Cyclic AMP-mediated regulation of vascular smooth muscle cell cyclic AMP phosphodiesterase activity

1. Rat cultured aortic vascular smooth muscle cells (VSMC) express both cyclic GMP-inhibited cyclic AMP phosphodiesterase (PDE3) and Ro 20-1724-inhibited cyclic AMP phosphodiesterase (PDE4) activities. By utilizing either cilostamide, a PDE3-selective inhibitor, or Ro 20-1724, a PDE4-selective inhibitor, PDE3 and PDE4 activities were shown to account for 15% and 55% of total VSMC cyclic AMP phosphodiesterase (PDE) activity.
2. Treatment of VSMC with either forskolin or 8-bromo-cyclic AMP caused significant concentration- and time-dependent increases in total cellular cyclic AMP PDE activity. Using cilostamide or Ro 20-1724, we demonstrated that both PDE3 and PDE4 activities were increased following forskolin or 8-bromo-cyclic AMP treatment, with a relatively larger effect observed on PDE3 activity. The increase in cyclic AMP PDE activity induced by forskolin or 8-bromo-cyclic AMP was inhibited by actinomycin D or cycloheximide, demonstrating that new mRNA synthesis and protein synthesis were required. An analogue of forskolin which does not activate adenylyl cyclase (1,9-dideoxyforskolin) or an analogue of cyclic GMP (8-bromo-cyclic GMP) did not affect total cyclic AMP PDE activity.
3. Incubation of VSMC with 8-bromo-cyclic AMP for 16 h caused a marked rightward shift in the concentration-response curves for both isoprenaline- and forskolin-mediated activation of adenylyl cyclase. A role for up-regulated cyclic AMP PDE activity in this reduced potency is supported by our observation that cyclic AMP PDE inhibitors (IBMX, cilostamide or Ro 20-1724) partially normalized the effects of isoprenaline or forskolin in treated cells to those in untreated cells.
4. We conclude that VSMC cyclic AMP PDE activity is increased following long-term elevation of cyclic AMP and that increases in PDE3 and PDE4 activities account for more than 70% of this effect. Furthermore, we conclude that increases in cyclic AMP PDE activity contribute to the reduced potency of isoprenaline or forskolin in treated VSMC. These results have implications for long-term use of cyclic AMP PDE inhibitors as therapeutic agents.

     

Possible role of cyclic AMP in the relaxation process of mammalian heart: Effects of dibutyryl cyclic AMP and theophylline on potassium contractures in cat papillary muscles

Summary The effect of dibutyryl cyclic AMP (DB-c-AMP; 3×10^–4–3×10^–3 M) on electrically induced twitch and high potassium (142.4 mM KCl)-induced contracture tension was studied in papillary muscles from normal and reserpinized cats ([Ca]₀ 1.8 mM; 25°C; pH 7.4). In both groups of preparations, the increase in twitch tension evoked by DB-c-AMP was accompanied by an abbreviation of the time to peak force and of relaxation time. In the same preparations, the high potassium contracture was markedly depressed by DB-c-AMP in a concentration-dependent manner. Similar results were obtained with the N⁶-monobutyryl derivative of cyclic AMP.The relaxing effects of the cyclic nucleotides on KCl contractures did not appear to be due to possible non-cyclic breakdown products: adenosine; 5-AMP and sodium butyrate did not attenuate contracture tension at concentrations up to 3×10^–3 M. The same applies to ATP and non-cyclic N⁶-2-O-3-O-tributyryl-adenosine-monophosphate. Theophylline (10^–2 M) was found to prolong the relaxation time of the twitch and to enhance the high KCl contracture.It is concluded that cyclic AMP may be capable of modulating the relaxation process of mammalian heart and that not only the positive inotropic but also the relaxant effects of catecholamines on myocardium described before may be mediated by the cyclic AMP system. The relaxant effects of cyclic AMP derivatives on intact myocardial preparations are attributed to a stimulation by cyclic AMP of the calcium transport of the sarcoplasmic reticulum (SR) and are interpreted to be a corollary to the effects of cyclic AMP previously obtained on isolated SR preparations.     

