Protein kinase nucleotide-dependent effects

The skeletal muscle Ca channels also can be phosphorylated in vitro by a protein kinase endogenous to the T-tubule membranes [111,115]. This kinase is neither Ca - nor cyclic nucleotide-dependent [115], and is interesting in that it phosphorylates primarily the P subunit while the ai subunit is a poor substrate. However, the amount of this kinase that co-purifies with the T-tubule membranes is variable, and consequently, very few studies have been performed. So far, only low levels of phosphorylation have been obtained (no more than 0.2 mol phosphate/ mol P subunit) and no functional effects of this phosphorylation have been observed in reconstitution studies. 

The changes in cAMP induced by glucagon in isolated hepatocytes are well correlated with the changes in the activation state of the protein kinase [58,59]. This is illustrated in Fig. 2. Careful examination of the correlations between the increases in cAMP and cAMP-dependent protein kinase activity induced by very low concentrations of glucagon illustrates some cooperativity in the effect of the nucleotide on the kinase [59] consistent with the synergistic interaction between the... 

Other evidence for the involvement of a G-protein in the action of insulin has come from studies by Walaas and co-workers [104]. They have demonstrated that insulin stimulated the activity of a cyclic AMP-dependent protein kinase activity in sarcolemma membranes. As this effect of insulin was enhanced if micromolar concentrations of GTP-binding protein were present, they suggested that a guanine nucleotide regulatory protein was involved in the hormonal control of this kinase. Indeed, cholera toxin also appeared to obliterate this action of insulin, as it did the effect of insulin on liver adenylate cyclase and the peripheral plasma membrane cyclic AMP phosphodiesterase in liver. 

Adenylate cyclase is considered as a second messenger that catalyzes the formation of cAMP (cyclic adenosine monophosphate) from ATP this results in alterations in intracellular cAMP levels that change the activity of certain enzymes—that is, enzymes that ultimately mediate many of the changes caused by the neurotransmitter. For example, there are protein kinases in the brain whose activity is dependent upon these cyclic nucleotides the presence or absence of cAMP alters the rate at which these kinases phosphorylate other proteins (using ATP as substrate). The phosphorylated products of these protein kinases are enzymes whose activity to effect certain reactions is thereby altered. One example of a reaction that is altered is the transport of cations (e.g., Na+, K+) by the enzyme adenosine triphosphatase (ATPase). 

Cyclic GMP, like AMP, can be inactivated by a specific cyclic nucleotide phosphodiesterase to 5 -GMP. Similarly, the biological effects of cGMP are believed to involve a cGMP-dependent protein kinase, also known as protein kinase G (PKG) or G-kinase. The cellular targets for G-kinase-mediated relaxation are unknown in ASM, and do not appear to involve phosphorylation of MLCK. However, in vascular smooth muscle cGMP and thus G-kinase may be involved in the extrusion of Ca " from the cytosol (Suematsu etal., 1984 Popescu etal., 1985) and in sr Ca sequestration (Twort and van Breemen, 1988), ultimately leading to relaxation. 

The studies described in this chapter indicate that there is simply no specific effect of a cyclic nucleotide analog or a cyclic nucleotide-dependent protein kinase in cells. In some instances cyclic nucleotide analogs (e.g.,... 

The frequency and amplitude of calcium sparks are modulated by SR calcium load, protein kinase C (PKC) and cyclic nucleotides/cyclic nucleotide-dependent protein kinases (Bonev et al. 1997 Porter et al. 1997). Calcium spark frequency decreases following activation of PKC (Bonev et al. 1997). This effect appears to be mediated by a direct action of PKC on... 

Matsuo M, Reardon S, Ikebe M, Kitazawa T (1994) A novel mechanism for the Ca -sensitizing effect of protein kinase C on vascular smooth muscle inhibition of myosin light chain phosphatase. J Gen Physiol 104 265-286 McDaniel NL, Rembold CM, Murphy RA (1994) Cyclic nucleotide dependent relaxation in vascular smooth muscle. Can J Physiol Pharmacol 72 1380-1385 Murphy RA (1994) What is special about smooth muscle The significance of covalent crossbridge regulation. FASEB J 8 311-318... 

Figure 32.2 Inhibitory effects of XI on human thyroid signaling intracellular cascades. R, receptor ATP, adenosine triphosphate nucleotide PuR, purinergic receptor Gs, stimulatory G protein of adenylyl cyclase Gi, inhibitory G protein of adenylyl cyclase Gq, stimulatory G protein of phospholipase C AC, adenylyl cyclase PLC, phospholipase C IPS, inositol 1,4,5-trisphosphate DAG, diacylglycerol PKC, protein kinase C DUOX, dual oxidase PGE, prostaglandin E1 TSHR, TSH receptor cAMP, cyclic 3 -5 adenosine monophosphate PDE, cAMP phosphodiesterase 5 AMP, adenosine monophosphonucleotide cA PK, cAMP-dependent protein kinase FK, forskolin ------> Stimulation inhibition — generation.

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