Myometrial protein kinase: hormone-stimulated translocation and membrane binding of the soluble enzyme

Partial characterization of cyclic AMP-dependent protein kinases in guinea-pig lung employing the synthetic heptapeptide substrate, kemptide. In vitro sensitivity of the soluble enzyme to isoprenaline, forskolin, methacholine and leukotriene D4

This paper describes the partial characterization of soluble cyclic AMP-dependent protein kinase (A-kinase) in guinea-pig lung using Kemptide, a synthetic serine-containing heptapeptide, and examines the sensitivity of this enzyme to drugs which are reported to increase and to decrease the intracellular concentration of cyclic AMP. Differential centrifugation of lung homogenates revealed that 78% of A-kinase was present in the 31,000 gmax x 15 min supernatant fraction. Both basal and cyclic AMP-stimulated phosphotransferase activity of this 'soluble' enzyme were abolished by the heat-stable inhibitor of A-kinase. Soluble A-kinase was Mg2(+)-dependent (apparent Km and and Kact 8.6 and 2.6 mM, respectively) and was stimulated nine-fold by saturating concentrations of both cyclic AMP (Kact: 131 nM) and cyclic GMP (Kact: 28.7 microM) at a protein (enzyme) concentration of 1.3 micrograms. Kinetic analysis of the effect of Kemptide and ATP revealed linear, Hanes plots with Michealis constants of ca. 12 and 13 microM, respectively. Chromatography of the soluble enzyme over DEAE-cellulose resolved three peaks of catalytic activity when fractions were assayed in the presence of cyclic AMP (10 microM): (i) free catalytic subunits (5%), (ii) Type I isoenzyme (5%) and (iii) Type II isoenzyme (90%). The A-kinase activity ratio was markedly increased in lung pre-treated with the smooth muscle relaxants isoprenaline and forskolin. This biochemical effect was both time- and concentration-dependent and was temporally associated with the ability of these drugs to reduce lung parenchymal tone. In contrast, the contractile agonists, methacholine (Mch) and leukotriene (LT) D4 exerted opposite effects on A-kinase activity. Thus, Mch significantly reduced cyclic AMP levels and lowered basal A-kinase activity whilst the converse was true for LTD4. For both drugs this biochemical effect accompanied contraction of the lung. Pre-treatment of lung tissue with flurbiprofen, an irreversible inhibitor of cyclo-oxygenase in vitro, abolished the ability of LTD4 to increase the A-kinase activity ratio suggesting that this biochemical response was mediated indirectly through the stimulated biosynthesis and release of a prostanoid(s) able to activate adenylyl cyclase; the increase in tension induced by LTD4, however, was not significantly affected by flurbiprofen pre-treatment. Collectively, these data support the concept that soluble A-kinase activity in guinea-pig lung can be regulated by changes in intracellular cyclic AMP and that activation and/or inhibition of this biochemical cascade may influence alterations in lung contractility.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclic AMP and mechanisms of vasodilation

Cyclic AMP and the mechanism of vasodilation have been reviewed by first discussing the enzymes involved (adenyl cyclase, cyclic nucleotide phosphodiesterases, cyclic AMP-dependent protein kinase) and then agents that increase cAMP in smooth muscle. Two mechanisms of vasodilation are described: (i) effects on contractile proteins; (ii) effects on Ca2+ levels. Evidence for compartments of cAMP is also presented.

Electrophysiological and other aspects of excitation-contraction coupling and uncoupling in mammalian airway smooth muscle

In this article the electrophysiological events which are believed to underly agonist-induced contraction and relaxation of airway smooth muscle are reviewed, with special emphasis on the indispensable role of the Ca ion. The contribution made by Na, K, Ca and Cl to, and the role that the electrogenic Na:K-dependent ATPase plays in, the maintenance of the resting membrane potential in both normal and sensitised airway smooth muscle cells is described together with the permeability changes that occur in the plasmalemma in response to excitatory and inhibitory agonists. In addition, the currently available evidence for the existence of potential-sensitive and receptor-operated Ca channels in respiratory smooth muscle, and how such channels may be involved in the regulation of airway calibre, is critically assessed.

Mechanisms of beta-adrenergic desensitization in rat myometrium

This study was performed in order to elucidate the mechanism behind the decreased responsiveness to beta-adrenergic stimulation occurring in uterine muscle after prolonged treatment with isoprenaline. Pretreatment of rats with isoprenaline, 20 nmol/kg, three times daily during four days, significantly decreased the myometrial relaxing effect of the beta-agonist. There was also a significant decrease of the beta-receptor binding capacity of the myometrial membranes measured by the (--)-(3H) DHA binding technique. In the animals pretreated with isoprenaline no significant increase of the adenylate cyclase activity could be observed after isoprenaline stimulation in vitro. The uterine cAMP level was diminished in the desensitized rats. The phosphodiesterase activity was increased. Thus both decreased production and increased degradation contribute to the lower level of uterine cAMP content. The activity of cAMP dependent protein kinase was also depressed. In this work, where low concentrations of isoprenaline have been administered in vivo, several biochemical parameters have been shown to contribute to the beta-adrenergic desensitization in myometrial tissue.

Smooth muscle cell sarcolemma. Purification and properties of plasma membranes from the rat uterus

A membrane fraction with sarcolemmal properties was purified from the smooth muscle layers (myometrium) of rat uterus by successive differential and equilibrium centrifugation in sucrose. The putative sarcolemmal fraction was identified by iodination with [125I]iodosulfanilic acid, had an equilibrium density of 1.15, and was enriched in enzyme activities usually associated with the plasma membrane including 5'-nucleotidase (EC 3.1.3.5) and (Na+ + K+) ATPase (EC 3.6.1.3). These membranes were free of mitochondrial or nuclear membrane contamination, suggesting the relative enrichment of sarcolemmal membranes in the fraction. Proteins of the membranes were heterogeneous with respect to molecular weight, but only a few were labelled when intact muscle was radioiodinated. Uniform resistance of sarcolemmal proteins to trypsin digestion and salt extraction suggested many are tightly bound or intrinsic membrane proteins and was a further indication of the homogeneity of membranes in this fraction.

Fibroblast growth factor causes an early increase in phosphorylation of a membrane protein in quiescent 3T3 cells

THE regulatory systems controlling cell division have not been identified, but it has been shown that growth factors such as epidermal growth factor, fibroblast growth factor (FGF), and serum initiate rapid changes in cellular metabolism, probably involving post-transcriptional control mechanisms(1-5). As phosphorylation has been shown to be an important regulatory mechanism in several metabolic pathways, we initiated experiments to determine whether factors which stimulate DNA synthesis also stimulate endogenous phosphorylation. We find mat, within 5 min of addition of FGF or serum to (32)P-labelled Swiss 3T3 cells, there is a specific increase in the phosphorylation of a membrane protein with an apparent molecular weight of 33,000. Experiments with isolated cell fractions demonstrate that the phosphorylation of this protein is stimulated by cyclic AMP. This rapid and specific response to mitogens raises the possibility that this phosphorylation might be part of the initial, cellular signal for DNA synthesis.