The effect of calcium oncardiac phosphorylase activation, contractile force and cyclic AMP in euthyroid and hyperthyroid rat hearts.

Calcium chloride injected into isolated perfused rat hearts produced a positive inotropic effect and increased the levels of phosphorylase a (EC 2.4.1.1). The increase in enzyme activity lagged behind the inotropic effect. Pretreatment of animals with thyroid hormone enhanced the ability of noradrenaline to activate phosphorylase but did not affect the inotropic or phosphorylase activating effect of calcium. Thyroid hormone pretreatment did enhance the chronotropic effect of calcium. Calcium did not affect the cardiac levels of cyclic AMP. It is concluded that calcium can activate phosphorylase by a mechanism other than cyclic AMP and that the enhancement of adrenergic amine-induced phosphorylase activation by thyroid hormone is not a calcium mediated event.     

PGE1-independent MDCK cells have elevated intracellular cyclic AMP but retain the growth stimulatory effects of glucagon and epidermal growth factor in serum-free medium

Prostaglandin E1 (PGE1), a component in the hormone-supplemented, serum-free medium for the Madin Darby canine kidney (MDCK) cell line, has been proposed to increase MDCK cell growth by increasing intracellular cyclic AMP levels. The association between increased intracellular cyclic AMP and the growth stimulatory effect of PGE1 has been examined in normal MDCK cells and in PGE1-independent variants of MDCK. These variant cells have lost the PGE1 requirement for long term growth in defined medium. Normal MDCK cells had almost twofold higher intracellular cyclic AMP levels during growth in Medium K-1 (9.0 pmol/mg protein) than in Medium K-1 minus PGE1. Furthermore, PGE1-independent clone 1 had higher intracellular cyclic AMP levels in Medium K-1 minus PGE1 than normal MDCK cells in Medium K-1. This latter observation suggests that the PGE1 requirement for MDCK cell growth is associated with the low intracellular cyclic AMP levels of this cell line. An involvement of cyclic AMP in the growth response to PGE1 is supported by these observations, as well as by the growth stimulatory effects of other agents that affect cyclic AMP metabolism in MDCK cells. These agents include glucagon, isobutyl methylxanthine (IBMX), and dibutyryl cyclic AMP. The growth of PGE1-independent clone 1 was inhibited rather than stimulated by PGE1. Similarly, PGE1-independent cell growth was inhibited by IBMX and dibutyryl cyclic AMP. However, the growth response to one agent which increases cyclic AMP (glucagon) was retained in PGE1-independent clone 1. This result suggests that the effect of glucagon is not associated with increases in intracellular cyclic AMP. The growth stimulatory effect of epidermal growth factor (EGF) on normal MDCK cells was also studied. Although EGF does not act via a cyclic AMP-mediated mechanism, EGF increased normal MDCK cell growth and substituted for PGE1 in Medium K-1. Thus, EGF and PGE1 could possibly affect similar growth-related functions in MDCK cells, although by different pathways. This possibility was examined further, using PGE1-independent clone 1. EGF, like glucagon, was still growth stimulatory to the PGE1-independent cells. Consequently, the biochemical pathways by which EGF and PGE1 increase MDCK cell growth probably do not converge.     

Modification of the positive inotropic effects of catecholamines, cardiac glycosides and Ca2+ by the orally active male contraceptive, gossypol, in isolated guinea-pig heart

Gossypol is an orally active male contraceptive with cardio-depressant side effects. To understand the mechanism of its cardiac actions, the interaction of gossypol with positive inotropic drugs was examined in isolated atrial muscle preparations obtained from guinea-pig heart. Gossypol delayed the onset of arrhythmias caused by digoxin. In the presence of gossypol, the positive inotropic effect of isoproterenol declined rapidly, and the effect of isoproterenol to increase tissue cyclic AMP concentrations was smaller. Pretreatment of atrial muscle with the combination of gossypol and isoproterenol markedly reduced effects of isoproterenol on developed tension and cyclic AMP concentrations when these effects were tested after the washout of the first dose of isoproterenol. These effects, however, were not specific to isoproterenol. The gossypol-isoproterenol pretreatment reduced the positive inotropic effect of ouabain or extracellular Ca2+. These results indicate that gossypol has pharmacodynamic interactions with several positive inotropic agents that are known to enhance developed tension by increasing intracellular Ca2+ transients.     

