Cholera toxin blocks glucagon-mediated inhibition of the liver plasma membrane (Ca2+-Mg2+)-ATPase

We have previously shown that liver plasma membrane (Ca2+-Mg2+)-ATPase activity is inhibited by glucagon. To investigate the possible involvement of a GTP-binding (G) protein in this regulation, we have examined the effects of pertussis toxin and cholera toxin on inhibition of (Ca2+-Mg2+)-ATPase by glucagon. Treatment of liver plasma membranes with pertussis toxin did not affect the sensitivity of (Ca2+-Mg2+)-ATPase to the hormone. In contrast, treatment of plasma membranes or prior injection of animals with cholera toxin prevented inhibition of the (Ca2+-Mg2+)-ATPase by glucagon. Even though adenylate cyclase activity was increased by cholera toxin treatment, addition of cyclic AMP did not mimic the effect of cholera toxin in blocking glucagon-mediated inhibition of (Ca2+-Mg2+)-ATPase activity. These data suggest that a cholera toxin-sensitive protein, perhaps Gs or a Gs-like protein, is involved in the regulation of liver (Ca2+-Mg2+)-ATPase activity. The results emphasize the possible role of Gs-like proteins in regulation of enzymes other than adenylate cyclase and suggest that the study of (Ca2+-Mg2+)-ATPase may provide a useful enzymatic system to examine such regulation.     

Quantitation of the guanine nucleotide binding regulatory protein Gs in S49 cell membranes using antipeptide antibodies to alpha s

Polyclonal antibodies reactive against the guanine nucleotide binding stimulatory protein, Gs, were affinity-purified from two rabbits immunized with a synthetic peptide corresponding to amino acids 28-42 in the alpha-subunit, alpha s. On immunoblots, these antibodies recognized alpha s, but not alpha-subunits from two other guanine nucleotide binding regulatory proteins, Gi and Go. A competitive enzyme-linked immunosorbent assay was developed in which inhibition of antibody binding to peptide-coated microtiter plates was used to quantitate purified Gs or Gs in cholate extracts of cell membranes. Plasma membranes derived from wild type S49 lymphoma cells contained 18.9 +/- 2.3 pmol/mg of membrane protein of alpha s. The same membranes bound 169 +/- 12 fmol/mg of protein of [125I]iodocyanopindolol to beta-adrenergic receptors, indicating that the amount of Gs is far in excess of the amount of beta-adrenergic receptors. Thus, even if every beta-adrenergic receptor molecule were to activate 10 Gs molecules, in order for Gs to be limiting for the receptors to reach their high affinity state, it is likely that compartmentation exists for target cell membrane receptors and Gs. Moreover, a comparison of beta-adrenergic receptor number and Gs levels in several different S49 lymphoma cell mutants having lesions in receptors or Gs argues against a coordinate regulation of beta-adrenergic receptors and Gs.     

Alpha 1- and beta 2-adrenergic receptors co-expressed on cloned MDCK cells are distinct glycoproteins

We have explored the molecular differences between alpha 1- and beta 2-adrenergic receptors that are co-expressed by a clonally-derived cell line, Madin-Darby canine kidney clone D (MDCK-D). MDCK-D membranes were pre-labeled with selective alpha 1- and beta-adrenergic radioligands and were then solubilized with the non-ionic detergent digitonin. Solubilized alpha 1- and beta 2-adrenergic receptors were retained by immobilized wheat germ agglutinin and were eluted following addition of N-acetyl-D-glucosamine or sialic acid. Both receptors were also retained by immobilized Limax flavus lectin, a sialic acid-binding lectin. Lectins that were specific for N-acetyl-D-glucosamine residues did not bind to these receptors. These results indicate that both alpha 1 and beta 2 receptors are sialylated glycoproteins. The solubilized alpha 1- and beta 2-adrenergic receptors migrated with different elution profiles from an Ultragel AcA 34 column. The apparent molecular sizes of the digitonin-receptor complexes were 68A for the alpha 1 receptor and 55A for the beta 2 receptor. These results show that alpha 1- and beta 2-adrenergic receptors can be present on the same cell as distinct sialic acid-containing glycoproteins.     

Neuroendocrine and behavioral effects of m-chlorophenylpiperazine administration in rhesus monkeys

The effects of m-chlorophenylpiperazine (mCPP), a serotonin receptor agonist, on the release of plasma prolactin (PRL), growth hormone (GH), and cortisol in the rhesus monkey were studied. mCPP was administered intravenously at doses of 0.5, 1.5, and 3.0 mg/kg. GH and cortisol were increased significantly at all doses whike PRL was significantly increased only following administration of 3.0 mg/kg mCPP. mCPP administration also produced behavioral alterations in each monkey, including sedation, penile erection, and defecation. PRL, GH and behavioral responses to mCPP were completely blocked by pretreatment with the serotonin anatgonist metergoline (MTG). However, pretreatment with MTG failed to entirely antagonize the cortisol response to mCPP. These data suggest that mCPP has prominent neuroendocrine and behavioral effects which are mediated, in part, by serotonergic mechanisms.     

