Mechanisms underlying angiotensin II-induced calcium oscillations

Abstract

To gain insight into the mechanisms that underlie angiotensin II (ANG II)-induced cytoplasmic Ca2+ concentration ([Ca]cyt) oscillations in medullary pericytes, we expanded a prior model of ion fluxes. ANG II stimulation was simulated by doubling maximal inositol trisphosphate (IP3) production and imposing a 90% blockade of K+ channels. We investigated two configurations, one in which ryanodine receptors (RyR) and IP3 receptors (IP3R) occupy a common store and a second in which they reside on separate stores. Our results suggest that Ca2+ release from stores and import from the extracellular space are key determinants of oscillations because both raise [Ca] in subplasmalemmal spaces near RyR. When the Ca2+-induced Ca2+ release (CICR) threshold of RyR is exceeded, the ensuing Ca2+ release is limited by Ca2+ reuptake into stores and export across the plasmalemma. If sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pumps do not remain saturated and sarcoplasmic reticulum Ca2+ stores are replenished, that phase is followed by a resumption of leak from internal stores that leads either to [Ca]cyt elevation below the CICR threshold (no oscillations) or to elevation above it (oscillations). Our model predicts that oscillations are more prone to occur when IP3R and RyR stores are separate because, in that case, Ca2+ released by RyR during CICR can enhance filling of adjacent IP3 stores to favor a high subsequent leak that generates further CICR events. Moreover, the existence or absence of oscillations depends on the set points of several parameters, so that biological variation might well explain the presence or absence of oscillations in individual pericytes.

Fig. 1.

Schematic representation of the cell, with its 3 compartments: bulk cytosol (Cyt), microdomains (MD), and sarcoplasmic reticulum (SR). Not shown are K_Ca, K_ir, K_ATP, and K_v channels and the chelating agents calmodulin and calsequestrin. A: common-store model. B: separate-store model.

Fig. 2.

Open probability (P_o) of ryanodine receptors (RyR) and inositol trisphosphate receptors (IP₃R) at steady state as a function of calcium concentration ([Ca], in M). In these calculations, [IP₃] is taken as 240 nM.

Fig. 3.

Effects of external KCl elevation on Ca²⁺ concentrations in cytosol ([Ca]_cyt, A), microdomains ([Ca]_md, B), and SR ([Ca]_SR, C) as a function of time (in s). For clarity of presentation, [K]_out is raised from 5.4 to 100 mM at t = 100 s for the common-store model and at t = 120 s for the separate-store model. D: signaling pathway after external KCl elevation.

Fig. 4.

Effects of external KCl elevation on microdomain and cytosolic Ca²⁺ currents as a function of time (in s), using the separate-store model. At t = 120 s, [K]_out is raised from 5.4 to 100 mM. A: IP₃R current is low because of its inhibition by [Ca]_md elevation while RyR current is large because of [Ca]_md-mediated stimulation. B: [Ca]_md elevation stimulates both IP₃R store- and RyR store-sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) pumps facing the microdomains to the point that they become saturated. NCX, Na⁺/Ca²⁺ exchanger. C: the increase in IP₃R-SR store Ca²⁺ leads to reversal of pumping by IP₃R-SERCA that face the cytosol (negative values mean flux from the SR to the cytosol). The RyR-SERCA facing the cytosol take up Ca²⁺ from the cytosol into the RyR-SR store. D: the large increase in [Ca]_SR-IP3R leads to elevation of Ca²⁺ current through those IP₃R that face the cytosol, and the rise in [Ca]_cyt stimulates elevation of current through the RyR that face the cytosol.

Fig. 5.

Effects of 90% and 75% K⁺ channel inhibition and 100% increase in IP₃ production at t = 100 s on [Ca]_cyt (A), [Ca]_md (B), and [Ca]_SR (C), with the common-store model.

Fig. 6.

Effects of 90% K⁺ channel inhibition and 100% increase in IP₃ production at t = 100 s on microdomain (A) and cytosolic (B) Ca²⁺ currents, with the common-store model. The scale in B has been reduced for clarity; the cytosolic SERCA and voltage-operated Ca²⁺ (VOCa) currents peak at +73 and −103 pA, respectively.