Cardiac effects of prostaglandins E1 and F1 alpha

The effects of prostaglandin E1 (PGE1) and prostaglandin F1 alpha (PGF1 alpha) were studied on perfused rat hearts and isolated rat atria. Both PGE1 and PGF1 alpha produced dose-dependent increases in right atrial rate but had no effect on left atrial tension development. PGE1 (10(-4) M) increased right atrial cyclic AMP content without changing phosphorylase a activity. PGF1 alpha (10(-4) M) did not change right atrial cyclic AMP or cyclic GMP content. Both prostaglandins had no effect on left atrial cyclic nucleotide content. When infused at a rate of 1 microgram/min, PGE1 produced a time-dependent increase in cyclic AMP content in the Langendorff perfused hearts but did not alter contractile force development or phosphorylase a activity. An infusion of PGF1 alpha produced a dose-dependent increase in tension development which was secondary to a negative chronotropic effect. PGF1 alpha (1 microgram/min) did not produce any changes in cyclic nucleotide levels or phosphorylase a activity in the Langendorff perfused hearts. These results show that PGE1 can selectively increase myocardial cyclic AMP content without altering contractile force or phosphorylase activity and that PGF1 alpha does not increase rat cardiac AMP levels.     

The new inotropic phosphodiesterase inhibitors

Compounds with phosphodiesterase inhibitory activity stimulate myocardial contractility by increasing the intracellular cyclic AMP concentrations. They can also increase Ca2+ entry and inhibit Ca2+ sequestration by the sarcoplasmic reticulum. Xanthines produce bronchodilation with associated venous and arteriolar dilation. However, their use is limited by their positive chronotropic effect and other side effects at high plasma levels. New phosphodiesterase inhibitors have been perfected: they are more specific with little chronotropic effect. Increasing the sensitivity of the myofilaments to Ca2+, and other unclear mechanisms may be involved in the inotropic action of these drugs. These new promising active compounds are described and discussed. They augment cardiac performance and improve regional distribution of blood flow and symptoms. However, their influence on the long-term outcome of severe heart failure has yet to be determined.     

Mechanism for amine modulation of the neurogenic Limulus heart: evidence for involvement of cAMP

The role of cyclic nucleotides as intracellular second messengers mediating the excitatory chronotropic and inotropic actions of octopamine (OCT) and dopamine (DA) on the neurogenic Limulus heart was investigated. Tissue levels of cAMP, but not cGMP, were significantly increased in isolated cardiac ganglia and cardiac muscle following 10 min exposure to 10(-5) M OCT or 10(-5) M DA. In both tissues, OCT elicited larger increases in cAMP than did DA. Amine-induced cAMP accumulation in the cardiac ganglion and in the cardiac muscle was prevented by the alpha-adrenergic blocker phentolamine. The adenylate cyclase activator forskolin and the phosphodiesterase inhibitor IBMX produced amine-like chronotropic and inotropic effects when applied to the isolated heart preparation. However, the kinetics of the responses differed for the two agents. Additional pharmacological agents (RO-20-1724, papaverine, SQ 20,009, and 8-parachloro-phenylthio cAMP) also had amine-like effects but to a lesser extent. The chronotropic, but not inotropic, effects of OCT and DA were potentiated in the presence of IBMX. These data suggest that a cAMP-dependent mechanism underlies the excitatory effects of the neuromodulators OCT and DA on the Limulus heart.     

