enkephalin containing peptides, Peptides F, E, and B, having physiological roles distinct from the enkephalins. The distribution profile of these [125I]peptides reported here gives insight into their potential effector sites.

Naloxone and [Met5]enkephalin effects on retention: attenuation by adrenal denervation

This study investigated the effect of posttraining administration (i.p.) of naloxone and [Met5]enkephalin on retention in intact and adrenal-denervated mice. Naloxone (0.3 mg/kg) enhanced and [Met5]enkephalin (0.5 or 2.0 micrograms/kg) impaired retention in both inhibitory avoidance and Y-maze discrimination tasks. Further, [Met5]enkephalin antagonized the facilitative effect of naloxone on retention while naloxone attenuated the retention deficit produced by [Met5]enkephalin. However, neither naloxone nor [Met5]enkephalin affected retention in adrenal denervated mice. These findings suggest that endogenous hormones of the adrenal medulla, including [Met5]enkephalin, may be involved in the modulating influence of opiate peptides on memory storage.

The opioid peptides. A role in hypertension?

This review is an attempt to highlight evidence that may implicate the endogenous opioid system in the pathogenesis of hypertension in humans. The evidence raised includes biochemical, physiological, pharmacological, and behavioral studies conducted in in vitro and in vivo systems, experimental models of hypertension, and humans with essential hypertension. While the compelling biochemical and pharmacological evidence in experimental animals clearly shows the presence of opioid peptides and their receptors in strategic sites of cardiovascular control and potent cardiovascular response to opioid peptides, opioid antagonists show no consistent blockade or reversal of hypertension in experimental animals or humans. One possible explanation for this phenomenon could be the vast redundancy in systems regulating blood pressure (i.e., the blockade of one system still leaves many other systems fully able to rapidly offset the eliminated system). Regarding the opioid system, the situation is much more complex, since some opioid receptors (mu-type) mediate pressor responses, while other receptors (kappa-type) mediate depressor responses. Therefore, nonselective opioid receptor antagonists (e.g., naloxone), which block both types of receptors, can be devoid of any cardiovascular activity, while a selective mu-receptor antagonist or a selective and potent kappa-receptor agonist may produce the desired antihypertensive effect. A combination of both actions (i.e., a drug that is both a mu-antagonist and a kappa-agonist) might be even more advantageous. Until such compounds are developed, this hypothesis will be hard to prove.

Enhancement of host resistance to viral and tumor challenge by treatment with methionine-enkephalin

Host resistance to disease is dependent upon a number of factors. Recent evidence indicates that natural killer (NK) cells play an important role in resistance to both neoplastic and virally induced disease. Treatment of C57Bl/6 mice with methionine-enkephalin (1, 3, 10, or 30 mg/kg body weight) results in significant increases in NK activity of splenic lymphocytes 20 hours after injection of the enkephalin. Enkephalin treatment also enhances host resistance. The short-term survival of A/J female mice after HSV-2 infection was significantly increased by daily subcutaneous injections (3 mg/kg body weight) of methionine-enkephalin. Similarly, daily doses of 50 micrograms of methionine-enkephalin for 7 to 14 days inhibit the local subcutaneous tumor growth of B15 melanoma in C57Bl/6 mice.

Plasma enkephalin-like peptide response to chronic nicotine infusion in guinea pig

Nicotine administration, using mini-osmotic pumps, to male guinea pigs (31 micrograms/h for 10 days) resulted in a significant elevation of plasma epinephrine and enkephalin-like peptides but not norepinephrine. The increase in plasma epinephrine and enkephalin-like peptides was not accompanied by corresponding alterations in either adrenal medullary synthesis or blood pressure and heart rate. These findings may provide a partial explanation for the observation that chronic smoking does not induce hypertension.

