PubMed Health⌕ Search

Biomedical subjects

J Keizer

Publications and source records attributed to J Keizer.

At least 37 records · Page 2Linked to original sources
PubMed1995

InsP3-induced Ca2+ excitability of the endoplasmic reticulum.

Oscillations in intracellular Ca2+ can be induced by a variety of cellular signalling processes (Woods et al., 1986; Berridge 1988; Jacob et al., 1988) and appear to play a role in secretion (Stojilković et al., 1994), fertilization (Miyazaki et al., 1993), and smooth muscle contraction (Iino and Tsukioka, 1994). Recently, great progress has been made in understanding the mechanisms involved in a particular class of Ca2+ oscillation, associated with the second messenger inositol 1,4,5-trisphosphate (InsP3) (Berridge, 1993). Working in concert with intracellular Ca2+, InsP3 controls Ca2+ release via the InsP3 receptor in the endoplasmic reticulum (ER) (Berridge and Irvine, 1989). The IP3 receptor is regulated by its coagonists InsP3 and Ca2+, which both activate and inhibit Ca2+ release (Finch et al., 1991; Bezprozvanny et al., 1991; De Young and Keizer, 1992). These processes, together with the periodic activation of Ca2+ uptake into the ER, have been identified as key features in the mechanism of InsP3-induced Ca2+ oscillations in pituitary gonadotrophs (Li et al., 1994), Xenopus laevis oocytes (Lechleiter and Clapham, 1992; Atri et al., 1993), and other cell types (Keizer and De Young, 1993). Earlier discussions and models of InsP3-induced Ca2+ oscillations focused on the nature and number of internal releasable pools of Ca2+ (Goldbeter et al., 1990; Swillens and Mercan, 1990; Somogyi and Stucki, 1991), the importance of oscillations in InsP3 (Meyer and Stryer, 1988), and other issues not based on detailed experimental findings in specific cells types.

Animals↗
PubMed1995

Mechanisms of calcium oscillations and waves: a quantitative analysis.

Oscillations and waves of increased intracellular free calcium concentration ([Ca2+]i) are observed in a wide range of cell types. Because of their inherent nonlinear nature and the consequent unreliability of intuitive approaches, mathematical modeling has an important role to play in the study of these phenomena. One important class of oscillations and waves is dependent on the presence of inositol (1,4,5)-trisphosphate (IP3), which releases Ca2+ from internal stores via the IP3 receptor/Ca2+ channel. With the minimum possible mathematical formalism, we review mechanistic models for IP3-dependent Ca2+ oscillations and waves. These models are based on the regulation of the IP3 receptor by both IP3 and Ca2+, and incorporate experimental data on the steady-state and kinetic properties of the receptor. The extension of the models to describe intracellular and intercellular Ca2+ waves is considered.

Animals↗
PubMed1995

Analysis of possible mechanisms for in vitro oscillations of insulin secretion.

We explore possible kinetic mechanisms responsible for the oscillatory (pulsatile) secretion of insulin observed in vitro when pancreatic islets or islet-derived cells are perifused with glucose. Three primary processes are included: 1) glucose stimulation of insulin secretion, controlled by glucokinase; 2) uptake of glucose through GLUT transporters; and 3) glucose metabolism. Perifusion is approximated as a limiting case of a chemical flow reactor. Using experimentally determined rate laws for the three processes, we examine the effects of exogenous insulin as a phenomenological activator and inhibitor of secretion. The resulting differential equations support oscillations using either direct or indirect inhibition by insulin. The oscillations have many of the characteristics observed in vitro, although the indirect model is in better overall agreement with experiment. We conclude that the mechanisms explored here may help explain insulin oscillations for HIT cells, but not for islets, and predict that oscillations can be modulated by including insulin in the perifusion medium.

Animals↗
PubMed1995

Ca2+ excitability of the ER membrane: an explanation for IP3-induced Ca2+ oscillations.

