Comparison of spectrophotometric and biological assays for nitric oxide (NO) and endothelium-derived relaxing factor (EDRF): nonspecificity of the diazotization reaction for NO and failure to detect EDRF

Endothelium-derived relaxing factor has been tentatively identified as nitric oxide (NO) partially on the basis of chemical assays. In the present study, saline solutions that were either bubbled continuously for 30 min with NO (NO/X) or prepared using 25 ml of NO/ml (NO/25) produced equivalent relaxations of segments of rabbit aorta which had the endothelium removed. NO solutions prepared using 0.1 ml of NO/ml (NO/0.1), and 3 mM sodium nitrite (NO2-) were significantly (P less than .05) less potent vasodilators than NO/X and NO/25 (order of potency: NO/X = NO/25 greater than NO/0.1 greater than NO2-). A novel automated method was developed to monitor nitrogen oxides using continuous-flow spectrophotometric detection (diazotization reaction). The absorbance readings for the solutions were NO/X greater than NO/25 = NO/0.1 = NO2-. Argon purging of NO/X, NO/25 and NO/0.1 solutions significantly (P less than .05) reduced (44-100%) the bioactivity of these solutions in inverse proportion to the initial volume of NO used in their preparation. In contrast, the absorbance values were unchanged, indicating that the chemical assay was not correlated with the bioassay. Varying the duration of NO gassing (1-30 min) significantly (P less than .05) increased the absorbance values, while having no effect on the vascular relaxations, elicited by the solutions. The diazotization assay did not detect nitrogen oxides released from cultured endothelial cells by bradykinin, ATP, or A23187, whereas the bioassay readily detected endothelium-derived relaxing factor release.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced sensitivity of heart cells to adenosine and up-regulation of receptor number after treatment of guinea pigs with theophylline

Experiments were carried out in hearts from guinea pigs that were fed either the adenosine receptor antagonist theophylline (0.6 mg/ml) or no drug. The A1 adenosine receptor radioligand [125I]aminobenzyladenosine bound to a single affinity class of receptors in heart cell membranes from control animals with Bmax and KD of 18.3 +/- 1.0 fmol/mg protein and 3.7 +/- 0.6 nM, respectively (n = 8). Heart cell membranes from animals fed theophylline for 2, 7, and 14 days bound the radioligand with about the same affinity, but the number of binding sites was significantly increased (p less than 0.01) to 30.6 +/- 1.7 (n = 3), 30.0 +/- 0.8 (n = 3), and 27.3 +/- 2.9 (n = 4), respectively. Nearly identical results were obtained with membranes prepared from enzymatically dispersed ventricular myocytes. Fourteen days of theophylline treatment also produced a small increase (12%, p less than 0.01) in the number of binding sites in membranes derived from cerebral cortexes. Isolated ventricular myocytes prepared from animals fed no drug or theophylline for 7 days were used to determine the effect of adenosine on 20 nM isoproterenol-stimulated calcium current (ICa) measured by the whole-cell patch-clamp technique. Adenosine reduced isoproterenol-stimulated ICa without affecting the activation or inactivation kinetics of the current; ICa density was reduced less by 5 microM adenosine in cells from control (25 +/- 3 to 21 +/- 3 microA/microF) than in cells from theophylline-fed animals (26 +/- 5 to 17 +/- 2 microA/microF). Although a high concentration (0.5 mM) of adenosine abolished isoproterenol-stimulated ICa in cells from control or theophylline-fed animals, the IC50 for adenosine was sixfold less in cells derived from theophylline-fed animals than in cells from control animals (4.6 +/- 0.6 microM and 28.3 +/- 1.4 microM, respectively, p less than 0.01). In contrast, the increase in ICa in response to isoproterenol alone and the potency of acetylcholine to antagonize this effect of isoproterenol were the same in both groups of cells. A maximally effective concentration of R-phenylisopropyladenosine (0.1 mM) inhibited isoproterenol-stimulated cyclic AMP accumulation less in cardiomyocytes from control than from theophylline-fed animals (28.7 +/- 1.8% vs. 42.0 +/- 4.2%, p less than 0.05). In summary, exposure of the myocardium to theophylline increases the number of adenosine receptors and the effects of receptor occupancy by agonists. These findings imply that the endogenous concentration of adenosine is high enough in the normoxic guinea pig heart to chronically maintain adenosine receptors in a partially down-regulated state.

