Influence of cyclization and acyl substitution on the inotropic effects of adenine nucleotides

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Dual effect of ATP and UTP on rat atria: which types of receptors are involved?

The effects of adenine compounds and UTP were examined in electrically driven rat left atria. ATP, ADP, AMP, adenosine and UTP caused a dual inotropic effect: first a rapid decrease in contractility, and second an increase in contractile tension. alpha,beta-Methylene ATP caused an increase in contractile tension only, whereas 2-methylthio-ATP only induced a negative inotropic effect, 1,3-Dipropyl-8-cyclopentylxanthine inhibited the negative effects of ATP and adenosine, whereas 3,7-dimethyl-1-propargylxanthine did not influence the effects of ATP. Suramin but not reactive blue 2 antagonized the positive inotropism induced by ATP and alpha,beta-methylene ATP. Suramin also abolished the positive inotropic effect induced by UTP. These results demonstrate that ATP may induce negative inotropism directly by an action on A1-adenosine receptors and positive inotropism by an action on P2x-purinoceptors. UTP induces a positive inotropic effect mediated by suramin-sensitive receptors.

Positive inotropic and chronotropic effects of 8-substituted derivatives of cyclic AMP and activation of protein kinase A

Inotropic and chronotropic effects of 8-substituted derivatives of cyclic AMP (8-SH, 8-SCH2C6H5, 8-N3, 8-SCH3, 8-Br, 8-N(CH3)2, 8-OCH3) were studied using guinea pig atrial and ventricular muscle preparations and correlated with the activation of the protein kinase A derived from the bovine myocardium. All the compounds produced positive inotropic and chronotropic effects. A good correlation was found between the chronotropic effect and the activation of the enzyme, while such a good correlation was not found between the enzyme activation and the positive inotropic effect. However, after treatment of the preparation with theophylline, the positive inotropic effects of some derivatives were potentiated to such a degree that the positive inotropic effects became well-correlated to the activation of the protein kinase. To elucidate the mechanism of the potentiation by theophylline, the effects of 8-phenyltheophylline and 3-isobutyl-1-methylxanthine on the positive inotropic effects of 8-Br and 8-OCH3 cyclic AMPs were studied. While 3-isobutyl-1-methylxanthine potentiated the effects of both compounds, 8-phenyltheophylline potentiated the effect of only 8-OCH3 cyclic AMP and only in the atria. These results suggest that the positive inotropic and chronotropic effects of 8-substituted cyclic AMP essentially due to the activation of the protein kinase A, with the hydrolysis of the compounds by phosphodiesterase and (in the atria) activation of adenosine R-receptor subserving the negative inotropic effect intervening.

The coexistence of adenosine A1 and A2 receptors in guinea-pig aorta

The effects of adenosine, 5'-(N-ethyl)carboxamidoadenosine (NECA), 2-chloroadenosine (2-CA), N6-cyclohexyladenosine (CHA) and N6(R-2-phenylisopropyl)-adenosine (R-PIA) on the tone of phenylephrine-constricted guinea-pig isolated aorta have been examined. For aortic relaxation the analogues exhibited the following rank order of potency: NECA greater than adenosine greater than 2-CA greater than R-PIA greater than CHA. This is consistent with previous reports that relaxation of this tissue is mediated by the adenosine A2 receptor. An unexpected finding was that R-PIA, 2-CA and CHA all induced contractions at concentrations lower than were required for relaxation, giving a biphasic dose-response curve. Neither NECA nor adenosine contracted the aorta. This is consistent with activation of vascular A1 receptors. An A1-selective concentration of the antagonist 1,3-dipropyl-8-cyclopentyl xanthine abolished the contraction elicited by R-PIA in the guinea-pig aorta. This further suggests that the contraction is mediated by A1 receptors.