Opioids and opioid receptors in peripheral tissues

Opioid peptides belonging to the enkephalin, beta-endorphin or dynorphin family, acting on specific opiate receptors may be found in peripheral tissues. Enkephalins have a widespread peripheral distribution, while beta-endorphin and dynorphin may be found locally in the enteric nervous system. The peptides of the various families are formed from specific precursor molecules. Apart from the enteric nervous system, opioids are also found in the adrenal medulla as well as in several autonomic ganglia. There is some evidence of three different classes of opioid receptors in peripheral tissues, i.e. mu-, delta- and kappa-receptors. These receptors are not only found on enteric nervous and mucosa cells but also on various cells in the immune system where opioid peptides seem to have important actions and appear to link the neuroendocrine and immune systems to control immunological functions. The physiological as well as the pathophysiological role of opioid peptides in the periphery is gradually being elucidated and, based on such knowledge, new therapeutic implications in gastrointestinal or immune diseases may be developed.

Adrenal enkephalin and catecholamine contents following subarachnoid hemorrhage in cats

A "closed space" subarachnoid hemorrhage (SAH) was produced experimentally in cats by rupture of the right middle cerebral artery to test the working hypothesis that a stressful event which provokes powerful sympathoadrenal discharge: causes a massive release of co-stored endogenous enkephalins together with catecholamines, induces an increased rate of opioid peptide precursor processing and/or synthesis, and eventually results in markedly elevated tissue levels of enkephalins relative to controls and to co-stored catecholamines. Adrenal medulla and other tissues were analyzed for met- and leu-enkephalins by RIAs and norepinephrine and epinephrine by HPLC-EC at 4 hrs, 3, 10, 16 and 30 days post-SAH. Catecholamines of adrenal medulla were already decreased at 4 hrs and by 3 days post-SAH depletion of epinephrine reached 86% and norepinephrine 53% compared to controls. Concurrently, at 4 hrs and 3 days post-SAH, the adrenal medulla was depleted 47% of met- and 53% of leu-enkephalins. By 10 days post-SAH, when catecholamines had regained control levels, met-enkephalin was elevated to 240% of control and 435% compared to the 3 day depletion; it remained elevated through 30 days post-SAH. In comparison, after 10 days reserpine treatment when catecholamines were markedly depleted, met-enkephalin rose to 970% and leu-enkephalin to 360% relative to controls, confirming recent reports in the literature. The data suggest that release of enkephalins originates primarily from epinephrine-type cells of the adrenal medulla in cat.

Generation of immunoreactive neurotensin(s) and enkephalin(s) by pepsin-treatment of plasma

Treatment of mammalian plasmas with pepsin yielded extraordinary quantities of immunoreactive neurotensin (iNT) and methionine5-enkephalin (iENK). The concentrations measured after pepsin-treatment (iNT, 1-5 microM and iENK, 0.1-0.5 microM) were 1-100 thousand times the normal circulating levels of these peptides. The reactions were shown to be time, temperature and pH dependent and to involve the action of pepsin on albumin-like proteins (Mr, ca, 65,000). Pepsin-generated iNT from rat plasma differed from NT since it reacted only with C-terminal directed antisera and eluted earlier than NT during HPLC on mu-Bondapak C-18. Partially purified iNT was active in two bioassays for NT, one which senses changes in vascular permeability to protein after intradermal injection into rats and another which measures release of histamine from isolated rat mast cells. Other biologic activities generated by pepsin-treating plasma included effects on systemic blood pressure in rats and on the contractility of the isolated guinea pig ileum. Some of these, however, were attributable to the formation of angiotensin- and bradykinin-related peptides. Pepsin-generated iENK gave three major peaks during HPLC, one of which (ca, 25%) co-eluted with oxidized ENK and also registered in a radioreceptor assay for opiate-related substances. In addition, this material produced ENK-like effects on the isolated guinea pig ileum and on vascular permeability in rat skin. The precursor-like protein(s) for iENK were distinguished from adrenal proenkephalins since it did not liberate iENK upon digestion with trypsin and carboxypeptidase B. Since pepsin can mimic renin these results suggest the existence of systems in blood (analogous to the renin/angiotensin system) for the generation of biologically active NT- and ENK-related peptides and they also raise the question as to whether other neuropeptides might be found circulating in precursor form(s).

