Cytosol activator protein from rat reticulocytes requires the stimulatory guanine nucleotide-binding protein for its actions on adenylate cyclase

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.

Analysis by cell hybridization of mechanisms that regulate beta-adrenergic responses in reticulocytes and in differentiating erythroid cells

In intact reticulocytes, but not in fragmented membranes, the loss of adenylate cyclase activity during cell maturation followed a biphasic time course. A rapid phase (t1/2 approximately 2 h) during which the initial activity was reduced by 40-50% was followed by a slow phase with t1/2 close to 3 days. The fast decay seemed to occur on the adenylate cyclase level since (-)isoprenaline- or forskolin-stimulated activities behaved similarly and bacterial toxin-monitored Gs and Gi proteins remained stable. The mechanism of the initial decrease in hormonal responsiveness was further analysed in hybrid cells prepared by fusing reticulocytes with Friend erythroleukemia (MEL) cells. The hybrids contained reticulocyte-derived beta-adrenoceptors and MEL cell-derived adenylate cyclase and G proteins. Fusion of reticulocytes to native MEL cells caused adenylate cyclase activity to drop by 30% at 2 h and 45% at 18 h after fusion. By contrast, hybrids prepared after dimethylsulfoxide-induced differentiation of MEL cells showed stable or increasing rates of receptor-coupled cAMP formation between 2 and 18 h after fusion, concomitant with the enhanced activity of the Gs protein in these cells. A cyclase-stimulating factor present in the cytosol of MEL cells and of reticulocytes appeared not to be involved in short-term regulation of hormonal responsiveness. We conclude that the strength of beta-adrenergic responses in erythroid progenitor cells is primarily regulated by modulating G protein-mediated receptor cyclase coupling while reticulocytes, during early maturation, seem to rely on direct inactivation of adenylate cyclase, probably via a cytosolic proteolytic pathway.

Apparent activation of rabbit lung membrane adenylate cyclase by cytosolic proteins possessing adenylate kinase activity

Lung cytosolic fraction (23500 x g supernatant) activates cAMP synthesis by lung membrane adenylate cyclase (AC). 23 kDa and 29 kDa proteins were isolated from rabbit lung cytosolic fraction in a homogeneous state, as 'activators' of lung membrane AC. Both of these proteins possess high adenylate kinase (AK) activity and are able to mimic the 'activating' effect of lung cytosol on the lung membrane AC in the standard incubation mixture devoid of adenylate kinase. The activating effect is abolished in the presence of adenylate kinase inhibitor DAPP and after heat- or trypsin-treatment of the cytosolic fraction. Commercial adenylate kinase or nonionic detergent Lubrol PX activate cAMP synthesis by lung membrane AC in a similar manner to that of cytosolic fraction. In the presence of commercial adenylate kinase or Lubrol PX no activating effect of the cytosolic fraction on lung membrane AC is revealed. The ability of cytosolic fraction, commercial adenylate kinase, Lubrol PX or purified 23 kDa and 29 kDa proteins to activate cAMP synthesis by lung membrane AC correlates with their ability to support the constant ATP (AC substrate) concentration in the AC assay mixture. Our data indicate that 'activation' of lung membrane AC in the presence of cytosolic fraction may be produced by cytosolic adenylate kinase activity which regenerates ATP from AMP in the presence of creatine kinase and creatine phosphate providing the substrate for cAMP synthesis by AC.

Developmental changes in the electrophysiological properties and the beta-adrenergic receptor-effector complex in atrial fibers of the canine coronary sinus

beta-Adrenergic stimulation induced delayed afterdepolarizations and triggered activity in atrial fibers of adult but not young canine coronary sinus. However, sensitivity to beta-adrenergic agonists with respect to maximum diastolic potential was identical at both ages, and delayed afterdepolarizations and triggered activity did occur in response to ouabain. Age-dependent lengthening of the action potential duration and plateau also were seen and were greater in the adult than the young. beta-Adrenergic receptor density and affinity and the stimulatory guanine nucleotide regulatory protein (Gs) were similar in adult and young tissues. In contrast, the inhibitory guanine nucleotide regulatory protein (Gi) was 2.5-fold greater in adult (15 fmol/mg) than in young (6.0 fmol/mg) tissues. Basal- and forskolin-stimulated adenylate cyclase activities were greater in adult than young coronary sinus although the increment in isoproterenol-stimulated adenylate cyclase activity in young tissue was greater when compared either with basal levels or expressed as a percentage of maximal catalytic activity. Both the traditional effector pathway of beta-adrenergic action, involving the stimulation of adenylate cyclase activity, and developmental changes in the action potential plateau may contribute to the developmental changes in delayed afterdepolarizations and triggered activity.

Reduced lymphocyte stimulatory guanine nucleotide regulatory protein and beta-adrenergic receptors in congestive heart failure and reversal with angiotensin converting enzyme inhibitor therapy

Adrenergic hyporesponsiveness in congestive heart failure has been understood previously in terms of a reduction in beta-adrenergic receptors. We have examined another hypothesis, one that states the stimulatory guanine nucleotide regulatory protein (Gs) that couples the beta-adrenergic receptor to adenylate cyclase activity is also decreased in congestive heart failure. In addition to the 40% decrease in lymphocyte beta-adrenergic receptors in patients in congestive heart failure (5.9 +/- 0.7 vs. 9.7 +/- 1.4 fmol/mg, p less than 0.05), we found an 80% decrease in levels of Gs compared with age- and sex-matched healthy control subjects (72.5 +/- 19 vs. 376 +/- 73 fmol/mg, p less than 0.05). Myocardial Gs levels correlated significantly with lymphocyte Gs levels. We also assessed the hypothesis that reductions in beta-adrenergic receptors and in Gs are reversible after successful therapy with angiotensin converting enzyme inhibitors. Treatment with either captopril or lisinopril was associated with clinical improvement, an increase in beta-adrenergic receptor density (from 5.5 +/- 0.7 to 8.7 +/- 1.5 fmol/mg), and a twofold increase in Gs levels (p less than 0.05). Thus, the data are compatible with Gs serving as an adaptable and reversible regulator of the adrenergic response in congestive heart failure. In view of the fact that Gs is a transducing element common to all hormones that stimulate cyclic adenosine 5'-monophosphate production, the observations could extend to other abnormal neurohumoral mechanisms in congestive heart failure.