Effects of 8-S-benzyl cyclic AMP on mechanical characteristics and cyclic AMP levels of the canine ventricular myocardium

Effects of 8-S-benzyl cyclic AMP on the mechanical characteristics of cardiac contradiction and on teh intracellular cyclic AMP level were studied on the isolated canine right ventricular myocardium. For comparison, inotropic effects of dibutyryl and 8-bromo cyclic AMP were also investigated. 8-S-Benzyl cyclic AMP caused a concentration-dependent positive intropic action. The action of 8-S-benzyl cyclic AMP developed and reached a steady level much faster thant hat of dibutyryl cyclic AMP (dbcAMP), while the time courses of the disappearance of the actions following washout of the compounds did not differ. The dose-response curve for 8-S-benzyl cyclic AMP lay substantially in the same concentration range as did that for dbcAMP. The duration of a single contraction was decreased by 8-S-benzyl cyclic AMP in a concentration-dependent manner, chiefly because of shortening of the relaxation time. The cyclic AMP phosphodiesterase (PDE) inhibitor, 3-isobutyl-1-methylxanthine (IBMX) enhanced the positive inotropic action of 8-S-benzyl cyclic AMP; 8-bromo cyclic AMP caused a definite positive inotropic action only in the presence of IBMX. The intracellular cyclic AMP levels determined 5, 15 and 30 min after the administration of 1 mM 8-S-benzyl cyclic AMP were not significantly different from the corresponding control values determined in the paired muscle. The present results indicte that the changes in mechanical characteristics caused by 8-S-benzyl cyclic AMP were very similar to those induced by β-aderenoceptor stimulation. Since the intracellular cyclic AMP level was not changed after administration of 8-S-benzyl cyclic AMP, this compound is probably not metabolized intracellularly to cyclic AMP but acts directly as the original compound on the protein kinase. This compound provides an excellent pharmacological tool to mimic the effect of β-adrenoceptor stimulation on myocardial contractility.     

Intracellular cyclic AMP level and intestinal smooth muscle relaxation

The intracellular level of cyclic AMP in the taenia from the guinea pig caecum was significantly increased by isoprenaline in a dose-dependent manner. Papaverine also increased the intracellular cyclic AMP level, but a synthetic antispasmodic agent, Aspaminol (1,1-diphenyl-3-piperidinobutanol hydrochloride) did not modify it. The results suggest that there are two mechanisms for the so-called papaverine-like actions. The increase in the intracellular cyclic AMP concentration produced by isoprenaline was prevented by propranolol. In taenia depolarized by KCl, papaverine caused relaxation but isoprenaline did not. This difference can be explained by the observation that the intracellular cyclic AMP level in the KCl-depolarized taenia is increased by papaverine but not by isoprenaline.     

The effect of histamine on cyclic AMP levels in guinea-pig tracheal smooth muscle

Histamine (10(-5)--3 x 10(-4) M) increased the cylic AMP content of guinea-pig tracheal smooth muscle, the maximal effect being a 3-fold increase after 2-min incubation with 10(-4) histamine. Histamine-induced accumulation of cyclic AMP was not affected by propranolol or atropine, but was reduced by mepyramine. Aspirin and indomethacin abolished the cyclic AMP response to histamine and potentiated histamine-induced contractions of the smooth muscle. These results suggest that the elevation of cyclic AMP levels in response to histamine is mediated by prostaglandins, and represents an important negative feedback regulatory mechanism which modulates the contractile response of guinea-pig tracheal smooth muscle to histamine.     

Cyclic nucleotide phosphodiesterase inhibition and vascular smooth muscle relaxation

The inhibitory effects of theophylline, papaverine and isoquinoleine derivatives and the activating action of imidazole were compared on purified 3′,5′-cylcic nucleotide phosphodiesterase and on isolated rat aorta. A quantitative correlation was found between inhibition of the enzyme activity (K_i) and prevention of isolated rat aorta contractions elicited by barium.     

Negative chronotropic action of cyclic AMP on the fetal rat heart in organ culture.

Dibutyryl cyclic AMP and cyclic AMP in a DMSO medium were administered to the fetal rat heart in organ culture. Changes in heart rate were observed. Dibutyryl cyclic AMP in concentrations of 5 X 10(-5) M and 1 X 10(-4) M produced significant negative chronotropism. At a concentration of 1 X 10(-3) M, cyclic AMP in a 3% or 0.5% DMSO medium produced significant negative chronotropism. These data were compared with other studies. The negative chronotropic action of cyclic AMP and its dibutyryl derivative are evidence against the involvement of cyclic AMP in positive cardiac chronotropism.     

