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C S Pace

Publications and source records attributed to C S Pace.

At least 37 records · Page 2Linked to original sources
PubMed1981

Glucose-induced electrical activity in the pancreatic beta-cell: effect of veratridine.

Veratridine was used in the presence of glucose to assess the role of the Na pump in the regulation of glucose-induced burst activity. In the presence of 8.4 mM glucose, veratridine elicited a silent hyperpolarization, followed by burst activity. The magnitude of depolarization to plateau potential and the duration of the silent phase were increased. The addition of tetrodotoxin (TTX) restored the pattern of electrical activity to that observed in the absence of veratridine. Similar results were observed when veratridine was used in the presence of 16.7 mM glucose and tetraethylammonium (blocks voltage-dependent potassium permeability). TTX or ouabain blocked the effects of veratridine, and produced depolarization and continuous spike activity. Quinine (blocks Ca-dependent potassium permeability) elicited continuous spike activity in the presence of 16.7 mM glucose. The addition of veratridine induced only a transient return to burst activity, followed by a return to continuous spike activity. These results suggest that an electrogenic Na pump is an important factor in maintaining the transmembrane potential at an optimum level for operation of a voltage- and Ca-sensitive potassium permeability: changes in potassium permeability operating on a background of electrogenic current may be responsible for the voltage transitions associated with burst activity.

Action Potentials↗
PubMed1981

Glucose-induced electrical activity in pancreatic beta-cell: modulation by pH.

Studies have been undertaken to examine the influence of changes in pH on the electrical activity in mouse islet beta-cells. Extracellular acidification or the presence of the permeable weak acid glycodiazine was found to alter the cyclic nature of glucose-induced electrical activity leading to depolarization and constant spike activity. Conversely, the presence of the permeable weak base imidazole was found to transiently lead to hyperpolarization with concomitant inhibition of electrical activity. Monensin, an electroneutral Na:H antiporter, was found to inhibit glucose-induced electrical activity, even in the presence of glycodiazine. These results suggest that protons may play an important role in the regulation and/or generation of the oscillatory pattern of electrical activity in the beta-cell.

Animals↗
PubMed1981

Somatostatin: mechanism of action in pancreatic islet beta-cells.

The widespread role of somatostatin (SRIF) as a mediator of function in the brain and gut has stimulated interest in it mechanism of action. We have examined the mode of action of SRIF in stimulus-secretion coupling in the pancreatic islet beta-cell to determine whether SRIF antagonizes the glucose-induced decrease in K+ permeability (PK). The influence of SRIF on 86Rb fluxes and insulin release in cultured rat islet cells, and also the electrical events recorded from cultured islets and microdissected mouse islets, was examined. In cultured islets, 100 ng/ml SRIF in the presence of 16.7 mM glucose inhibited the incidence of spike activity and evoked hyperpolarization. This effect was counteracted by 0.1 mM quinine and 20 mM tetraethylammonium (TEA), drugs that inhibit the Ca2+-sensitive or voltage-sensitive increase in PK, respectively. These agents also counteracted the inhibitory influence of SRIF on glucose-induced insulin release in cultured islets. SRIF disrupted the typical glucose-induced oscillatory pattern of electrical activity (burst activity) during continuous microelectrode recordings in mouse beta-cells, resulting in a transient 5mV hyperpolarization and a decrease in the frequency of generation of burst activity. The presence of 20 mm TEA prevented the influence of SRIF on the electrical activity. SRIF had no effect on the accumulation of 86Rb into islet cells obtained in the presence of 16.7 mM glucose. However, SRIF enhanced the rate of 86Rb efflux from cells exposed to glucose. SRIF-induced enhancement of 86Rb efflux was antagonized by TEA or quinine. These results indicate that SRIF may activate PK as its primary mode of action, an event that may be sufficient to reduce the accumulation of intracellular Ca2+ thereby disrupting glucose-induced stimulus-secretion coupling.

Animals↗
PubMed1980

ATP dependence of H+ secretion.

Cells in isolated rabbit gastric gland were made permeable to ATP by high voltage discharge across a gland suspension. In both normal (5.4 mM K+) and high K+ (108 mM) medium, this electrical shock resulted in a marked reduction in the ability of the parietal cell to produce and accumulate acid. Acid production was monitored both microscopically by acridine orange accumulation in the secretory canaliculus and by accumulation of the weak base [14C]aminopyrine. In 108 mM K+ solutions but not in 5.4 mM K+ solutions 5, mM ATP was able to restore the accumulation of these probes to control (unshocked) levels. When shocked glands had been previously stimulated by secretagogues, the aminopyrine accumulation ratio was only partly restored by ATP. Inhibition of mitochondrial respiration by cyanide, azide, or Amytal abolished acid secretion; the subsequent addition of ATP to shocked glands increased the aminopyrine accumulation ratio to 47 and resulted in an acridine orange fluorescence indistinguishable from that of histamine-stimulated, unshocked glands. We conclude that ATP can act as a substrate for H+ secretion in the parietal cell, and that perhaps no additional energy source is necessary.

