Differential modulation of pancreatic beta-cell bursting by intracellular pH in the presence and absence of a K-ATP channel blocker

Regulation by glucose of oscillatory electrical activity and 5-HT/insulin release from single mouse pancreatic islets in absence of functional K(ATP) channels

The glucose sensitivity of bursting electrical activity and pulsatile insulin release from pancreatic islets was determined in absence of functional K(ATP) channels. Membrane potential, [Ca(2+)](i) and 5-HT/insulin release were measured by intracellular recording, fura-2 fluorescence and 5-HT amperometry, respectively. Single mouse islets, bathed in tolbutamide or glibenclamide and high extracellular Ca(2+) (Ca(2+)(o)), displayed bursting activity and concomitant fast [Ca(2+)](i) and 5-HT/insulin oscillations. Sulphonylurea block of K(ATP) channel current was unaffected by raising Ca(2+)(o). Raising glucose or alpha-ketoisocaproic acid (KIC) concentration from 3 to 30 mM increased spiking activity and burst plateau duration. Staurosporine did not impair glucose potentiation of electrical activity, ruling out the involvement of serine/threonine kinases. Glucose enhanced both [Ca(2+)](i) and 5-HT/insulin oscillatory activity, causing a approximately 3-fold increase in overall 5-HT release rate. Cells lacking bursting activity in high Ca(2+)(o) and low glucose (or KIC) developed a pattern of intensified spiking in response to 11 mM glucose. It is concluded that beta-cells exhibit graded oscillatory electrical and secretory responses to glucose in absence of functional K(ATP) channels. This suggests that, under physiological conditions, early glucose sensing may involve other channels besides the K(ATP) channel.

Oscillation of gap junction electrical coupling in the mouse pancreatic islets of Langerhans

1. Pancreatic beta-cells oscillate synchronously when grouped in islets. Coupling seems essential to maintain this oscillatory behaviour, as isolated cells are unable to oscillate. This allows the islet to be used as a model system for studying the role of coupling in the generation of oscillatory patterns. 2. Pairs of beta-cells were intracellularly recorded in islets. beta-Cells oscillated synchronously. Propagated voltage deflections were observed as a function of glucose concentration and of the distance between the recording electrodes. Space constants were smaller in the silent than in the active phases, suggesting a higher intercellular connection in the active phases. 3. Coupling coefficients and estimated coupling conductances were larger in the active than in the silent phases. 4. Coupling coefficients and coupling conductances changed dynamically and in phase with the membrane potential oscillations, pointing to an active modulation of the gap junctions. 5. We hypothesize a role for coupling in the generation of the oscillatory events, providing different levels of permeability dependent on the state of conductance during the oscillatory phases.

Multiphasic action of glucose and alpha-ketoisocaproic acid on the cytosolic pH of pancreatic beta-cells. Evidence for an acidification pathway linked to the stimulation of Ca2+ influx

Glucose stimulation raises the pHi of pancreatic beta-cells, but the underlying mechanisms are not well understood. We have now investigated the acute effects of metabolizable (glucose and the mitochondrial substrate alpha-ketoisocaproic acid, KIC) and nonmetabolizable (high K+ and the K-ATP channel blocker tolbutamide) insulin secretagogues on the pHi of pancreatic beta-cells isolated from normal mice, as assessed by BCECF fluorescence from single cells or islets in the presence of external bicarbonate. The typical acute effect of glucose (22-30 mM) on the pHi was a fast alkalinization of approximately 0.11 unit, followed by a slower acidification. The relative expression of the alkalinizing and acidifying components was variable, with some cells and islets displaying a predominant alkalinization, others a predominant acidification, and others yet a mixed combination of the two. The initial alkalinization preceded the [Ca2+]i rise associated with the activation of voltage-sensitive Ca2+ channels. There was a significant overlap between the glucose-evoked [Ca2+]i rise and the development of the secondary acidification. Depolarization with 30 mM K+ and tolbutamide evoked pronounced [Ca2+]i rises and concomitant cytosolic acidifications. Blocking glucose-induced Ca2+ influx (with 0 Ca2+, nifedipine, or the K-ATP channel agonist diazoxide) suppressed the secondary acidification while having variable effects (potentiation or slight attenuation) on the initial alkalinization. KIC exerted glucose-like effects on the pHi and [Ca2+]i, but the amplitude of the initial alkalinization was about twice as large for KIC relative to glucose. It is concluded that the acute effect of glucose on the pHi of pancreatic beta-cells is biphasic. While the initial cytosolic alkalinization is an immediate consequence of the activation of H+-consuming metabolic steps in the mitochondria, the secondary acidification appears to originate from enhanced Ca2+ turnover in the cytoplasm. The degree of coupling between glucose metabolism and Ca2+ influx as well as the relative efficacies of these processes determines whether the acute pHi response of a beta-cell (or of a tightly coupled multicellular system such as an islet of Langerhans) is predominantly an alkalinization, an acidification, or a mixed proportion of the two.

Changes in 2',7'-bis(carboxyethyl) 5'(6')-carboxyfluorescein-, fura-2 and autofluorescence in intact rat pancreatic islets in response to nutrients and non-nutrients

Intracellular pH (pHi) was measured in intact rat islets loaded with the dye 2',7'-bis(carboxyethyl) 5'(6')-carboxyfluorescein. Raising the concentration of glucose from 3 to 13 mM caused a modest, gradual increase in pHi (500:450 fluorescence ratio). The addition of 20 mM lactate caused a gradual decline in pHi which reversed upon withdrawal of lactate. In contrast, the weak acids propionate and acetate (20 mM) induced a rapid, pronounced fall in pHi followed by a gradual recovery. Upon removal of the weak acid, a marked, reversible alkalinization occurred. The addition of 20 mM NH4Cl caused a pronounced intracellular alkalinization, followed by recovery. The subsequent removal of NH4Cl induced a rapid, reversible acidification. The addition of 20 mM KCl did not affect pHi. Epifluorescence at 350 and 380 nm excitation, and the 350:380 fluorescence ratio, an index of cytosolic [Ca2+] ([Ca2+]i), were measured in islets loaded with the calcium indicator fura-2. Approximately 30% of the total fluorescence was estimated to be derived from NAD(P)H autofluorescence. Addition of KCl or acetylcholine to fura-2 loaded islets raised and lowered, respectively, the 350 and 380 signals, thereby causing marked increases in the 350:380 ratio. Neither KCl nor acetylcholine affected cellular NAD(P)H autofluorescence in non-loaded islets. An increase in glucose concentration caused an increase in both the 350 and 380 fluorescence signals and also in the 350:380 ratio. Qualitatively similar, although smaller changes were observed when Ca2+ was omitted from the medium.(ABSTRACT TRUNCATED AT 250 WORDS)