Dual control by ATP and acetylcholine of inwardly rectifying K+ channels in bovine atrial cells

Whole-cell and single-channel recordings were used to study an ionic current activated by extracellular adenosine 5'-triphosphate (ATP) applied to calf atrial cells. ATP (Kd approximately 10 microM) elicited an inwardly rectifying current that reversed near EK and was blocked by external Cs+ (10 mM). Under identical conditions, adenosine had no effect. Cell-attached patch recordings revealed an ATP-activated channel with a slope conductance of about 30 pS. At both the whole-cell and single-channel levels, the channels activated by ATP seemed nearly identical to the potassium channels activated by acetylcholine (ACh) in the same cells. However, the effects of ATP were not affected by atropine, suggesting that ATP does not interact with the same receptors as ACh. In some cells, whole-cell currents of similar magnitude were activated by ACh alone, ATP alone, or ACh and ATP applied together. These results suggest that calf atrial cells possess a population of inwardly rectifying potassium channels that are controlled jointly by two populations of receptors selective for ACh and ATP.

Pronounced cholinergic but only moderate purinergic effects in isolated atrial and ventricular heart muscle from cats

1. The effects of cholinergic and purinergic stimulation on action potential, force of contraction and 86Rb efflux were investigated in cat atrial and/or ventricular heart muscle. 2. Acetylcholine and carbachol exerted a concentration-dependent negative inotropic effect in cat atrial heart muscle. Carbachol 10 mumol l-1 completely abolished the force of contraction and increased the rate constant of 86Rb efflux 2-3 fold, whereas the action potential duration was shortened to about 1/10 of its length under control conditions. 3. The effects of acetylcholine and carbachol in cat atrial heart muscle were mimicked, qualitatively, by adenosine and its analogues 5'-(N-ethyl)-carboxamido-adenosine (NECA) and (-)-N6-(R-phenyl-isopropyl)-adenosine (R-PIA). Maximal purinergic effects, however, amounted to about 15-50% in comparison to those of cholinergic stimulation. 4. In cat ventricular heart muscle, cholinergic or purinergic stimulation had no significant effects on the force of contraction in the absence of a cyclic AMP-dependent positive inotropic effect. Carbachol antagonized the positive inotropic effect elicited by either 3-isobutyl-1-methylxanthine, isoprenaline or cyclic 8-(4-chlorphenylthio)adenosine-3':5'-monophosphate; NECA and R-PIA were less effective. The inhibition by carbachol of the effects of isoprenaline was not related to a change in the rate constant of 86Rb efflux. 5. It is concluded that the effects of cholinoceptor and purinoceptor agonists in the cat heart involve a change in the potassium conductance in the atrium, whereas the effects in the ventricle may be related to changes of intracellular cyclic AMP levels. It seems reasonable to assume that, in comparison to cholinergic stimulation, a low density of purinoceptors in the cat heart is responsible for the relatively weak effects of adenosine agonists in this species.

Inotropic and chronotropic effects of N6-substituted derivatives of cyclic AMP as assessed in guinea-pig isolated right atria and papillary muscle

1. The inotropic and chronotropic actions of N6-substituted adenosine 3':5'-cyclic monophosphate (cyclic AMP) derivatives (N6-R cyclic AMPs) were studied in guinea-pig isolated right atrial preparations and in the papillary muscle preparations from guinea-pig right ventricle. 2. All the N6-R cyclic AMPs except N6-C14H29 produced positive inotropic effects in papillary muscle. The C5H11, C6H13, C7H15, C8H17 and C9H19 compounds were the most potent as inotropic agents, the potency being lower with compounds having longer or shorter N6-side chains than these. 3. In right atria N6-C2H5-C7H15 cyclic AMPs produced negative chronotropic effects. However, after treatment of the preparations with 8-phenyltheophylline the negative chronotropic effects were either much attenuated or abolished, indicating the involvement of adenosine receptors. 4. All the N6-R cyclic AMPs except N6-C14H29 were more potent activators of bovine myocardial protein kinase than cyclic AMP. The partition coefficients between octanol and an aqueous phase of N6-R cyclic AMPs became greater as the numbers of carbon atoms increased, and there appeared to be a relationship between partition coefficient and inotropic potency. It was concluded that membrane penetrativeness rather than potency as activators of protein kinase determined the potencies of N6-R cyclic AMPs as positive inotropic agents. 5. Derivatives such as N6-C7H15 cyclic AMP, which have positive inotropic activity without any marked negative chronotropic effect, may be useful as cardiotonic agents in heart failure.