Co-purification of A1 adenosine receptors and guanine nucleotide-binding proteins from bovine brain

A1 adenosine receptors and guanine nucleotide-binding proteins (G proteins) solubilized with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate have been co-purified from bovine cerebral cortex. A portion of solubilized receptors which displays high affinity GTP-sensitive agonist binding (40-50%) adheres tightly to agonist affinity columns composed of N6-aminobenzyladenosine-agarose. A1 adenosine receptors and G proteins are rapidly and selectively coeluted from agonist columns by the addition of 8-p-sulfophenyltheophylline, but only in combination with Mg2+-GTP or N-ethylmaleimide, agents which lower the affinity of receptors for agonists. Purified receptors and G protein alpha-subunits can be detected with the potent A1-selective antagonist radioligand, [125I]3-(4-amino-3-iodo)phenethyl-1-propyl-8-cyclopentylxanthine (125I-BW-A844U) and [35S]guanosine 5'-3-O-(thio)triphosphate [( 35S]GTP gamma S), respectively. Pretreatment of solubilized receptors with 0.1 mM N-ethylmaleimide or 0.1 mM R-phenylisopropyladenosine abolishes adsorption of receptors and G proteins to affinity columns. Following removal of 8-p-sulfophenyltheophylline and GTP, purified receptors bind agonists (2 sites) and antagonists (1 site) with affinities similar to crude soluble receptors and typical of A1 receptors. Some receptors may be denatured as a result of purification since only 23% of the radioligand binding sites which adhere to the affinity column can be detected in the eluate. The Bmax of purified receptors, 820 +/- 100 pmol/mg protein (n = 3) is 1800-fold higher than crude soluble receptors. The specific activity of [35S]GTP gamma S binding sites in affinity column eluates is 4640 pmol/mg protein. Assuming a 1:1 stoichiometry, this specific activity indicates that receptor-G protein complexes are greater than 50% pure following affinity chromatography. The photoaffinity labeled purified receptor was identified by polyacrylamide gel electrophoresis as a single band with a molecular mass of 35 kDa which when deglycosylated undergoes a characteristic shift in molecular mass to a sharp band at 32 kDa. In addition to the receptor, silver staining revealed polypeptides with molecular masses of 39 and 41 kDa, which are ADP-ribosylated by pertussis toxin, and 36 kDa corresponding to G protein beta-subunits.

Human retinal pigment epithelial cells in culture possess A2-adenosine receptors

Adenosine agonists cause a marked stimulation in cyclic AMP accumulation in whole human retinal pigment epithelial (RPE) cells in the presence of adenosine deaminase and papaverine, a phosphodiesterase inhibitor. N-Ethylcarboxamidoadenosine (NECA) stimulates cyclic AMP accumulation 16.1-fold above basal with an EC50 of 2.5 x 10(-7) M. It is also an effective (1.9-fold) stimulator of adenylate cyclase activity in RPE membrane preparations and a modest (1.22-fold) stimulator in the presence of forskolin in RPE cell membranes prepared from freshly isolated porcine RPE. N6-Cyclopentyladenosine (CPA) and N6-phenylisopropyladenosine (PIA) also increase cyclic AMP levels with EC50s of 4.9 x 10(6) M (8.9-fold above basal) and 3.5 x 10(-6) M (8.0-fold above basal) respectively. This potency order (NECA greater than PIA greater than CPA) is typical of A2-adenosine receptors. The relatively A1-selective agonists 10(-7) M indicating that RPE cells do not have A1-receptors which inhibit adenylate cyclase. Three adenosine receptor antagonists, BW-A1433U, 8-cyclopentyltheophylline and 8-sulfophenyltheophylline, blocked the NECA-induced stimulation of cyclic AMP accumulation with IC50s of 0.36 microM, 1.5 microM, and 75 microM respectively. Since alteration of cAMP levels has been demonstrated to affect several RPE functions, including cell migration, resorption of subretinal fluid, and phagocytosis, adenosine may play a significant regulatory role in RPE.