Effects of opioid peptides containing the sequence of Met5-enkephalin or Leu5-enkephalin on nicotine-induced secretion from bovine adrenal chromaffin cells

Eighteen endogenous opioid peptides, all containing the sequence of either Met5- or Leu5-enkephalin, were tested for their ability to modify nicotine-induced secretion from bovine adrenal chromaffin cells. ATP released from suspensions of freshly isolated cells was measured with the luciferin-luciferase bioluminescence method as an index of secretion. None of the peptides affected 5 microM nicotine-induced ATP release at 10 nM. Three peptides inhibited secretion at 5 microM: dynorphin1-13, dynorphin1-9, and rimorphin inhibited by 65%, 37%, and 29% respectively. Use of peptidase inhibitors (bestatin, thiorphan, bacitracin, or 1,10-phenanthroline) did not result in any of the other peptides showing potent actions on the nicotinic response, although bestatin and thiorphan did enhance the inhibitory actions of dynorphin1-13 and dynorphin1-9 by 20-30%. Nicotine-induced secretion of endogenous catecholamines from bovine chromaffin cells cultured for 3 days was also studied to assess any selective actions of the peptides on adrenaline or noradrenaline cell types. Dynorphin1-13 was 1,000-fold more potent than Leu5-enkephalin at inhibiting endogenous catecholamine secretion. Dynorphin1-13 was slightly more potent at inhibiting noradrenaline release than adrenaline release whereas Leu5-enkephalin showed the opposite selectivity. The structure-activity relationships of opioid peptide actions on the chromaffin cell nicotinic response are discussed in relation to the properties of the adrenal opioid binding sites.

Changes in plasma proenkephalin peptide F and catecholamine levels during graded exercise in men

Proenkephalin peptide F immunoreactivity, epinephrine, and norepinephrine were measured in the plasma of endurance-trained and untrained male subjects riding on a bicycle ergometer at 28%, 54%, 83%, and 100% of maximum oxygen consumption (VO2). At rest the trained group had peptide F levels almost twice the level of the untrained group, whereas all other variables measured were the same. The maximum epinephrine and norepinephrine levels were found at 100% exercise intensity, with a precipitous drop in the levels at 5 min of recovery. In contrast, the peptide F immunoreactivity reached a maximum at 5 min of recovery and was still substantially above the initial level after 15 min of rest. In addition, the trained subjects showed another peak of peptide F immunoreactivity at 54% VO2max. Possible explanations for the different patterns of catecholamine and peptide F levels are presented.

[Leu5]enkephalin inhibits norepinephrine-induced contraction of rat aorta

Leucine-enkephalin produces a dose related decrease in the contractile response of spirally cut strips of rat aorta to norepinephrine (NE). The maximal effect (55.0 +/- 1.0 percent reduction in developed tension) occurs at a leucine-enkephalin concentration of 10(-10) M. The attenuation of contractile response is reversed by naloxone.

Neuroimmunomodulation with enkephalins: enhancement of human natural killer (NK) cell activity in vitro

To further define the effects of enkephalins on immune function, the effect of methionine-enkephalin and leucine-enkephalin on natural killer cell (NK) activity in isolated human peripheral blood lymphocytes was investigated. Incubation of lymphocytes with either enkephalin resulted in significant increases in natural killer cell activity. At effector:target cell ratios of 11:1 methionine-enkephalin significantly (P less than 0.05) enhanced NK activity at dilutions of 10(-6), 10(-8), 10(-10), and 10(-14) mg/ml, while leucine-enkephalin significantly (P less than 0.05) enhanced NK activity at dilutions of 10(-4), 10(-6), 10(-8), 10(-10), and 10(-14) mg/ml. Cells from individuals with low NK activity showed greater percentage increases in NK activity following enkephalin than did cells from individuals with high NK activity.