Fig. 7.

Effects of 90% K⁺ channel inhibition and 100% increase in IP₃ production at t = 100 s on Ca²⁺ concentrations in cytosol (A), microdomains (B), and SR stores (C), with the separate-store model. The additional effects of IP₃R and RyR block on [Ca]_cyt are shown in D.

Fig. 8.

Effects of 90% K⁺ channel inhibition and 100% increase in IP₃ production at t = 100 s on [Ca]_cyt (nM), [Ca]_md (nM), and [Ca]_SR (×10² mM), with the separate-store model. Note that the [Ca]_md peak is off-scale. Phase A: RyR-mediated Ca²⁺-induced Ca²⁺ release (CICR) in microdomains, sharp [Ca]_md ascent. Subsequent depletion of RyR stores and filling up of IP₃R stores. Rapid membrane depolarization and repolarization, [Ca]_cyt peak. Phase B: activation of NCX current, sharp [Ca]_md descent. Ca²⁺ content in RyR and IP₃R stores reaches minima and maxima. Phase C: slower [Ca]_md descent. Subsequent replenishment of RyR stores and depletion of IP₃R stores as I_RyR^md and I_IP3R^md-SERCA decrease. Phase D: slowly rising RyR- and IP₃R-mediated Ca²⁺ release into microdomains, progressive [Ca]_md increase. Stabilized Ca²⁺ concentration in cytosol and SR stores.

Fig. 9.

Effects of 90% K⁺ channel inhibition and 100% increase in IP₃ production at t = 100 s on microdomain and cytosolic Ca²⁺ currents, with the separate-store model. A: microdomain RyR current rises up to 60 pA during CICR, whereas microdomain IP₃R current remains low because its inhibition by [Ca]_md elevation. B: CICR-mediated [Ca]_md increase stimulates IP₃R store- and RyR store-SERCA pumps facing the microdomains, as well as Ca²⁺ export from the microdomains via NCX (minimal value, −14 pA). C: immediately after CICR, the IP₃R-SERCA and RyR-SERCA that face the cytosol both take up Ca²⁺ from the cytosol to replenish SR stores. Soon thereafter, the increase in IP₃R-SR store Ca²⁺ leads to reversal of pumping by IP₃R-SERCA that face the cytosol. D: cytosolic IP₃R current rises and falls in conjunction with [Ca]_SR-IP3R. Cytosolic RyR current is stimulated during the depolarization-induced rise in [Ca]_cyt.

Fig. 10.

Effects of 90% K⁺ channel inhibition and either 100% or 50% increase in IP₃ production at t = 100 s on microdomain Ca²⁺ (A) and IP₃ (B) concentrations, with the separate-store model. Note that the [Ca]_md peak is off-scale.

Fig. 11.

Effects of parameter variations on the frequency (A) and amplitude (B) of oscillations induced by 90% K⁺ channel inhibition and 100% increase in IP₃ production. Parameters are given relative to their baseline value. Results are obtained for the separate-store configuration. Oscillations persist when Ca²⁺ conductivity of IP₃R (ν_IP3R) equals up to 18 times its baseline value.

Fig. 12.

A and B: frequency histograms of 2,062 spontaneous transient inward currents (STICs) recorded in 73 cells exposed to 10 nM ANG II (52). Most values are well below 1 Hz. The reason that the mean is 1.15 Hz is that a few frequencies are as high as 25, so that SD is very large and the mean is skewed away from the predominant frequency, near 0.09 Hz. C and D: amplitude histograms of the same data from Ref. . The predominant amplitude was near −10 pA, but the mean is much larger because of rare cells that exhibited large currents.

Fig. 13.

Predicted membrane current during voltage clamp at −80 mV and exposure to angiotensin II (ANG II), with the separate-store model. We assume that exposure to ANG II results in 90% K⁺ channel inhibition and 100% increase in IP₃ production. A: current above cytosol. B: current above microdomains. C: overall current, calculated as the sum of the currents crossing the cytosol and the microdomains, and the leak current (taken as −8 pA, assuming a 10-GΩ seal).