Receptors that inhibit phosphoinositide breakdown

Calcium-mobilizing receptors are believed to activate phospholipase C. Joel Linden and Thérèse Mary Delahunty summarize recent reports which indicate that activation of some receptors that inhibit the accumulation of Ca2+ within cells - notably receptors for adenosine, dopamine and several other neurotransmitters - can inhibit phosphoinositide metabolism. Two types of mechanism may be involved in these responses. Many instances of receptor-mediated inhibition of phosphoinositide breakdown can be detected only after a period of several minutes and may be secondary to receptor-mediated events that lower [Ca2+]i or activate certain protein kinases. In other instances the activation of receptors rapidly (within seconds) inhibits phosphoinositide breakdown, possibly via the activation of guanine nucleotide binding proteins that either directly, or by a rapid indirect action, inhibit phospholipase C. Putative mechanisms for direct and indirect regulation of phospholipase C are discussed.

Downregulation and desensitization of A1-adenosine receptors in embryonic chicken heart

Downregulation of cardiac A1-adenosine receptors and desensitization of the negative inotropic response to adenosine were examined 44 h after pretreatment of 15-day-old chick embryos with phenylisopropyladenosine (R-PIA) or vehicle. The number of A1-adenosine receptors, assessed by 125I-aminobenzyladenosine (125I-ABA) binding to crude cardiac membranes, was decreased by 63% (P less than 0.001) after pretreatment of embryos with 1 mumol of R-PIA, from 16.7 +/- 2.4 to 6.2 +/- 1.4 (SD) fmol/mg protein (n = 6). The KD of 125I-ABA binding to the remaining receptors did not differ significantly from control (0.71 +/- 0.45 vs. 0.79 +/- 0.57 nM). A maximally effective concentration of R-PIA (10(-5) M) decreased the developed tension of electrically paced right ventricular (RV) muscle strips from control embryos by 76 +/- 6% (mean +/- SD, n = 7) from its forskolin-stimulated plateau level, whereas the developed tension of strips from R-PIA-pretreated embryos was decreased significantly less, by 38 +/- 4% (P less than 0.001). The potency of R-PIA and the efficacy and potency of carbachol to decrease contractile tension were not changed. The data suggest that chronic exposure of the heart to R-PIA can produce downregulation of A1-adenosine receptors and homologous desensitization of the negative inotropic response to adenosine.

Regulation of GH3-cell function via adenosine A1 receptors. Inhibition of prolactin release, cyclic AMP production and inositol phosphate generation

We examined the mechanism by which adenosine inhibits prolactin secretion from GH3 cells, a rat pituitary tumour line. Prolactin release is enhanced by vasoactive intestinal peptide (VIP), which increases cyclic AMP, and by thyrotropin-releasing hormone (TRH), which increases inositol phosphates (IPx). Analogues of adenosine decreased prolactin release, VIP-stimulated cyclic AMP accumulation and TRH-stimulated inositol phospholipid hydrolysis and IPx generation. Inhibition of InsP3 production by R-N6-phenylisopropyladenosine (R-PIA) was rapid (15 s) and was not affected by the addition of forskolin or the removal of external Ca2+. Addition of adenosine deaminase or the potent adenosine-receptor antagonist, BW-A1433U, enhanced the accumulation of cyclic AMP by VIP, indicating that endogenously produced adenosine tonically inhibits adenylate cyclase. The potency order of adenosine analogues for inhibition of cyclic AMP and IPx responses (measured in the presence of adenosine deaminase) was N6-cyclopentyladenosine greater than R-PIA greater than 5'-N-ethylcarboxamidoadenosine. This rank order indicates that inhibitions of both cyclic AMP and InsP3 production are mediated by adenosine A1 receptors. Responses to R-PIA were blocked by BW-A1433U (1 microM) or by pretreatment of cells with pertussis toxin. A greater amount of toxin was required to eliminate the effect of R-PIA on inositol phosphate than on cyclic AMP accumulation. These data indicate that adenosine, in addition to inhibiting cyclic AMP accumulation, decreases IPx production in GH3 cells, possibly by directly inhibiting phosphoinositide hydrolysis.