Negative inotropic effects of ATP on the isometric contractions of isolated rat heart muscle

The effects of ATP on the isometric contractions of isolated rat left ventricular papillary muscle were studied. Exogenously administered ATP had an immediate onset, an abrupt response, progressive recovery and produced dose-related depression in the peak developed tension, maximum rate of tension development and relaxation, which were statistically significant. There were no significant changes in the resting tension, time to peak tension and relaxation time, except for a significantly prolonged relaxation time at the highest concentration of ATP. In the studies of interactions of ATP and either epinephrine or Ca(++), we observed that ATP seemed to interfere with the inotropic effect of epinephrine, while Ca(++) antagonized the negative inotropic action of ATP. We conclude that the site of negative inotropic action of ATP is most likely on the cell membrane, where ATP interferes with Ca(++) flux, and that ATP interferes with the positive inotropic action of epinephrine.

[Cardiac effects of adenosine. Mechanism of action, pathophysiologic and clinical significance]

Adenosine has a negative inotropic effect in cardiac atrial preparations ("direct" negative inotropic effect). This effect is probably due to an activation of a potassium outward current which shortens the action potential duration and hence reduces the force of contraction. A pertussis toxin-sensitive N-protein is involved in the signal transduction from the adenosine receptor to atrial potassium channels. In ventricular cardiac preparations adenosine has no negative or even a weak positive inotropic effect, but it reduces the force of contraction in the presence of cAMP-increasing agents such as isoprenaline ("indirect" negative intropic effect). This effect is due to an inhibition of the slow Ca2+ inward current which has previously been enhanced by an increase in the cellular cAMP content. This "indirect" negative inotropic effect of adenosine is also present in the human heart. Since increased amounts of adenosine are released during cardiac stimulation via beta-adrenoceptors, the "indirect" effect might protect the heart against excessive stimulation by catecholamines. In addition, adenosine has negative chronotropic actions and prolongs AV conduction by an activation of potassium channels or an inhibition of the slow Ca2+ inward current (AV node). Cardiac bradyarrhythmias in hypoxia have been attributed to an increased formation and release of adenosine. Furthermore, adenosine has been shown to terminate supraventricular tachycardias involving the AV node. Since it has a very short duration of action it might prove safe and hence advantageous to conventional therapy in the treatment of supraventricular tachycardias.

Inotropic and electrophysiological effects of 8-substituted cyclic AMP analogues on guinea-pig papillary muscle

The inotropic potencies of 8-substituted cyclic AMP analogues, applied as sodium salts and in form of benzyl esters, were determined in isolated guinea-pig papillary muscles contracting isometrically at a frequency of 0.2 Hz. Half-maximally effective concentrations, EC50, for the positive inotropic effect of 8-substituted cyclic AMP (sodium salt) increased in the order 8-(4-chloro-phenyl)thio-cyclic AMP, 8-tertiary-butyl-thio-cyclic AMP, 8-benzyl-seleno-cyclic AMP, 8-benzyl-thio-cyclic AMP, 8-methyl-thio-cyclic AMP, 8-bromo-cyclic AMP. Neutralization of the phosphate hydroxyl residue of 8-substituted cyclic AMP by a benzyl group yielded cyclic AMP benzyl esters (cAMP-O-Bn) which were 30 to 100 times more potent than the respective cyclic AMP salts. Cyclic AMP derivatives with a 8-(4-chloro-phenyl)thio- or a 8-tertiary butyl-thio substituent showed comparatively high inotropic potencies. The intrinsic activity was uniformely the same for all 8-substituted cyclic AMP derivatives and equalled that of isoprenaline. As measured by octanol/water partitioning (log P), the increase in lipophilicity of 8-substituted cyclic AMP by esterification with a benzyl group was 7000-fold for 8-bromo-cyclic AMP, 5000-fold for 8-methyl-thio-cyclic AMP, and approximately 1000-fold for the other derivatives. Within the series of benzyl esters, differences in lipophilicity were small. The positive inotropic effect of 8-substituted cyclic AMP analogues was accompanied by a shortening of contraction duration, mainly due to an abbreviation of relaxation time.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for adenosine receptor-mediated isoprenaline-antagonistic effects of the adenosine analogs PIA and NECA on force of contraction in guinea-pig atrial and ventricular cardiac preparations