Direct actions of prostaglandins E1, A1, and F2α on myocardial contraction

The inotropic and chronotropic actions of prostaglandin (PG) types PGE₁, PGA₁, and PGF_2α were studied in isolated guinea pig right and left atria, and papillary muscles; rabbit atria; and toad ventricular strips in order to more completely define the cardiac contractile properties of PG. All three prostaglandins, in muscle bath concentrations of 10μg/ml, exerted positive inotropic and chronotropic actions on guinea pig atrium. These contractile effects were persistent after removal of PG from the muscle bath and appeared to limit the relative response to a subsequent dose of PG. The inotropic action of PGE₁ was present over a wide range of bath calcium concentrations (1.1 to 4.4 mM/L). Beta adrenergic receptor blockade, histamine blockade, and pretreatment with reserpine failed to significantly affect the inotropic actions of PG. Norepinephrine and histamine produced more potent inotropic and chronotropic effects on guinea pig atria than did PG and these contractile effects did not exhibit persistence or tachyphylaxis. The prostaglandins did not significantly affect dose response curves for norepinephrine inotropic and chronotropic actions. The prostaglandins had no effect on the force or frequency of contraction in rabbit atria. PGE₁ exerted a positive inotropic effect on toad ventricular myocardium whereas PGA₁ had no effect and PGF_2α had a negative inotropic action.     

Effect of Forskolin and Prostaglandin E1 on Stimulus Secretion Coupling in Cultured Bovine Adrenal Chromaffin Cells

Treatment of adrenal chromaffin cells with forskolin (0.1-10 microM) stimulated cyclic AMP levels, reduced the maximal stimulation of release of noradrenaline by nicotine, and increased release in response to elevated external potassium and the calcium ionophore A23187. The presence of the phosphodiesterase inhibitor Ro 20-17-24 with forskolin potentiated both the stimulation of cyclic AMP and the inhibition of nicotine-induced noradrenaline release. Dibutyryl cyclic AMP, and the elevation of cyclic AMP with prostaglandin E1, also attenuated nicotine-stimulated release. However, when the stimulation of intracellular cyclic AMP production by prostaglandin E1 was potentiated by low levels of forskolin, there was not a concomitant potentiation of effect on noradrenaline release. Dideoxyforskolin, an analogue of forskolin which does not stimulate adenylate cyclase, inhibited both potassium- and nicotine-stimulated release, probably by a mechanism unrelated to the action of forskolin in these experiments. Using Fura-2 to estimate free intracellular calcium levels, both forskolin and dideoxyforskolin (at 10 microM) reduced the calcium transient in response to nicotine. These results support a model in which elevation of cyclic AMP inhibits the activation of nicotinic receptors, but augments stimulus secretion coupling downstream of calcium entry. The data, however, do not indicate a simple relationship between total intracellular cyclic AMP levels and the attenuation of nicotinic stimulation of release.     

Oppositional effects of acetylcholine and isoproterenol on isometric tension and cyclic nucleotide concentrations in rabbit atria.

The effects of acetylcholine chloride and isoproterenol on myocardiial cyclic GMP, cyclic AMP and on isometric tension were studied in isolated electrically driven rabbit atria. Acetylcholine (0.5 muM) produced a significant decrease in isometric force that was associated with a significant elevation in atrial cyclic GMP. Cyclic AMP was significantly lowered at 15 seconds after the addition of acetylcholine, but was only slightly decreased at earlier time periods. Both the negative inotropic action and increase in cyclic GMP after addition of acetylcholine were blocked by atropine. Isoproterenol (0.1 muM) produced a significant increase in isometric tension that was associated with a significant elevation in atrial cyclic AMP levels, whereas cyclic GMP levels were not changed. These effects were blocked by practolol. The increases in atrial cyclic GMP and cyclic AMP following addition of acetylcholine and isoproterenol, respectively, preceded the changes in isometric tension in response to these agents. These data support the hypothesis that changes in intracellular levels of cyclic AMP and cyclic GMP may mediate the positive and negative inotropic effects of adrenergic and cholinergic agents.     

13-Propylberberine reduces response of guinea-pig myocardium to inotropic interventions including changes in extracellular Ca2+

Berberine has been shown to increase developed tension in cardiac muscle but its derivatives have been reported to inhibit the catalytic subunit of adenylate cyclase. In the present study, the cardiac actions of the most potent derivative, 13-propylberberine, were examined. It produced a transient increase followed by a sustained decrease in developed tension in paced left atrial muscle preparations isolated from guinea-pig heart. In the presence of 13-propylberberine, isoproterenol caused only a transient increase in developed tension; marked desensitization to the positive inotropic effect of isoproterenol occurred within 20 min. After washout of isoproterenol and an additional 15-min incubation in the presence of 13-propylberberine, the muscle lost its sensitivity to isoproterenol. Moreover, the positive inotropic effect of ouabain or effects of decrease or increase in extracellular Ca2+ concentration on the force of muscle contraction were markedly attenuated. Isoproterenol-induced elevation of tissue cyclic AMP concentration was inhibited by 13-propylberberine; however, 13-propylberberine did not alter the basal cyclic AMP concentration and its effects on inotropic actions of ouabain or extracellular Ca2+ appear unrelated to tissue cyclic AMP concentration.     