Leucine-enkephalin increases norepinephrine-stimulated chronotropy and 45Ca++ uptake in guinea-pig atria

Norepinephrine (NE) (10(-5) M) increases the beating rate of isolated, spontaneously beating guinea-pig atria 78 +/- 3 beats per minute. Leucine-enkephalin (10(-7) M) increases the maximal chronotropic response to NE by 38%, i.e., by 30 beats per minute. In the presence of 10(-7) M leucine-enkephalin and 10(-7) M naloxone, the chronotropic response to NE is reduced to 38 +/- 3 beats per minute, a value observed in the presence of naloxone alone. Neither naloxone nor leucine enkephalin significantly altered the inotropic response to NE, or significantly altered the basal beating rate. Parallel effects were observed when 45Ca++ uptake by atrial tissue was examined. Incubation of atrial slices with 10(-5) M NE for 10 minutes minimally stimulated 45Ca++ uptake from 1.27 +/- 0.04 to 1.45 +/- 0.17 nmol/mg tissue. In the presence of 10(-7) M leucine-enkephalin, 45Ca++ uptake was increased to 1.95 +/- 0.14 nmol/mg tissue. 45Ca++ uptake was reduced to control values (1.19 +/- 0.09 nmol/mg tissue) in the presence of NE, leucine-enkephalin and naloxone (10(-7) M). The data are consistent with a leucine-enkephalin augmentation of NE-induced chronotropy in guinea-pig atria due to an enhancement of NE-dependent Ca++ accumulation. This effect of leucine-enkephalin is opposite that previously reported for rat atria, in which leucine-enkephalin inhibits both NE-induced positive chronotropy and Ca++ influx.

Acupuncture relief of chronic pain syndrome correlates with increased plasma met-enkephalin concentrations

In twenty patients with chronic pain syndrome, acupuncture treatment resulted in significant improvement of both pain and psychiatric symptoms and higher plasma concentrations of metenkephalin. Plasma beta-endorphin concentrations were unchanged. The degree of symptom relief was correlated with the increase in plasma met-enkephalin.

Leucine enkephalin antagonizes norepinephrine-induced 45Ca++ accumulation in rat atria

Exposure of rat atrial slices to 10(-5) M norepinephrine (NE) for 10 minutes increases 45Ca++ accumulation from 1.64 +/- 0.10 to 2.23 +/- 0.06 nmol/mg tissue. In the presence of leucine enkephalin (10(-8) M), NE-stimulated 45Ca++ uptake is reduced to 1.44 +/- 0.10 nmol/mg tissue. The effect of leu-enkephalin is reversed in the presence of 10(-7) M naloxone, NE-stimulated 45Ca++ uptake being increased to 2.17 +/- 0.15 nmol/mg tissue. The results support a direct interaction of leu enkephalin with beta-agonist-stimulated Ca++ flux in rat atria, and correlate with the previously reported enkephalin antagonism of NE-induced positive chronotropy in the same tissue.

Met5-enkephalin-arg6-phe7 content of human and rabbit plasma

Using an adsorption technique combined with high pressure liquid chromatography and a specific radioimmunoassay, the met5-enkephalin-arg6-phe7 (YGGFMRF) content was measured in human and rabbit plasma. This heptapeptide content was 0.16 +/- 0.03, 0.20 +/- 0.05 pmol/10 ml plasma, for human and rabbit plasma, respectively. The degradation of YGGFMRF injected intravenously (rabbit) or that of the opioid heptapeptide added to rabbit plasma is rapid. The biological half life (T 1/2) of 125I-YGGFMRF in rabbit plasma was about 45 seconds (in vivo) and 1 minute and 8 seconds (in vitro). The metabolic clearance rate of YGGFMRF is slower than that of met5-enkephalin. The YGGFMRF content of rabbit plasma increased following inhibition of dipeptidyl peptidase activity by an intravenous injection of captopril. The presence of met5-enkephalin-arg6-phe7 into circulation indicates that it may have some physiological role, however it is unknown whether circulating YGGFMRF originates from adrenal medulla, intermediate lobe of pituitary or some other site.