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At least 1,153 records · Page 64Linked to original sources
PubMed1990

Stimulation of DNA synthesis by an inositol polyphosphate-activated protein phosphatase in calcium-deprived rat liver cells.

Specific stages of the prereplicative G1 phase of the cell cycle in nonneoplastic cells requires extracellular Ca2+ for successful transition. These are the G0-G1 and the G1-S transitions. A variety of agents are able to replace Ca2+ and to at least partially stimulate cells to replicate their chromosomes. One of these agonist, inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4], has been demonstrated by us to also stimulate the activity of a phosphoprotein phosphatase. The addition of a purified preparation of the protein phosphatase to Ca2(+)-deprived G1-S-blocked T51B rat liver cells stimulates a rapidly responding fraction of cells to enter their S phase, and this effect is blocked by protein phosphatase inhibitors heparin and inhibitor 2.

Animals↗
PubMed1985

Subcellular localization and some properties of the enzymes hydrolysing inositol polyphosphates in rat liver.

The hydrolysis of inositol [32P]trisphosphate (IP3) and inositol [32P]bisphosphate (IP2) has been examined in subcellular fractions of rat liver. IP3 was degraded by an enzyme located in the plasma membrane which did not degrade IP2. Cytosolic fractions were found to degrade both IP2 and IP3. The IP3 phosphatase activity of liver plasma membranes displayed a neutral pH optimum, was Mg2+ dependent and was not inhibited by high concentrations of Li+. Half-maximal activity of the enzymes hydrolysing IP3 and IP2 was obtained with substrate concentrations in the range 1-2 microM. The significance of these observations to the proposed Ca2+-mobilizing role of IP3 is discussed.

Animals↗
PubMed1986

Alpha 1-adrenergic activation of brown adipocytes leads to an increased formation of inositol polyphosphates.

alpha 1-Adrenergic activation of isolated brown adipocytes causes a rapid mobilization of intracellular Ca2+. The cells also respond with an increased turnover of inositol lipids. The present work demonstrates that alpha 1-adrenergic stimulation of brown adipocytes results in phospholipase C-mediated breakdown of phosphatidylinositol bisphosphate to form inositol trisphosphate. The rate of appearance of inositol trisphosphate is sufficiently rapid for it to mediate or contribute to Ca2+ mobilization in these cells.

Adipose Tissue, Brown↗
PubMed1987

Electrophysiological responses to bradykinin and microinjected inositol polyphosphates in neuroblastoma cells. Possible role of inositol 1,3,4-trisphosphate in altering membrane potential.

Addition of bradykinin to mouse N1E-115 neuroblastoma cells evokes a rapid but transient rise in cytoplasmic free Ca2+ concentration ([Ca2+]i). The [Ca2+]i rise is accompanied by a transient membrane hyperpolarization, due to a several-fold increase in K+ conductance, followed by a prolonged depolarizing phase. Pretreatment of the cells with a Ca2+-ionophore abolishes the hormone-induced hyperpolarization but leaves the depolarizing phase intact. The transient hyperpolarization can be mimicked by iontophoretic injection of IP3(1,4,5) or Ca2+, but not by injection of IP3(1,3,4), IP4(1,3,4,5) or Mg2+ into the cells. Instead, IP3(1,3,4) evokes a small but significant membrane depolarization in about 50% of the cells tested. Microinjected IP4(1,3,4,5) has no detectable effect, nor has treatment of the cells with phorbol esters. These results suggest that, while IP3(1,4,5) triggers the release of stored Ca2+ to hyperpolarize the membrane, IP3(1,3,4) may initiate a membrane depolarization.

Animals↗
PubMed1989

Molecular recognition of inositol polyphosphates by intracellular receptors and metabolic enzymes.

The discovery that inositol lipids are fundamentally involved in cell signalling has been one of the most significant recent advances in cell biology. In particular, there is now evidence that certain products of polyphosphoinositide metabolism play second messenger roles in most cells. Inositol 1,4,5-trisphosphate and perhaps inositol 1,3,4,5-tetrakisphosphate bind to specific receptors and regulate Ca2+ release from, and movement between, intracellular stores. The synthesis of novel analogues of these second messengers is now providing clues to the structural requirements at such receptors as well as for molecules with stability towards metabolic enzymes. Stefan Nahorski and Barry Potter discuss these developments with a view to future pharmacological intervention at these sites.

Animals↗