Vasoactive intestinal peptide evokes endothelium-dependent relaxation and cyclic AMP accumulation in rat aorta

We have investigated VIP-induced relaxation and cyclic AMP accumulation in rat thoracic aorta strips, and the importance of endothelium to both actions. The relaxation was greatly attenuated by removal of endothelium, but was unaltered by cyclo-oxygenase or lipoxygenase inhibitors. Similarly, cyclic AMP formation was nearly abolished with loss of endothelium, but was largely unaffected by inhibitors of arachidonate pathways, cytochrome P450 or guanylate cyclase. VIP may stimulate the release of a diffusible factor from endothelium (an EDRF), which activates adenylate cyclase and relaxes aortic smooth muscle.

125I-BW-A844U, an antagonist radioligand with high affinity and selectivity for adenosine A1 receptors, and 125I-azido-BW-A844U, a photoaffinity label

3-(4-Amino)phenethyl-1-propyl-8-cyclopentylxanthine (BW-A844U) has been synthesized and shown to bind with high affinity to adenosine A1 receptors of bovine brain membranes (KD = 0.23 nM). This compound is highly selective for A1 receptors; the KI for binding to A2 receptors of human platelet membranes is 2.0 microM (A2/A1 ratio = 8700). Radioiodination of the 3-aminophenethyl group resulted in 125I-BW-A844U, a radioligand that retains high affinity for A1 receptors in bovine brain membranes (KD = 0.14 nM) and to 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate-solubilized receptors (KD = 0.34 nM). Specific binding of 125I-BW-A844U represented greater than 90% of the total binding at the KD. From the association constant (K1 = 5.0 X 10(8) M-1min-1) and the dissociation constant (K-1 = 0.064 min-1), the kinetic KD (K-1/K1) in membranes was calculated to be 0.13 nM. NaCl (1 M) had little effect on the binding affinity of 125I-BW-A844U, in contrast to the large effect of salt on the binding affinity of acidic antagonist radioligands. 8-Sulfophenyltheophylline inhibited radioligand binding with a Hill coefficient of 1.0, indicative of a single affinity binding state for the antagonist. By comparison, two distinct agonist affinity states of A1 receptors for the agonist (R)-phenylisopropyladenosine could be resolved, a high affinity state (62%, KH = 74 pM) and a low affinity state (KL = 3.83 nM). The addition of 0.1 mM guanylylimidodiphosphate converted all receptors to the low affinity state. Addition of NaCl (0.5 M) decreased the fraction of receptors in the high affinity state and increased both KH and KL, suggesting that NaCl alters coupling of receptors to G proteins and influences the conformation of the receptor polypeptide, whether or not the receptor is coupled to a G protein. Conversion of the arylamine on the 3-position of 125I-BW-A844U to an aryl azide resulted in a photoaffinity label, 125I-azido-BW-A844U. Upon photoactivation, the photoaffinity label was specifically photoincorporated into the 34,000-dalton polypeptide of the adenosine A1 receptor.

Photoaffinity labeling adenosine A1 receptors with an antagonist 125I-labeled aryl azide derivative of 8-phenylxanthine

We have derivatized a series of 125I-labeled 8-phenylxanthines with photoactive aryl azide groups on the 1- or 3-position of the xanthine ring. A 3-azidophenethyl derivative was found to be optimal for use as an antagonist photoaffinity label for adenosine A1 receptors. Following photoactivation, radioactivity was covalently and specifically incorporated into a 34,000-dalton and, to a lesser extent, into a 24,000-dalton polypeptide of rat brain membranes. Photoincorporation into both polypeptides was competitively inhibited by adenosine analogues with a potency order typical of adenosine A1 receptors, but the 24,000-dalton polypeptide bound both agonists and antagonists with lower affinity than the 34,000-dalton polypeptide. Specific photolabeling of receptors in brain membranes of rat, guinea pig, dog, and cow did not show any variation in the 34,000-dalton adenosine receptor binding subunit. The adenosine agonist photoaffinity label [125I]N6-(4-azido-3-iodobenzyl)adenosine also specifically photolabeled the 34,000-dalton polypeptide, but photoincorporation of the agonist was less efficient than the antagonist and, unlike the antagonist, was greatly reduced by guanosine 5'-(beta,gamma-imidotriphosphate). The results indicate that the antagonist photoaffinity label may be more useful than agonists particularly for labeling uncoupled receptors.

