Calcium regulatory systems

However, the total regulatory system is not so simple and linear. In skinned muscle preparations especially, it can be shown that there are calcium stores which cannot be released by IP3 but which are released by elevated levels of calcium itself That is, by the mechanism of calcium induced calcium release (CICR). The CICR... 

The biosynthesis of peptide hormones and proteohormones, as well as their secretion, is controlled by higher-order regulatory systems (see p. 372). Calcium ions are among the substances involved in this regulation as second messengers an increase in calcium ions stimulates synthesis and secretion. 

Superimposed on these records are markings of environmental perturbations which sometimes inhibit, and sometimes accelerate carbonate deposition. In the final analysis, however, there has to be an answer, why organisms respond in such a systematic fashion. In all probability these phenomena are connected to the high demand for calcium at all levels of the cell regulatory system. Only at times of lower activities or when the calcium pool runs over will the excretory system package and export the surplus calcium to the periphery of the cell where it may be utilized in carbonate deposition. 

The cellular mechanism of lead nephrotoxicity appears to be due to an alteration of calcium homeostasis. Lead (Pb ) competes with calcium (Ca " ") for transport, binding to calmodulin and at other cell calcium regulatory sites. Lead can accumulate in mitochondria using the calcium transporter and disrupt respiration. Interactions of lead with calmodulin can result in a disruption of the calcium messenger system to adversely affect normal cell function. The nuclear inclusion bodies may also alter the cellular function of DNA, although this interaction has not been fully elucidated. 

Yamaguchi M. 1993. Regulatory effects of zinc and copper on the calcium transport system in rat liver nuclei Relation to SH-groups in the releasing mechanism. Biochem Pharmacol 45(4) 943-948. 

Calmodulin. As mentioned in the introduction it has recently been demonstrated that calcium regulation of a number of enzyme systems and other cellular events is mediated by a low molecular weight protein termed calmodulin. There is presently good evidence that calmodulin is an ubiquitous calcium regulatory protein whose primary sequence is highly conserved throughout all eucaryotic cells(3). 

Before discussing effects of the retinoids on the cyclic nucleotide/calcium-dependent regulatory systems, it is necessary to identify the various components of these systems. The cyclic nucleotide system contains many elements (1) the substrate ATP (or GTP) (2) the synthetic and degradative enzymes adenylate or... 

This of course all happens very quickly, and contraction is driven by thousands of heads in each muscle fibre acting in concert. The instruction from the brain, by way of the nerve, to contract is implemented by the release of calcium ions from the membrane that surrounds the muscle fibres. The calcium ions bind to a protein that is part of the regulatory system associated with the actin filaments. Once again a structural distortion ensues, and a second protein component of that system is moved out of the way to allow the myosin heads to bind to the actin and start to work. 

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