The influence of detergents on the availability of pertussis toxin substrates

Treatment in hypertensive cardiac hypertrophy, II. Postreceptor events

We investigated the effect of pharmacological treatment with captopril, nitrendipine, and captopril plus nitrendipine on myocardial heterologous adenylyl cyclase desensitization and the underlying postreceptor defects in spontaneously hypertensive rats (SHR). In myocardial membranes from SHR, stimulation of adenylyl cyclase with guanylylimido-diphosphate (P < .001) and forskolin (P < .05) was significantly reduced, whereas no difference with forskolin was obtained in the presence of manganese chloride. Reconstitution of Gs alpha into Gs alpha-deficient S49 cyc- mouse lymphoma cells revealed no difference between SHR and control rats. In contrast, pertussis toxin labeling of Gi alpha was significantly increased in SHR. The reduction of adenylyl cyclase in SHR was abolished after pertussis toxin treatment of membranes. Treatment with captopril, nitrendipine, or both reduced Gi alpha and increased guanylylimidodiphosphate-stimulated adenylyl cyclase activity in SHR. In summary, heterologous adenylyl cyclase desensitization due to an increase of Gi alpha but in the presence of an unchanged activity of Gs alpha or the catalyst occurs in SHR. This alteration, which could contribute to the progression of contractile dysfunction by producing adrenergic subsensitivity, is sensitive to pharmacological treatment most likely because of a reduction of sympathetic activity.

Lymphocyte G proteins reflect response to treatment in congestive heart failure

Congestive heart failure is associated with chronotropic and inotropic hyporesponsiveness to adrenergic stimulation. A decrease in Gs alpha or an increase in Gi alpha is associated with a decrease in adenylyl cyclase activity. The current study assessed G proteins in response to treatment with direct-acting vasodilators and correlated changes in lymphocyte beta-adrenergic receptor components with changes in hemodynamic variables. Twenty-three patients with severe chronic congestive heart failure (New York Heart Association functional classes III and IV) were studied. Patients were grouped as responders (n = 10) or nonresponders (n = 13) on the basis of clinical assessment of functional status from questionnaires. Therapy was associated with an increase in cardiac index, a decrease in mean arterial pressure, and a decrease in systemic vascular resistance in all patients. Left ventricular filling pressure significantly decreased in responders (26 +/- 2 mm to 13 +/- 3 mm, p < 0.05) but did not change significantly in nonresponders. Similarly, mean right atrial pressure significantly decreased in responders (11 +/- 2 mm Hg to 4 +/- 1 mm Hg, p < 0.05) but did not change in nonresponders. Plasma norepinephrine increased significantly only in nonresponders (679 +/- 100 pg/ml to 1233 +/- 201 pg/ml, p < 0.05). Whereas lymphocyte beta-adrenergic receptor density and Gs did not significantly change, Gi increased after treatment only in the nonresponder group (23 +/- 5 to 51 +/- 11 fmol/mg, p < 0.05). A poor response to direct-acting vasodilators can be distinguished by reactive increases in plasma norepinephrine and lymphocyte Gi in the absence of a decrease in either left- or right-sided filling pressures.

C-terminal modifications of pertussis toxin-sensitive G-protein alpha-subunits differentially affect immunoreactivity. Evidence against endogenous ADP-ribosylation in human heart, lung, thrombocytes and adipose tissue

Immunochemical detection of pertussis toxin-sensitive guanine-nucleotide binding proteins has been suggested to represent the most direct approach to quantitate the protein than pertussis toxin-catalysed [32P]ADP-ribosylation. The latter technique is potentially hampered by pre-existing covalent modification of the C-terminus. However, limited data exist as to whether and in what way modifications of the C-terminus affect immunoreactivity of Gi alpha (alpha-subunit of the inhibitory G-protein of adenylyl cyclase). Membranes from human myocardium, thrombocytes, adipose tissue and lung were treated with pertussis toxin or N-ethylmaleimide. Both, conditions prevented high affinity agonist binding to m-cholinoceptors and inhibited [32P]ADP-ribosylation by pertussis toxin consistent with the notion that the modifications took place at the C-terminus. Pertussis toxin treatment increased immunoreactivity to different antisera raised against the C-terminal decapeptide of transducin alpha (KENLKDCGLF, DS 1-4, AS). N-Ethylmaleimide reduced immunoreactivity towards all antisera studied. Pertussis toxin reduced the mobility of Gi alpha on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) depending on the presence of the toxin and sensitivity to inhibition of ADP-ribosylation by nicotinamide. In native membranes from none of the tissues studied, immunoreactive material comigrating with pertussis toxin-modified form of Gi alpha was detected. It is concluded that modification of the C-terminus by pertussis toxin or N-ethylmaleimide resulting in the same functional consequence, i.e. prevention of high affinity agonist receptor binding, is capable of producing opposite changes of immunoreactivity. Pertussis toxin treatment reduces the electrophoretic mobility on SDS-PAGE. Separation of the native and pertussis toxin-modified form of Gi alpha on SDS-PAGE demonstrates that endogenously ADP-ribosylated Gi alpha is lacking in membranes from human myocardium, thrombocytes, lung and adipose tissue.

Occurrence of the alpha subunits of G proteins in cerebral cortex synaptic membrane and postsynaptic density fractions: modulation of ADP-ribosylation by Ca2+/calmodulin

We have examined the isolated postsynaptic density (PSD) fraction for the presence of a G protein. First, we found specific binding of guanosine 5'-[gamma-[35S]thio]triphosphate to the PSD. Second, pertussis toxin-activated ADP-ribosylation of the isolated PSD fraction resulted in the appearance of a G protein with an apparent molecular mass of 41 kDa, and two G proteins with apparent molecular masses of 41 kDa and 39 kDa in synaptic membrane (SM) fraction and total homogenate (H). The amount of the 41-kDa G protein per unit protein was in the order of SM greater than H greater than PSD. Anti-G(i0 antibodies recognized the 41-kDa G protein in both PSD and SM, whereas anti-G(o) antibodies reacted with the 39-kDa G protein in the SM. The absence of G(o) protein in the PSD suggested that there was no contamination with SM. Moreover, unlabeled PSD incubated with an extract of SM that contained the labeled G proteins resulted in no label in the subsequently reisolated PSD, suggesting that the G protein found in the PSD was not due to adsorption of the G protein onto the PSD during its isolation from the SM. PSD pretreated with EGTA gave an 11-fold increase in the ADP-ribosylation reaction of the G(i) protein; similar effects on the G(i) and G(o) proteins of SM were obtained. Restoration of Ca2+/calmodulin to the PSD, but not of either Ca2+ or calmodulin alone, removed the effect of EGTA, indicating a strong complex formation between G(i) and Ca2+/calmodulin that decreased the ADP-ribosylation reaction. Preincubation with the Ca(2+)-channel blocker nifedipine decreased the ADP-ribosylation reaction in the PSD. We conclude that G(i) is present in the PSD, that it may interact with calmodulin and that it is involved in the regulation of voltage-dependent Ca2+ channel. We present a theory of the involvement of the G protein and calmodulin in postsynaptic neurophysiological events.