Recent research dealing with experiments and theoretical models of Ca2+ excitability of the endoplasmic reticulum (ER) membrane induced by inositol 1,4,5-trisphosphate (IP3) is reviewed. Ca2+ excitability refers to the ability of a small increment of cytoplasmic Ca2+ concentration ([Ca2+]i) to trigger a large [Ca2+]i pulse or oscillations. Such nonlinear regenerative behavior is conferred by the existence of IP3 channels and Ca(2+)-ATPase transporters on the ER membrane, which extends throughout the cytoplasm. Ca2+ excitability resembles the plasma membrane electrical excitability of neurons and other cells: it is driven by the ionic concentration gradient across the ER membrane (higher Ca2+ concentration inside the ER); each [Ca2+]i spike partially consumes the prestored energy that is reestablished through ATP-dependent active transport; and [Ca2+]i, the excitation variable, controls the nonlinear dynamic release rate of ER Ca2+. This review focuses on the kinetic models based on these features and on experiments dealing with the kinetic properties of [Ca2+]i-dependent gating of the IP3 receptor channel. We summarize evidence in favor of two roles for [Ca2+]i in gating the channel's opening: activation at a rapid time scale and inactivation on a slower time scale. Exploiting an analogy to the well-known Hodgkin-Huxley model for neuronal electrical excitability, we show how Ca2+ excitability of the ER membrane can be explained by these gating properties combined with the ER Ca2+ pump activity. The theory's ability to predict is illustrated by comparing calculated with experimental [Ca2+]i responses for pituitary gonadotrophs under various stimulus conditions.

Animals↗
PubMed1994

Diffusion of inositol 1,4,5-trisphosphate but not Ca2+ is necessary for a class of inositol 1,4,5-trisphosphate-induced Ca2+ waves.

Combining a realistic model of inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ oscillations with the diffusion of IP3 and buffered diffusion of Ca2+, we have found that diffusion of Ca2+ plays only a minor role in a class of agonist-induced Ca2+ wave trains. These waves are primarily kinematic in nature, with variable wavelengths and speeds that depend primarily on the phase differences between oscillators at different spatial points. The period is set by the steady-state value of IP3, while the wave speed approximately equals the wavelength/period. Ca2+ diffusion, which is much slower than that of IP3 because of endogenous buffers, is shown to have only a small effect on the wave trains and not to be necessary for the apparent wave propagation. Diffusion of IP3 sets the phase gradient responsible for these wave trains, which consist primarily of localized cycles of Ca2+ uptake and release. Our results imply a possible previously undisclosed role for IP3 in cell signaling.

Animals↗
PubMed1994

Calcium oscillations in pituitary gonadotrophs: comparison of experiment and theory.

We have developed a mathematical model that describes several aspects of agonist-induced Ca2+ signaling in single pituitary gonadotrophs. Our model is based on fast activation of the inositol 1,4,5-trisphosphate (InsP3) receptor Ca2+ channels at low free cytosolic Ca2+ concentration ([Ca2+]i) and slow inactivation at high [Ca2+]i. Previous work has shown that these gating properties, when combined with a Ca(2+)-ATPase, are sufficient to generate simulated Ca2+ oscillations. The Hodgkin-Huxley-like description we formulate here incorporates these different gating properties explicitly and renders their effects transparent and easy to modulate. We introduce regulatory mechanisms of channel opening which enable the model, both in the absence and in the presence of Ca2+ entry, to give responses to a wide range of agonist doses that are in good agreement with experimental findings, including subthreshold responses, superthreshold oscillations with frequency determined by [InsP3], and nonoscillatory "biphasic" responses followed occasionally by small-amplitude oscillations. A particular added feature of our model, enhanced channel opening by reduced concentration of Ca2+ in the lumen of the endoplasmic reticulum, allows oscillations to continue during pool depletion. The model predicts that ionomycin and thapsigargin can induce oscillations with basal [InsP3] and zero Ca2+ entry, while Ca2+ injection cannot. Responses to specific pairings of sub- or superthreshold stimuli of agonist, ionomycin, and thapsigargin are also correctly predicted. Since this model encompasses a wide range of observed dynamic behaviors within a single framework, based on well-established mechanisms, its relevance should not be restricted to gonadotrophs.

Calcium↗
PubMed1994

Effects of rapid buffers on Ca2+ diffusion and Ca2+ oscillations.