Stimulation of calcium uptake into aortic microsomes by cyclic AMP and cyclic AMP-dependent protein kinase

Summary Enhancement of calcium uptake into rabbit aortic microsomes was seen at a cyclic AMP concentration of 10^–6M in the presence of cyclic AMP-dependent protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37). Protein kinase alone also increased calcium uptake, but cyclic AMP alone was without effect. The results suggest that stimulation of calcium sequestration may be the mechanism of cyclic AMP involvement in vascular smooth muscle relaxation.     

Expression of cyclic GMP-inhibited phosphodiesterases 3A and 3B (PDE3A and PDE3B) in rat tissues: Differential subcellular localization and regulated expression by cyclic AMP

1. A combination of pharmacological, molecular biological and biochemical approaches were used to investigate the differential expression of two cyclic GMP-inhibited cyclic nucleotide phosphodiesterase genes (PDE3A and PDE3B) in the rat.
2. RT–PCR using PDE3A- or PDE3B-specific oligonucleotide primers allowed amplification of products encoding PDE3A (508 bp) or PDE3B (499 bp) sequences from several rat tissues (heart, aorta, liver, kidney and epididymal fat), from primary cultures of aortic vascular smooth muscle cells (VSMC) as well as from an SV40 large T-antigen immortalized aortic VSMC line.
3. Immunoblotting experiments with PDE3-selective antisera allowed the detection of both PDE3A and PDE3B immunoreactive proteins in several rat tissues, including tissues of the cardiovascular system, in primary cultures of aortic VSMC and in an SV40 large T-antigen immortalized aortic VSMC line. In all cases, PDE3A was expressed as a 120 kDa protein which was only detected in the cytosolic fraction. PDE3B was expressed as a 135 kDa protein and its expression was limited to the particulate fraction of all tissues and cells studied.
4. Prolonged incubation of cultured aortic VSMC with agents that increase VSMC cyclic AMP (forskolin or 8-bromo-cyclic AMP) produced marked time-dependent increases in PDE3 activity which correlated with increases in PDE3A and PDE3B RT–PCR signals and a marked increase in particulate PDE3 activity and PDE3B protein.
5. The physiological, pharmacological and biochemical implications of these findings are discussed based on previous reports of the effects of PDE3 inhibitors in the cardiovascular system and the relevance of our findings are presented in the context of the development of PDE3A and/or PDE3B-selective pharmacological agents.

     

Inhibition of intestinal secretion without reduction of cyclic AMP levels

Net water transport by the jejunum of anaesthetized rats was measured together with jejunal cyclic AMP content. Vasoactive intestinal peptide stimulated secretion and elevated the cyclic AMP level. Morphine (10 mg/kg s.c.) inhibited the stimulated secretion but failed to depress the cyclic AMP accumulation. It is suggested that cyclic AMP may not be the mediator of intestinal fluid secretion. Alternatively, morphine may block the secretory effect of cyclic AMP elevated by vasoactive intestinal peptide.     

Comparative actions of substances affecting cyclic AMP metabolism on the isometric contractions of fast and slow skeletal muscle during single-pulse and subtetanic stimulation

Increasing concentrations of isoprenaline (0.5-4 muM) produced a dose-dependent increase in both TD and dT/dtmax during direct single-pulse stimulation of hemidiaphragm of the rat. The same drug during the same type of stimulation produced an insignificant change in these parameters of the isometric contraction of the isolated guinea-pig soleus muscle. On the contrary, isoprenaline produced a dose-dependent decrease of the isometric contraction during subtetanic stimulation of the soleus muscle. Contrary to the results obtained on hemidiaphragm, there was no interaction between halothane and aminophylline on the soleus muscle. In the soleus muscle, aminophylline (0.3-3.2 mM) produced a dose-dependent increase in TD and dT/dtmax during single-pulse stimulation, whereas isoprenaline failed to do so under the same experimental conditions, in spite of the fact that both substances are activators of cyclic AMP system. The beta2-selective adrenoceptor agonist terbutaline acted in the same way as isoprenaline. During subtetanic stimulation aminophylline (0.3-3.2 mM) produced a dose-dependent decrease of both parameters of the isometric contraction of hemidiaphragm, which is opposite to the results obtained during single-pulse stimulation. It is concluded that various types of electrical stimulation can produce different responses in slow and fast-contracting muscles, depending on the fundamental biochemical differences of two types of muscle, but some of these responses are the same irrespective of the method of muscle activation.     

