Calcium-activated sodium and chloride fluxes modulate platelet volume: role of Ca2+ stores

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Modifications by Olmesartan medoxomil treatment of the platelet protein profile of moderate hypertensive patients

Olmesartan medoxomil is a new angiotensin II receptor blockers (ARB) which exhibits pleiotropic effects that are not fully understood. Our aims were: i) to determine the effect of Olmesartan medoxomil on blood pressure, lipid profile and renal functionality in moderately hypertensive patients with non-controlled blood pressure, ii) to determine if Olmesartan medoxomil may exert anti-inflammatory effects and modify the expression profile of platelet proteins. Thirteen moderate hypertensive patients with non-controlled systolic blood pressure (SBP) and renal function classified as Kidney Disease Outcome Quality Initiative stage 2-3 were included. Patients were treated with Olmesartan medoxomil (20 mg/day) for 6 months. SBP, proteinuria and the plasma levels of cholesterol and low density lipoprotein (LDL)-cholesterol were reduced after the treatment. Olmesartan medoxomil did not modify the circulating plasma levels of a number of proteins associated with inflammation, but reduced the expression level of different platelet proteins including tropomyosin-β chain isotypes 3 and 4, serotransferrin isotypes 1 to 5, the leukocyte elastase inhibitor and the chloride intracellular channel-protein isotype 1. The expression of the gelsolin precursor isotype 4 was increased in the platelets after the treatment. In summary, Olmesartan medoxomil reduced SBP, total and LDL-cholesterol plasma levels and urinary protein excretion and induced changes in the expression of platelet proteins which may be related to some action of the drug at the megakaryocyte level.

Bimodal control of a Ca(2+)-activated Cl(-) channel by different Ca(2+) signals

Ca(2+)-activated Cl(-) channels play important roles in a variety of physiological processes, including epithelial secretion, maintenance of smooth muscle tone, and repolarization of the cardiac action potential. It remains unclear, however, exactly how these channels are controlled by Ca(2+) and voltage. Excised inside-out patches containing many Ca(2+)-activated Cl(-) channels from Xenopus oocytes were used to study channel regulation. The currents were mediated by a single type of Cl(-) channel that exhibited an anionic selectivity of I(-) > Br(-) > Cl(-) (3.6:1.9:1.0), irrespective of the direction of the current flow or [Ca(2+)]. However, depending on the amplitude of the Ca(2+) signal, this channel exhibited qualitatively different behaviors. At [Ca(2+)] < 1 microM, the currents activated slowly upon depolarization and deactivated upon hyperpolarization and the steady state current-voltage relationship was strongly outwardly rectifying. At higher [Ca(2+)], the currents did not rectify and were time independent. This difference in behavior at different [Ca(2+)] was explained by an apparent voltage-dependent Ca(2+) sensitivity of the channel. At +120 mV, the EC(50) for channel activation by Ca(2+) was approximately fourfold less than at -120 mV (0.9 vs. 4 microM). Thus, at [Ca(2+)] < 1 microM, inward current was smaller than outward current and the currents were time dependent as a consequence of voltage-dependent changes in Ca(2+) binding. The voltage-dependent Ca(2+) sensitivity was explained by a kinetic gating scheme in which channel activation was Ca(2+) dependent and channel closing was voltage sensitive. This scheme was supported by the observation that deactivation time constants of currents produced by rapid Ca(2+) concentration jumps were voltage sensitive, but that the activation time constants were Ca(2+) sensitive. The deactivation time constants increased linearly with the log of membrane potential. The qualitatively different behaviors of this channel in response to different Ca(2+) concentrations adds a new dimension to Ca(2+) signaling: the same channel can mediate either excitatory or inhibitory responses, depending on the amplitude of the cellular Ca(2+) signal.

Ca2+-dependence of the depolarization-inducible Na+ current of Xenopus oocytes

The role of Ca2+ on the depolarization-induced appearance of a Na+ current in Xenopus oocytes was studied. Oocytes were voltage-clamped and the induction of the Na+ current was tested under various conditions. In oocytes pre-injected with 400 pmol EGTA to increase the intracellular Ca2+ buffering power, the current was significantly reduced. Conversely, when intracellular Ca2+ was made to increase by injecting an analogue of inositol 1,4,5-trisphosphate (3-F InsP3), to cause Ca2+ release from internal stores, the induction of the Na+ current was potentiated. The depolarization-inducible Na+ channels of the Xenopus oocyte membrane appear, therefore, to be Ca2+ sensitive, as well as depolarization-activated.

Chloride channels in excised membrane patches from human platelets: effect of intracellular calcium

Human platelets were studied by patch clamp recordings from inside-out membranes; there were formed by briefly dipping the platelet, in cell-attached mode, into silicone grease. At 20 degrees C in symmetrical 150 mM NaCl, spontaneous channel openings were rarely observed at negative potentials, whereas depolarised potentials (+ 60 to + 100 mV) elicited sustained channel activity in 38% of patches. The single channel conductance was 53 +A- 1 pS at + 80 mV (outward current), decreasing to 20 +/- 2 pS at -80 mV (inward current). Ion substitution experiments indicated that this channel conducts Cl- and not Na+. Furthermore, 5-nitro-2-(3-phenylpropylamino)benzoate (100 microM), a recognized inhibitor of anion channels, induced a reversible 'flickery' channel block. We estimate that each platelet possesses < or = 30 such channels. Kinetic analysis suggested at least two open channel states (tau = 0.8 +/- 0.2 ms, tau = 22 +/- 14 ms, n = 4) and two closed states (tau = 0.8 +/- 0.2 ms, tau = 12 +/- 0.6 ms, n = 4). Increasing [Ca2+]i to 10 microM, following channel activation by depolarisation, had no significant effect on channel kinetics or open probability, however, elevated [Ca2+]i (300 nM-10 microM) increased the number of anion channels activated by subsequent depolarisation. This study represents the first recordings of ionic currents in excised, inside-out membrane patches from human platelets, and provides further evidence for the existence of chloride channels in these cells.