125I-labeled 8-phenylxanthine derivatives: antagonist radioligands for adenosine A1 receptors

A series of 8-phenylxanthine derivatives has been synthesized with oxyacetic acid on the para phenyl position to increase aqueous solubility and minimize nonspecific binding and iodinatable groups on the 1- or 3-position of the xanthine ring. The structure-activity relationship for binding of these compounds to A1 adenosine receptors of bovine and rat brain and A2 receptors of human platelets was examined. The addition of arylamine or photosensitive aryl azide groups to the 3-position of xanthine had little effect on A1 binding affinity with or without iodination, whereas substitutions at the 1-position caused greatly reduced A1 binding affinity. The addition of an aminobenzyl group to the 3-position of the xanthine had little effect on A2 binding affinity, but 3-aminophenethyl substitution decreased A2 binding affinity. Two acidic 3-(arylamino)-8-phenylxanthine derivatives were labeled with 125I and evaluated as A1 receptor radioligands. The new radioligands bound to A1 receptors with KD values of 1-1.25 nM. Specific binding represented over 80% of total binding. High concentrations of NaCl or other salts increased the binding affinity of acidic but not neutral antagonists, suggesting that interactions between ionized xanthines and receptors may be affected significantly by changes in ionic strength. On the basis of binding studies with these antagonists and isotope dilution with the agonist [125I]N6-(4-amino-3-iodobenzyl)adenosine, multiple agonist affinity states of A1 receptors have been identified.

Purification of 125I-labeled succinyl cyclic nucleotide tyrosine methyl esters by high-performance liquid chromatography

Carrier free 125I-labeled succinyl cyclic adenosine monophosphate (ScAMP) and succinyl cyclic guanosine monophosphate (ScGMP) tyrosine methyl esters (TME) were purified by reversed phase high-performance liquid chromatography (HPLC) or descending paper chromatography. Using an isocratic buffer for HPLC, mono-ScAMP-125I-TME and mono-ScGMP-125I-TME were eluted from a C18 column at 8.9 and 6.9 min, respectively. Both of the mono-iodinated radioligands were completely separated from their noniodinated precursors and other iodinated products. The radioligands purified by HPLC or paper chromatography were used for the radioimmunoassay (RIA) of cAMP and cGMP. Cyclic AMP or cGMP inhibited binding of the HPLC purified radioligands at three- to fivefold lower concentrations than the paper chromatography purified radioligands. The sensitivity of the RIA decreased with time if paper chromatography purified radioligands were used, but remained stable for 4 months if the HPLC purified compounds were used, even with storage at 4 degrees C. We attribute these results to better purification of radioligands by the HPLC than by the paper chromatography. Using optimal conditions the HPLC method takes only 10 min and results in a high yield (greater than 95%) of added 125I into the monoiodinated products.

Effect of adrenergic and muscarinic cholinergic agonists on atrial natriuretic peptide secretion by isolated rat atria. Potential role of the autonomic nervous system in modulating atrial natriuretic peptide secretion

Stretching of the atrial wall is a known stimulant for atrial natriuretic peptide (ANP) secretion. Little is known about other factors that may influence ANP secretion. We examined the effects of the neurotransmitters of the autonomic nervous system on ANP secretion from isolated rat left atria. Superfusion with 10 muM norepinephrine produced a biphasic rise in ANP secretion with a peak response 2.5-fold above baseline secretion. To determine whether the response to norepinephrine primarily reflected alpha- or beta-adrenergic receptor stimulation, atria were superfused with 0.1 muM isoproterenol or 10 muM phenylephrine and 1 muM propranolol. ANP secretion in response to isoproterenol was biphasic, similar to the response to norepinephrine. Phenylephrine evoked a monophasic ANP secretory response, which was delayed in onset relative to that of isoproterenol or norepinephrine. Superfusion with 10 muM methacholine alone had no effect on ANP secretion, but rapidly attenuated norepinephrine-stimulated secretion by 67%. From these observations we conclude: (a) Both alpha- and beta-adrenergic agonists directly and distinctively stimulate ANP secretion; (b) Norepinephrine stimulates ANP secretion by both alpha- and beta-adrenergic mechanisms, however the secretory response pattern of norepinephrine reflects a predominence of beta-adrenergic activity; (c) Under basal conditions, methacholine does not influence ANP secretion; and (d) Methacholine inhibits norepinephrine-stimulated ANP secretion. Thus, in vivo, activation of the sympathetic nervous system may enhance ANP secretion, whereas a rise in parasympathetic tone may lower ANP secretion.