Adenosine Triphosphate↗
PubMed1979

The differential effect of thyrotropin on the electrical responses of thyroid cells in monolayer cultures of varying duration.

The acute effects of thyrotropin on the membrane potential of thyroid cells maintained in the presence or absence of thyrotropin (0.2 U/ml) in the culture medium was determined. Monolayer cultures were prepared from porcine thyroid glands and cultured for 4--17 days after which the culture medium was exchanged for a buffered salt solution for intracellular measurements of the membrane potential. Cells were serially impaled with a microelectrode, first in the absence and then in the presence of 10 mU/ml thyrotropin. Cells cultured for 4--9 days depolarized from --29.6 +/- 1.7 (mean +/- S.E.) to --19.3 +/- 1.3 mV within 10 min after acute addition of 10 mU/ml thyrotropin. From 11 to 17 days of culture, basal membrane potentials were lower and, in most instances, cell hyperpolarization occurred within 30 min in response to thyrotropin. There was no difference in the electrical response of cells maintained in culture with or without thyrotropin. However, cells cultured with thyrotropin formed follicle-like structures in contrast to the monolayer formation of cells cultured without thyrotropin. The changes in the basal and stimulated electrical responses occur within a time frame similar to that reported for changes in the biosynthetic capacity of thyroid cells in culture. The data further emphasize the possible regulatory role of the cell membrane in stimulus-secretion coupling in the thyroid.

Animals↗
PubMed1978

Electrical responses of rat islets maintained in culture with varying levels of glucose.

The electrical responses of rat islet cells maintained in culture with varying levels of glucose were determined. After culture with 2.8 or 4.2 mM glucose, the beta-cells did not respond electrically to 27.8 mM glucose. However, after culture in 5.6 mM glucose, the cells responded to 27.8 mM glucose in short term experiments with depolarization and an increase in the incidence of spike activity. Islet cells maintained in 8.4 or 14.0 mM glucose had increasingly greater (negative) membrane potentials. The incidence of glucose-induced spike activity was the greatest for cells which had been cultured in 14.0 mM glucose. Raising glucose in the culture medium from 2.8 to 14.0 mM for the last 2 days of culture restored the ability of the islets to respond electrically to glucose in short term experiments. These observations show that the electrical phenomena of the beta cell membrane are affected by alterations in the level of glucose in the culture medium and serve as reliable indicators of functional glucose sensitivity of the islet cells.

Animals↗
PubMed1977

Electrophysiological evidence for the autoregulation of beta-cell secretion by insulin.

Electrophysiological studies of cultured rat pancreatic beta-cells using intracellular microelectrodes show that exogenous insulin over the range of 0.1 -- 10.0 microng/ml inhibits the electrical activity due to 27.8 mM glucose in a dose-related manner. This inhibitory effect is manifested by a mean increase of the membrane potential from about --20 to --30 mV and inhibition of the number of cells impaled showing spike activity from 60 to less than 10%. The inhibitory influence of insulin is rapid occurring within 5 min for the highest level used. The results provide evidence for a negative feedback role of insulin in regulating its own release.

Animals↗
PubMed1977

Somatostatin inhibition of glucose-induced electrical activity in cultured rat islet cells.

Electrophysiological studies of rat islet cells in monolayer culture were undertaken to determine the role of transmembranous ionic fluxes in the inhibitory action of somatostatin on insulin release. In the presence of somatotropin release inhibiting factor (SRIF) (2.5 nM), hyperpolarization occured with or without glucose (16.6 mM) in the medium. SRIF also inhibited the incidence of glucose-induced spike activity. The inhibitory action of SRIF occurred within 5 min and was readily reversible. An increase in extracellular K+ (5-13 mM) or Ca2+ (2.3-4.6 mM) prevented SRIF inhibition of glucose-induced electrical activity. The secretory response of cultured islets to glucose (16.6 mM) was completely inhibited by SRIF (2.5 nM). The presence of high [Ca2+]o or [k+]o enhanced insulin release in the presence of SRIF and glucose. Although phentolamine (5.0 microgram/ml) did not block the inhibition of glucose-induced electrical responses by SRIF, it prevented the inhibitory action of epinephrine (0.2 microgram/ml). It is concluded that the primary action of SRIF is to alter transmembranous cationic fluxes, as manifested by hyperpolarization and a decrease in the incidence of spike activity, which may prevent glucose from eliciting a normal secretory response.