The effects of the adenosine agonists (-)-N6-phenylisopropyladenosine (PIA) and 5'-N-ethylcarboxamideadenosine (NECA) on force of contraction, adenylate cyclase activity and normal as well as slow action potentials were studied in guinea-pig isolated atrial (left auricles) and ventricular preparations (papillary muscles). In auricles PIA and NECA exerted concentration-dependent negative inotropic effects with similar potencies (mean EC50:0.05 mumol l-1 for PIA and 0.03 mumol l-1 for NECA). Similar results were obtained in the presence of isoprenaline. In papillary muscles PIA and NECA alone had no effect on force of contraction but produced negative inotropic effects in the presence of isoprenaline (mean EC50:0.19 mumol l-1 for PIA and 0.10 mumol l-1 for NECA). In both preparations, the negative inotropic effects of PIA and NECA in the presence of isoprenaline were antagonized by the adenosine receptor antagonist 8-phenyltheophylline. In both preparations, PIA and NECA did not affect adenylate cyclase activity, both in the absence and presence of isoprenaline. In auricles the negative inotropic effects of both nucleosides were accompanied by a shortening of the action potential. This effect was also observed in the presence of isoprenaline. In papillary muscles the adenosine analogs did not detectably alter the shape of the normal action potential. Ca2+-dependent slow action potentials elicited in potassium-depolarized preparations also remained unaltered in the presence of PIA or NECA alone. However, the isoprenaline-induced enhancement of the maximal rate of depolarization of slow action potentials was attenuated by PIA or NECA.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine and adenosine analogs inhibit phosphodiesterase activity in the heart

Adenosine and its analogs (-)-N6-phenylisopropyladenosine and 5'-N-ethylcarboxamideadenosine inhibit cAMP and cGMP phosphodiesterase activity in guinea-pig atrial and ventricular preparations at concentrations of 100 mumol l-1 and higher. These effects are probably unrelated to the inotropic effects of these substances. However, inhibition of cAMP breakdown may compensate for the adenosine-induced inhibition of adenylate cyclase and may thus at least partially explain why with this drug no changes in cAMP or cGMP content have previously been observed in intact cardiac tissue.

Phosphorylation of C-protein in intact amphibian cardiac muscle. Correlation between 32P incorporation and twitch relaxation

The molecular mechanisms by which neurotransmitters modulate the force of contraction of cardiac muscle are incompletely understood. Hartzell and Titus (1982. J. Biol. Chem. 257:2111-2120) have recently reported that C-protein, an integral component of the thick filament, is reversibly phosphorylated in response to ionotropic agents. In this communication, C-protein phosphorylation (as measured by isotopic labeling with 32P) is correlated with changes in the rate of relaxation of twitch tension. On the average, isoproterenol simultaneously increases peak systolic tension twofold, decreases twitch relaxation time from a control value of approximately 450 to approximately 300 ms, and increases C-protein phosphorylation two- to threefold, with a maximum effect occurring less than 60 s after addition of 1 microM isoproterenol. Carbamylcholine, in contrast, decreases peak systolic tension more rapidly than it affects relaxation or C-protein phosphorylation. The maximum decrease in peak tension (60%) occurs within 1 min of addition of 0.5 microM carbamylcholine, but relaxation time increases slowly to 800 ms over approximately 6 min. The increase in relaxation time correlates well with the decrease in 32P incorporation into C-protein (r = 0.94). Changing beat frequency between 0.2 and 1/s has no effect on C-protein phosphorylation but does alter relaxation time (relaxation time decreases approximately 100 ms when beat frequency is changed from 0.5 to 1/s) and thus alters the quantitative relationship between C-protein phosphorylation and relaxation rate. These results suggest that two separate processes affect relaxation. It is proposed that the level of C-protein phosphorylation sets the boundaries over which relaxation is regulated by a second process that is dependent upon beat frequency and probably involves changes in intracellular Ca.