Negative functional effects of cyclic GMP are altered by cyclic AMP phosphodiesterases in rabbit cardiac myocytes

In this study, we tested the hypothesis that the negative functional effects of cyclic GMP on cardiac myocytes would be affected by the actions of cyclic GMP on cyclic AMP phosphodiesterases. Ventricular myocytes from eight rabbits were used to determine the functional and cyclic AMP changes caused by 10(-7), 10(-6), 10(-5) M 8-Bromo-cGMP alone and after the administration of 10(-6) M milrinone (cyclic GMP-inhibited cyclic AMP phosphodiesterase inhibitor) or 10(-6) M erythro-9-(2-Hydroxy-3-3-nonyl)adenine (EHNA, cyclic GMP-stimulated cyclic AMP phosphodiesterase inhibitor). 8-Br-cGMP dose-dependently reduced %shortening by 35+/-4% of baseline at 10(-5) M. This effect was significantly blunted by EHNA at all doses. The maximum rate of shortening was reduced by 31+/-3% by 10(-5) M 8-Br-cGMP. This effect of 8-Br-cGMP was significantly enhanced (42+/-4%) in the milrinone group. A similar pattern was observed in the maximum rate of relaxation data. Cyclic AMP levels were significantly increased from a baseline level of 4.0+/-0.8 pmol/10(5) myocytes by milrinone (+60%), EHNA (+61%) and 8-Br-cGMP (+47%). The combination of EHNA plus 8-Br-cGMP increased cyclic AMP levels significantly more that the combination of milrinone plus 8-Br-cGMP. Exogenous cyclic GMP reduces myocyte function, while raising cyclic AMP possibly through cyclic GMP-inhibited cyclic AMP phosphodiesterase effects. Blocking cyclic GMP-inhibited cyclic AMP phosphodiesterase enhances the functional effects cyclic GMP, while blocking cyclic GMP-stimulated cyclic AMP phosphodiesterase reduced these effects. The study demonstrated a functional interaction between cyclic GMP and cyclic AMP related to the cyclic GMP affected cyclic AMP phosphodiesterases.     

Effects of adenosine and related compounds on adenylate cyclase and cyclic AMP levels in smooth muscle.

The hypotheses were tested that the relaxant effect of adenosine and related compounds in the longitudinal muscle of the rabbit small intestine involves interaction with adenylate cyclase and/or the elevation of tissue cAMP levels. Adenylate cyclase was prepared by gentle homogenization of an isolated smooth muscle cell fraction obtained after collagenase digestion of longitudinal muscle strips. A number of analogs and derivatives of adenosine possessing a primary or secondary 6-amino group were found to inhibit the enzyme similarly to adenosine; however, there was no correlation between compounds known to relax the intact tissue and the existence, or the degree of, cyclase inhibition. Isolated muscle strips were exposed to adrenaline, isoprenaline, adenosine or ATP, at doses causing 30-60% relaxation, for 60 sec prior to sampling and analysis of cAMP content. While small increments in cAMP levels were found after administering adrenaline or isoprenaline, no change was found with adenosine in the absence or presence of theophylline of 1-methyl-3-isobutylxanthine. Neither adenylate cyclase inhibition nor changes in cAMP levels appear to be part of the mechanism of the smooth muscle relaxant action of adenosine or ATP.     

The effects of amrinone on contractility, Ca2+ uptake and cAMP in smooth muscle

Amrinone, a known positive inotropic agent in the heart, was found to cause a dose-dependent (10--100 micrograms/ml) inhibition of norepinephrine (NE) or high-K+-induced contractions of rabbit aorta. Amrinone also inhibited carbachol or high-K+-induced contractions of guinea-pig taenia coli. Neither total tissue 45Ca uptake nor the rate of 45Ca uptake induced by 80 mM K+ in rabbit aorta was altered by pretreatment with amrinone. On the other hand, a similar pretreatment with amrinone inhibited NE (10(-6) or 10(-5) M) induced tissue 45Ca uptake. Amrinone (100 micrograms/ml) caused about a 70% increase in cAMP concentration over resting levels. It is concluded that amrinone causes a nonspecific inhibition of smooth muscle contractility by acting probably at multiple sites to decrease the availability of Ca2+ required for activation. One or more of these mechanisms may involve cAMP.     

Papaverine, cyclic AMP and the dependence of the rat aorta on extracellular calcium.