Prostaglandin E2 inhibition of growth in a colony-stimulating factor 1-dependent macrophage cell line

The effects of prostaglandin E2 (PGE2) were examined in a murine macrophage cell line (BAC1.2F5) that was completely dependent on colony-stimulating factor-1 (CSF-1) for both growth and survival. The addition of PGE2 to cultures of BAC1.2F5 cells resulted in the inhibition of CSF-1-induced [3H]thymidine incorporation and cell proliferation. The inhibitory effects of PGE2 were mimicked by the addition of dibutyryl-cyclic AMP, and the effectiveness of PGE2 was markedly potentiated by 1-methyl-3-isobutylxanthine, a potent inhibitor of cyclic nucleotide phosphodiesterase activity. PGE2 caused a 10-fold elevation of the intracellular cyclic AMP concentration, whereas CSF-1 neither increased cyclic AMP levels nor attenuated the rise in cyclic AMP promoted by PGE2. However, CSF-1 may indirectly regulate cyclic AMP levels since in the absence of CSF-1, BAC1.2F5 cells actively synthesized PGE2, whereas PGE2 production was abruptly terminated by the addition of CSF-1. In BAC1.2F5 cells, PGE2 increases the intracellular cyclic AMP concentration, thereby blocking cell proliferation, but does not down-regulate the CSF-1 receptor or abrogate the functions of CSF-1 necessary for cell survival.     

The effects of hormones on cyclic adenosine 3':5'-monophosphate accumulation in transitional epithelium of the urinary bladder.

Transitional epithelium lining rabbit urinary bladders was isolated and studied in vitro. The homogeneity of the isolated epithelium was demonstrated by light and electron microscopical monitoring as well as cell culture studies. Transitional epithelium responded to epinephrine and prostaglandin E1 (PGE1) in the presence of 2mM 1-methyl, 3-isobutylxanthine (MIX) with increases in intracellular levels of cyclic adenosine 3':5'-monophosphate (cyclic AMP). Corticotropin, aldosterone, insulin, parathyroid hormone and vasopressin were slightly but significantly stimulatory under similar conditions. Glucagon and oxytocin were not stimulatory at the concentrations tested. The effects of epinephrine and PGE1 were potentiated by 2mM MIX 20-fold or greater. The cells were slightly more sensitive to PGE1 then to epinephrine. The prostaglandin produced a noticeable response at about 10nM, while effects of epinephrine were discernible at 0.1muM. Maximal responses to both effectors were seen at about 10muM. The action of 10muM epinephrine, but not 10muM PGE1, was completely abolished by 0.1mM propranolol. Responses to combinations of epinephrine and PGE1 were additive. Cyclic AMP accumulated in the incubation medium of transitional epithelial cells exposed to epinephrine, PGE1, MIX, or combinations of the agonists. The appearance of cyclic AMP in the medium was slow compared to the rate of intracellular accumulation, but reached significant levels following prolonged stimulation.     

Inhibition of cyclic nucleotide phosphodiesterase during exposure to WI-38 cells to prostaglandin E1.

Short term incubation of WI-38 cultures with 5.7 micron prostaglandin E1 (PGE1) caused cyclic AMP phosphodiesterase activity in fibroblast homogenates to fall by 25 to 35% as compared to controls. The PGE1-induced decline in phosphodiesterase activity coincided with a rapid increase in intracellular cyclic AMP levels in response to the hormone and was rapidly reversed by washing the cultures free of the prostaglandin before homogenizing the cells. The effect of PGE1 on WI-38 phosphodiesterase activity was localized to the enzyme form(s) present in 27,000 times g supernatant fractions of cell homogenates. These data suggest that the pattern of cyclic AMP accumulation in WI-38 fibroblasts exposed to PGE1 may be related, at least in part, to decreased phosphodiesterase activity during hormone stimulation.     

