Cytoplasmic gradient

Voltage-gated Ca2+ channels are Ca2+-selective pores in the plasma membrane of electrically excitable cells, such as neurons, muscle cells, (neuro) endocrine cells, and sensory cells. They open in response to membrane depolarization (e.g., an action potential) and permit the influx of Ca2+ along its electrochemical gradient into the cytoplasm. 

Intracellular Ca2+-levels are controlled by release into, and removal from, the cytoplasm (Fig. 1). Ca2+-pumps in the plasma membrane and endoplasmic reticulum (ER the Ca2+-store in a cell) keep cytoplasmic Ca2+-levels low (about 0.1 pmol/L in resting cells) and generate a 10,000-fold concentration gradient across membranes (because extracellular Ca2+ is in the millimolar range). Upon stimulation, Ca2+ enters the cytosol of the cell via Ca2+-channels (plasma membrane) or via Ca2+-channels in the ER, leading to the activation of a great variety of Ca2+-dependent processes in the cell. 

The exocytotic release of neurotransmitters from synaptic vesicles underlies most information processing by the brain. Since classical neurotransmitters including monoamines, acetylcholine, GABA, and glutamate are synthesized in the cytoplasm, a mechanism is required for their accumulation in synaptic vesicles. Vesicular transporters are multitransmembrane domain proteins that mediate this process by coupling the movement of neurotransmitters to the proton electrochemical gradient across the vesicle membrane. 

PNMT catalyzes the N-methylation of norepinephrine to form epinephrine in the epinephrine-forming cells of the adrenal medulla. Since PNMT is soluble, it is assumed that norepinephrine-to-epinephrine conversion occurs in the cytoplasm. The synthesis of PNMT is induced by glucocorticoid hormones that reach the medulla via the intra-adrenal portal system. This special system provides for a 100-fold steroid concentration gradient over systemic arterial blood, and this high intra-adrenal concentration appears to be necessary for the induction of PNMT. 

As described above, because MAO is bound to mitochondrial outer membranes, MAOIs first increase the concentration of monoamines in the neuronal cytosol, followed by a secondary increase in the vesicle-bound transmitter. The enlarged vesicular pool will increase exocytotic release of transmitter, while an increase in cytoplasmic monoamines will both reduce carrier-mediated removal of transmitter from the synapse (because the favourable concentration gradient is reduced) and could even lead to net export of transmitter by the membrane transporter. That MAOIs increase the concentration of extracellular monoamines has been confirmed using intracranial microdialysis (Ferrer and Artigas 1994). 

The PemB cellular localisation was determined both in E. chrysanthenu and in an E. coli recombinant strain by Western blot of the cell fractions with a PemB-antiserum. No PemB was detected in the culture supernatant and only trace amounts were found in the soluble cell fractions - periplasm and cytoplasm (Figure 2). PemB was found mostly in the total membrane fraction from which it could be completely extracted by Triton X-100/Mg2+ and partially extracted by Sarkosyl (Figure 2). This behaviour is typical of inner membrane proteins, but since some exceptions have been noticed it does not positively indicate the PemB localisation (15). We performed cell membrane fractionation in sucrose density gradient centrifugation both by sedimentation and flotation, using several markers of inner and outer membrane vesicles. PemB was found in the outer membrane vesicles (data not shown). 

Most living cells, including muscle, maintain the cytoplasmic Ca concentration at submicromolar levels, against steep gradients of [Ca ], both at the cell surface and across the endoplasmic reticulum membrane [17]. In the musele cell two membrane systems are primarily involved in this function the sarcoplasmic reticulum and the surface membrane. 

Another situation is found for the Na+ ions. When the membrane is permeable to these ions, even if only to a minor extent, they will be driven from the external to the internal solution, not only by diffusion but when the membrane potential is negative, also under the effect of the potential gradient. In the end, the unidirectional flux of these ions should lead to a concentration inside that is substantially higher than that outside. The theoretical value calculated from Eq. (5.15) for the membrane potential of the Na ions is -1-66 mV. Therefore, permeabihty for Na ions should lead to a less negative value of the membrane potential, and this in turn should lead to a larger flux of potassium ions out of the cytoplasm and to a lower concentration difference of these ions. All these conclusions are at variance with experience. 

A well-known example of active transport is the sodium-potassium pump that maintains the imbalance of Na and ions across cytoplasmic membranes. Flere, the movement of ions is coupled to the hydrolysis of ATP to ADP and phosphate by the ATPase enzyme, liberating three Na+ out of the cell and pumping in two K [21-23]. Bacteria, mitochondria, and chloroplasts have a similar ion-driven uptake mechanism, but it works in reverse. Instead of ATP hydrolysis driving ion transport, H gradients across the membranes generate the synthesis of ATP from ADP and phosphate [24-27]. 

When the action potential reaches the synaptic bouton, depolarisation triggers the opening of voltage-operated calcium channels in the membrane (Figure 2.5). The concentration gradient for Ca2+ favours the passive movement of this ion into the neuron. The subsequent rise in cytoplasmic Ca2+ ion concentration stimulates the release of neurotransmitter into the synaptic cleft, which diffuses across this narrow gap and binds to receptors located on the postsynaptic neuronal membrane (Figure 2.5). 

Rejection of Na+, Cr and Ca2+ (and Mn2+) to control cytoplasmic ionic solutions, with uptake of K+, and other elements by pumps or exchangers, mechanical catalysts the sodium gradient was used to assist nutrient uptake, the gradient itself being driven by the bioenergetic proton gradient... 

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