The spasmolytic effects of papaverine and dibutyryl cyclic AMP (db-cAMP) were compared on isometric contractile responses induced by addition of increasing amounts of external calcium to K+-depolarized or noradrenaline-stimulated rat aorta strips. Papaverine at a concentration active on depolarized strips (3 times 10(-5) moles/1) reduced the maximal contraction (Emax) elicited by Ca2+ in these preparations, while db-cAMP did not. Contrary to what was observed on depolarized aortae, the degree of inhibition of noradrenaline-stimulated strips did not decrease with increasing extracellular calcium concentration (Ca)e. Both db-cAMP and papaverine at a concentration which did not depress Emax (5 times 10(-6) moles/1) potentiated the relaxing effect of high (Ca)e on contractions elicited by noradrenaline. In conclusion, cyclic AMP is probably implicated in the mode of action of papaverine on the noradrenaline-stimulated rat aorta. At a concentration active on depolarized strips, papaverine is also able to impair contractility directly.     

Prostaglandin E2 and the stimulation of rat gastric acid secretion by dibutyryl cyclic 3',5'-AMP

In the anaesthetised rat, i.v. injection of dibutyryl cyclic 3′,5′-AMP (dbcAMP) caused a dose-dependent increase in gastric acid secretion during pentagastrin, histamine or carachol stimulation. This was accompanied by a rise in gastric mucosal blood flow. During basal secretion, dbcAMP caused by a relatively small stimulation, atlhough this response was greatly enhanced by threshold doses of pentagastrin or histamine. Cyclic AMP (cAMP) did not stimulate acid secretion, but increased mucosal blood flow and decreased blood pressure. Theophylline had no consistent effect on acid secretion. Prostaglandin E₂, administered i.v. or perfused through the gastric lumen, reduced basal acid secretion but failed to inhibit the secretory response to dbcAMP. These results in the rat support the role of cAMP in gastric acid secretion and suggest that prostaglandins inhibit other secretogogues at a stage prior to cAMP.     

Ocular hypotensive,vasorelaxant and cyclic AMP intermediation activities of clozapine displaying antiglaucoma properties

Clozapine, an atypical antipsychotic agent with multiple receptor antagonist activities, was investigated in vivo and in vitro to discover its effects on intraocular pressure and blood flow, on the contractility of ciliary muscle and nonophthalmologic blood vessels, on calcium concentration in A7r5 smooth muscle cells, and on cyclic AMP intermediation. The adrenergic and muscarinic mechanisms involved in the above effects were also examined. In rabbits, clozapine (0.1, 0.25, and 0.5%) prolonged (IOP) recovery time and inhibited IOP response. Clozapine (0.1 and 0.25%) also produced a significant increase in ocular blood flow in this iris, ciliary, retina, and choroid at 30 to 180 min after drug administration. In isolated guinea pig thoracic aorta, clozapine relaxed phenylephrine (10 μM)- and KCl (75 mM)-induced contractions, the estimated IC₅₀ values being 20.4 ± 3.1 nM and 10.6 ± 1.8 μM, respectively. Clozapine (0.1–100 μM) inhibited phenylephrine (10 μM)-induced influx of Ca²⁺, the estimated IC₅₀ value being 0.4 ± 0.1 nM. In isolated pig eye ciliary muscles, clozapine (1.0–100 μM) inhibited carbachol (10 μM)-induced contractions, the estimated IC₅₀ value being 5.8 ± 1.2 μM. Clozapine (0.1–100 μM) increased cyclic AMP accumulation in pig's ciliary bodies, including ciliary process, ciliary muscle, and trabecular meshwork. Pretreatment with carbachol (100 μM) first decreased, then increased, clozapine-induced cyclic AMP accumulation. Studies of pretreatments with various muscarinic receptor antagonists at 100 μM revealed that pirenzepine significantly enhanced clozapine (100 μM)-induced cyclic AMP accumulations in trabecular meshwork, while 4-DAMP methiodide inhibited it in ciliary bodies, and methoctramine decreased it in ciliary process. The ocular hypotensive effects of clozapine may be mostly related to its muscarinic agonist/antagonist activities and associated cyclic AMP increasing activities, which lead to ciliary muscle relaxation and a possibly associated increase in uveoscleral outflow. Clozapine's ability to increase blood flow and relax vessels may be attributed to its ability to block α-adrenergic and decrease intracellular calcium. IOP is considered the major cause of glaucoma, and compounds which are capable of reducing IOP are considered useful for glaucoma treatment. Based on the results above, clozapine may potentially be important in the development of new antiglaucoma agents. Drug Dev. Res. 44:163–173, 1998. © 1998 Wiley-Liss, Inc.     