Uptake and catabolism of N6, 2'-O-dibutyryl cyclic AMP by the perfused heart

Dibutyryl cyclic AMP when administered to perfused rat hearts produced increased contractile tension and increased intracellular levels of cyclic AMP. When [³H]dibutyryl cyclic AMP was administered, it was taken up by the heart in extremely small amounts. The material was present in the tissue throughout the duration of the inotropic response. The compound was slowly degraded to monobutyryl cyclic AMP and traces of cyclic AMP. It is suggested that N⁶-monobutyryl-cyclic AMP, or possibly cyclic AMP, may mediate the effects of the dibutyryl analog.     

Cyclic AMP and platelet prostaglandin synthesis.

The present study has investigated the influence of agents which elevate intracellular levels of endogenous platelet adenosine 3'5'-cyclic monophosphate (cyclic AMP), and the effect of the exogenous cyclic AMP analog, dibutyryl cyclic AMP, on the conversion of 14C-arachidonic acid by washed platelets. Prostaglandin E1 (PGE1), PGE1 with theophylline, or dibutyryl cyclic AMP incubated with washed platelets prevented arachidonic acid induced platelet aggregation, but had no effect on the conversion of arachidonic acid to 12L-hydroxy-5,8,10, 14-eicosatetraenoic acid (HETE), 12L-hydroxy-5,8,10 heptadecatrienoic acid (HHT), or thromboxane B2. Ultrastructural studies of the platelet response revealed that agents acting directly or indirectly to increase the level of cyclic AMP inhibited the action of arachidonic acid on washed platelets and prevented internal platelet contraction as well as aggregation. The influence of PGE1 with theophylline, and dibutyryl cyclic AMP on the thrombin induced release of 14C-arachidonic acid from platelet membrane phospholipids was also investigated. These agents were found to be potent inhibitors of the thrombin stimulated release of arachidonic acid from platelet phospholipids, due most likely to an inhibition of platelet phospholipase A activity. The results show that dibutyryl cyclic AMP and agents which elevate intracellular cyclic AMP levels act to inhibit platelet activation at two steps 1) internal contraction and 2) release of arachidonic acid from platelet phospholipids.     

Elevation of prostaglandin and cyclic AMP levels by arachidonic acid in primary epithelial cell cultures of C3H mouse mammary tumors.

Arachidonic acid causes a sharp transient increase in cyclic AMP levels in primary epithelial cell cultures obtained from C3H mouse mammary tumors. The effect is evident within two minutes and is enhanced by theophylline or 3-isobutyl-1-methylxanthine. Maximum increase in cyclic AMP levels are observed with a dose of 100 mug/ml of arachidonic acid (AA). At higher dose levels the increase in cyclic AMP levels is reduced. Naproxen, an inhibitor of prostaglandin synthesis in this system markedly reduces the stimulation of cyclic AMP by arachidonic acid but it does not affect the increase in cyclic AMP levels observed after the addition of prostaglandin E's, epinephrine or cholera enterotoxin. Arachidonic acid, under the same conditions, also causes a significant elevation of PGE and PGF media levels which is slower and more sustained than the cAMP response. The data strongly suggest that a metabolic of arachidonic acid is responsible for the cyclic rise, however, it is not certain whether this is due to PGE2 or some other product.     

The cardiac sarcoplasmic reticulum - glycogenolytic complex, an internal beta adrenergic receptor.

Agents which stimulate the formation of cyclic AMP are well known to cause positive inotropic effects on the heart. This review suggests that the myocardial cell is multicompartmented and has several effector sites for cyclic AMP-mediated events and therefore several possible receptor sites for cyclic AMP-mediated agents. It is possible that some of the complex relationships between cyclic AMP-mediated agents and cardiac physiologic functions result from differences among agonist receptors and enzymes in the intracellular compartments. Examination of the characteristics of intracellular drug receptors may yield a new frontier for cardiac pharmacology.     

