Prestimulatory Values Research Articles

Ca2+-dependent desensitization of insulin secretion by strong potassium depolarization

Depolarization by a high K(+) concentration is a widely used experimental tool to stimulate insulin secretion. The effects occurring after the initial rise in secretion were investigated here. After the initial peak a fast decline occurred, which was followed by a slowly progressive decrease in secretion when a strong K(+) depolarization was used. At 40 mM KCl, but not at lower concentrations, the decrease continued when the glucose concentration was raised from 5 to 10 mM, suggesting an inhibitory effect of the K(+) depolarization. When tolbutamide was added instead of the glucose concentration being raised, a complete inhibition down to prestimulatory values was observed. Equimolar reduction of the NaCl concentration to preserve isoosmolarity enabled an increase in secretion in response to glucose. Unexpectedly, the same was true when the Na(+)-reduced media were made hyperosmolar by choline chloride or mannitol. The insulinotropic effect of tolbutamide was not rescued by the compensatory reduction of NaCl, suggesting a requirement for activated energy metabolism. These inhibitory effects could not be explained by a lack of depolarizing strength or by a diminished free cytosolic Ca(2+) concentration ([Ca(2+)](i)). Rather, the complexation of extracellular Ca(2+) concomitant with the K(+) depolarization markedly diminished [Ca(2+)](i) and attenuated the inhibitory action of 40 mM KCl. This suggests that a strong but not a moderate depolarization by K(+) induces a [Ca(2+)](i)-dependent, slowly progressive desensitization of the secretory machinery. In contrast, the decline immediately following the initial peak of secretion may result from the inactivation of voltage-dependent Ca(2+) channels.

Differential behavioural and hormonal responses to two different stressors (footshocking and immobilization) in sheep.

Two different stressors footshocking and immobilization applied for 3 days induced in sheep differential behavioural and hormonal responses in cortisol and prolactin secretion. Immobilization drastically disturbed rumen motoric activity (significantly attenuated its contractions), caused loss of appetite and severe general depression. Footshocking did not induce any of these symptoms. Immobilization induced 2-3 fold higher rise of plasma cortisol mesors on the days of stressing as compared to footshocking. On the poststressing days after footshocking as well as after immobilization plasma cortisol mesors fell to 50% of the prestimulatory values. The rise of plasma prolactin mesors in footshocked and in immobilized animals on the days of stressing was nearly of the same magnitude. However, a significant difference in the response of prolactin secretion between footshocked and immobilized animals occurred in the circadian rhythmicity of the hormone secretion. Footshocking induced circadian rhythmicity with characteristic acrophases, while immobilization did not induce the circadian pattern of prolactin secretion. An attempt of interpretation of the described phenomena has been undertaken.

Effects of spinal cord stimulation on the flexor reflex and involvement of supraspinal mechanisms: an experimental study in mononeuropathic rats.

The physiological mechanisms responsible for pain relief caused by spinal cord stimulation (SCS) are essentially unknown and recent experimental data are sparse. In the present study the authors explored the possible involvement of supraspinal mechanisms in the effects of SCS applied in rats with experimental mononeuropathy produced by sciatic nerve ligation according to the method of Bennett and Xie or that of Seltzer, et al. Confirming results of a previous study undertaken by the authors, the thresholds of the early component of the flexor reflex (latency 8-12 msec), which is mediated by A fibers, were significantly lower in the nerve-ligated than in the intact leg. In halothane-anesthetized animals the spinal cord was exposed and SCS was applied with parameters similar to those used in clinical SCS. Ten minutes of SCS produced a significant elevation of the lowered threshold of the early flexor component only in the nerve-ligated leg, and this augmentatory effect of SCS persisted for 30 to 40 minutes after cessation of the stimulation. The threshold elevation amounted to between 50% and 80% of the prestimulatory value and it was related to the intensity of SCS. The threshold of the late, C-fiber-mediated component of the flexor reflex was not influenced in either of the legs. After transection of the spinal cord at the T-6 level, there was a moderate threshold increase in both the early and late components in both legs, but the threshold of the early component in the nerve-ligated leg remained lower. Spinal cord stimulation produced an almost identical threshold increase in the early component in the nerve-ligated leg with the same time course as before the transection. There was no effect on the late component of the reflex in either leg. The results indicate that this effect of SCS in mononeuropathic rats does not necessarily involve supraspinal mechanisms; instead SCS is operative at a spinal, segmental level. In view of the similarities between the effects of therapeutic SCS on cutaneous hypersensibility in patients with peripheral neuropathic pain and the effects demonstrated in neuropathic rats, the clinical pain relief achieved with SCS may be produced, at least partially, by intraspinal mechanisms.