Based on realistic mechanisms of Ca2+ buffering that include both stationary and mobile buffers, we derive and investigate models of Ca2+ diffusion in the presence of rapid buffers. We obtain a single transport equation for Ca2+ that contains the effects caused by both stationary and mobile buffers. For stationary buffers alone, we obtain an expression for the effective diffusion constant of Ca2+ that depends on local Ca2+ concentrations. Mobile buffers, such as fura-2, BAPTA, or small endogenous proteins, give rise to a transport equation that is no longer strictly diffusive. Calculations are presented to show that these effects can modify greatly the manner and rate at which Ca2+ diffuses in cells, and we compare these results with recent measurements by Allbritton et al. (1992). As a prelude to work on Ca2+ waves, we use a simplified version of our model of the activation and inhibition of the IP3 receptor Ca2+ channel in the ER membrane to illustrate the way in which Ca2+ buffering can affect both the amplitude and existence of Ca2+ oscillations.

Animals↗
PubMed1993

Effect of voltage-gated plasma membrane Ca2+ fluxes on IP3-linked Ca2+ oscillations.

We present a theoretical investigation of the effect of Ca2+ influx through ion channels in the plasma membrane on Ca2+ oscillations induced by agonist stimulation of Ca2+ release from internal stores. We expand a recent model of internal Ca2+ oscillations based on activation and inhibition of the IP3-receptor in the endoplasmic reticulum by introducing plasma membrane voltage-gated Ca2+ and K+ channels based on patch-clamp experiments on mouse pancreatic beta cells. Simulations of voltage- and current-clamped experiments are carried out. The amplitude and frequency of the oscillations as well as the sensitivity to agonist are strongly affected by altering the value of the voltage-clamp. We show that the fundamental quantity governing voltage clamp measurements is the inward flux of Ca2+, which acts as a control parameter to alter characteristics of the oscillations. Under current clamp conditions (I(applied) = 0) that lead to continuous spiking electrical activity, we examine the coupling between electrical activity and Ca2+ by introducing a small whole cell Ca(2+)-activated K+ conductance. Results of calculations are similar to those observed in agonist-stimulated beta cells and gonadotrophs. We conclude, however, that glucose-induced bursting and agonist induced bursting in beta cells are distinct phenomena and suggest a mechanism by which agonist-induced Ca2+ oscillations might potentiate insulin secretion. This leads to a general principle for selective signal transduction by Ca2+ oscillations.

Animals↗
PubMed1992

A single-pool inositol 1,4,5-trisphosphate-receptor-based model for agonist-stimulated oscillations in Ca2+ concentration.

Relying on quantitative measurements of Ca2+ activation and inhibition of the inositol 1,4,5-trisphosphate (IP3) receptor in the endoplasmic reticulum, we construct a simplified kinetic model to describe the properties of this channel. Selecting rate constants to fit key kinetic and equilibrium data, we find that the model reproduces a variety of in vivo and in vitro experiments. In combination with Ca(2+)-ATPase activity for Ca2+ uptake into the endoplasmic reticulum, the model leads to cytoplasmic oscillations in Ca2+ concentration at fixed IP3 concentration and only a single pool of releasable Ca2+, the endoplasmic reticulum. Incorporation of a positive-feedback mechanism of Ca2+ on IP3 production by phospholipase C enriches the properties of the oscillations and leads to oscillations in Ca2+ concentration accompanied by oscillations in IP3 concentration. We discuss the possible significance of these results for the interpretation of experiments.

Calcium↗
PubMed1992

Slow voltage inactivation of Ca2+ currents and bursting mechanisms for the mouse pancreatic beta-cell.

Recent whole-cell electrophysiological data concerning the properties of the Ca2+ currents in mouse beta-cells are fitted by a two-current model of Ca2+ channel kinetics. When the beta-cell K+ currents are added to this model, only large modifications of the measured Ca2+ currents will reproduce the bursting pattern normally observed in mouse islets. However, when the measured Ca2+ currents are modified only slightly and used in conjunction with a K+ conductance that can be modulated dynamically by ATP concentration, reasonable bursting is obtained. Under these conditions it is the K-ATP conductance, rather than the slow voltage inactivation of the Ca2+ current, that determines the interburst interval. We find that this latter model can be reconciled with experiments that limit the possible periodic variation of the K-ATP conductance and with recent observations of intracellular Ca2+ bursting in islets.

Animals↗
PubMed1992

The regioselective binding of CHCl3 reactive intermediates to microsomal phospholipids.