ADP- and epinephrine-elicited release of [3H]guanylylimododiphosphate from platelet membranes: Implications for receptor-Ni stoichiometry

Epinephrine-promoted release of [³H]guanylylimidodiphosphate ([³H]Gpp(NH)p) from human platelet membranes has been used to probe the interactions between alpha₂-adrenergic recpetors and N_i, the guanine nucleotide binding protein that couples those receptors to an inhibition of adenylate cyclase activity. We show here that ADP, which also acts through specific platelet receptors to inhibit adenylate cyclase activity, also promotes the release of [³H]Gpp(NH). The amount of [³H]Gpp(NH)-release elicited by epinephrine and by ADP together is equal to the sum of the amounts released by the two agents acting individually. Furthermore the maximal amounts of [³H]Gpp(NH)-release elicited by each of the two agents approximates the numbers of receptors for ADP and epinephrine present in the platelet membranes. These results suggest that the two receptor types interact with distinct portions of the pool of N_i molecules and that each receptor initiates guanine-nucleotide exchange on a single molecule of N_i.     

alpha 1- and beta 2-adrenergic receptor expression in the Madin-Darby canine kidney epithelial cell line

The Madin-Darby canine kidney (MDCK) cell line, derived from distal tubule/collecting duct, expresses differentiated properties of renal tubule epithelium in culture. We studied the expression of adrenergic receptors in MDCK to examine the role of catecholamines in the regulation of renal function. Radioligand-binding studies demonstrated, on the basis of receptor affinities of subtype-selective adrenergic agonists and antagonists, that MDCK cells have both alpha 1- and beta 2- adrenergic receptors. To determine whether these receptor types were expressed by the same cell, we developed a number of clonal MDCK cell lines. The clonal lines had stable but unique morphologies reflecting heterogeneity in the parent cell line. Some clones expressed only beta 2-adrenergic receptors and were nonmotile, whereas others expressed both alpha 1- and beta 2-receptors and demonstrated motility on the culture substrate at low cell densities. In one clone, alpha- and beta- receptor expression was stable for more than 50 passages. Catecholamine agonists increased phosphatidylinositol turnover by activating alpha- adrenergic receptors and cellular cyclic adenosine monophosphate accumulation by activating beta-adrenergic receptors. Guanine nucleotide decreased the affinity of isoproterenol for the beta 2- receptor but did not alter the affinity of epinephrine for the alpha 1- receptor. These results show that alpha 1- and beta 2-receptors can be expressed by a single renal tubular cell and that the two receptors behave as distinct entities in terms of cellular response and receptor regulation. Heterogeneity of adrenergic receptor expression in MDCK clones may reflect properties of different types of renal tubule cells.     

Comparative effects of trichothecene mycotoxins on bovine platelet function: Acetyl T-2 toxin,a more potent inhibitor than T-2 toxin

The effects of the trichothecene mycotoxins (acetyl T-2 toxin, T-2 toxin, HT-2 toxin, palmityl T-2 toxin, diacetoxyscirpenol (DAS), deoxynivalenol (DON), and T-2 tetraol) on bovine platelet function were examined in homologous plasma stimulated with platelet activating factor (PAF). The mycotoxins inhibited platelet function with the following order of potency: acetyl T-2 toxin > palmityl T-2 toxin = DAS > HT-2 toxin = T-2 toxin. While T-2 tetraol was completely ineffective as an inhibitor, DON exhibited minimal inhibitory activity at concentrations above 10×10^?4M. The stability of the platelet aggregates formed was significantly reduced in all mycotoxin treated platelets compared to that of the untreated PAF controls. It is suggested that the increased sensitivity of PAF stimulated bovine platelets to the more lipophilic mycotoxins may be related to their more efficient partitioning into the platelet membrane compared to the more hydrophilic compounds.     

Studies of cyclic AMP action using mutant tissue culture cells

Summary S49 mouse lymphoma cell mutants, each with a specific defect in its ability to generate or respond to cyclic AMP, have been isolated. Analysis of the properties of these cells has begun to provide information on complex and significant biologic problems related to the cyclic AMP system. Presented in the Opening Symposium on Nutritional Factors and Differentiation at the 28th Annual Meeting of the Tissue Culture Association, New Orleans, Louisiana, June 6–9, 1977. The work was supported in part by National Science Foundation Grant BMS 75-06764 and National Institutes of Health Grants GM 16496 and GM 00001. P.C. is the recipient of National Institutes of Health Research Career Development Award GM 00308. P. A. I. is an Established Investigator of the American Heart Association.     

Studies of cyclic AMP action using mutant tissue culture cells.

S49 mouse lymphoma cell mutants, each with a specific defect in its ability to generate or respond to cyclic AMP, have been isolated. Analysis of the properties of these cells has begun to provide information on complex and significant biologic problems related to the cyclic AMP system.