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  • br Calcium permeability pathway It has


    Calcium permeability pathway It has been demonstrated that physiological shear stress in the circulation causes a reversible increase in Ca2+ permeability [29], [30], [31], [32], [33] and electrophysiological evidence has been recently reported for transient activation of the Ca2+-sensitive K+ channel upon membrane deformation by suction into a glass pipette [8]. This study confirmed that activation is due to a fast increase in [Ca2+]i resulting from sudden increase in calcium permeability (PCa) driven by a steep inward gradient. It remains to be elucidated whether Ca2+ entry occurs via an electrodiffusional conductive pathway with a finite Ca2+ conductance, or via a mechanosensitive stretch-activated non-selective cation channel (NSC), or via other permeability pathways. The transient nature of the response may reflect a declining time-course of PCa, a delayed response of the plasma membrane Ca2+-ATPase slowly restoring [Ca2+]i to sub-activation levels of the K+ channels, or both. This work also provided electrophysiological evidence for the parallel activation of anion channels, consecutive to massive Gardos channel activity, never observed before in experiments with red blood cell suspensions. This phenomenon raises the question of its potential relevance in the microcirculation in vivo where even a brief local membrane deformation has the potential to induce significant dehydration. A physiological process based on frequent volume loss is not conceivable in the human RBC known to possess a slow volume regulation [34]. Rather, a physiological role could be sought in reversible alterations of membrane deformability properties by brief variation of intracellular calcium level in a range below the activation threshold for the Gardos channel [34]. Another possibility is the stochastic activation of high calcium permeability in a very small population of DL-AP5 triggering transient anion release with immediate vasodilatory effect in the narrowest of capillaries. A pre-requisite for this hypothesis is that the anion conductance is a pathway for molecules likely to induce nitric oxide (NO) release from capillary endothelium such as ATP. It has been well documented that mechanical deformation, decreased oxygen partial pressure and reduced pH, such as those occurring in microvessels of exercising muscle, induce ATP release [35], [36], [37], [38], [39], [40]. This release activates purinergic receptors of vascular endothelial cells resulting in synthesis of NO and metabolites of arachidonic acid known as powerful relaxation factors of smooth muscle vasculature. ATP is a large molecule which cannot diffuse through the membrane. It has been frequently assumed that the pathway for ATP release is a CFTR (cystic fibrosis transmembrane regulator) channel [36], [39], [41] but the presence of CFTR protein has never been clearly demonstrated electrophysiologically. In addition, transcripts for CFTR protein were not found in the cDNAs prepared from human erythroid progenitor cells and use of different specific antibodies did not allow detection of CFTR protein in RBCs ghosts [42].
    Peripheral benzodiazepine receptor Two recent works have shed a new light on the molecular nature of the anion conductive pathway. The first study shows that maxi-anion channels with multiple conductance levels mediates band 3-independent anion conductance across the erythrocyte membrane [12]. Hence, the diversity of anion channel activities recorded in human erythrocytes in previous studies [2], [9], [10], [11], [13], [43], [44], [45], [46] corresponds to different kinetic modalities of a unique channel with multiple conductance levels and multiple gating properties and pharmacology, depending on conditions. The second study demonstrates the presence of a voltage-dependent anion channel (VDAC) expressed in plasma membranes (Bouyer et al., submitted) as a component of the peripheral-type benzodiazepine receptor (PBR) complex composed of at least three components [47]: a 32-kDa VDAC, a 18-kDa translocator protein (TSPO, also called isoquinoline-binding protein IBP), and a 30-kDa adenine nucleotide transporter (ANT). The PBR is characterized by a nanomolar affinity for the ligands PK 11195>Ro5-4864>diazepam. All blood cells are endowed with large populations of receptors displaying such affinity [48], [49]. Their number varies from 750,000 sites in lymphocytes to 100–200 in RBCs which is in keeping with the presence of a good hundred copies of VDAC mediating the maxi-anion conductance [12].