The effect of adenyl compounds on the rat heart

1 The effects of adenyl compounds were examined on the rat atrium and ventricle. 2 Adenosine, adenosine 5'-monophosphate, adenosine 5'-diphosphate, adenosine 5'-triphosphate (ATP) and beta, gamma-methylene ATP (APPCP) produced negative inotropic effects on the rat atrium. These inhibitory effects were antagonized by 8-phenyltheophylline (8-PT), a P1-purinoceptor antagonist, and potentiated by erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA), an adenosine deaminase inhibitor, but were not affected by dipyridamole, which blocks adenosine uptake. 3 alpha, beta-Methylene ATP (APCPP), which is resistant to degradation, did not produce a similar inhibitory response in the rat atrium. 4 Adenosine did not affect the basal developed force of the rat ventricle nor did it affect the beta-adrenoceptor-mediated positive inotropic effect. 5 Very high concentrations of ATP (0.1-3 mM) produced negative inotropic effects in the rat ventricle. APPCP (0.3 mM) also produced an inhibitory response, which was significantly smaller than that produced by ATP (0.3 mM). APCPP elicited excitation rather than the expected inhibitory response. 6 The inhibitory effect of ATP in the rat ventricle was not blocked by indomethacin, 8-PT or atropine. 7 It is possible that the action of ATP in the rat ventricle is mediated via P2-purinoceptors and that the lack of inhibitory action of APCPP on the rat ventricle is due to the difference in structural conformation between ATP and APCPP. 8 It seems likely that inhibitory P1-purinoceptors are present in the rat atrium and that the inhibitory responses produced by ATP in the rat ventricle may be mediated via P2-purinoceptors.

Mechanical response of rat myocardium to dibutyryl cyclic AMP in relation to effects of alpha-and beta-adrenoceptor stimulators

Dibutyryl cyclic AMP, and alpha- and beta-adrenoceptor stimulators are all able to elicit inotropic effects. alpha- and beta-Adrenoceptor stimulation are known to change each myocardial contraction-relaxation cycle differently. In order to elucidate the myocardial function of cyclic AMP the effects of dibutyryl cyclic AMP on the contraction-relaxation cycle of isolated rat heart papillary muscle were examined and compared to the effects of alpha-and beta-adrenoceptor stimulation, respectively. Dibutyryl cyclic AMP (in the presence of propranolol) increased developed tension (Tmax) by 18%, rate of tension rise (T'max) by 46%, rate of tension fall (T'min) by 62% and onset-rate of relaxation (T"min) by 136%. These changes in the contraction-relaxation cycle were strikingly similar to those produced by isoprenaline (beta-adrenoceptor stimulation). The response to dibutyryl cyclic AMP, however, developed much more slowly than did the response to isoprenaline. The latter effect was associated with cyclic AMP elevation in a way indicating a trigger function for cyclic AMP. The alpha-adrenoceptor stimulation (by phenylephrine combined with propranolol), however, increased measures both for contraction and for relaxation by about the same degree, and the effects occurred without changes of cyclic AMP contents. Phenylephrine alone (combined alpha-and beta-adrenoceptor stimulation) elicited a substantial cyclic AMP elevation but gave mechanical effects only slightly different from the pure alpha-adrenergic response. Thus cyclic AMP effects did not seem to be fully expressed in this case. As a whole, the results indicate that the effects of both dibutyryl cyclic AMP and of isoprenaline are mediated by the cyclic AMP-system while alpha-adrenoceptor stimulation involves other mechanisms.

Inotropic responses of the frog ventricle to adenosine triphosphate and related changes in endogenous cyclic nucleotides