Influences of variation in total energy intake and dietary composition on regulation of fat cell lipolysis in ideal-weight subjects

Weight-maintaining fat-rich, "prudent," carbohydrate-rich, as well as energy-restricted diets (300 kcal/d) were fed in succession for 7 d to 12 healthy males of ideal body weight under metabolic ward conditions. At the end of each period isolated fat cells were prepared from subcutaneous abdominal adipose tissue and incubated in vitro in the absence or presence of adenosine deaminase, either alone or in combination with various lipolytic or antilipolytic hormones and agents. Variations in total energy intake and dietary composition had characteristic and specific effects on fat cell lipolysis in vitro. High carbohydrate and prudent diets resulted in low rates of nonstimulated glycerol release and impaired insulin action in the presence of adenosine deaminase (320 mU/ml). High-fat and energy restricted diets were characterized by high rates of nonstimulated glycerol release. Sensitivity of antilipolysis to insulin and prostaglandin E2 was 10 to 200 times lower respectively on energy-restricted than on fat-rich diets. The effects of alpha 2- and beta-adrenergic catecholamines and of N6-phenylisopropyladenosine were not affected by the preceding diets.

A comparison of the properties of the cardiac beta-adrenergic receptor adenylyl cyclase system in the rat and the marmoset monkey

1. A comparison was made between adrenergic receptor binding properties and catecholamine-stimulated adenylyl cyclase activity in cardiac membrane fractions from the rat and the marmoset monkey. 2. [125I]HEAT and [125I]ICYP were used to determine respectively, the alpha- and beta-adrenergic receptor binding in cardiac membrane fractions. 3. Greatest adrenergic receptor density and degree of specific binding was evident using membranes sedimenting between 6000 and 46,000 g. 4. In rat heart, the ratio of beta- to alpha-adrenergic receptors was 57:43, while for the marmoset this ratio was 92:8. 5. Basal, isoproterenol, sodium fluoride and forskolin-stimulated adenylyl cyclase activities in the rat and marmoset monkey were investigated in several different cardiac membrane fractions. 6. The highest-fold stimulation of adenylyl cyclase activity was present in membranes sedimenting between 0 and 500 g. 7. Adenylyl cyclase activities were higher in the marmoset heart membrane preparations, however the rat heart adenylyl cyclase exhibited greater sensitivity to isoproterenol; ED50 3.8 X 10(-7) M compared with 7.5 X 10(-7) M for the marmoset. 8. Differences between rat and marmoset catecholamine-sensitive adenylyl cyclase activity were apparent when a variety of adrenergic agonists and antagonists were tested. 9. In the marmoset but not the rat, adrenergic antagonists alone stimulated basal adenylyl cyclase activity. 10. Differences in the activation of cardiac adenylyl cyclase by GTP and GMP-PNP were also evident between the rat and the marmoset monkey, particularly with regard to basal and isoproterenol-stimulated activity.

Enhancement of parathyroid hormone-responsive renal cortical adenylate cyclase activity by a cytosol protein activator from rat reticulocytes

The effects of the cytosol activator protein obtained from rat reticulocytes (RCAP) were investigated in a heterologous membrane system--partially purified cell membranes from dog renal cortex. RCAP enhanced the response of dog renal cortical adenylate cyclase to bovine parathyroid hormone (1-34) [bPTH (1-34)] from two- to three-fold. RCAP also enhanced the response to 5 microM arginine vasopressin, 10 microM glucagon, and 10 microM isoproterenol. Analysis of double-reciprocal plots of substrate concentration and enzyme activity indicated that bPTH (1-34) alone and together with RCAP increased the Vmax of the adenylate cyclase enzyme and did not alter the apparent Km of the enzyme for MgATP. Membranes from dog renal cortex contain 42K and 39K proteins that are ADP-ribosylated by cholera toxin and pertussis toxin, respectively, and appear to be the stimulatory (Ns) and inhibitory (Ni) guanine nucleotide binding proteins described in many other hormone-responsive membrane preparations. Similar to its effects in rat reticulocytes, RCAP inhibited ADP-ribosylation of Ns and enhanced ADP-ribosylation of Ni. The muscarinic agonist, carbachol, inhibited PTH-responsive adenylate cyclase activity in dog renal cortical membranes and this inhibition was reversed by RCAP. These results indicate that RCAP enhances stimulation of adenylate cyclase by a variety of hormones in a heterologous membrane preparation and supports the hypothesis that RCAP's site of action is common to all adenylate cyclase systems. RCAP may facilitate coupling between Ns and the catalytic unit of adenylate cyclase by a pertussis toxin-like effect to inactivate Ni. The dual effects of RCAP upon ADP-ribosylation of Ni and Ns alpha subunits suggest that a binding site for RCAP may exist at a site of homology between Ns alpha and Ni alpha.