Effect of a novel inhibitor of cyclic AMP phosphodiesterase,E-1020, on cytosolic Ca++ level and contraction in vascular smooth muscle

Summary 1. The effects of a novel cyclic AMP phosphodiesterase inhibitor, E-1020 (1,2-dihydro-6-methyl-2-oxo5-(imidazo[1,2-a]pyridin-6-yl)-3-pyridine carbonitrile hydrochloride monohydrate), on cytosolic Ca⁺⁺ level ([Ca⁺⁺]_cyt) and muscle tension were examined in rat aorta using a fluorescent Ca⁺⁺ indicator, fura-2. 2. The sustained contraction induced by norepinephrine was more strongly inhibited by E-1020 than that induced by high K⁺. The contraction induced by a higher concentration of the stimulant was less sensitive to E-1020 than that due to a lower concentration. 3. Contractions induced by high K⁺ and norepinephrine followed the increase in [Ca⁺⁺]_cyt. E-1020 inhibited the increments in [Ca⁺⁺]_cyt and muscle tension. A Ca⁺⁺ channel blocker, verapamil, inhibited the norepinephrine-stimulated [Ca⁺⁺]_cyt more strongly than the contraction. E-1020 inhibited the verapamil-insensitive portion of the norepinephrine-stimulated [Ca⁺⁺]_cyt and contraction. 4. Norepinephrine transiently increased [Ca⁺⁺]_cyt and muscle tension in Ca⁺⁺-free solution. E-1020 inhibited the transient contraction but not the stimulated [Ca⁺⁺]_cyt. 5. E-1020 increased the cyclic AMP content of the muscle. The effects of E-1020 on cyclic AMP content and contraction were potentiated by an activator of adenylate cyclase, forskolin. 6. These results suggest that E-1020 inhibits the vascular contractility by the decrease in [Ca⁺⁺]_cyt and decrease in Ca⁺⁺ sensitivity of contractile elements. These effects may be mediated by the increase in cyclic AMP content of the muscle. Send offprint requests to M. Täjimi at the above address     

Positive dromotropic effect of dibutyryl cyclic adenosine 3',5'-monophosphate on the atrioventricular node.

The effect of atrioventricular (A-V) conduction of N6-2'-0-dibutyryl cyclic 3',5'-adenosine monophosphate (dibutyryl cyclic AMP) was investigated in comparison with those of norepinephrine, cyclic 3',5'-adenosine monophosphate (cyclic AMP), adenosine-5'-monophosphate (5'-AMP), and adenosine by the use of the isolated, blood-perfused A-V node preparation of the dog. Single injections of dibutyryl cyclic AMP (3-300 micronmol) and norepinephrine (0.1-1 nmol) into the posterior septal artery of the preparation (the upper part of the A-V node is mainly perfused through this artery) produced a dose-dependent decrease in the A-V conduction time. The time to the peak effect and the duration of the effect of dibutyryl cyclic AMP were much longer than with norepinephrine. The positive dromotropic effect of dibutyryl cyclic AMP was resistant to the beta-adrenoceptor blocking action of propranolol. Unlike dibutyryl cyclic AMP, cyclic AMP (above 30 nmol), 5'-AMP and adenosine (above 1 nmol) injected into the posterior septal artery prolonged the A-V conduction time in a dose-dependent manner. The results indicate that dibutyryl cyclic AMP has a mode of action on A-V nodal cells which differs distinctly from that of either norepinephrine or cyclic AMP, 5'-AMP, and adenosine.     

Dibutyryl cyclic AMP and adrenaline increase contractile force and 45Ca uptake in mammalian cardiac muscle

Summary The effects of dibutyryl cyclic AMP (DB-AMP; 10^–3M) and adrenaline (2.2×10^–6 M) on contractile force, ⁴⁵Ca uptake, and total myocardial Ca concentration were investigated in electrically driven left auricles isolated from rat hearts. The experiments were performed at an extracellular Ca concentration of 0.45 mM and at low frequency of stimulation (15 beats/min). ⁴⁵Ca exposure was 5 min. Under the conditions used, both drugs increased contractile force and enhanced ⁴⁵Ca uptake (expressed as relative specific activity) by about 30% (DB-AMP) and 40% (adrenaline), respectively. Thus, the results provide evidence that the effects of adrenaline on ⁴⁵Ca uptake in mammalian cardiac muscle can be mimicked by DB-AMP.