The electrical activity of mouse pancreatic beta-cells recorded in vivo shows glucose-dependent oscillations.

1. The characteristics of the electrical activity of beta-cells from islets of Langerhans recorded in vivo are described. For blood glucose concentrations from 4 to 11 mM, the electrical activity of pancreatic beta-cells is oscillatory, with alternating depolarized and hyperpolarized phases. During the depolarized phases, action potentials are triggered. 2. The main effect of increasing glucose concentration consists of an increase in the duration of the depolarized phase. The relationship between blood glucose concentration and the percentage of time in the depolarized phase can be described by a sigmoidal function with half-activation at 6.8 mM glucose. The equivalent value obtained in parallel experiments in vitro is 13.3 mM, a significant rightward shift in the activation curve that suggests a role for other neural or humoral factors in determining the islet sensitivity to glucose. 3. The injection of glucose into the bloodstream produces a transitory phase of continuous electrical activity that is recorded within seconds after the change and that leads to a decrease of the glycaemia to the prestimulatory value. 4. The results demonstrate that under physiological conditions the electrical response of beta-cells to glucose consists of membrane potential oscillations, validating previous data obtained with isolated preparations. Furthermore, the electrical response occurs at lower levels of glycaemia than those predicted from recordings in isolated preparations and is maximal within the physiological range of blood glucose.

Peptidergic nerves mediate post-nerve stimulation hyperemia in rat gut.

Cessation of perivascular nerve stimulation (NS) elicits a transient increase in intestinal blood flow above the prestimulatory value. This enhancement of blood flow constitutes the phenomenon of post-nerve stimulation hyperemia (PSH). We investigated the involvement of peptidergic sensory nerves in intestinal PSH. In anesthetized rats the velocity of blood flowing through the anterior mesenteric artery (VBF) was measured with a pulsed Doppler velocimeter. PSH was induced by 4 min of postganglionic electrical NS (5 Hz). PSH was abolished by distal periarterial application of tetrodotoxin and intra-arterial lidocaine, which suggests a peripheral sensory nervous mechanism for PSH. The increase in conductance at peak PSH was blocked by pretreatment with the selective, primary afferent neurotoxin capsaicin administered as 1) subcutaneous injection in neonatal life, 2) topical application to periarterial nerves, or 3) injection into the jejunal lumen. In rats pretreated with reserpine, NS evoked a hyperemic response, which was blocked by capsaicin. Treatment with adenosine deaminase inhibited PSH considerably less than capsaicin, suggesting a lesser role for adenosine in PSH. Our findings support the hypothesis that postganglionic NS activates both adrenergic and peptidergic nerves and that the latter release vasodilator peptides in the gut during PSH.

EGF receptor down-regulation attenuates ligand-induced second messenger formation

Epidermal growth factor (EGF)-induced increases in cytosolic Ca 2+ and inositol polyphosphate production were compared in a human hepatocellular carcinoma-derived cell line, PLC/PRF/5, and in an EGF receptor-overexpressing subline, NPLC/PRF/5. Formation of these second messengers was correlated to EGF receptor display at the cell surface by monitoring ligand-induced EGF receptor down-regulation. Both cell lines exhibited a strikingly similar cytosolic Ca 2+ increase upon exposure to EGF. The initial inositol phosphate responses were also similar in the two cell lines; inositol 1,4,5-trisphosphate increased within 10–15 s and returned to prestimulatory values after 2 min in both cell lines, while inositol tetrakisphosphate and inositol 1,3,4-trisphosphate were elevated after a 2-min exposure to EGF. At later times the responses were markedly different; NPLC/PRF/5 cells exhibited prolonged production of inositol 1,3,4-trisphosphate and inositol tetrakisphosphate (maximum at 1–3 h) but PLC/PRF/5 cells showed decreased levels of these isomers after 10 min and a return to basal values by 1 h. Exposure of PLC/PRF/5 cells to EGF caused a progressive decrease in the amount of EGF receptor at the cell surface whereas such treatment did not change the surface receptor levels in NPLC/PRF/5 cells. Kinetic analysis of EGF receptor down-regulation showed that receptor internalization was rapid enough to account for the transient nature of the inositol phosphate response in PLC/PRF/5 cells. Thus, the divergent patterns of signaling exhibited by the two cell lines may reflect differences in the efficiency of EGF-induced down-regulation of surface receptors.