1. A study has been made of a well documented but poorly understood response of the isolated frog ventricle to treatment with exogenous adenosine 5' triphosphate (ATP). Measurements of membrane potential, isometric twitch tension and levels of endogenous 3',5'-cyclic nucleotides have been made at various times during the ATP-induced response. 2. ATP elicits a characteristic triphasic response, which comprises an initial, abrupt increase in contractility, rising to a maximum within a few beats (first phase); followed by a period when the twitch amplitude falls, sometimes to below the control level (second phase); and superceded by a more slowly developing and longer-lasting increase in contractile force (third phase). The response is unaffected by atropine, propranolol or phentolamine. However, the prostaglandin synthetase inhibitor indomethacin depresses the first phase and entirely suppresses the third phase. 3. The inotropic effects of ATP are accompanied by changes in the shape of the action potential. These effects are dose-related. The duration of the action potential (D-30mV) and its positive overshoot (O) are increased during all phases of the response, for [ATP]o's up to 10(-5) M. However, at higher [ATP]o's, D-30mV and O ar both reduced during the second phase (but not the first or third phase), when isometric twitch tension is also depressed. The relationship between action potential duration and twitch tension (P) for different [ATP]o's is linear for all three phases of the response, but the slopes of the curves (delta P/delta D) are markedly different, indicating that the sensitivity of the contractile system to membrane depolarization is not constant, but varies continuously throughout the response. 4. ATP has a potent stimulatory effect on the metabolism of endogenous 3',5'-cyclic nucleotides. The time courses of the changes in adenosine 3','5-cyclic monophosphate (3',5'-cyclic AMP) and guanosine 3',5'-cyclic monophosphate (3',5'-cyclic GMP) are complex, but the accompanying change in isometric twitch tension is paralleled closely by corresponding changes in the ratio 3',5'cyclic AMP:3',5'-cyclic GMP. 5. It is concluded that ATP exerts a dual effect on the ventricle and that the contractile response is regulated by changes in the metabolism of 3',5'-cyclic nucleotides. The effects of indomethacin indicate a possible involvement of prostaglandins in mediating the ATP response. It is suggested that the initial effect of ATP on the ventricle is to increase the permeability of the fibres to Ca2+. 6. The relationship between 3',5' cyclic nucleotide levels and ventricular contractility is discussed. It is postulated that the antagonistic effects of 3',5'-cyclic AMP and 3',5'-cyclic GMP are expressed at the level of certain phosphoproteins which regulate both the availability of Ca2+ and the sensitivity of the contractile proteins to Ca2+.

Hypothermia-induced potentiation of histamine H2-receptor-mediated relaxation and cyclic AMP increase in the isolated mesenteric artery of the rabbit

On helically cut strips of the rabbit's mesenteric artery, a temperature decrease from 42 degrees C to 25 degrees C reduced the contractile responses to histamine. Metiamide shifted the dose-response curve of the histamine-induced contraction towards higher values at 25 degrees C, but not at 42 degrees C. Furthermore, on arterial strips contracted by phenylephrine histamine evoked a dose-dependent relaxation at 25 degrees C whereas at 42 degrees C only slight relaxing responses to histamine occurred. Metiamide was capable of preventing the relaxation induced by histamine in a competitive manner. At 25 degrees C the relaxation as produced by histamine was accompanied by increases in cyclic AMP which occurred prior to the relaxing effects. Metiamide abolished the cyclic AMP increase in response to histamine. At 42 degrees C histamine was unable to elevate the cyclic AMP content. Thus, it is concluded that a cyclic AMP-mediated relaxation due to stimulation of H2-receptors counteracts the histamine-induced contraction and reduces the contractile responses to histamine at low temperatures. In addition, clear-cut evidence exists from the present study that also on artery smooth muscle the H2-receptor-mediated responses are closely associated to cyclic AMP.

Effect of theophylline on myocardial adenylate cyclase activity

Theophylline (0.001-10.0 mM) did not increase but rather decrased adenylate cyclase activity (AC) of guinea-pig auricles. Isoprenaline (1-100 microgram) and sodium fluoride (0.3-10.0 mM) stimulated AC in a concentration-dependent manner.

Electrophysiological responses of cardiac muscle to isoproterenol covalently linked to glass beads

We investigated the effects of isoproterenol aryl glass beads on the electrical properties of cardiac muscle and related these to our previous results concerning biochemical and contractile effects (Ingebretsen et al., Circ, Rs., 40: 474-484, 1977). Beads (10-15) were placed near one end to guinea pig papillary muscles mounted horizontally in a bath perfused with Krebs-Henseleit solution at 30 degrees C and stimulated at 0.2 Hz. The beads produced increased tension and elevation and slight lengthening of the plateau potential when [k+]o = 3.8 mM. After depolarization to a resting potential of -49 mV with [K+]o = 22 mM, isoproterenol beads restored contraction to a comparable extent as occurred with 10(-8) M soluble drug. During field stimulation, action potentials were initiated at the site of bead application and spread decrementally. When beads were placed distal to the site of point stimulation, virtually no excitation could be obtained from cells in the vicinity of the beads. When they were placed close to the stimulating electrode, the beads increased excitability and typical slow action potentials spread to the other end of the muscle. These potentials had the characteristics associated with the slow inward Ca2+ current. The slow channel blocker, D-600, blocked responses to isoproterenol beads. Tetrodotoxin caused responses similar to those obtained with K+ depolarization. The beads probably act by stimulating only a small fraction of the papillary muscle catecholamine receptors. Spread of action potentials from these sites and propagated tension depend on Ca2+ influx, but the nature of an intermediate messenger involved in the propagation of contractions is unknown.

