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J T Tarvin

Publications and source records attributed to J T Tarvin.

15 recordsLinked to original sources
PubMed1991

Changes in cellular membrane and paracellular conductances by amphotericin B in the epithelium of the bullfrog cornea.

In the isolated bullfrog cornea, measurements of DC electrical parameters in conjunction with AC impedance and ultrastructural analyses were used to determine the effects of 10(-5) M amphotericin B on epithelial cellular membrane and paracellular conductances. In NaCl Ringers, amphotericin B elicited a 3.5-fold increase in the specific apical membrane conductance (Ga/Ca); where Ga and Ca are the apical membrane conductance and capacitance, respectively. The basolateral membrane conductance (Gb) and the basolateral membrane capacitance (Cb) fell by 57% and 50%, respectively. In the paracellular pathway, the tight junctional complex (Gj) was unchanged whereas the lateral intercellular space resistance (Rp) decreased by 55%. The declines in Gb and Cb were suggestive of cell volume shrinkage because these changes were consistent with a previously described decline in intracellular K+ content and reduction in exposed basolateral membrane area to current flow. Ultrastructural analysis validated that amphotericin B caused cell volume shrinkage because there was: (1) increased folding of the basolateral membrane and waviness of the basal aspects- of the plasma membrane; (2) dilatation of the lateral intercellular spaces. This agreement suggests that intracellular activity decreased following exposure to amphotericin B which resulted in cell volume shrinkage and an impairment of Cl- uptake across the basolateral membrane.

Amphotericin B↗
PubMed1986

Amiloride-sensitive regulation of intracellular pH in B-cells: activation by glucose.

Alterations in intracellular pH (pHi) generated by metabolism of glucose has been proposed to be a transduction device for controlling changes in K+ conductance in the plasma membrane of the B-cell leading to depolarization and cyclic variations in the membrane potential associated with spike activity. The influence of permeable weak acids or bases and amiloride inhibition of H+ extrusion by a Na:H exchanger on glucose-induced electrical activity has suggested that the electrical events are pH-sensitive. In order to document that these conditions alter pHi, we determined the influence of glucose, propionic acid, and NH4Cl, in the presence or absence of amiloride on pHi of rat islets using [14C] DMO. Glucose, 2.8 mmol/L decreased pHi by .09 unit compared to the absence of glucose (pHi = 7.08 +/- .01, M +/- SEM) and 16.7 mmol/L glucose reduced pHi by .19 unit. The glucose dose-related decrease in pHi yielded a half-maximal response at 4 mmol/L. The addition of 0.1 mmol/L amiloride had no influence on pHi without glucose and decreased pHi in the presence of 2.8 mmol/L glucose by .14 unit. The addition of 20 mmol/L propionic acid to 2.8 mmol/L glucose reduced pHi to 6.85 +/- .05, whereas 20 mmol/L NH4Cl increased pHi to 7.27 +/- .07. The addition of amiloride did not further lower the reduction in pHi elicited by 20 mmol/L propionic acid or 16.7 mmol/L glucose. These results suggest that the amiloride-sensitive Na:H exchanger plays a major role in regulation of pHi, but another modality for pHi regulation exists to compensate for inhibition of Na:H exchange under conditions of an acid load.

Amiloride↗
PubMed1983

pH modulation of glucose-induced electrical activity in B-cells: involvement of Na/H and HCO3/Cl antiporters.