The effect of splanchnic nerve stimulation and neuropeptide Y on cholera secretion and release of vasoactive intestinal polypeptide in the feline small intestine.

The effect of sympathetic nerve stimulation and intra-arterial infusion of neuropeptide Y (NPY) on net fluid secretion and release of vasoactive intestinal polypeptide (VIP) was studied in the cat small intestine during a secretion due to cholera toxin. Activation of the splanchnic nerves (4 Hz, 5 ms, 5 V) decreased net fluid secretion to 57 +/- 10% of control. Concomitantly, the release of VIP was reduced to less than 50%. Furthermore, close i.a. infusion of NPY (estimated increase in plasma concentration 75 nmol l-1) reduced the net fluid secretion and VIP release to 27 +/- 5 and 28 +/- 4% of the pre-stimulatory value. The correlation between the decrease in net fluid secretion and reduction in VIP release showed a strong positive correlation (r = 0.83). These results strongly indicate that the antisecretory effect of sympathetic nerve stimulation during cholera diarrhoea is mediated by inhibition of secretory VIP neurons in the intestinal mucosa. A similar mechanism is also proposed for the intravascularly administered NPY.

Role of phosphoinositide metabolism in induction of memory in isolated perifused rat islets.

Prior exposure of isolated perifused rat islets to 20 mM glucose or 10 mM glyceraldehyde amplifies their subsequent insulin secretory response to 10 mM glucose. The involvement of phosphoinositide turnover in the induction of this "time-dependent potentiation" (TDP) was investigated. In islets in which inositol-containing phospholipids were prelabeled with myo-[2-3H]inositol, the addition of 20 mM glucose augments the efflux of 3H. This effect persists for approximately 50 min after the cessation of stimulation. Direct measurements of labeled inositol phosphate accumulation confirmed that this increase in 3H efflux is primarily the result of a persistent increase in phosphoinositide (PI) hydrolysis and not due to the slow efflux and/or degradation of performed [3H]inositol phosphates. The duration of the increase in 3H efflux parallels the duration of TDP. Mannoheptulose abolishes both the increase in 3H efflux evoked by 20 mM glucose and TDP. The omission of extracellular calcium plus 0.5 mM ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid also abolishes both of these effects of high glucose. D-Glyceraldehyde (10 mM) addition to 3H-inositol-prelabeled islets results in an acute efflux of 3H, a persistent efflux after removal of the D-glyceraldehyde from the perifusion medium, and the induction of TDP. Similar to the results obtained with high glucose, the return of 3H efflux rates to prestimulatory values is accompanied by the abolition of TDP. These results suggest that events associated with persistent stimulant-induced increases in phosphoinositide hydrolysis may participate in the induction and maintenance of TDP.

Effect of electric stimulation of the celiac vagus on gastric acid secretion and plasma concentrations of somatostatin and gastrin in the portal and gastroepiploic veins of dogs.

Electric stimulation (ES) of the celiac vagus during tetragastrin infusion reduced significantly the portal plasma concentration of somatostatin (SS) from 113 +/- 11.3 pg/ml to 87.8 +/- 5.8 pg/ml (P less than 0.05) in anesthetized dogs, in parallel with marked decrease of gastric acid secretion (59.9 +/- 7.1% of the prestimulatory value; P less than 0.01). A similar change in the portal plasma SS concentration was observed by ES of the celiac vagus on infusion of saline, with a concomitant significant increase in the portal plasma level of gastrin from a basal value of 65.9 +/- 7.0 pg/ml to a peak value of 129 +/- 29.9 pg/ml (P less than 0.05). However, no fluctuation of the plasma SS or gastrin level in the gastroepiploic vein was detected during or after ES of the celiac vagus. These findings indicate that gastric SS and gastrin are not of primary importance in the mechanism of inhibition of gastric acid secretion induced by ES of the celiac vagus in the dog.