Enhanced cAMP accumulation after termination of cholinergic action in the heart

Cholinergic agents decrease myocardial contractility in part by inhibiting adenylate cyclase (EC 4.6.1.1) activity. We have found that after a prolonged preincubation period (greater than 6 h), washout of cholinergic agents from embryonic chick hearts or cultured heart cells results in a persistent increase in their basal and catecholamine-stimulated cAMP content. Membranes prepared from pretreated cells have elevated basal, forskolin-, and catecholamine-stimulated adenylate cyclase activities. This myocardial adaptation to cholinergic agents is analogous to changes in nerve cells and other cell types after prolonged exposures to narcotics or other inhibitors of adenylate cyclase, respectively. A rapid (less than 5 min) adaptation response to cholinergic agents can also be demonstrated in heart cells by quickly blocking agonist action with atropine. Atropine alone has no effect, but after a brief preincubation period with agonists (methacholine or oxotremorine), the addition of atropine transiently enhances catecholamine-stimulated cAMP accumulation by 2.5-fold. These responses are absent in heart cells pretreated with pertussis toxin. The data indicate that the response is not mediated by the phosphoinositide pathway, which has been demonstrated to be insensitive to pertussis toxin in chick heart. Enhanced cAMP accumulation after termination of muscarinic agonist action may provide an explanation for the observation that acetylcholine sometimes produces biphasic contractile responses.

Antagonism of the effects of adenosine and hypoxia on atrioventricular conduction time by two novel alkylxanthines: correlation with binding to adenosine A1 receptors

Adenosine has been shown to have a negative dromotropic effect and has been implicated in mediating atrioventricular conduction disturbances induced by hypoxia. This study was designed to determine the ability of various alkylxanthines including two novel derivatives, i.e., BW A533U and BW A1433U, to 1) attenuate adenosine- and hypoxia-induced atrial to His bundle (AH) interval prolongation, 2) compete for binding of 125I-aminobenzyladenosine to ventricular membranes and 3) inhibit myocardial phosphodiesterase. In normoxic isolated perfused hearts (n = 20) instrumented for measurement of atrioventricular conduction time and left ventricular pressure, BW A1433U (0.1 microM) or BW A533U (5 microM) attenuated AH interval prolongation induced by adenosine (5 microM) by 90%, but neither xanthine derivative attenuated the AH interval prolongation induced by acetylcholine (0.11 microM), digoxin (0.91 microM) or D600 (1.3 microM). In four additional hearts, BW A1433U at concentrations of up to 10 microM had no effect on left ventricular pressure or AH interval. BW A1433 or BW A533U (50 microM) inhibited myocardial cyclic AMP phosphodiesterase by only 11.5 +/- 1.6 and 26.6 +/- 2.6%, respectively. Schild analysis of adenosine concentration-response curves obtained in the absence and presence of BW A533U and BW A1433U (n = 14) yielded pA2 values of (mean +/- S.E.M.) 6.32 +/- 0.10 and 7.70 +/- 0.08, respectively. pKd values for BW A533U and BW A1433U binding to adenosine receptors on ventricular membranes were 6.36 and 6.94, respectively. In a separate series of 19 hearts, BW A533U and BW A1433U were shown to attenuate hypoxia-induced AH interval prolongation.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of glutaraldehyde-induced cross-links in calcification of bovine pericardium used in cardiac valve bioprostheses