Microsomal phospholipids (PL) are a good target for the reactive intermediates produced by either the oxidative or the reductive biotransformation of CHCl3 (Testai et al. (1990), Toxicol. Appl. Pharmacol. 104, 496-503). In order to preliminarily characterize the different PL with CHCl3 reactive intermediates, two common methods of PL breakdown have been exploited: the acid-catalyzed transmethylation and the enzymatic hydrolysis with phospholipase C. The results indicated that radioactivity derived from the adducts of PL with the oxidation metabolite, phosgene, partitioned preferentially in the aqueous phase (the ratio of aqueous to organic phase radioactivity contents was about 10); the opposite occurred (ratio about 0.1) when the PL adducts were produced by the reductive process metabolites (dichloromethyl radicals). Therefore, the two methods of PL adduct breakdown can be used to detect and quantitate selectively the two reactive intermediates of CHCl3 biotransformation. The use of phospholipase C, which specifically cleaves the bond between the glyceryl-oxygen and the phosphor atom of PL also gave some structural information. Indeed, the radioactivity partitioning in the aqueous phase after enzymatic hydrolysis of CHCl3 oxidation-associated PL adducts, indicated the selective covalent binding of phosgene residues with the PL polar heads. The clear-cut different partition of radioactivity observed after hydrolysis of PL adducts with CHCl3 reduction intermediates, analogously indicated that dichloromethyl radicals were selectively bound to the PL fatty acyl chains. Using this method we could confirm that in in vitro experimental conditions resembling the physiological status of the liver, both metabolic pathways were concurrently active in hepatic microsomes of B6C3F1 mice. Extents of reactive metabolites similar to those found in B6C3F1 mouse liver microsomes, could be measured in Sprague-Dawley rat liver microsomes only after pretreatment of the animals with PB and incubation with higher CHCl3 concentrations. The toxicological implications of these findings are discussed.

Animals↗
PubMed1992

Conditional probability analysis for a domain model of Ca(2+)-inactivation of Ca2+ channels.

The domain model of Ca2+ inactivation of Ca2+ channels, which has been used to explain rapid inactivation of whole cell Ca2+ currents in pancreatic beta cells, is applied to single-time and conditional open probability measurements on guinea pig ventricular myocyte Ca2+ channels. These two measurements greatly constrain the choice of kinetic constants in the model. Calculations with the model provide a simple quantitative explanation of recent experimental results, including a slow increase in the inactivation rate.

Animals↗
PubMed1992

Two roles of Ca2+ in agonist stimulated Ca2+ oscillations.

We propose a mechanism for agonist-stimulated Ca2+ oscillations that involves two roles for cytosolic Ca2+: (a) inhibition of inositol-1,4,5-trisphosphate (IP3) stimulated Ca2+ release from the endoplasmic reticulum (ER) and (b) stimulation of the production of IP3 through its action on phospholipase C (PLC), via a Gq protein related mechanism. Relying on quantitative experiments by Parker, I., and I. Ivorra (1990. Proc. Natl. Acad. Sci. USA. 87:260-264) on the inhibition of Ca2+ release from the ER using caged-IP3, we develop a kinetic model of inhibition that allows us to simulate closely their experiments. The model assumes that the ER IP3 receptor is a tetramer of independent subunits that can bind both Ca2+ and IP3. Upon incorporation of the action of Ca2+ on PLC that leads to production of IP3, we observe in-phase-oscillations of Ca2+ and IP3 at intermediate values of agonist stimulation. The oscillations occur on a time scale of 10-20 s, which is comparable to the time scale for inhibition in Xenopus oocytes. Analysis of the mechanism shows that Ca(2+)-inhibition of IP3-stimulated Ca2+ release from the ER is an essential step in the mechanism. We also find that the effect of Ca2+ on PLC can lead to an indirect increase of cytosolic Ca2+, superficially resembling "Ca(2+)-induced Ca(2+)-release." The mechanism that we propose appears to be consistent with recent experiments on REF52 cells by Harootunian, A. T., J. P. Y. Kao, S. Paranjape, and R. Y. Tsien. (1991. Science [Wash. DC]. 251:75-78.) and we propose additional experiments to help test its underlying assumptions.

Animals↗