Paradoxical response of sphincter of Oddi to intravenous injection of cholecystokinin or ceruletide. Manometric findings and results of treatment in biliary dyskinesia.

Sixty two patients with a clinical suspicion of biliary dyskinesia were investigated with endoscopic manometry of the sphincter of Oddi before and after intravenous injection of cholecystokinin or ceruletide. In 52 patients injection was followed by decreased pressure in the sphincter of Oddi; 43 of these had normal prestimulatory values (group I), while the values were raised in the other nine patients (group II). A paradoxical response to intravenous injection was observed in 10 women (group III): increased baseline sphincteric pressure occurred in eight and increase in the amplitude of phasic contractions in four patients. The prestimulatory sphincteric pressure was raised in five and normal in the remaining patients. Eight patients were treated with papillotomy (seven) or balloon dilatation of the sphincter (one). They experienced relief of pain during a follow up period of 11-16 months. Intravenous injection of cholecystokinin or ceruletide may disclose a special type of biliary dyskinesia even in patients with normal prestimulatory manometric findings. Hormone injection increases the diagnostic yield of endoscopic manometry in patients suspected of biliary dyskinesia.

DIFFERENT ACTIONS OF INTRAVENOUS ETHANOL ON BASAL (= INTERDIGESTIVE) SECRETION OF GASTRIC ACID, PANCREATIC ENZYMES AND BILE ACIDS AND GASTROINTESTINAL MOTILITY IN MAN

The action of an intravenous infusion of ethanol (10% v/v; given in a dose of 300 mg kg-1 body weight for 30 min followed by 3 mg kg-1 min-1 for 2 hr) on the basal (= interdigestive) gastrointestinal motor activity and the basal gastric acid, pancreatic amylase and bile acid secretion was determined in 6 healthy human volunteers. Ethanol did not affect the duration of the interdigestive motor complex and the output of bile acids into the duodenum. Ethanol significantly (P less than 0.05) stimulated the gastric acid output by about 2.2-fold and inhibited the pancreatic amylase output by about 43% as compared to control experiments in which an intravenous infusion of 0.15 M NaCl was given. Ethanol did not alter the mean plasma levels of gastrin and pancreatic polypeptide as compared to prestimulatory values and to control experiments. In conclusion, these results show that intravenous ethanol given in a moderate dose stimulates gastric acid output and inhibits pancreatic amylase output in fasting non-alcoholic human beings. The mechanism(s) of these different actions of ethanol is unknown since release of gastrin or pancreatic polypeptide by ethanol does not account for the observed effects of intravenous ethanol.

A stimulating effect of glucose on somatostatin release is impaired in noninsulin-dependent diabetes mellitus.

The effect of oral glucose (1 g/kg BW) on levels of immunoreactive somatostatin (SLI) in peripheral venous plasma was investigated in young and older nondiabetic subjects (mean ages, 25.8 and 56.7 yr, respectively) as well as in subjects with decreased iv glucose tolerance (K value less than 1.0) and in diet-treated diabetic patients with fasting hyperglycemia (blood glucose, greater than 7.0 mmol/liter). SLI was assayed after extraction of plasma on silica glass beads. In control experiments (glucose omitted), SLI levels tended to decline. Ingestion of glucose was followed by a moderate (52% or less) increase in SLI levels in subjects with normal or decreased iv glucose tolerance. The stimulating effect was sustained for 90 or 120 min after glucose ingestion and the increase in plasma SLI was significant (P less than 0.05-0.02) whether in relation to prestimulatory values or control experiments. In contrast, in overtly diabetic patients, glucose ingestion was not followed by increased SLI levels. It is concluded that oral glucose stimulates SLI secretion in individuals of different ages and with varying degrees of glucose tolerance, but the response is impaired in type 2 diabetic patients with fasting hyperglycemia.

Motor responses to serotonin in isolated guinea pig fundus and antrum.

The effects of serotonin on gut motility have been both excitatory and inhibitory. The purpose of the present study was to elucidate how serotonin influences motor activity in two functionally different parts of the stomach. Pressure recordings were made from fundic and antral pouches of isolated guinea pig whole stomach preparations. In the fundus serotonin induced relaxation, which developed gradually within 3 min. In the antrum serotonin initiated phasic contractions, which culminated within 3 min and then returned to near or below prestimulatory values. Whereas tetrodotoxin did not significantly reduce the fundic relaxation, the antral excitation was strongly inhibited by both atropine and tetrodotoxin, indicating different effect mechanisms in fundus and antrum.