Calcification is the principal cause of failure of tissue-derived cardiac valve replacements pretreated with glutaraldehyde (GLUT). The objective of this study was to determine the role of GLUT-induced cross-links in bovine pericardial tissue calcification. Various levels of 3H-GLUT incorporation were obtained by varying incubation pH, and protein modification was determined by amino acid analysis and resistance to collagenase digestion. Calcification of cross-linked tissue was studied using subdermal implants in rats. Low GLUT uptake (less than 150 nm/mg) resulted in minimal calcification (Ca2+, 12.8 micrograms/mg) and stability (4% residual weight following digestion) due to a limited crosslinking (lysine + hydroxylysine = 26.1 residues/1000 amino acids [AA]). In contrast, higher GLUT uptake induced more cross-links (Lys + Hyl = 8.2 residues/1000 AA) and consequent higher stability (95% residual wt); such tissues calcified severely (Ca2+, 93.5 micrograms/mg). Incorporation of GLUT two to three times beyond a critical level did not further enhance calcification. It is concluded that the amount of GLUT incorporated controls the extent of cross-links, which in turn directly determines tissue stability and calcification.

Inhibition of cyclic AMP-dependent protein kinase activity by the cardiotonic drugs amrinone and milrinone

Amrinone and milrinone are new cardiotonic drugs that have potent inotropic and vasodilatory properties. The mechanism of action of these agents is controversial, but the positive inotropic component is thought to be due to the inhibition of phosphodiesterase. Because amrinone and milrinone have been shown to be involved primarily in cyclic AMP-mediated processes, we examined the effect of these agents on cyclic AMP-dependent protein kinase. The results indicate that amrinone and milrinone inhibit cyclic AMP-dependent protein kinase activity by competing with ATP but not cyclic AMP binding sites. Dissociation constants (Ki) of amrinone and milrinone for ATP binding sites on protein kinase were calculated to be 100-300 microM and 842 microM, respectively. The phosphodiesterase inhibitor isobutylmethylxanthine (1 mM) had no effect on protein kinase activity. Amrinone and milrinone inhibited the catalytic subunit of protein kinase to the same degree as the holo-enzyme by competitively inhibiting the binding of ATP. Amrinone and milrinone had no effect on phospholipid-sensitive, calcium-dependent protein kinase indicating that there may be differences in the ATP binding sites on these two protein kinases. Inhibition of cyclic AMP-dependent protein kinase by amrinone and milrinone occurs at concentrations higher than those used clinically. However, because amrinone and milrinone are lipophilic drugs, they may be useful tools for the investigation of protein kinase mediated reactions.

Effect of atrial contraction frequency on atrial natriuretic peptide secretion

Paroxysms of atrial tachycardia are oftentimes associated with polyuria. The plasma levels of the potent diuretic hormone, atrial natriuretic peptide (ANP), are elevated during episodes of atrial tachycardia, suggesting that ANP may play a role in mediating the diuresis. The mechanism of enhanced ANP secretion associated with atrial tachycardia is not known. We examined the effect of altering the pacing frequency of isolated left rat atria on ANP secretion. Atria were suspended between an electrode and hook connected to a force transducer and superfused with medium 199. The ANP content of the superfusate was measured by radioimmunoassay. Increasing the frequency of pacing from 2 to 4 Hz resulted in a 46 +/- 5% (means +/- SE, n = 9) rise in immunoreactive ANP secretion above base line (P less than 0.01). Lowering the frequency from 4 to 2 Hz lowered immunoreactive ANP secretion by 36 +/- 3% (n = 6) relative to base line (P less than 0.02). Incremental increases in the pacing frequency from 2 to 8 Hz resulted in a continual rise in immunoreactive ANP with a peak increase of 191 +/- 6% of base line (n = 8) at 8 Hz. To examine the possibility that release of norepinephrine or acetylcholine from endogenous nerves mediated this effect, the atria were superfused with the combination of 0.1 microM propranolol, 1.0 microM phentolamine, and 10 microM atropine. The concentrations of the antagonists were 125-fold or higher than the dissociation constant for binding to receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of guanylate cyclase by lipoxygenase inhibitors