Action Potentials↗
PubMed1976

Hormone secretion and glucose metabolism in islets of Langerhans of the isolated perfused pancreas from normal and streptozotocin diabetic rats.

The glucose responsiveness of alpha- and beta-cells of normal as well as untreated and insulin-treated streptozotocin diabetic rats was tested in the extracorporeal perfusion system. Also assessed was the possible in vitro effect of added insulin on the glucose sensitivity of islets from untreated diabetic animals. Insulin and glucose responsiveness of the two cell types. The rate of glucose entry islet tissue was estimated, and the effect of glucose on the tissue supply of ATP and lactate and the cyclic 3':5'-AMP level of islets was measured under the above in vitro conditions. It was demonstrated that beta-cells are more accessible to glucose than alpha-cells, that glucose entry into islet cells is not significantly modified by insulin and that glucose had no effect on ATP, lactate and cyclic 3':5'-AMP levels of islet tissue under any of the conditions investigated. High insulin in vitro elevated ATP levels of alpha-cell islets independent of extracellular glucose. Glucose caused insulin release from normal but not from diabetic islets and rapidly and efficiently suppressed stimulated glucagon secretion of the pancreas from normal and insulin treated diabetic rats. Glucose was less effective in inhibiting stimulated glucagon secretion by the pancreas from untreated diabetic rats whether insulin was added to the perfusion media or not. Therefore, profound differences of glucose responsiveness of alpha-cells fail to manifest themselves in alterations of basic parameters of glucose and energy metabolism in contrast to what had been postulated in the literature. It is however, apparent that the glucose responsiveness of alpha-cells is modified by insuling by an as yet undefined mechanism.

Adenosine Triphosphate↗
PubMed1975

Multiple metabolic functions of glucose in rat pancreatic islets.

Metabolic interactions between glucose and amino acids were studied with isolated rat islets using glucose utilization and lactate formation as indicators. Certain amino acids (8-10 mM) are capable of greatly stimulating lactate formation from 5mM glucose. On a molar basis L-isoleucine is the most potent stimulator in a group of twenty-six amino acids. Aphysiological amino acid mixture (7.5-14 mM) or L-isoleucine (8 mM) profoundly altered the basic sigmoidal relation between glucose concentration in the medium and the rate of glucose utilization and lactate formation: with basal glucose (5 mM) both glucose utilization and lactate production were stimulated by the amino ACID MIXTURE and by L-isoleucine; at high glucose levels utilization was decreased by the amino acid mixture, but was unaffected by L-isoleucine, whereas lactate formation was decreased by both additions. The data indicate that amino acids may play a significant role in regulating the extent to which glucose serves as a fuel of pancreatic islet cells and in determining the pathways of glucose metabolism. In order to elucidate the mechanisms of the amino acid effect, studies with phloridzin, ouabain, iodoacetate, cytochalasin B, and Na+-deficiency were performed with the most effective amino acid, L-isoleucine. Each of these agents and Na+-deficiency substantially reduced or completely blocked the extra lactate formation induced by L-isoleucine (8-10 mM). The intracellular uptake of 14-CL-isoleucine by isolated islets was found to be Na+-independent, and uphill transport of this amino acid was not detectable, whether basal glucose was present in the medium or not. The action of iodoacetate in blocking glycolysis was reinvestigated. After forty-five minutes of exposure, 0.2mM iodoacetate completely blocks lactate formation as well as glucose utilization. Thisconfirms and extends earlier data for this laboratory and suggests that this SH-reagent indeed allows dissociation of the fuel and releasing functions of glucose.

Amino Acids↗
PubMed1975

Electrical and secretory manifestations of glucose and amino acid interactions in rat pancreatic islets.

Interactions between glucose and amino acids in rat pancreatic islets were studied by recording the intracellular membrane potential and spike discharges from the isolated perfused pancreas. It was found that L-isoleucine requires the presence of basal glucose (5 mM) in order to increase spike discharge from islet cells and depolarize the cell membrane. Similarly basal glucose is needed for insulin release by L-isoleucine. A physilolgical mixture of twenty amino acids also required the presence of basal glucose in order to increase spike activity and insulin release. In contrast to L-isoleucine the amino acid mixture did not depolarize the beta-cells. Iodoacetate, at concentrations previously shown to block glycolysis completely, did not interfere with any of these permissive actions of glucose, nor did iodoacetate alter the well known electrical manifestations of high levels of glucose itself (i.e. depolarization and increased spike discharge). These data show that glucose plays a pre-eminent role as regulator of islet cell function, governing the efficacy of amino acids as beta-cells stimulants. The results are most easily interpreted if one assumes that glycolysis is not required for glucose to exert its action.

Amino Acids↗