Vasoactive intestinal polypeptide in vagally mediated pancreatic secretion of fluid and HCO3.

The role of nerves that liberate vasoactive intestinal polypeptide (VIP) in the porcine pancrease as mediators of the atropine-resistant action of the vagus on flow and bicarbonate (HCO3) secretion was examined. Efferent electrical stimulation of the vagus in atropinized pigs produced a profuse flow of pancreatic juice with high HCO3 content concomitantly with a significant increase in pancreatic VIP output from 13 to 113 fmol/min. Intravenous administration of somatostatin (SRIF) during continuous electrical vagal stimulation caused a parallel suppression of the VIP release and the pancreatic fluid and HCO3 secretion to prestimulatory values. The SRIF-induced reduction in fluid and HCO3 secretion seemed to be mediated via an inhibition of the VIP release rather than through a direct effect on the exocrine cells, inasmuch as SRIF did not influence the VIP-provoked exocrine response from the in vitro isolated perfused porcine pancreas. The results support the view that VIP is transmitter in the vagally induced atropine-resistant water and HCO3 secretion from the porcine pancreas.

Effects of α- and β-Adrenergic Inhibition and Somatostatin on Plasma Glucose, Free Fatty Acids, Insulin, Glucagon, and Growth Hormone Responses to Prostaglandin E1in Man*

It has been previously shown that prostaglandin E1 (PGE1) induced a significant increase in plasma glucose, FFA, glucagon, and GH levels when infused in normal man. In the present study, we evaluated the effects of α- and β-adrenergic blockers and somatostatin on these metabolic and hormonal responses to PGE1. The pretreatment with propranolol (0.08 mg/min) reduced basal plasma glucagon from a mean value of 86 ± 6 to 70 ± 8 pg/ml (mean ± SEM; P < 0.05) and completely blunted glucagon and FFA responses to PGE1 (0.2 μg/kg-min), but did not interfere with the rise in plasma glucose or GH levels. Phentolamine pretreatment (0.5 mg/min) caused a significant increase in basal glucagon and insulin (P < 0.05). Phentolamine had no effect on the responses of the various parameters to PGE1. Plasma insulin levels remained essentially unchanged during the development of hyperglycemia during both α and β blockade. Somatostatin (250 μg/h) lowered basal plasma insulin and glucagon (P < 0.01) and prevented glucagon and GH increases by PGE1. By contrast, plasma glucose rose from a mean basal value of 85 ± 4 to 135 ± 7 mg/100 ml (P < 0.001), and plasma FFA rose from a prestimulatory value of 600 ± 70 to 775 ± 76 μeq/liter (P < 0.05) after PGE1. Plasma glucose and FFA levels were significantly higher during PGE1 and somatostatin administration than when PGE1 was given alone. These results suggest that some (glucagon and FFA) but not all (glucose and GH) responses to PGE1 are mediated by the β-adrenergic system, that PGE1 inhibits insulin secretion not mediated by α stimulation, and that somatostatin blocks the increases in glucagon and GH resulting from PGE1 and also potentiates its hyperglycemic and lipolytic effects.

The effect of gonadotropins on the "androgen status" in boys with undescended testes.

The investigation was carried out in order to evaluate the stimulating effect of human chorionic gonadotropins (HCG) on the “androgen status” in a group of boys with undescended testes. The “androgen status” included the values found for plasma concentrations of testosterone, 5α-dihydrotestosterone (Gupta, McCafferty, Rager, 1972), plasma testosterone-binding affinity (Gupta, Huenges, Rager, 1971), and urinary excretion of testosterone and androsterone (Gupta and Butler, 1969). In 15 boys aged 1 to 12 years with incompletely descended testes the “androgen status” was found to be low as compared to that of normal boys of the same age group. With HCG stimulation, the “androgen status” of the subjects shifted to a higher level, although showing marked individual differences depending on the chronological age of the patient and the position and volume of the testes. The post-stimulation level of the “androgen status” in two anorchic boys, although different from the pre-stimulatory values, was not as elevated as in the other subjects. These results suggest that examination of the “androgen status” under HCG is valuable in the differential diagnosis of the existence of functional testicular tissue.