Drugs that inhibit endothelium-dependent relaxation were tested to determine their effect on soluble guanylate cyclase purified from dog aorta. Basal, arachidonic acid (10(-5) M)-stimulated, and nitroprusside (5 X 10(-5) M)-stimulated guanylate cyclase activities were inhibited by methylene blue and the lipoxygenase inhibitors nordihydroguaiaretic acid and eicosatetraynoic acid. The effective inhibitory doses were in the range of those that have been reported to inhibit endothelium-dependent relaxation. Other compounds known to inhibit endothelium-dependent relaxation had little or no effect on guanylate cyclase activity. Basal guanylate cyclase activity was more resistant to inhibition than were activated states of the enzyme. The data suggest that reported inhibition of endothelium-dependent relaxation by some lipoxygenase inhibitors may be the result, at least in part, of their direct effect on guanylate cyclase activity.

The influence of resting tension on immunoreactive atrial natriuretic peptide secretion by rat atria superfused in vitro

Atrial natriuretic peptide is a potent diuretic hormone secreted by the atria in response to volume expansion. We examined the effect of resting tension on atrial natriuretic peptide secretion by rat atria superfused in vitro. Left atria were hooked between an electrode and force transducer and superfused with medium 199. The atria were studied at a pacing frequency of 0 or 3 Hz. Atrial natriuretic peptide content of the superfusate was measured by radioimmunoassay. In nonpaced and paced atria, increasing resting tension three- to five-fold caused immunoreactive atrial natriuretic peptide secretion to increase by 35 +/- 5% (mean +/- SEM, n = 6, p less than 0.01) and 30 +/- 3% (n = 4, p less than 0.01), respectively. Lowering resting tension by 50% in nonpaced and paced atria lowered immunoreactive atrial natriuretic peptide secretion by 30 +/- 3% (n = 7, p less than 0.01) and 24 +/- 3% (n = 6, p less than 0.01), respectively. To exclude the possibility that release of norepinephrine or acetylcholine from endogenous nerve endings was mediating this effect, the atria were superfused with the combination of propranolol 0.1 microM, phentolamine 1.0 microM, and atropine 10 microM. These concentrations of the antagonists were 125-fold or higher than their Kd for binding to their respective receptors. The antagonists did not block the rise in immunoreactive atrial natriuretic peptide secretion; neither did they inhibit an established rise in immunoreactive atrial natriuretic peptide secretion induced by increasing the resting tension.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical mechanisms for the inotropic effect of the cardiotonic drug milrinone

Milrinone is a new inotropic agent for the treatment of refractory congestive heart failure. Our understanding of the mechanisms(s) of action of this synthetic cardiotonic drug is incomplete. We examined the effects of milrinone and the parent compound amrinone on sarcoplasmic reticulum function (45Ca-uptake and Ca-ATPase); radioligand binding to adenosine, beta-adrenergic, and cholinergic muscarinic receptors; cyclic AMP accumulation; and inhibition of various forms of cyclic AMP phosphodiesterases. Comparisons were made to observe how these effects correlate with the inotropic response of heart. Milrinone was shown to be a potent phosphodiesterase inhibitor that was 40 times more potent than amrinone and 10 times more potent at inhibiting the high-affinity (Km = 0.23 microM) form (Ki = 22 microM) than the low-affinity (Km = 140 microM) form (Ki = 225 microM) of cyclic AMP phosphodiesterase in heart. The potency of milrinone as a phosphodiesterase inhibitor was the same in the presence and absence of calcium. Concentrations of milrinone that increased cyclic AMP accumulation also produced positive inotropy. A comparison of milrinone with amrinone and methylxanthines revealed the order of potency to be isobutylmethylxanthine greater than milrinone greater than theophylline greater than caffeine greater than amrinone. Milrinone and amrinone had no effect on 45Ca-uptake or Ca-ATPase activity in myocyte sarcoplasmic reticulum. However, milrinone did bind weakly to adenosine receptors (KD = 466 microM) but not to cholinergic muscarinic or beta-adrenergic receptors. Also, in combination with isoproterenol high concentrations of milrinone blocked the negative inotropic response to the adenosine agonist phenylisopropyladenosine.(ABSTRACT TRUNCATED AT 250 WORDS)