Regulation of intracellular pH is an essential function and may be especially significant in the B-cell in which the influence of glucose on electrical activity is modulated by alterations in pH. Two possible regulatory processes have been examined: Na/H and HCO3/Cl exchange, by using inhibitors, an ionophore, and changes of ionic concentrations. In the presence of 11.1 mM glucose we found that DIDS, an inhibitor of anion exchange, elicited a dose-response increase in the relative duration of the active phase with an ED50 of 99 microM. Probenecid (0.5 mM), an inhibitor of anion fluxes, also augmented the electrical activity (EA) due to glucose. Withdrawal of HCO-3 elicited constant spike activity followed by a resumption of burst activity with a greater duration of the active phase compared to control. These data are consistent with predicted cellular acidification. However, reduction of Cl-o by isethionate substitution produced no marked effect on EA. In contrast, SO-4- substitution for Cl- resulted in variable effects characterized by constant spike activity or a decrease in the duration of the active and silent phases along with silent hyperpolarization. Tributyltin, a Cl/OH, ionophore enhanced EA at 0.25 microM with 120 mM Cl-o, but reduced EA with 10 mM Cl- as would be predicted with either cellular acidification or alkalinization, respectively. Amiloride at 100 microM elicited constant spike activity perhaps due to inhibition of Na/H exchange. Reduction of Na+o from 142.8 to 40.8 mM had a similar effect and enhanced the influence of amiloride. It appears therefore that interference with putative pH regulatory mechanisms in the B-cell are consistent with the hypothesis that cell pH is involved in regulation of EA.

Animals↗
PubMed1983

Stimulus-secretion coupling in beta-cells: modulation by pH.

We have examined the influence of changes in pH on the oscillatory pattern of electrical activity (EA) in the beta-cell by altering medium pH (pHo) and using permeable weak buffers to alter intracellular pH (pHi). A decrease in pH in the presence of glucose elicited depolarization to the active phase and constant spike activity, whereas an increase in pH elicited a decrease in spike activity or silent hyperpolarization. On inhibition of HCO3:Cl antiport by addition of DIDS (4,4'-diisothiocyano-2,2'-stilbene disulfonic acid), probenecid, or withdrawal of medium HCO-3, there was an increase in the duration of the active phase. A similar result was obtained on the inhibition of Na:H antiport by the addition of amiloride or the reduction of medium [Na+]. The influence of H+ and glucose has been proposed to decrease K+ permeability (PK). However, the influence of pH on 86Rb+ efflux was most effective at subthreshold or 4.2 mM glucose; only a moderate decrease in PK occurred at 8.3 mM glucose, and no effect was obtained at 16.7 mM glucose. Alteration of pHi, and not pHo, induces similar effects on glucose-induced electrical and secretory events. There is a clear dissociation between the influence of inhibitors of the Na:H and HCO3:Cl antiporters on the electrical and secretory events. DIDS and amiloride increased glucose-induced EA, but markedly inhibited the secretory response to glucose. It is evident that pH modulates the electrical events and cationic fluxes and ultimately influences the transduction of information to the mechanisms controlling the secretory process in the beta-cell.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1982

Influence on anion transport on glucose-induced electrical activity in the B-cell.

We have previously shown that the effect of glucose on electrical activity (EA) in islet B-cells is altered by modification of pH. The regulation of intracellular pH (pHi) in nerve and muscle cells is coupled to anion exchange. In the present study we have examined the involvement of HCO3:Cl exchange across the plasma membrane in the maintenance of glucose-induced EA in B-cells. 4,4'-diisothiocyanostilbene-2,2' disulfonic acid (DIDS), an inhibitor of anion exchange, elicited a dose-related stimulation of EA in the presence of 11.1 mM glucose. The increase in the relative duration of the active phase (constant spike activity) was first observed at 20 microM DIDS, and a nearly maximal effect was obtained at 200 microM. The substitution of HCO3- by a Hepes buffer elicited constant spike activity. The application of 0.25 microM tributyltin, an electroneutral Cl:OH exchanger, also enhanced EA as indicated by an increase in the duration of the active phase. The influence of HCO3- withdrawal, DIDS, and tributyltin all elicited electrical events similar to that obtained by a decrease in pHi. Our results suggest that anion exchange may be involved in the regulation of electrical events in the B-cell by influencing pHi, as has been documented to occur in invertebrate nerve and muscle.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
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↗