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

The effect of dibutyryl cyclic AMP (DB-c-AMP; 3 X 10(-4)-3 X 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 degrees 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 X 10(-3) M. The same applies to ATP and non-cyclic N6-2'-0-3'-0-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.

Stimulation of cartilage macromolecule synthesis by adenosine 3',5'-monophosphate

The role of cyclic AMP in the regulation of cartilage macromolecule synthesis in vitro was studied in pelvic cartilage from 10-12 day chick embryos. Incubation of cartilages in medium containing 0.5 mM cyclic AMP resulted in a 30% inhibition of 35SO4-2, [3H]leucine and [3H]uridine incorporation into proteoglycan, total protein and RNA, respectively. Higher concentrations of cyclic AMP had no greater effects. In contrast, butyrylated cyclic AMP derivatives (0.5-5.0 mM) added to the incubation medium stimulated (50-100%) the incorporation of these radiolabeled precursors into cartilage macromolecules. Theophylline, in concentrations (0.1-0.5 mM) which raise intracellular cyclic AMP, also increases the incorporation of radiolabeled precursors into macromolecules. The data indicate that exogenous cyclic AMP and butyrylated cyclic AMP derivatives have paradoxical effects on cartilage macromolecule synthesis. Butyrylated cyclic AMP derivatives, not exogenous cyclic AMP, mimic the effects of intracellular cyclic AMP. Incubation of embryonic chicken cartilage with exogenous cyclic AMP results in the extracellular degradation of the cyclic AMP to adenosine. Adenosine (0.125 mM) inhibits precursor incorporation into cartilage macromolecules. The metabolism of exogenous cyclic AMP generates sufficient adenosine to account for the observed inhibitory effects of exogenous cyclic AMP on cartilage macromolecule synthesis. Butyrylated cyclic AMP derivatives are not degraded during incubation with cartilage. The data indicate that cartilage is a tissue in which the effect of cyclic AMP is to stimulate anabolic processes.

Localization of beta adrenergic receptors, and effects of noradrenaline and cyclic nucleotides on action potentials, ionic currents and tension in mammalian cardiac muscle

1. Isoprenaline and noradrenaline were applied iontophoretically to cardiac Purkinje fibres. Intracellular application of the drugs had no effect, while extracellular application of the same amounts of charge caused acceleration of pace-maker activity and a shift of the plateau level of the action potential. These results indicate that beta-adrenergic receptors are located at the outside of the cardiac cell membrane.2. A systematic comparison of the effects of cyclic AMP derivatives and noradrenaline on action potentials and isometric tension of ventricular myocardial preparations showed that the nucleotides and the catecholamine increase the plateau height and the duration of the action potential and also increase tension. However, there are quantitative differences in the action of these drugs.3. Cyclic AMP derivatives and noradrenaline increase the slow inward current, I(Ca), in ventricular myocardial preparations. Voltage clamp analysis of I(Ca) showed that the kinetic parameters of this membrane current are not affected by these drugs. However, the membrane conductance to Ca ions is greatly increased by noradrenaline and to a smaller extent by dibutyryl cyclic AMP.4. Concentration-response relations of the membrane effects of noradrenaline on plateau height of the action potential and on I(Ca) could be fitted by the same theoretical log concentration-response curve. The Hill plot of this concentration-response curve had a slope of 2. The half maximal response occurred at 5 x 10(-7)M.5. The results are compared with other membrane effects of catecholamines and cyclic nucleotides in cardiac muscle. The effects on I(Ca) are related to the positive inotropic effect of the drugs.