Two Possibly Distinct Prostaglandin E1 Receptors in N1E-115 Clone: One Mediating Inositol Trisphosphate Formation, Cyclic GMP Formation, and Intracellular Calcium Mobilization and the Other Mediating Cyclic AMP Formation

Prostaglandin E1 (PGE1)-mediated transmembrane signal control systems were investigated in intact murine neuroblastoma cells (clone N1E-115). PGE1 increased intracellular levels of total inositol phosphates (IP), cyclic GMP, cyclic AMP, and calcium ([Ca2+]i). PGE1 transiently increased inositol 1,4,5-trisphosphate formation, peaking at 20 s. There was more than a 10-fold difference between the ED50 for PGE1 at cyclic AMP formation (70 nM) and its ED50 values at IP accumulation (1 microM), cyclic GMP formation (2 microM), and [Ca2+]i increase (5 microM). PGE1-mediated IP accumulation, cyclic GMP formation, and [Ca2+]i increase depended on both the concentration of PGE1 and extracellular calcium ions. PGE1 had more potent intrinsic activity in cyclic AMP formation, IP accumulation, and cyclic GMP formation than did PGE2, PGF2 alpha, or PGD2. A protein kinase C activator, 4 beta-phorbol 12 beta-myristate 13 alpha-acetate, had opposite effects on PGE1-mediated IP release and cyclic GMP formation (inhibitory) and cyclic AMP formation (stimulatory). These data suggest that there may be subtypes of the PGE1 receptor in this clone: a high-affinity receptor mediating cyclic AMP formation, and a low-affinity receptor mediating IP accumulation, cyclic GMP formation, and intracellular calcium mobilization.     

Cholera toxin-induced changes in force of contraction and cyclic AMP levels in canine ventricular myocardium: Inhibition by carbachol

Cholera toxin (1–10 μg/ml) had a biphasic inotropic action on the isolated canine ventricular muscle: it produced a transient negative and a long lasting positive inotropic effect. The negative effect reached a maximum 43 + 2 min (n = 12) after administration of the toxin, while it took 3–5 hrs for the positive effect to reach a steady level. The positive inotropic effect of cholera toxin was accompanied by a prominent abbreviation of the time to peak tension and the relaxation time of individual contractions. The level of adenosine 3′,5′-cyclic monophosphate (cyclic AMP) of the tissue was elevated by cholera toxin in a time- and concentration-dependent manner. Carbachol (1 μmol/l) administered 3 or 5 hrs after the administration of cholera toxin (10 μg/ml) reversed the increase in force of contraction and the elevation of cyclic AMP levels induced by cholera toxin. These results indicate that cholera toxin exerts a cyclic AMP-dependent positive inotropic effect and a negative inotropic effect which is not related to cyclic AMP levels in canine ventricular myocardium.     

1983 Upjohn Award lecture. Endocrine dysfunction and cardiac performance

Studies were carried out to study the effect of endocrine changes on rat cardiac performance, biochemistry, and responses to drugs. Hyperthyroidism increased contractility in rat hearts and enhanced the phosphorylase response to catecholamine. The inotropic response may be due to an increase in cardiac mass while the enzyme changes may be due to several factors. Hypothyroidism decreased force of contraction, enhanced alpha-adrenergic inotropic and chronotropic responses, and decreased beta-adrenergic responses in isolated atrial preparations. An interaction between cyclic AMP and cyclic GMP is suggested as a possible explanation. Diabetes induced by alloxan or streptozotocin produced a decrease in cardiac performance after 42 days which was correlated with a decrease in sarcoplasmic reticulum (SR) Ca2+ uptake. Insulin treatment reversed or prevented both SR and functional changes; other treatments were not as successful. Responses to cardiotonic drugs were altered by the diabetic state. The phosphorylase response to isoproterenol was enhanced while the inotropic response was not affected. An initial subsensitivity to carbachol at 30-100 days of diabetes subsequently converted to a supersensitivity to the muscarinic agent. Ouabain responses were decreased in atrial and papillary preparations from diabetic animals. Studies are continuing to elucidate the mechanisms involved in the altered pharmacological responses seen in hearts from diabetic animals.